Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/15/19

Area Forecast Discussion
National Weather Service Des Moines IA
536 PM CST Sat Dec 14 2019 .DISCUSSION.../Tonight through Saturday/ Issued at 341 PM CST Sat Dec 14 2019 ...Cold Temperatures Likely North Tonight... ...Light Snow Event Possible in Southern Iowa Sunday PM-Monday... ...Quiet and Warmer Weather Upcoming Week... Cold Temperatures likely north tonight: Key Points: High confidence that wind speeds will diminish through the night tonight, eventually reaching less than 5 mph after midnight. High confidence in air temperatures within a few degrees +/- of minus 5 degrees across the northern two tiers of Iowa counties. Ultimately, wind chill values should bottom out in the minus 15 to minus 20 degree range around midnight. Through noon Sunday, wind chill values will be below zero north of Highway 20, so bundle up! Scientific Discussion: All models, including hi-res models, have done awful today with handling cloud cover across Iowa as skies have tried to clear from northwest to southeast throughout Saturday. In advance of the Sunday PM-Monday system, expecting a few hours of clearing after sunset this evening. Subsequently, temperatures will plummet fairly rapidly (in particular across northern Iowa). Top- down cloud increases expected through the overnight (after midnight) will steady the rate of temperature decline. With winds being the lightest near sunrise, lowest wind chill values will likely occur closer to midnight tonight as winds should still be near 10 mph or slightly under at that time. Given diminishing winds and forecast wind chill values, Wind Chill Advisory likely not warranted. Sunday PM-Monday Snowfall Potential: Key Points: Forecast has trended to push highest snowfall totals south, into Missouri. High confidence that winds should be in the 5 mph or so range during any snow... which will mitigate visibility reductions during periods of snow. Additionally, with snow arriving late Sunday morning and lasting through Sunday night... snowfall likely to be spread out over the course of 24 hours or so. With forecast totals generally in the trace to 2" range, the snowfall rates will be very light, mitigating impacts as well. Ultimately, feel the minimal impacts do not warrant Winder Weather Advisory Criteria. Scientific Discussion: As hi-res models have come into play, southward trend in precip definitely manifested. Have leaned in particular towards the HRRR output. Canadian a high outlier. GFS interestingly has precip spreading across most of central Iowa, nearing as far north as Highway 20/30. Have bought into this slightly... interrogation of the 285K sfc does pick up on widespread isentropic adiabatic ascent processes and some intermittent pockets slight moisture flux convergence near the DGZ. Large negative for accumulations is the sub-cloud dry air. Forecast soundings have almost 1 km of "dry" air to punch through before reaching the sfc... which is forecast to sport a dwpt depression of +10F at times. By the time lower stratus arrives on Sunday evening, dry air aloft from trailing high pressure already being advected down into Iowa... nixing any semblance of saturation through the DGZ. If anything, current forecast snowfall may be too high. There is a non-zero chance next forecast update will remove snowfall accumulation >1" or so from south-central Iowa... but for now, did not want to make too drastic of a change in case forecast track shifted north. Quiet, Warmer Weather Next Week: Huge thermal ridge building behind this system as sfc high slides into the central Midwest through early next week. By mid- week, long- range models coming into agreement with crashing this ridge over Iowa... potentially bringing 850mb temps near +7C to Iowa by 12z Thu. Accompanied with sunshine, this would support highs into the 40s across central to southern Iowa. If this current pattern holds, may even be looking at highs in the 50s. Tuesday and beyond has no widespread, significant precipitation forecast. && .AVIATION.../For the 00Z TAFS through 00Z Sunday evening/ Issued at 533 PM CST Sat Dec 14 2019 VFR conditions prevail for much of central Iowa over the next 24 hours. Isolated pocket of stratus remains over south central Iowa, just south of KDSM. Satellite imagery shows this area of stratus slowly drifting south, and may impact KOTM this evening with MVFR level cigs. Snow spreads across the region tomorrow but much of it should stay south of the state. -SN is possible at KOTM and KDSM after 18z, but confidence is not high enough to include in the current TAF. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION...Kotenberg AVIATION...Martin
becomes more zonal (see short-term forecast discussion below).

Big changes will arrive early Monday morning. A strong shortwave trough, which is currently located about 150 miles off the Oregon coast will dive southeast across the western CONUS and into the Great Plains by late Sunday night. This shortwave trough will help drive a surface cold front through our region. The cold front should reach our northwestern counties by late Sunday evening. Winds may become a bit northerly a little ahead of the front, as a pre-frontal trough looks like it may become established during the afternoon hours. The true cold front will be marked by a stronger push of northerly winds, and colder temperatures. High temperatures on Monday are likely to remain in the 40s northwest of the Metroplex, in the 50s across DFW and into Central Texas, and in the 60s in our far southeastern counties (where the cold front will not arrive until later). Strong ascent ahead of the cold front, supported by differential cyclonic vorticity advection will allow for some showers and a few thunderstorms to develop. Rain chances will increase the farther south and east one goes. At this time, severe weather looks unlikely, and rainfall totals should remain light. Monday night will be cold with most of our CWA likely to experience freezing temperatures (the exception being our southeastern two tiers of counties). Tuesday night will be even colder as clear skies and light winds should allow for excellent radiational cooling. The entire CWA will see below freezing temperatures on Tuesday night, with temperatures in the low to mid 20s possible in our western counties where it will be a little drier and the elevation is slightly higher. For the second half of the week, weak upper-level ridging will return, bringing temperatures back up to seasonal normals (mid to upper 50s) for Wednesday and through the end of the week. Another upper-level shortwave trough will approach the region on Friday, but a lack of moisture will keep rain chances low, and confined to areas east of I-35. Looking way ahead to Christmas, the suite of deterministic and ensemble guidance seems to be in good agreement that a deep trough or upper-level low will be in place over the western U.S. While this pattern could result in some unsettled weather for the Southern Plains and into the Southwest, this is not a pattern that typically results in substantial cold air deliveries into our region. It is far too early to get into any kind of details right now, and it should be noted that this type of pattern typically has low predictability this far out (even if the models agree on the larger scale pattern). There`s not a whole lot to say about the weather as we get close to Christmas other than there is great uncertainty, and we probably won`t get a decent handle on even the major details for another 3-5 days. 37 && .SHORT TERM... /Issued 1213 PM CST Sat Dec 14 2019/ /This afternoon through Sunday afternoon/ Zonal flow aloft will continue to bring in moisture from the Pacific today, resulting in scattered high clouds. There will still be plenty of sun across much of the region, allowing afternoon temperatures to warm into the 60s and lower 70s. The only exception will be along the Red River where cloud cover will be the thickest and most persistent, keeping temperatures in the upper 50s. Surface high pressure will build southeast out of the Northern Plains today bringing a quick reinforcement of dry air followed by the return of low level moisture tonight once the surface ridge axis moves farther east and surface low pressure deepens across the southern High Plains. The surge of moisture over the relatively cool ground will result in the development of advection fog and even some drizzle. The fog could be dense at times, especially south of I-20 and east of I-35. Although low level warm advection will increase through the day Sunday, some morning cloud cover and fog should slow daytime heating a bit, especially in the southeast zones. We also expect a cold front to arrive across the northwest zones by late Sunday afternoon which will also noticeably lower temperatures. Afternoon highs Sunday will range from the mid 60s in the north to near 80 across the southwest zones where daytime heating will get a boost from southwest winds/adiabatic warming. 79 && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 48 74 42 52 32 / 0 0 5 10 0 Waco 48 78 46 57 32 / 0 0 10 20 0 Paris 42 69 45 52 29 / 0 5 40 40 0 Denton 45 73 40 49 29 / 0 0 5 5 0 McKinney 45 72 42 51 30 / 0 0 10 20 0 Dallas 48 75 43 52 32 / 0 0 5 10 0 Terrell 46 75 45 57 30 / 0 0 20 30 0 Corsicana 49 76 49 60 32 / 5 0 20 30 0 Temple 50 79 48 60 33 / 0 0 5 20 0 Mineral Wells 43 74 36 49 27 / 0 0 0 5 0 && .FWD WATCHES/WARNINGS/ADVISORIES... None. && $$ 05/92
National Weather Service Hastings NE
934 PM CST Sat Dec 14 2019 .UPDATE... Issued at 925 PM CST Sat Dec 14 2019 The snow has been expanding across the forecast area as expected, but with perhaps a few heavier snow bands a bit further south than previously thought. Therefore, we have added two rows of counties to the advisory to bridge the gap between the previous 2 separate advisories making one continuous winter weather advisory. Lowered forecast snowfall amounts a bit across our northeastern zones and increased amounts a bit south of I-80 in southern Nebraska due to the slight shift further south with this first wave of snow tonight. Also tweaked snowfall forecast to lower Kansas snowfall totals from the 2nd wave of snow on Sunday as this 2nd round may also be shifting a bit further south. && .SHORT TERM...(This evening through Sunday) Issued at 255 PM CST Sat Dec 14 2019 Although the first batch of snow arriving this evening into Sunday morning is still more uncertain than the second round of snow coming Sunday into Monday morning, I decided to add a row of three counties along I-80 from Highway 281 to the west. Solid evidence of a couple of mid-level vort maxes to possible give us a few inches of snow tonight. The NAM and ECMWF are backing off a bit with snow amounts, but have shifted farther south, and it looks like there may be just enough snow to warrant a warning. The GFS trended heavier, with a solution positioned near the NAM, ECMWF, and many CAMs. The HRRR indicates that a few inches of snow is a distinct possibility tonight along the I-80 corridor west of Highway 281. Still looks like the separation of two advisories in space and time (north, south)and (Saturday/Sunday, Sunday, Monday)is a good bet, rather than going with a blanket advisory over the entire CWA. Also, this still looks like an advisory situation, as snow amounts and wind speed would not warrant anything more at this time. The HRRR indicates a possible loss of saturation in the dendritic production layer over parts of the western CWA toward 10Z or 11Z early Sunday morning, but this appears to be an outlier, and I will not introduce any other precip type but snow at this time. We could be cold enough that if this occurred, we may get something more like sleet. Anyway, with the HRRR being an outlier, I did not put any other precip type but snow, but if this signal becomes more consistent, the evening/midnight shift may have to stick something else besides snow in there, especially for our southwest CWA from maybe 09Z to 12Z or 13Z or so. .LONG TERM...(Sunday night through Saturday) Issued at 255 PM CST Sat Dec 14 2019 Looks like the forecast for the second round of snow Sunday into early Monday morning is more in question on the northern side of the main deformation snow band, as the 700 mb consolidates over the south central Plains. Models have been generally tightened up the gradient on the northern side of the band, although the NAM seems to be dumping heavier qpf/snowfall within the heaviest part of the band. The GFS has shifted snowfall farther south from previous runs is not so bullish with QPF. The main deformation band looks like it should remain mostly southeast of the CWA and any precip type issues with freezing drizzle/rain looks like it will remain south of our CWA. Still looks like an advisory situation with snow amounts and expected wind speed, although it would not be completely out of the question to have amounts approaching 6 inches in a heavier band on Sunday over Mitchell County, especially. The 12Z run of the ECMWF would indicate this. After the system leaving Monday, the forecast is dry, but we do have a closed mid-level system within west-northwest flow coming through the Plains toward late work week, so I`m not completely confident in the dry forecast. && .AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Sunday) Issued at 550 PM CST Sat Dec 14 2019 We continue to expect snow at our KGRI and KEAR TAF sites this evening mainly between 8 or 9 PM through 6 to 8 AM Sunday morning. Snow showers will become more hit and miss by later Sunday morning and afternoon. Expecting generally 1 to 2 inches at these airports tonight. MVFR conditions will be likely through most of the night with perhaps periods of IFR ceilings/visibility within the heavier snow bands. && .GID WATCHES/WARNINGS/ADVISORIES... NE...Winter Weather Advisory until 9 AM CST Sunday for NEZ039-040-046- 047-060>062-072>076-082>085. Winter Weather Advisory from 6 AM Sunday to 9 AM CST Monday for NEZ086-087. KS...Winter Weather Advisory from 6 AM Sunday to 9 AM CST Monday for KSZ005>007-017>019. && $$ UPDATE...Wesely SHORT TERM...Heinlein LONG TERM...Heinlein AVIATION...Wesely
Area Forecast Discussion
National Weather Service Grand Junction CO
359 PM MST Sat Dec 14 2019 .SHORT TERM...(Tonight through Sunday night) Issued at 233 PM MST Sat Dec 14 2019 Messy forecast this afternoon as a cold front drops southward through the CWA. Right now, the front is just south of the I-70 corridor as indicated by the heavier rain and snow showers occurring. The jet stream aloft is supporting the front as the jet has set up in a west to east orientation but this will also cause the front to weaken as the day progresses. Latest radar mosaic does appear to show a bit of weakening but some stronger echos as well. Models also showing a definitive turning of the winds to more northerly which follows as the front pushes through. Guidance continues to suggest the front to drop down to just north of the San Juans and then stall. The orientation of the jet just won`t provide enough energy for the front to make it over the mountains. For the next few hours then, expect some heavier rain/snowfall with a quick inch or two possible. As far as models go, the HRRR did really well this morning with the placement of the front and it`s forward propagation so following it`s lead. A noticeable decrease in precip should occur this evening with only the higher terrain really seeing any accumulating snow but this looks to occur from the northern mountains down to the San Juans. A few bands are possible over Steamboat Springs and also the central mountains so the current highlights will stay in effect. Of note, did drop the advisory for the La Sals in Utah. Snow just didn`t want to materialize there and latest guidance shows a quick burst of snow as the front moves through but not enough to warrant any highlight. Tomorrow morning, the jet will round the base of the upstream trough and provide enough support to get the front moving again and provide more widespread lift. QG ascent will also start increasing early Sunday morning which, along with the front and upper level support, will bring a return to heavier snowfall. By daybreak Sunday, some heavier precip will form along the front from the Four Corners region into the San Juans. Interestingly, as the upper trough approaches, it`ll force this area of precip northward, possibly as far north as Vail Pass and the Grand Mesa so again, another reason to keep the highlights in place. The upper level trough finally pushes through Sunday evening as precip slowly becomes more showery in nature. .LONG TERM...(Monday through Saturday) Issued at 233 PM MST Sat Dec 14 2019 By Monday morning, almost all snow showers will be moved east of the San Juans as the colder airmass settles over the area following the trough and associated cold front`s passage. As a transient ridge sets up over the west CONUS coast, we then experience continued northwest flow through Tuesday morning before the ridge axis moves over us Tuesday night. Meanwhile, the trough moves onshore and is over the Great Basin by mid-day Wednesday. Some disagreement exists between deterministic models but this trough axis moves across our CWA on Thursday morning. At any rate, models keep this system fairly dry and quick moving likely bringing the upper elevations some gusty winds. Not expecting much in the way of precipitation across our forecast area. Beyond that, models generally show another fairly large trough moving toward the west CONUS coast by next weekend into the following week. Stay tuned to future forecasts on that system and how it could impact Christmas travel plans. Temperatures start out cold (several degrees below normal) with the northwest flow behind the first trough on Monday. In fact, this looks like some of the coldest air we`ve seen so far this season...possibly slightly colder than the October outbreak in a few places especially on Monday night. Expect a very slow warming trend through next week, reaching near normal temperatures by Thursday, and continuing to gain a few degrees each day. && .AVIATION...(For the 00Z TAFS through 00Z Sunday evening) Issued at 355 PM MST Sat Dec 14 2019 Several bands of rain and snow have developed across the area. The bands will gradually shift south and east this evening with mountain snow continue. Some sites will improve above ILS by later this evening. Another round of snow moves in from the west later tonight. MVFR/IFR conditions are most likely across the southern sites especially KTEX, KGUC and KDRO. Mountains will remain obscured through the next 24 hours in many places. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...Winter Weather Advisory until midnight MST Sunday night for COZ014. Winter Storm Warning until midnight MST Sunday night for COZ012- 018-019. Winter Weather Advisory until 5 PM MST Sunday for COZ009-017. Winter Storm Warning until 5 PM MST Sunday for COZ010. Winter Storm Warning until 8 AM MST Sunday for COZ004-013. Winter Weather Advisory until 8 AM MST Sunday for COZ005-008. UT...None. && $$ SHORT TERM...TGR LONG TERM...SS AVIATION...KJS
Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
936 PM CST Sat Dec 14 2019 .UPDATE... Issued at 919 PM CST Sat Dec 14 2019 Only some slight changes needed to the tonight portion of the forecast to account for southeastward clearing of stratus. However high and mid clouds from the Plains will spread back over the area tonight in the wake of the diminishing lower clouds. I have slightly tweaked lows down. Tonight is tranquil. The anticipated winter storm is still on track for Sunday into Monday. The new deterministic 00Z NAM and latest RAP are still on target with snow spreading west to east across the area on Sunday, beginning in central MO from 8-10 am, far eastern MO from 10 am to noon, and across western IL during the afternoon. These deterministic model runs have also shifted the axis of heaviest snow a bit further south centered along the I-70 corridor. Impressive isentropic lift and frontogenetic forcing is expected to support a rather wide snow band with rates at times from 0.5 to 1 inch per hour. If these solutions are in fact correct with the intensity and location then are current snowfall amounts during the afternoon would need to be doubled, and closer to 4-5 inches along this axis. I hesitate making alot of changes to the forecast at this time based on 2 deterministic models when a closer evaluation of the full model suite and ensembles may not fully support this decided south shift. None the less, hazardous travel is expected on Sunday and into Sunday night. The current winter storm watch still is reasonable and will be converted overnight to a series of winter storm warnings and advisories. It still appears that the first wave of snow will exit by Sunday evening and many areas may lose ice nuclei thus light freezing rain or drizzle would occur. Another round of snow would then impact the region on Monday/Monday evening. Glass && .SHORT TERM... (Through Late Monday Night) Issued at 337 PM CST Sat Dec 14 2019 Early this afternoon, a very broad-based TROF extended over much of the CONUS with a W-SW flow aloft over our region. The strong storm system that we have been watching carefully has come onshore and was located over northern Arizona. At the surface, a cold front was to the southeast of our region with colder air spilling in on NW winds. An extensive area of clouds existed over much of the Midwest with clearing in Iowa but additional cloud cover in Kansas already sliding east towards our region that will likely limit any clearing over our way. Temperatures were in the 30s for most of the area. Quiet weather is expected tonight as we will be in-between systems. In the meantime, colder air will settle in from a 1025mb area of high pressure, resulting in min temps early Sunday morning mainly in the lower to mid 20s. This will be key for what happens next. The well-advertised next storm system continues to look set for Sunday through Monday night. Confidence continues to increase on the general characteristics: (1) Two main waves of precipitation: one focused on Sunday afternoon-evening and the other one on Monday, lingering into Monday night. This longer duration into Monday night is a key change. (2) The first wave on Sunday will primarily be in the form of snow and will be driven by frontogenesis in a weak but slowly strengthening broad WAA regime. The trend for this area seems to have settled on a region near the Interstate 70 corridor, especially for what could be focused bands of snow. If this materializes, these bands of snow will be capable of heavy bursts of snow with rates in excess of 1"/hour. The cold surfaces will allow snow to accumulate on any untreated surfaces and result in hazardous travel. This episode is expected to exit by late Sunday evening. Where it lingers the longest will be capable of several inches of snow. (3) The interlude between the two main waves of precipitation will begin moving in from the southwest Sunday evening and continue into early Monday morning and will feature intermittent freezing drizzle or light freezing rain and a resultant ice accretion for most areas. Only where cloud ice can hang on better in far northern MO will precipitation types of snow linger. (4) Cloud ice will be re-introduced enough with the concluding wave of precipitation after sunrise on Monday and looks to now linger into Monday night with the upper level system moving slower, but with the broad moderate WAA regime continuing. Two upper level impulses, one lead one Monday morning will bring cloud ice back into the equation for much of the area near and north of I-70, should transition the freezing drizzle back over to snow, with the main system set for Monday afternoon and evening, expanding coverage of snow at the expense of freezing drizzle into the southern portions of the forecast area (southeast MO and far southern IL). This second wave of precipitation, especially where it looks to transition back to and continue as snow looks now to intersect much of the area that will benefit most from the first wave of snow on Sunday. (5) Adding it all up, there continues to be a threat for several inches of snow for areas near and north of I-70. Much of this area, in fact, is flirting with Warning (6") criteria and with the trends shaping up, this threat for higher snow totals is edging closer to the I-70 corridor. For this reason, we are continuing the Watch, but beginning it sooner and extending it longer in time. The southern portion of the Watch will be converted to an Advisory where lower snow totals and icing will be an issue. TES .LONG TERM... (Tuesday through Next Saturday) Issued at 337 PM CST Sat Dec 14 2019 Medium range guidance has been very consistent in keeping the mid Mississippi Valley in cold, dry northwest flow Tuesday and into early Wednesday behind the storm. It`s unlikely that temperatures will rise above freezing across northeast Missouri and west central and south central Illinois with fresh snow on the ground, however some mid and upper 30s look likely across parts of southeast Missouri. Wednesday morning continues to look like the coldest of the week due to a clear sky, light wind, and the snow cover. Lows in the low to mid teens across most of the forecast area still looks probable. Models show the low level ridge moving east of the Mississippi Valley Wednesday as the upper level ridge builds into the Great Plains. The combination of increasing southerly low level flow and rising heights will bring warmer air back to Missouri and Illinois for the latter half of the week. Areas with snow on the ground will still be chilly Wednesday with heights struggling up to near freezing, but parts of southeast and central Missouri should warm up to the low to mid 40s. The upper level ridge axis is forecast to move east of the Mississippi River on Thursday. Low level warm advection will continue with southwest flow Thursday and Friday and highs are expected to climb above normal values for Thursday and Friday. ECMWF has trended toward the GFS which brings enough low level moisture out of the Gulf to produce some light rain across Arkansas and southern Missouri ahead of a short wave trough on Friday. There`s still little if any precip indicated in our area, so have kept Friday`s forecast dry for the time being. Saturday looks dry as well with the trough to our east...though it`s a low confidence temperature forecast with GFS showing slightly different surface patterns with the GFS being the cooler of the two. Carney && .AVIATION... (For the 00z TAFs through 00z Sunday Evening) Issued at 535 PM CST Sat Dec 14 2019 Stratus resulting in MVFR flight conditions blanketed the region early this evening. The northern edge of the stratus stretched across northern MO into west central IL, and this clearing line is expected to progress southeastward this evening. Accordingly the terminals should see flight conditions improve to VFR with the southeastward clearing of the stratus. VFR flight conditions are then expected to persist until around mid-morning Sunday. Thereafter snow will develop/spread from western MO across the region during the later part of the morning and through the afternoon. Flight conditions will deteriorate after the onset of snow eventually lowering to IFR. SPECIFICS FOR KSTL: Stratus will persist this evening resulting in MVFR flight conditions. The stratus should clear around 05-06z resulting in VFR flight conditions. VFR flight conditions are then expected to persist until late morning Sunday. Snow will develop/spread into the terminal from the west near midday, with intensity increasing during the afternoon. Flight conditions will deteriorate after the onset of snow eventually lowering to IFR during the afternoon. Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Winter Weather Advisory from 9 AM Sunday to midnight CST Monday night for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. Winter Storm Watch from 9 AM CST Sunday through Monday evening for Audrain MO-Boone MO-Callaway MO-Cole MO-Franklin MO- Gasconade MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO- Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO-Shelby MO-Warren MO. IL...Winter Weather Advisory from 9 AM Sunday to midnight CST Monday night for Randolph IL. Winter Storm Watch from 9 AM CST Sunday through Monday evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL- Monroe IL-Montgomery IL-Pike IL-Saint Clair IL-Washington IL. && $$ WFO LSX
Area Forecast Discussion
National Weather Service Marquette MI
703 PM EST Sat Dec 14 2019 .SHORT TERM...(This evening through Sunday) Issued at 340 PM EST SAT DEC 14 2019 Water vapor imagery and RAP analysis show a shortwave trof extending from Hudson Bay to ne MN. This feature is supporting some areas of -sn across the fcst area this aftn. Earlier today, a sfc trof was extending basically w to e across southern Lake Superior. Convergence on the w end of this trof over western Lake Superior was much stronger with sfc obs and models suggesting there may have been a meso low on the trof there. In any event, this trof has moved s and inland during the morning/early aftn. With its passage, there has been mdt lake enhanced snow into far western Upper MI over the last couple of hrs, generally vcnty of KIWD to the Porcupine Mtns. Farther e, weaker convergence and 850mb temps a couple of C higher than over the w has led to nothing of note in terms of shsn off Lake Superior. With passage of the aforementioned shortwave, colder air will spread over the area tonight. 850mb temps are fcst to fall from -10C e and -15C w this evening to around -17C across Lake Superior by 12z Sun. Fcst soundings indicate inversions generally at 5-6kft with the DGZ becoming well positioned in the lake induced convective layer under the expected caa. As a result, will be able to fluff up a few inches of snow tonight into Sun morning in the NNW wind snow belts. Overall, looks like a 1-3 inch snowfall with up to around 4 for some locations that have more persistent shsn. Given the heavier snow that is falling this aftn far w, will maintain advy for Ontonagon/Gogebic counties, though heaviest snow will fall during the remainder of the aftn into the early evening with shsn then becoming lighter tonight. Snowfall tonight will be low impact based on increasing snow-to-water ratios in excess of 20 to 1. In exposed areas close to Lake Superior, there will be enough wind to cause some blsn/drsn. Lows tonight will range from the single digits above zero interior w to the upper teens east along Lake Superior. During Sun, low-level winds will steadily back, becoming wsw over western Lake Superior and w over eastern Lake Superior by late aftn. As a result, lingering LES bands and lake stratocu will shift n and ne thru the day, leaving behind mostly sunny skies. By the end of the aftn, the last of the shsn should be confined to Keweenaw County and east of Grand Marais. High temps Sun will range from the lower teens w to the lower 20s e. .LONG TERM...(Sunday night through Saturday) Issued at 301 PM EST SAT DEC 14 2019 General troughing across the CONUS will continue through the weekend before ridging builds across the West Coast Sunday night into Monday. As this ridge builds and moves inland across the West, steeper troughing will occur downstream and cause a period of NW flow Tuesday into Wednesday as a shortwave crosses the area bringing another quick shot of Arctic air. As the previously mentioned ridge continues east across the CONUS, heights will begin to rise Wednesday afternoon, gradually cutting off LES as winds back more southerly Wednesday evening. Models begin some disagreement going into Thursday, as heights continue to rise and 850 temps begin to warm. Models suggest a very subtle shortwave to pass over Lake Superior Thursday afternoon which may bring a period of -SHSN across the NW LES belts and eastern UP. On Friday going into the weekend, models suggest a low pressure wave to develop across the Plains and move east across the CONUS, staying south of the cwa. Sunday night into Monday, 850 temps remain plenty warm for LES; however, sounding profiles in westerly wind belts show a steep inversion with a lack of moisture also a concern. Sounding profiles have some differences between models, but winds begin to back more WSW to SW for some. This backing will limit most snow showers across only the Keweenaw for Sunday night. Shifting winds and limited moisture will limit accumulations to under an inch. Monday into Tuesday, winds will veer slowly to the NW as troughing amplifies and a shortwave approaches Lake Superior. Models suggest 850mb temps will fall from -13 to -15C on Monday night, to -24 to - 26C by Tuesday night. NW LES belts are expected to produce snow through the day Tuesday, increasing through the night. As 850 temps fall blo -20C, DGZ saturation becomes limited again. The limited saturation through the DGZ will limit snowfall accumulations, but could also produce the smaller, more visibility-restricting snowflakes often seen in arctic air. A weak shortwave will brush the Upper Lakes on Mon, resulting in veering winds bringing light LES back to the nw fcst area. This will be followed by the next stronger shortwave dropping se from the n central Canada mid-level low Tue morning. Arctic air will surge into the area with this wave, bringing blustery conditions and increasing nw flow LES for Tue thru Tue night. If the arctic air arrives early in the day per GFS/CMC, temps will likely fall thru the day Tue as 850mb temps fall to at least -20C. Overall, I increased PoPs and QPF Tuesday/Tuesday night compared to the NBM to account for this, but didn`t go too crazy as model differences still are evident and high- res models will become more available for this event in the next suite of models. Heights begin to rise again Wednesday night as winds back more from the SW. This shift should cut off any lingering LES; however, GFS and EC show a quick-moving shortwave that may bring light snow showers across the Keweenaw and far eastern UP Thursday. On Friday, the UP should remain dry. GFS and GEM suggest the development of a low pressure system across the Plains that is progged to pass to the south of Upper Michigan across IL. There is a chance if this solution is correct for some snow showers across mainly the far south, but these chances remain low. Models suggest heights continue to rise into the weekend, with 850 temps rising above 0C. This should lead to a drier weekend as medium-range models continue to show some disagreements. && .AVIATION...(For the 00Z TAFS through 00Z Sunday evening) Issued at 652 PM EST SAT DEC 14 2019 Expect IFR conditions at both IWD and CMX this evening in lake effect snow, but then conditions at IWD will gradually improve to MVFR late tonight and to VFR Sunday morning as winds gradually back west and then sw and push lake effect clouds and showers northward. Conditions at CMX will improve to MVFR Sunday afternoon as lake effect -shsn push north. MVFR conditions at SAW will also improve to VFR late Sunday morning as winds back more westerly. && .MARINE...(For the 4 PM Lake Superior forecast issuance) Issued at 340 PM EST SAT DEC 14 2019 With colder air flowing into the Upper Great Lakes tonight, expect some freezing spray with northerly winds of 20-30kt across Lake Superior, strongest over the e half. Winds will diminish to mostly under 20kt on Sun as they become w to sw. Winds will then increase back to 20-30kt Sun night, strongest over the w half, as a trof approaches. After winds diminish a little later Mon/Mon evening, a stronger cold front will pass across Lake Superior on Tue. Behind it, expect nw winds to 30kt into Tue night/early Wed morning with gales to around 35kt a good possibility, at least over the e half of Lake Superior. With arctic air flowing across the area, heavy freezing spray will also be possible. Winds will then quickly diminish to under 20kt from w to e on Wed as sfc high pres ridge arrives. After the ridge passes, expect some increase in winds heading into Thu. && .MQT WATCHES/WARNINGS/ADVISORIES... Upper Michigan... Winter Weather Advisory until 7 AM EST /6 AM CST/ Sunday for MIZ002-009. Lake Superior... None. Lake Michigan... None. && $$ SHORT TERM...Rolfson LONG TERM...JAW AVIATION...Voss MARINE...Rolfson
Area Forecast Discussion
National Weather Service Pocatello ID
838 PM MST Sat Dec 14 2019 .UPDATE... Very minor, almost transparent, update to the forecast for cloud cover tonight. As a whole, forecast is reasonable. Still seeing a few residual snow showers but additional accumulations look very light. Might see some patchy fog develop, otherwise expect low stratus to expand across the Snake Plain and into portions of the Magic Valley tonight. && .PREV DISCUSSION... /issued 144 PM MST Sat Dec 14 2019/ SHORT TERM...Tonight through Tuesday. Early afternoon radar imagery was showing the current storm system transitioning slowly east through the region and gradually dissipating with time. We are expecting at least a few more hours of light snowfall from Pocatello into the SE mountains through early this evening but the threat of moderate snowfall has ended. A temporary break in the action should follow tonight before the next upstream storm system positioned along the Oregon/British Columbia coast shears inland Sunday. The numerical weather models show pieces of energy from this system advancing through the region Sunday (mainly Sunday afternoon/night) with periods of light snowfall ongoing through Monday morning until that disturbance clears the region and a ridge of high pressure and associated drying build east across the NW states Monday afternoon through Tuesday. The clearing aloft anticipated with the incoming ridge will allow temps to plummet well below normal across the region while valley fog and low clouds develop Tuesday morning. Huston LONG TERM...TUESDAY NIGHT THROUGH NEXT SATURDAY. A ridge of high pressure is forecast to remain in control Tues night into at least the first part of Wed, with dry and cold weather but increasing clouds. Long-range models then push another trough of low pressure across the northern Rockies. The GFS/ECMWF/Canadian all hint at a bit of a split-feature trough as it approaches, with one weakening piece of energy pushing across OR/WA into Canada to our north, and a second and stronger piece of energy pushing NE across NV, UT, and WY Wed/Wed night, becoming a dominant closed low as it spills into the Plains. All 3 models are interestingly dry in their deterministic solutions with the passage of this trough (even to our south just ahead of the axis of the dominant piece of the trough), but we see no reason to argue with them just yet as we monitor trends. A moist westerly flow is expected to become established in the wake of the trough with a chance of rain/snow showers Thurs through Sat, although confidence is low on placement and coverage, and we ran with the fairly broad-brushed and reasonable NBM solution for now. The best chance of additional accumulating snowfall during this period will be in the Central Mountains west of Challis and north of Hailey, especially again in the Sawtooths/Smokys, but at an early glance this setup doesn`t look quite a nice or long-duration as what we saw with our most recent storm as a ridge of high pressure tries to potentially intrude and cut off the flow by Sat. Temps look to creep up a few degrees each day, with highs reaching the 30s again by Fri and Sat. - KSmith AVIATION...It`s been a difficult day at many of the airports across the southern half of our forecast area, with low cigs/vsbys due to widespread snow working through the region. Snow has ended at KBYI/KIDA/KDIJ, and should finally be ending for KPIH by about 3 PM. As the clouds associated with the snow push out on satellite, they are revealing some low stratus in the eastern Magic Valley, Snake Plain, and adjacent valleys. While some of this has perhaps been reflected in the ASOS observations (contaminated due to the snow), this has otherwise been hard to fully diagnose so far due to those lower clouds being blocked on satellite by the retreating higher clouds, but we expect some of this stratus will linger for the rest of the day, and then lower and intensify tonight for at least KPIH/KIDA/KDIJ. Have started working this thinking into the TAFs, and have also added some BR and VCFG to a few terminals with a very moist low-level airmass, recent snowpack, and light winds, all largely based on MOS guidance and the HRRR cig/vsby products. We will try to continue to refine the timing and impact of this activity with the 00z TAF package. - KSmith && .PIH WATCHES/WARNINGS/ADVISORIES... None. && $$
Area Forecast Discussion
National Weather Service Riverton WY
401 PM MST Sat Dec 14 2019 .SHORT TERM...This evening through Sunday night Issued at 1248 PM MST Sat Dec 14 2019 Isolated snow showers linger across the area, as the upper level system exits to the east. Activity is beginning to pick up in eastern Fremont and southern Natrona counties much the way the HRRR has been indicating. Southern Natrona looks like it will have the best chance of getting measurable snow with an inch or two possible. These showers should begin to weaken around 00Z, with all areas largely dry around 03Z or so. With some warm advection aloft, inversions will likely set up in the basins. Many areas will see single digit temperatures overnight with the Green River Basin looking to bottom out in the negatives. Sunday still looks like a quieter day. A weak wave will bring some snow showers to western Wyoming, but with limited moisture to work with any amounts look small. Any chance for showers east of the Divide will likely be confined to the mountains. Basin inversions will keep temperatures cold Sunday afternoon, with many areas struggling to get out of the teens. Decreasing cloud cover will allow for more widespread negative temperatures across the area Sunday night. .LONG TERM...Monday through Saturday Issued at 1248 PM MST Sat Dec 14 2019 Unseasonably cold Monday, as a strong 1040mb surface high builds into the Intermountain West. Highs are expected to be 10 to 20 degrees below average. Cannot rule out a few snow showers over the far west. On Tuesday, an upper level ridge will build into the area with 700mb temperatures warming by around 10 Celsius. Trapped basins and valleys will remain well below average, while the mountains and some adjacent foothills will see a decent warm up Tuesday. The best potential for valley/basin fog in the extended will be Tuesday night, as a very strong surface inversion sets up with a very dry air mass just above the surface. Models indicate a trough pushing over California Tuesday night, but diverge on the track Wednesday into Thursday. The GFS has been fairly consistent in swinging this trough northeast across the region late Wednesday into Wednesday night, while the ECMWF has been farther south and a bit slower. However, both models have been dry ahead and along the trough axis as models have a very dry atmosphere ahead of the trough. In wake of the trough, models are showing a moist westerly flow impacting the area with chances of snow over the far west Thursday into Friday. The GFS has been more bullish on this solution than the ECMWF. Have trended PoPs upward over the far west/northwest. && .AVIATION...For the 00Z TAFS through 00Z Monday West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals Decreasing cloudiness will continue this evening into the overnight hours in most locations, except for the far western valleys including KJAC where a ceiling between 3-6kft agl is expected to persist. There could be patchy morning fog Sunday, but limited areal coverage and overall low confidence of it forming, precludes any mention at any terminal for now. A few snow showers will be possible over the northwest Sunday afternoon, and will have VCSH at the KJAC terminal. Otherwise, VFR conditions and light winds Sunday. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Areas of light snow across central areas with MVFR ceilings and MVFR/IFR visibility will end from west to east this evening. After 04Z, the snow should have exited the area with clouds lifting to VFR or scattering out. Although there will be a small chance of fog Sunday morning, the limited areal coverage and overall low confidence of it forming, precludes any mention at any terminal for now. Sunday looks to be VFR with light winds with some mid-high level cloudiness overspreading the area Sunday afternoon. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued AT 1248 PM MST Sat Dec 14 2019 The current weather system will exit to the east this evening with isolated snow showers lingering in western Sweetwater and Fremont counties though 6 pm and southern Natrona County until 9 pm. Humidity will remain well above critical levels. Gusty winds will decrease this evening in southern Wyoming and remain light overnight. Winds will be moderately gusty in the mountains Sunday afternoon and across the Wind Corridor from Sweetwater to Natrona counties. Mixing and smoke dispersal will be generally poor through Sunday. Inversions are likely to set up in the basins tonight and into Sunday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 5 PM MST this afternoon for WYZ023>025-027>030. && $$ SHORT TERM...Hulme LONG TERM...Murrell AVIATION...Murrell FIRE WEATHER...Hulme