Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/09/17

National Weather Service Jackson KY
954 PM EST Fri Dec 8 2017 .UPDATE... Issued at 954 PM EST FRI DEC 8 2017 The latest observing and model trends continue to point to little to no snow across the southeast tonight. The outlier has been the HRRR and that model is now showing a drier trend in the latest runs. This continues to fall more in line with the radar and satellite trends as well as the latest downstream observations. That said, did keep a slight in the fringe higher terrain spots otherwise think most areas will remain dry. Another challenge has been the mid to high level clouds that continue stream in from the southwest and the impact on the temperature curve. Overall have kept with the previously modified low temperature grid, but this may have to be modified further based on the cloud cover trends. The latest grids have been finalized and sent. Also updates were made to HWO and ZFP to reflect latest forecast thinking. UPDATE Issued at 642 PM EST FRI DEC 8 2017 The evening surface analysis is showing a small influence from surface high nosing northward. Otherwise we remain stuck between two systems this evening. This keeps us mostly driven by the strong jet energy aloft and disturbance to the east that continue to render mid to upper level clouds across eastern Kentucky. This being to case did increase the overall cloud coverage for portions of the night. There remains some concern on weaker moisture making it into the far southeast tonight, however, the short term models are overdone based on current trends. These models have been most robust with the QPF and given this confidence remains low this evening for any snow. Therefore, did lower POPs and QPF in the southeast and this subsequently leads to lower snowfall potential. The cloud cover also leads to less confidence in the low temperatures tonight and did lean slightly higher based on that idea. Otherwise loaded the grids with latest obs and trends. && .SHORT TERM...(This evening through Saturday night) Issued at 247 PM EST FRI DEC 8 2017 We have two weather systems we are monitoring this afternoon. The first one is pushing northeast across the southeastern states with a large plume of moisture extending across the southern Appalachians. The second system is diving southeastward across the far western great lakes. Presently, eastern Kentucky is sandwiched between these two systems, with nothing more than a bit of mid to high level cloud cover. This will change for a few folks tonight as moisture associated with the system to our south brushes the Kentucky counties bordering Virginia. RAP/HRRR have been wavering around with the northward extent of precipitation over the past 3 or 4 runs, but its looking more likely that some light snow will push into our counties bordering Virginia late this evening or overnight. RAP/HRRR/GFS all indicate about 0.05 to 0.10 inch of liquid equivalent tonight, which would support around an inch of snowfall with a snow ratio near 15:1. The highest totals will likely reside in the highest terrain, with drier air eating away at totals at lower elevations. For all other locations, expect skies to gradually clear overnight. The clearing skies and expected light winds will allow for temperatures to plummet tonight. Lows will fall well into the teens in many areas. The exception will be in the far southeast where precipitation and clouds keep temperatures in the 20s. Any snowfall will end around daybreak Saturday with a mostly sunny start expected. However, the second system will roll into eastern Kentucky from the northwest by midday or early afternoon. This will bring an increase in cloud cover and increased chances for snow showers. Low level lapse rates will steepen up quite a bit through the early evening hours, allowing for a few intense snow showers that could lay down a quick coating of snow. This is despite highs expected to reach into the mid 30s. We will likely see temperature falls within the snow showers. However, given the air temperatures near or above freezing, travel conditions should not be all that impacted. Expected liquid equivalent is only going to be a few hundredths of an inch for most, so this won`t support that much snow. Perhaps up to a half inch in favored westerly upslope regions. A few snow showers or flurries may linger into Saturday evening before coming to an end. Most of the moisture with this system will fall in the dendritic growth zone, so we could see some decent dendrites with the snow on Saturday. This could aid in some instances of reduced visibilities, impacting travel in the heavier snow showers. Temperatures Saturday night will drop back into the teens again with another strong push of cold air moving in behind the clipper system. .LONG TERM...(Sunday through Friday) Issued at 410 PM EST FRI DEC 8 2017 The long term period is expected to begin with a western Conus ridge and an eastern Conus/eastern North American trough. The axis of this trough is expected to have moved east of the area by the start of the period or shortly thereafter. At that point, surface high pressure is expected to be centered over the Southern Plains and southeastern states. The clipper system/shortwave that is expected to affect the region in the near term will have moved east of the area with another clipper system working through the Great Lakes at that point. Height rises are generally anticipated from Sunday into Monday with the eastern Conus trough becoming more broad in general and the airmass modifying. In fact, 850 mb temperature are expected to recover from near -9C or -10C to start the period to around 3C or so by Monday evening with southerly flow between high pressure departing to the south and east and the next clipper system dropping into the MS Valley on Monday. Like the previous clipper system, the one for Monday night through Tuesday is again expected to track north of the area a track typical of minimal wintry precipitation. The cold front associated with this system should move across the area on Monday night with a few snow showers and flurries with additional coverage of snow showers possible on west to northwest flow and increased instability due to solar insolation on Tuesday. At this point, isolated to scattered coverage of this is expected with any accumulations mainly above 2000 feet and limited to an inch or less as vertical motions should peak below the dendritic growth zone with limited moisture. The coldest of the airmass should move across the area on Tuesday night with 850 mb temperatures bottoming out near -15C. The period from late Tuesday night through Wednesday night appears to be in between clipper systems at this time, with cold high temperatures and a fair amount of mid and high level clouds. There is uncertainty regarding the track and timing of the last clipper system of the extended period which should arrive late Thursday or Thursday night with light precipitation chances lingering into Friday. What falls, if anything with this system should primarily be snow. After a cold start, highs should average 10 to 15 degrees below normal on Sunday. Clouds and some mixiness due to a moderate pressure gradient should keep temperatures from bottoming out on Sunday night. Highs should moderate to near normal levels for Monday. As colder air moves in, daytime highs may struggle to climb above the mid 30s on Tuesday in the cold advection pattern. With the heart of the cold airmass in place for much of the night on Tuesday night, widespread teens are expected for lows. The cold airmass will still be in place for Wednesday with many locations likely struggling to climb above freezing. Temperatures will likely moderate ahead of the the last clipper system of the long term period on Thursday to around 5 degrees below normal, with similar highs are expected on Friday. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) ISSUED AT 622 PM EST FRI DEC 8 2017 We continue to see mid and upper level clouds from strong upper level jet and disturbances aloft. The good news is we remain VFR and this will be the story for the TAF sites through much of the period. There is a chance of seeing moisture sneak into our far southeast tonight which could lead to light snow. While this could lead to localized issues, the models are not handling this very well based on current observations leading to less confidence. Another upper level disturbance drops southeast Saturday afternoon, and we will see snow showers develop and move across the area. This could lead to localized drops in CIGs and VIS, but kept the TAF sites VCSH for now. Given some influence by nosing high pressure think the winds will remain calm tonight, and then winds will increase through the day Saturday out of the west at 10 knots with gusts of 15 to 20 knots. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...DJ SHORT TERM...KAS LONG TERM...JP AVIATION...DJ
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CST Fri Dec 8 2017 .UPDATE... 902 PM CST No big changes planned to going forecast. Did low temps upward just a bit tonight. Potent jet is diving south into the Upper MS Valley region this evening with RAP analysis suggesting 150kt+ 300mb winds at the core with 125kt sampled on MSP`s evening sounding. Strong ascent in the left exit region of this jet is resulting in strengthening ascent and blossoming area of virga over northern IL. Radar presentation would suggest that snow is very close to reaching the ground if it isnt already over the northwest suburbs. Recent ACARS soundings suggest the dry layer is only on the order of about 3-4kft deep and virga will likely be able to penetrate through and saturate the low levels this evening. Once low levels are saturated, expect light snow to develop withe some bursts of occasionally moderate snow showers. Most intense snow showers should occur near and just ahead of the approaching cold front later tonight. See no reason to make any adjustments to forecast snow amounts tonight with most areas likely to see a dusting to an inch; though areas of NE IL and NW IN could see an inch to perhaps 2" on the high end. One change to forecast for Saturday morning was to add post frontal snow showers and flurries in the CAA regime, particularly eastern CWA in closer proximity to the cold upper low. Moisture will be limited, but what little moisture there is should be squeezed out fairly easily given the steep low level lapse rates and strong CAA. Not expecting much more than isolated amounts of a few tenths to maybe a half inch, but that should be spotty given the expected isold-wdly sctd nature of the activity. The same steep low level lapse rates and high RH in the low level conducive for CAA driven snow showers over land will make for a stellar lake effect snow (LES) set-up tomorrow. LES parameters are quite impressive and it looks like an intense single band could develop. Tropopause drops to near 500mb which will also serve as the equilibrium level for lake induced convection (very high for LES) and combined with 850-lake delta Ts approaching 20C, suggests a very potent LES band should develop. Given the progged parameters, if single LES band develops as expected, would expect snowfall rates of at least 2" per hour (probably higher) in the heart of the snow band. The unseasonably warm lake and deep instability also raise the specter of potential isolated thunderstorms with the LES band tomorrow. Big question for snowfall amounts will be residence time of the snow band at any given location. Hires models strongly suggest that lake induced mesoscale enhancement of the surface low will take place Saturday. The primary LES band is expected to develop to the west of this low, with the low likely to result in some meandering/wavering of the position of the band. Gave very strong consideration to upgrading Porter to a winter storm warning, as such an intense band could drop 6" of snow in just a couple hours. If rates of that magnitude occur, impacts could be very significant, particularly in light of the expected 30-45 mph winds near the lake which could result in white-out/blizzard conditions at times in the more intense snow squall(s). The primary factor keeping me from upgrading to a warning at this time is the uncertainty in location of the LES band. The operational HRRR keeps it primarily east of Porter County, but many other models have it farther west and keep Porter County well within the line of fire. - Izzi && .SHORT TERM... 112 PM CST Through Saturday... Expect a burst of dry snow tonight area wide, mainly after midnight. Gusty northwest winds will arrive Saturday morning creating low lying drifting snow. Northwest Indiana, particularly Porter County, will likely see some lake effect snow, along with blowing snow on Saturday. We did not see any big changes in the 12z guidance to change the message of a quick hitting band of accumulating snow overnight. This system is still across southern Canada, but will make a bee- line southeast into our area tonight, then quickly exit east on Saturday. This will be a drier snow with fairly decent -12 to -18C omega shifting through the area just ahead of a cold front overnight. Areas east stand chance of a bit more accumulation. Expect a several hour period of snow, and with cold pavement, accumulation will happen quickly once the initial top down saturation occurs this evening. The main snow will occur after midnight. This looks like a solid 1-2" event across the center of the CWA, with lower totals west (north central Illinois and across Central Illinois) and higher numbers possible east across Indiana. A few showers will linger Saturday morning, though accumulations would be limited to a dusting or a few tenths. Concerns on Saturday will shift to gusty northwest winds, drifting/blowing snow. In northwest Indiana, lake effect snow is expected. Earlier in the shift, we added a winter weather advisory for Porter County beginning tonight and ending late Saturday. The synoptic snow does not appear to be significantly different than other areas, although northwest Indiana is in a bit more favored track for initial synoptic snows. After these few inches of snow fall, strong winds (gusts to 30 mph or so) will occur behind the snow as a compact surface low will pass right over southern Lake Michigan. Forecast soundings across northwest Indiana depict several hundred J/Kg of lake induced CAPE, with this CAPE square through the dendritic growth region, and reasonably high equilibrium levels. The heaviest lake effect snow (which is typically highly variable dependent on location) will fall into LaPorte and counties east where stronger convergence is depicted, but there is a decent signal into Porter county for at least a time. These combined concerns of strong winds, especially for northern portions of the county seem to warrant an advisory. KMD && .LONG TERM... 147 PM CST Saturday night through Friday... A fairly active weather pattern is still expected for the next several days, with periodic chances for pcpn and generally below normal temperatures. By Saturday evening, any remaining snowfall will likely be some lingering lake effect snow for northwestern Porter County as low level winds back from nwly early in the evening to swly by morning. Wind speeds should also be on a diminishing trend as a weak ridge of high pressure builds across the midwest. The upper level pattern will continue to trend toward high amplitude and slow progression with upper ridging building over the wrn 1/3 of the CONUS and broad troughing over the ern 2/3 CONUS. The slowly progressive, high amplitude long-wave pattern lends relatively higher confidence in the overall trends of the long term forecast period, with persistent nwly flow aloft keeping the region under persistent, deep layer cold advection. This would suggest that the trend of below normal temperatures will continue through the period. Toward the end of the forecast period, from Wednesday through Friday, the long term models are trending a bit warmer than the large scale pattern would suggest and feel that model guidance temperatures are a bit too high, so have trended the temperature forecast a few degrees lower than much of the model guidance would suggest. The other, more complicated, problem for the extended forecast period will be pcpn chances. Under nwly flow aloft, pcpn chances will largely be driven by shortwaves dropping out of Canada and tracking across the midwest. After a dry period Saturday night and sunday, general forecast confidence is relatively high in periods of pcpn, likely snow, from Monday through the remainder of next week. There is relatively good agreement among the models in the idea of a quick moving clipper bringing another round of snow to the region. The fast nature of the system would suggest that any new snow accumulations should be light. After Monday, confidence in the exact timing, track and intensity of individual shortwaves diminishes as model solutions of the finer scale details diverge. So, will continue the idea of periodic chances for pcpn, which should mostly be snow. Each shortwave dropping out of Canada will bring a reinforcing shot of cold air, helping to maintain the general trend of below normal temperatures through next week. && .AVIATION... For the 00Z TAFs... The main concern is the period of snow tonight. Light accumulations of up to an inch or so of dry/fluffy snow are expected for the eastern terminals, with less west. There is some uncertainty on the snow start time later this evening, which will depend on the low levels fully saturate. However, confidence is medium-high in timing of the highest rates, which will be up to about 3 hours after 06z. Looking at upstream observations, visibility of 1 mile or less is probable during the period of highest rates, along with brief low MVFR to IFR CIGs. The steadier snow will quickly taper off prior to 12z, with on and off snow shower activity likely for the eastern terminals through mid day/early afternoon. There looks to be a window where 2-4 hours of more robust snow showers are possible, starting in the mid to late AM. In addition, lake effect snow could set up near GYY, but most likely just east. Southwest winds will continue in advance of the clipper low pressure system this evening and much of the overnight. Winds will then shift to strong northwest Saturday morning as the low tracks to western lower Michigan, with gusts in the 25-30 kt range, possibly higher at times especially at GYY. Any lingering MVFR CIGs will clear by Saturday evening, along with diminishing west- northwest winds. Castro && .MARINE... 200 PM CST Headlines...A Gale Warning is now in effect for the southern 2/3 of Lake Michigan, from Saturday morning into Saturday evening. North to northwest winds are expected to increase rapidly late tonight as deepening low pressure lifts through the eastern Great Lakes and high pressure builds across the Upper Missouri Valley. The far northern portions of the lake will likely remain below Gale Force as the gradient should be a little weaker there. The overall period of gale potential should be relatively short as the ridge builds across the western Great Lakes by Saturday evening, allowing winds to diminish and back to southwesterly. The series of low pressure systems will continues through next week. Another weak low will form near Lake Superior Sunday morning followed by a stronger low that moves over Western Great Lakes Monday afternoon. Guidance differs if the low will pass over central Lake Michigan or south of the lake entirely, which would have some impact on the forecast wind trends. However, at this time, it looks like Gales are again possible behind the low Monday night into Tuesday. Another high pressure ridge will quickly pass over the western Great Lakes Wednesday bringing a short period of diminishing winds, before ramping up again as the next low approaches the Great Lakes late next week. && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Winter Weather Advisory...INZ002...midnight Saturday to 6 PM Saturday. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Saturday to 9 PM Saturday. Gale Warning...LMZ743-LMZ744-LMZ745...9 AM Saturday to 9 PM Saturday. Small Craft Advisory...IN nearshore waters until 9 AM Saturday. Gale Warning...LMZ777-LMZ779...6 AM Saturday to 9 PM Saturday. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO
Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
420 PM MST Fri Dec 8 2017 .DISCUSSION...(This Evening Through Friday) Issued at 219 PM MST Fri Dec 8 2017 Strong upper ridging persists across the western CONUS with deep upper trough across the eastern half of the country. Surface low sits over northern MN with a strong pressure gradient continuing across western SD. Winds across the area are generally northwest 20 to 30 mph with gusts to 40 mph. The exception is across the plains just east of the Black Hills where winds obs show 25 to 40 mph with gusts up to 55 mph. Winds will decrease over the next two hours as the overnight inversion begins to set up. Upper ridging over the next several days will move very little...resulting in not much change in the weather pattern through much of next week. The baroclinic zone will stay east of the area with temperatures generally in the 40s to near 50. However warmer temps are still progged to push into the region through Saturday and Sunday...with highs in the 50s (possibly even low 60s Sunday). Winds will also be persistent through the next several days. Breezy northwest winds will continue through the weekend. A clipper system will cross the area Monday driving in colder temps in the 30s and 40s...along with strong northwest winds. It looks like winds Monday could reach advisory criteria...and possibly high wind warning. Will have to keep an eye on models trends for that system. Rest of the week will be quiet with highs in the 40s and not-as-breezy winds. Next chance for snow showers will be toward the end of the work week. && .AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening) Issued At 417 PM MST Fri Dec 8 2017 Gusty west/northwest winds will continue through the period, esp around the eastern Black Hills. Low-level wind shear is expected at RAP later this evening into early Saturday morning with northwest winds at 50 knots around 2000 feet as surface winds lessen to under 20 knots. Otherwise, VFR conditions will prevail. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Wind Advisory until 5 PM MST this afternoon for SDZ026-031-072- 073. WY...None. && $$ DISCUSSION...13 AVIATION...JC