Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/06/20

Area Forecast Discussion
National Weather Service Caribou ME
1018 PM EST Sat Dec 5 2020 .SYNOPSIS... Strong low pressure will cross the Gulf of Maine tonight, then exit across the Maritimes Sunday through Monday. High pressure will cross the region Tuesday, then exit across the Maritimes Wednesday. && .NEAR TERM /THROUGH SUNDAY/... 1010 PM Update: Wx types ovr the FA updated based on latest sfc obs and precip types estimated from radar ref/CCs, which indicates the transition line is traversing a little further N and W then originally thought, currently running from Bridgewater southwest to just west of Dover Foxcroft. Subsequently, we adjusted the most northward xtnt of the transition zone a little further N to about Caribou SW to alg the ern periphery of Baxter State Park. Snow totals will likely be reduced somewhat alg the ern periphery of the wntr stm wrng area with a little more in sleet accumulation possible. In addition, far NE areas may see a reduction of intensity of precip rates when precip goes back to all sn due dry slotting wrapping back from the NE due to the maturity of the sfc-upper low system entering into the occluded stage. Orgnl Disc: Low pres moving toward Cape Cod late this afternoon per latest sfc analysis. A coastal front extending from the low up into the Maine coast. This coastal front is forecast by the HRRR and NAM to move into the coast up into eastern Washington County and then stall for a time, before drifting back off the coast early Sunday. This will allow for colder air to work back in to allow any rain/mixed precip to switch back over to snow. Further n, snow overnight mixed w/a period of sleet as WAA moves in aloft. The NAM soundings show that warm layer setting up overnight into the early mrng hrs as far n as Caribou and Presque Isle. Heavy banding potential is there and confidence is high that a deformation band will set up portions of eastern and central Maine where snfl rates of 1-2 inches per hr can be expected. The upper level low is forecast to lift along and just off the Maine coast. An upper front/trof will extend back from the low across the aforementioned areas. This feature will be the mechanism for that banding. Further s, along the Downeast region, this deformation banding will enhance the heavy rainfall. Temps will be a challenge overnight especially across eastern sections of the CWA due to the proximity of the coastal front and the warmer air trying to push northward. Therefore, snow will be mixing with and changing to rain east of Bangor to Lee line. N and west of that line, precip should be in the form of snow w/the sleet mentioned above. So, we could be looking at a foot or more of snow from parts of the Penobscot Valley up into the St. John Valley. This would include the Maine Central Highlands which could easily see 15" if the banding sets up just right. The low will continue to intensify as it lifts to the nne overnight into Sunday. Snow will be winding down from s to n during the day. An interesting feature that was pointed out this afternoon is that there could be some dry slotting working across the NE Sunday morning which will shutdown the intensity of the snowfall. The 12Z ECMWF and GFS show the banding to pivot back the St. John Valley and North Woods into late morning which will allow the snow to pile up in those areas before winding down by the afternoon. Winds will be increasing during the day leading to a good deal of blowing and drifting of snow, despite the high water content. Not much drop in temps on Sunday w/upper 20s to lower 30s north and low to mid 30s across central and Downeast areas. In regards to the hazards, Winter Storm Warnings remain in place. A Winter Weather Advisory was issued to Nrn Hancock and Nrn Washington Counties til 4 PM Sunday. A Flood Watch remains in place for the Downeast. See Hydrology section below. A Wind Advisory remains in place for the coast as gusts could hit 45 mph. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... Broad low pressure will be entering the Maritimes and exiting New Brunswick Sunday night into Monday. Expect some residual snow showers across the north as moisture remains in the area amid cool NW flow. These will taper early in the morning as dry air slowly pivots in. Clouds will remain overhead through Tuesday with high temperatures widely in the upper 20s across the north and lower 30s Downeast. Winds will be a bit breezy, up to 25 mph at times Monday afternoon. This may continue to blow around recent snow accumulations for the northern half of the CWA. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... After the departure of this weekend`s storm, high pressure will move over the region and bring a period of relatively quiet weather. There is some discrepancy in the models whether a trough will move in from the northwest and bring a period of showers in the Weds/Thurs time period. Decided that a period o showers during this time is likely, although stay tuned for further details on timing. By Saturday the ridge starts to move eastward, allowing another system to approach from the west. However the models are showing a lot of disagreement on this next system, so details are difficult to refine at the moment. && .AVIATION /03Z SUNDAY THROUGH THURSDAY/... NEAR TERM: Mod to hvy snow will pivot across Aroostook TAF sites this evening and overnight. This will keep ceilings and vis LIFR. Gusty winds will also cause blowing snow. HUL will see a period of PL mixing in with SN tonight. BGR will see a period of heavy precip in the vicinity this evening with RNSNPL. LLWS will move across all of northern and eastern Maine through early Sunday morning. Locations will trend to IFR mid-Sunday morning with some remaining snow showers across the far north in breezy NW winds. SHORT TERM: Sunday night: IFR to MVFR north, MVFR Downeast. Light snow showers for Aroostook terminals. NW wind. Monday: MVFR, possibly improving to VFR Downeast. Light NW wind. Monday night: VFR. Light N wind. Tuesday: VFR. Light N Wind . Tuesday night: VFR. Light N wind becoming calm. Wednesday: VFR with MVFR possible across the north with low clouds and snow showers. Light SW wind. Wednesday night: MVFR to IFR north, VFR Downeast. SW wind. Thursday: MVFR to VFR north. VFR Downeast. Gusty W wind. && .MARINE... NEAR TERM: A Gale Warning stay in place through Sun. NE winds 25-30 w/gusts hitting 40+ kt later tonight as the low intensifies as it moves across the Gulf of Maine waters. Winds will go N later tonight and then shift to the NW on Sun as the low pulls into the Maritimes. Winds will start to subside by later in the day. Seas will build to a range of 12-16 ft, but start to get knocked back by Sun morning as the winds turn more offshore. SHORT TERM: SCA criteria continues Sunday night through Monday morning with winds gusting to 25 kt, subsiding below 20 kt during the afternoon. Waves will fall from 4 to 6 ft to 2 to 3 ft Monday afternoon as offshore NW winds decrease. && .HYDROLOGY... Looking for heavy rain w/upwards of 2.0 to 2.5 inches. This could lead to some flooding of roads and washouts across southeast portions of the region, especially and the Downeast region later tonight into Sunday. A Flood Watch remains in effect for the Hancock and Washington County areas. These areas received 2 to 5 inches of rain earlier this week, and the soils are saturated and streams are running high. && .CAR WATCHES/WARNINGS/ADVISORIES... ME...Winter Storm Warning until 7 PM EST Sunday for MEZ002-005-006- 011. Winter Storm Warning until 4 PM EST Sunday for MEZ001-003-004- 010-015-031. Flood Watch through Sunday afternoon for MEZ016-017-029-030. Wind Advisory until 7 AM EST Sunday for MEZ029-030. Winter Weather Advisory until 4 PM EST Sunday for MEZ016-032. MARINE...Gale Warning until 4 PM EST Sunday for ANZ050>052. && $$ Near Term...VJN Short Term...Cornwell Long Term...LF Aviation...VJN/Cornwell Marine...VJN/Cornwell Hydrology...VJN
Area Forecast Discussion
National Weather Service Des Moines IA
538 PM CST Sat Dec 5 2020 .DISCUSSION.../Tonight through Saturday/ Issued at 200 PM CST Sat Dec 5 2020 Main Points: 1. Chance for light snow in northeast cwa this evening. 2. Well above normal temperatures middle of next week. 3. High uncertainty with pattern evolution late next week. Active northwest flow will highlight the short term forecast period. The first shortwave to cross the region is already diving south into Minnesota this afternoon, evident on satellite as a wide swath of stratus clouds Chances for light snow will increase in a zone of weak thermal ascent ahead of the wave which may clip the northeast cwa this evening. Model soundings struggle to fully saturate in the presence of a dry airmass, but some hi-res members such as the RAP generate enough lift and saturation within and below the DGZ to support the idea of some light snow reaching the surface. Low end PoPs highlight this potential with little to no accumulation or impacts expected. A second wave dives southeast on Sunday although with a more easterly trajectory over the Great lakes, which should keep the area dry. However, the wave may help reinforce stratus coverage over the area and hinder warming. Western conus ridge slightly translate east over the first half of next week. This will further continue our stretch of dry conditions with above average temperatures. The warmest period occurs as the thermal axis pushes east off the high Plains Tuesday into Wednesday. High confidence that temperatures will easily exceed model progs, and have continued the trend of boosting forecast highs into the 75th to 90th percentile of the guidance spread. Some locations cracking into the 60s cannot be ruled out on Wednesday, especially in western Iowa. Forecast confidence quickly evaporates late next week as the pattern becomes much more active again. A series of shortwave troughs diving southeast out of the Pac NW work to flatten the western conus ridge. GFS has shown some run-to-run consistency with a much more dynamic and amplified pattern evolution and the potential for rain/snow next weekend. After several more benign runs the 12z Euro has also shifted trending toward the more aggressive GFS solution. Stark discrepancies exist between the deterministic and ensemble QPF outputs through next weekend, so it is tough to deviate away from the drier NBM solution just yet. && .AVIATION.../For the 00Z TAFS through 00Z Sunday evening/ Issued at 534 PM CST Sat Dec 5 2020 Area of light snow showers forecast for far northeastern Iowa. Flurries possible near MCW and ALO, and thus will state VCSH in the TAFs for those locations, as the bulk of the activity will remain further north and east. MVFR CIGs possible across the east. Will need to monitor satellite trends at OTM closely for low-end MVFR CIGs, most models paint an overcast/clear sky line just east of OTM. For now, will keep OTM TAF at VFR, but should clouds begin to work westward a bit, may need to put OTM in MVFR with 06z TAF issuance. Overall impact though should still be low. Elsewhere at the other terminals, VFR conditions are forecast. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION...Martin AVIATION...Krull
Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
613 PM CST Sat Dec 5 2020 ...New Short Term, Aviation... .SHORT TERM... /NEW/ /Through Sunday Night/ While the radar scope will be quite active over the next 12 hours, the large amount of dry air will mean that most areas will only see a few sprinkles. However, some model guidance has suggested that a better risk for measurable rainfall may take place across parts of North Texas during the pre-dawn hours on Sunday through mid-morning. Ascent continues to overspread portions of North and Central Texas this evening as a strong upper low across West Texas churns and lifts northeastward. As an aside, this upper low actually produced about 2-6" of snow across West Texas earlier today. NO winter weather is expected for North/Central Texas. Our presently ascending 00 UTC weather balloon indicates a plethora of dry air below 700mb and this dry air will likely mean that most locations only experience sprinkles at best despite what the radar imagery may depict. However, as the upper low ventures closer to our region, the combination of evaporative cooling/sublimation and strong ascent will moisten the column for parts of the area. In fact, RAP forecast soundings near Stephenville and western parts of the D/FW Metroplex suggest that by 0900/1000 UTC the profile will become nearly saturated from top to bottom. Hi-res guidance appears to hint at a mid/low-level deformation zone along the north/northwesterly periphery of the low. This frontogenetic band is reflected in 1 hr. probability match mean (PMM) precipitation products from the HREF and suggests that the best chance for measurable rainfall (very light and less than 0.05") will be along and north of a line from Eastland to Hillsboro to Corsicana after midnight. The most concentrated ascent and thus 30% PoPs reside from near a Graham to Cleburne line. This "banded" precipitation structure will likely move northeastward toward the D/FW area around 3-4 am. By 6-7 am, the "band" will likely be focused along the U.S. HWY 380 corridor. Again, this band will just be very light rain. I cannot rule out a brief rumble of thunder or two nor can I rule out reports of graupel or very small hail (which we have already seen near Lake Whitney) given the steep lapse rates. I expect the instances of either will be on a very isolated basis, unless we can generate more instability. Something to monitor overnight. By mid-morning Sunday, most of the ascent will have waned and shifted to the east, so I expect certainly by noon onward, we`ll be dealing with precipitation-free conditions across the entire forecast area. Once the upper low exits stage right, it`ll be a breezy and cool day. However, sunshine will help temperatures climb into the 50s and 60s. Tomorrow night (Sunday Night) should feature a good radiational setup with clear skies, light winds and dry air allowing temperatures to fall into the 30s. A light freeze is possible for parts of western North Texas and the more rural areas. Bain && .LONG TERM... /Issued 153 PM CST Sat Dec 5 2020/ /Sunday Night through Saturday/ The week will start off with a deep trough over the eastern US and a cutoff low spending some time near Baja. This will place North and Central Texas beneath some weak ridging and nebulous flow aloft. Generally clear skies and light winds are expected which means low temperatures will benefit from radiational cooling and drop into the 30s, while high temperatures will be mild and reach the 60s and 70s. The Baja upper low will get kicked eastward by mid to late week and bring a cold front into the area around Thursday night. The lift associated with the upper trough moving across and the front will lead to a low chance of showers Thursday night and Friday. Moisture and instability will be somewhat limited and right now we only expect any rainfall to be less than a half inch. Temperatures behind the front will be cool and drop back to near normal. Cluster analysis of all of the ensemble systems suggest guidance has not yet settled on a solution for the upper level pattern late in the week and next weekend. About half of the members keep upper level troughing back to our west, while the other half are more progressive. Meanwhile the 12z deterministic guidance has all trended more progressive. The progressive scenario would lead to clearing skies and dry weather by Friday afternoon and into the weekend, but the inherent uncertainty in the forecast means low PoPs need to be mentioned Friday night and Saturday. Any rain would be light and of the over-running variety. However if trends continue these may be dropped from the forecast and next weekend may end up being another nice one. TR.92 && .AVIATION... /NEW/ /00 UTC TAF Cycle/ Concerns---VCSH and low end turbulence potential. Otherwise VFR. VFR is anticipated through the entire 00 UTC TAF cycle. The radar scope will remain illuminated through Sunday morning, but dry air between the surface and 7,000 feet will keep most precipitation from reaching the surface through the evening hours. Given the potential for turbulence below (TB SEV reported near ACT around 2330 UTC) cloud base, however, have advertised a rather long period of VCSH at all TAFs. Between 0900-1400 UTC, however, stronger lift will spread across North TX and offers the greatest opportunity for drops to reach the surface. Otherwise, cigs between 7,000 and 10,000 feet AGL are advertised. Light/variable winds will attain more westerly component through the nighttime hours with stronger northerly winds anticipated after daybreak Sunday. Bain && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 43 58 37 63 38 / 30 30 0 0 0 Waco 40 60 37 64 38 / 10 10 0 0 0 Paris 39 55 35 58 35 / 10 20 0 0 0 Denton 40 59 31 65 31 / 30 30 0 0 0 McKinney 41 57 34 62 34 / 30 30 0 0 0 Dallas 44 59 39 64 40 / 30 30 0 0 0 Terrell 41 57 34 61 35 / 20 20 0 0 0 Corsicana 44 59 39 62 40 / 20 10 0 0 0 Temple 38 61 36 65 37 / 10 10 0 0 0 Mineral Wells 40 59 34 65 35 / 30 20 0 0 0 && .FWD WATCHES/WARNINGS/ADVISORIES... None. && $$
Area Forecast Discussion
National Weather Service Key West FL
953 PM EST Sat Dec 5 2020 .DISCUSSION... A rope cloud feature associated with the leading edge of a shallow front is drifting southward through Gulf waters, currently located about 10 nm north of Smith Shoal through New Ground Rocks Light. Patchy stratus was noted north of the boundary. Isolated showers were present in the western Straits, and recently focusing along the western portion of the boundary. Winds were very light or calm, favoring a divergent offshore component from the mainland. Temperatures were in the lower 70s throughout most of the Keys, with 69F reported at the Key Deer Refuge RAWS toward the north end of Big Pine Key. The evening sounding showed westerlies had dried out the 750-450 mb layer, as a small mid-level trough axis in the same layer has moved east to near Andros Island, just slightly perceptable in the 10-7.3 micron difference GOES-16 imagery this evening. The main weather issue ovenight will be the possible expansion of the stratus field across the Gulfside/Bayside waters and advecting southwest from the Everglades as high pressure moves east along the northern Gulf coast. HRRR and other convective- allowing guidance suggests some redevelopment of isolated, perhaps scattered, showers over the Straits as the initial rope cloud/boundary slides south of the Keys overnight. This scenario along with the variable winds favoring north-northeast later tonight is already accounted-for in the public forecasts; no changes are planned this evening. && .MARINE... Light and variable winds this evening are expected to becoming north to northeast at light to gentle speeds during the early morning hours, then clocking around to southeast later Sunday. The previous discussion from this afternoon states the weather following Sunday night and onward well, and is repeated here: A low pressure and strong cold front will result in a period of showers and thunderstorms likely Sunday night into at least Monday morning. Small Craft Advisories may be required during that time, as southwesterly flow increases ahead of the front. After a lull in wind speeds Monday afternoon, the front is forecast to sweep through the Florida Keys coastal waters Monday evening, with northwest to north fresh to strong breezes commencing. Small Craft Advisories will likely be needed for some or all coastal waters at that time. Northerly breezes will gradually diminish Tuesday through Wednesday and begin to veer to the northeast to east late in the week. && .AVIATION... A stalling, shallow frontal boundary bringing light northerly winds, along with rich low level moisture overtopping a seasonably cool Gulf and Bayside will mean a significant potential for stratus with IFR ceilings possible during the early morning hours. KAPF (Naples Airport) and KMKY (Marco Island Airport) had reported ceilings in the FL003-005 range for a couple hours earlier this evening. The arrival could be as early as 05Z at KEYW and following 06Z-07Z at KMTH. These conditions could linger after 12Z, but at this time opted for a slow lift to low MVFR before breaking up a few hours after sunrise. Light, mostly north to northeast winds will become easterly midday Sunday. && .KEY WATCHES/WARNINGS/ADVISORIES... FL...None. GM...None. && $$ Public/Marine/Fire...JR Aviation/Nowcasts....JR Data Acquisition.....DR/Haner Visit us on the web at Follow us on Facebook and Twitter at:
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
840 PM CST Sat Dec 5 2020 .UPDATE... 839 PM CST Currently monitoring a rather vigorous, but moisture-starved, mid-level vort max moving through southeast Minnesota. Earlier guidance was a bit too far west with the eastern extent of the higher 850-700 hPa moisture represented by a 5-8kft cloud deck entering northwest Illinois. Given upstream obs of snow showers and the presence of a decent area of mid-level cloud cover over developing low-level clouds, seeding from the mid-level deck should result in some isolated snow showers across western portions of the CWA (west of a Rockford to Morris to Paxton line) during the overnight hours. Also, inversion heights rising to around 5kft with appreciable low-level omega profiles over and around Lake Michigan should support isolated to scattered lake effect showers late tonight through Sunday morning. Though 850 hPa temps will lower tonight, delta Ts with the water temps will be marginal for lake effect and should limit coverage and/or intensity. Synoptic low-level E to NE flow with some minor land breeze enhancement from Lower Michigan will likely result in an enhanced convergent mid-lake band shifting westward through the overnight hours. Given the overall weak flow, a kink or two in the form of a subtle mesovortex in the band is a distinct possibility. This activity will likely move onshore into far northwest Indiana and central/southern Cook County by around daybreak Sunday. Thermal profiles do support the potential for some rain to mix in, but convective layer min temps of -8 to -9C should be sufficient to introduce ice given the turbulent PBL. Kluber && .SHORT TERM... 136 PM CST Through Sunday... Generally low impact weather remains in the forecast, though there are some low chances for a dusting of snow mainly lakeside, with flurries possible at times. And while this is not the most likely scenario, there could some light sprinkles with close to freezing surface temperatures briefly late tonight near the lake. The morning sounding from International Falls, MN, located close to where tonight`s clipper system is currently located, depicts saturation down to about 5kft and mid level lapse rates in excess of 7 deg/km. To the immediate east of the upper low track, snow has been falling with visibility as low as 2SM. Model agreement is strong that this low will dig across the Mississippi River valley and into Central Illinois. It is across this axis where better chances for snow will be, but with the lapse rate plume moving overhead and some mid level saturation, a few flurries cannot be ruled out -- especially west of Chicago. Behind this wave, a secondary surge of slightly deeper and colder air will filter into the lower Great Lakes region. Low level flow will remain similar to today but with an onshore component to northeast IL at first, then across northwest Indiana. This will lead to a slightly better, but still somewhat marginal lake effect precipitation setup. Lake induced inversion heights will increase to around 5,000 feet or so, and with slightly deeper cloud depths. More marginal parameters this morning led to some spotty light precipitation across northwest Indiana. There would anticipate with some upper level support too, that lake effect precipitation is likely in spots. The question is how widespread, how long, and what precipitation type. The lowest levels of the forecast sounding suggest ice crystal formation will be marginal (as was the case today), such that snow or some liquid precipitation will be possible. Overnight into early Sunday there could be some seeding from the mid level moisture layer though the dry layer between the mid and lower cloud may be a bit too deep. So what`s the message here? Lake effect precipitation is likely into northeast Illinois overnight into Sunday morning, with a slow but gradual shift into northwest Indiana with time. It will likely be light, spotty, not lasting long in one spot, but better coverage than today. And it will likely be a mix of snow/rain/and maybe freezing drizzle (for elevated surfaces) in the morning with colder surface temperatures. With the shorter duration, the impacts should be low, but a light dusting in some areas cannot entirely be ruled out. With more clouds, highs Sunday could struggle to reach 40. The next system is still taking shape across north central Canada, and will take southward dip through the western Great Lakes Sunday. Any precip with this should hold off until Sunday evening. KMD && .LONG TERM... 300 PM CST Monday through Saturday... Most of the period will be quiet with two exceptions. First, on Sunday evening and night, looking at potential for isolated snow showers and marginal lake effect setup into northwest Indiana, but impacts probably minimal at most. Then late in the work week into early next weekend timeframe, probably our best precip chance though confidence is low on timing. Temperatures will be near seasonable Monday into Tuesday morning, then mild thereafter. In the rather odd blocky, meridional mid and upper level pattern we`ve been in thanks to anomalously and amplified western ridging and downstream eastern troughing, another potent short-wave disturbance is progged to track due south from western Hudson Bay to overhead Sunday evening-night. Despite antecedent lacking moisture in roughly 850-700 mb layer, think that there will be enough large scale forcing (PVA and height falls) to support isolated to possibly widely scattered snow showers. Guidance continues to suggest a plume of steep mid-level lapse rates will accompany the short-wave and aid development of convective snow showers. With the above in mind and support from RAP and HREF high-res sim-ref, added in some slight chance PoPs. Footprint of snow showers will be small and they`ll be moving south at a good clip, would think nothing more than a localized dusting on colder surfaces in the offing. Regarding the marginal lake effect setup, modest cold advection accompanying the short-wave could allow lake induced inversion heights to get as high as 5-6kft with some minor lake induced CAPE. It appears that convergence could set up and favor primarily northeast Porter County for activity, followed by loosely organized band or bands *possibly* drifting west towards Illinois shore early Monday before weakening. Lake remains rather mild so modification of air mass over it could mean that if any patchy coating of snow occurs, it might be a bit inland of the lake. For temperatures Sunday night, expecting extensive lower cloud cover, so temperature drop will limit temperature drop. Monday should start partly to mostly cloudy and then northwesterly dry advection aloft should scour out the cloud cover, with lake induced clouds latest to erode. Forecast highs are a bit above normal at upper 30s to around 40. Monday night will have a surface high pressure ridge extending northeast across the area, so assuming clouds aren`t an issue, probably the last near normal lows of the week (low-mid 20s inland and around 30 Chicago). Tuesday will start the warming trend though slower departure of ridge axis and weak warm advection should keep highs primarily in lower 40s, locally mid 40s points west and southwest. The flattening of the western ridging will bring a surge of unseasonably mild warm advection for the time of year in downslope west-northwest flow from higher terrain in western US. Again assuming no significant cloud cover issues, Wednesday will likely reach or get a few degrees above 50F in most of the area. Most recent guidance did speed up the passage of a weak cold front to Wednesday night with winds flopping over to easterly by Thursday. It will remain quite mild aloft, so especially for areas south of I-80, 50+F highs appear probable again, with slightly cooler conditions north (including rare for December minor lake cooling). Have been noting that medium-long range guidance has been indicating ejection of short-wave energy from a western trough. If another wave can dig in behind lead energy and phase, that will increase chances of a stronger system affecting the region. Overall, confidence in evolution is low so low NBM PoPs appear reasonable, but note that the ECMWF and GFS suites did speed up timing of possible system to mainly Friday-Friday night for primarily rain, possibly changing to snow before ending. Castro && .AVIATION... For the 00Z TAFs... Aviation weather concerns this period relate mainly to MVFR cigs and the potential for periodic chances for light snow showers (possibly mixed with some rain). VFR cigs prevail at the Chicago-area terminals early this evening, but anticipate a gradual expansion to high-end MVFR through the overnight hours as flow aloft switches a bit more northeasterly and helps to push some of the lake-effect stratus inland again. Regional TDWRs are picking up a smattering of light echoes just east of GYY where some off-and-on flurries or drizzle will be possible. Monitoring a compact but strong disturbance rotating southward across Minnesota which is expected to swing in overhead later tonight. Forcing with this system is strong, but the depth of lower-level moisture remains in question, as does the exact track of the upper low even at this short range. Given surface observations indicating cigs around 5-7 kft across western Wisconsin, thinking is that guidance is a bit too dry in the low- mid levels at this time. As a result, have added in a brief PROB30 for -SHSN at RFD overnight as the core of this upper-level feature pivots overhead. This feature will also result in slightly deepening inversion heights over the lake on Sunday morning with the potential of lake-effect precipitation increasing a bit. Still uncertain as to the orientation/location of any lake convergence region, but latest guidance is trending westward a bit, which would bring chances into the MDW and GYY sites. Some question as well regarding the potential for ice nucleation, so for now have introduced some TEMPO groups for VFR -SHSNRA Sunday morning, even though surface wetbulb temperatures would be cold enough for all snow. Will continue to monitor guidance trends this evening. Otherwise, high-MVFR cigs look to prevail tomorrow with light surface winds which may oscillate between north-northwest and north-northeasterly. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at:
Area Forecast Discussion
National Weather Service Medford OR
222 PM PST Sat Dec 5 2020 .SHORT TERM...Through Tuesday Morning...One light rain band from the approaching front made its way onshore a few hours ago and no rain was reported with it. A second, more concentrated band, is beginning to move onshore now. A few embedded showers are showing convective activity with 40 dBZ returns on radar. No lightning has been reported yet though. The threat for offshore and coastal lightning should cease around 7 PM PST, as the cold pool moves inland and the upper level trough shears apart. The front has come in at a more southerly approach than suggested by earlier model runs. The most recent runs of the HRRR show the 850 mb and surface flow from the south and this indicates stronger winds in the Shasta valley, near Weed, than previously forecast. Gusts are not expected to exceed advisory criteria, but could approach 30 mph. Snow levels will remain mostly above 5000 feet, dropping to around 4500 feet as the showers taper off Sunday morning. The high Cascades will get one to three inches of snow from this system. The showers will end Sunday morning as the upper low departs, moving south into Nevada. High pressure will build in behind this system and is likely to be the most significant front until the day eight through ten timeframe. Fog/freezing is expected in west side valleys tomorrow night into Monday morning and could develop to be quite dense, possibly warranting Dense Fog/Dense Freezing Fog Advisories. There is uncertainty as to which valleys the fog will lift and erode away enough for the valley floors to see sunshine. It is very possible none of the west side valleys fully clear. Some hi-res models also suggest patchy fog in the Klamath Basin and Klamath Marsh. This is possible with less snow and more liquid and mixed phase precipitation with this front. However, confidence is very low and so elected to exclude it from the forecast for now. More fog is expected Tuesday morning. The ridge won`t have much time to build as another system approaches. -Miles .LONG TERM...Tuesday through Saturday...The long term begins with a large cut-off low to the south of the western coast of Baja, and broad, flat ridging over the western US. Upstream, a blocking ridge will reside over the Bering Strait, or in that general vicinity. Being features that models have been known to have frequent difficulties with, the cut-off low and the blocking high appear to be the wrenches in the model inner workings, keeping the GFS and ECMWF suites from coming to any consistent solution. Given the wide range of solutions in the ensemble members, have trended the forecast towards a blend of the operational models, which surprisingly enough, have fair agreement between them. The ridge overhead Tuesday should shift east under pressure from a weak ridge expected to move onshore late Tuesday into early Wednesday morning. The front is likely to wash out as it moves onshore, leaving chances for light precipitation confined to the coast, the coastal mountains, and the Umpqua Basin. This front appears to be the sacrifice necessary to allow a second trough to dig into the region form the northwest on Thursday. Precipitation will be a bit more widespread with this event, although still mostly confined to areas along and west of the Cascades. Despite the lack of precipitation, temperatures will be noticeably cooler after passage, dropping to values more typical of early December. Heading into next weekend, the overall pattern appears to become more progressive, with the cut-off low opening up and re-joining the westerlies, and numerous shortwaves lined up and headed our way out of the eastern Pacific. The one caveat is the persistent blocking ridge mentioned above, still holding position somewhere between Siberia and western Alaska, and still keeping the upstream pattern, and therefore the downstream details, uncertain. The operational EC and GFS both resolve a significant frontal passage next weekend, something more typical of our December storms, but confidence is low, and the timing and/or strength of this system will likely change over the coming forecast cycles. -BPN && .AVIATION...For the 06/18Z TAFs...Most of the lower level clouds have cleared up this morning. Upper level ceilings will build ahead of a cold front. This cold front will bring rain to locations west of the Cascades during the late afternoon and evening hours. Look for widespread MVFR ceilings and terrain obscuration once the cold front moves through the region. Spotty LIFR and IFR visibilities are possible in the valleys Saturday night. -Smith && .MARINE...Updated 200 PM PST Saturday 05 Dec 2020...Seas will become high, steep and wind wave dominated this afternoon. South winds will approach and may briefly reach gale force during the afternoon with the strongest winds expected north of Port Orford. The combination of swell and wind driven seas this afternoon and evening will result in a very steep and hazardous warning level seas beyond 20 nm from shore and north of Cape Blanco with conditions hazardous to small craft expected for all other areas. Behind the front, high pressure briefly builds Sunday with seas remaining high and steep into Monday morning due to west swell. Another heavy, very long period swell builds early Monday with seas likely becoming very high and very steep Monday afternoon into Monday night. Guidance has been consistent in showing seas of 17 to 20 ft at 18 to 21 seconds Monday afternoon into Tuesday morning, likely peaking Monday evening. This will lead to dangerous bar crossings as well as hazardous surf conditions. Seas are expected to remain high and steep, but trend lower Tuesday into Wednesday with the next cold front moving through on Tuesday night. Additional fronts are possible late in the week with seas likely to remain high and steep into the week. /BR-y/Smith && .MFR WATCHES/WARNINGS/ADVISORIES... OR...High Surf Advisory from 10 AM Monday to 4 PM PST Tuesday for ORZ021-022. CA...None. Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Monday for PZZ350-356-370-376. Hazardous Seas Warning until 10 PM PST this evening for PZZ370. $$
Area Forecast Discussion
National Weather Service Mobile AL
918 PM CST Sat Dec 5 2020 .NEAR TERM /Now Through Monday/...An upper low currently ejecting from the southern Rockies out over the South Plains will open into a positively tilted shortwave trough as it continues eastward over the lower Mississippi River Valley through Sunday. Ahead of this feature, synoptic ascent improves beneath the right entrance region of a strengthening upper level jet that extends from the Texas coastline northeast over the Deep South and Southeastern states. With significant surface cyclogenesis lacking, low-level winds remain out of the north and we don`t see the typical onshore flow and moisture return ahead of this system. Thus, vertical moisture profiles remain quite dry, which will play a large factor in who sees rainfall from this system on Sunday and how much falls. Per the 00Z LIX sounding, seeing a deep layer of dry air extending from the surface up to around 600 mb (which matches well with the 00Z HRRR analysis soundings). It`s therefore no surprise that surface obs are reporting little to no rain beneath the stratiform shield over SE LA right now (as seen on the KMOB WSR-88D). It`s going to take some time for the dry air near the surface to moisten via sub-cloud evaporation alone, likely all of tonight and through Sunday morning. However, by Sunday afternoon and evening, we should begin seeing instances of light rain across primarily the southern half of our local area as the low levels finally moisten enough for rain to reach the ground. Maintained 30-50% PoPs across these areas during that time to reflect this, with lower chances (~20%) farther inland. Accumulations will be light and likely measured on the order of hundredths of an inch. Rain tapers off from west to east Sunday night as the upper shortwave pushes east of the area and the forcing departs with it. Highs on Sunday remain mostly in the 50s (some locations could see temps right around 60) due to the thick cloud cover and sub-cloud evaporative cooling. Lows Sunday night dip into the upper 30s to low 40s. A dry, deep-layer northwesterly flow then sets up Monday, with clear skies and temps holding steady in the 50s. /49 && .MOB WATCHES/WARNINGS/ADVISORIES... AL...None. FL...None. MS...None. GM...None. && $$ This product is also available on the web at: