Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 01/19/22

Area Forecast Discussion
National Weather Service Denver/Boulder CO
833 PM MST Tue Jan 18 2022 .UPDATE... Issued at 833 PM MST Tue Jan 18 2022 All the ingredients are in place for freezing drizzle early Wednesday morning. As is nearly always the case in these events, uncertainty is very high in timing, duration, and amounts. That`s because minor fluctuations in temperature of the saturated layer, and how moist the near surface layer can become, determine a large part of how much FZDZ can occur and thus, how much ice to expect on bridges, overpasses, sidewalks, and some roads (or most roads if enough drizzle falls). The cold front responsible for all of this is still up in Nebraska and east-central Wyoming. It should arrive by midnight but saturation in the low levels will take a few hours after the winds shift northeast. By 3 AM saturation may be reached across the northeast plains, and by 5 AM pretty much all of our area along the east slope foothills below 8,000 ft, the I-25 corridor, and all of the plains could start to see drizzle due to saturation in the lowest several thousand feet, shallow upslope flow, and the fact that the temperatures in the saturated layer continue to only be around -8 degC through midday. Thus, freezing drizzle could occur across the I-25 corridor from 4 or 5 AM through about noon. If that happens, and the lift/moisture is enough to generate decent drizzle, significant impacts to travel could occur for the morning commute. Confidence in those impacts is very low, and focused on how much drizzle might occur. Temperatures will be plenty cold for it to freeze to elevated surfaces and even some roads and sidewalks. That is the reason for the Winter Weather Advisory: There are enough ingredients in place, and the ensuing impacts could be significant if enough drizzle falls. The HRRR has recently picked (or technically re-picked) up on the FZDZ for our area, perhaps providing a tiny bit of increased confidence that impacts could materialize. Normally even CAMs have a hard time generating FZDZ because of the shallow nature and extreme sensitivity to temp/moisture profiles in time and space. The overall forecast from this afternoon hasn`t changed much at this point. We will know much more about drizzle amounts a few hours after FROPA late tonight via surface obs, spotters, and webcams. Snow amounts for the afternoon/evening hours are much more certain, as the signal remains for some QG lift to overspread the drizzle layer, and seeder/feeder processes will quickly win out to generate light snow across much of the area, but additional travel impacts (or for areas that don`t see any drizzle) due to snow are expected to be limited. && .SHORT TERM...(This evening through Wednesday) Issued at 348 PM MST Tue Jan 18 2022 Main concern for impacts will be the potential for areas of freezing drizzle and light snow late tonight into Wednesday extending from the plains into the urban corridor. A light glaze is possible on roads and other hard surfaces which may impact the AM commute. A Winter Weather Advisory is in effect for the plains and foothills early Wednesday morning into Wednesday evening. Cross-sections show the post-frontal airmass further cooling and moistening overnight. Light shallow upslope and weak synoptic lift in combination with a moist layer to the surface will support light precipitation and low ceilings. This will start in the NE corner of CO and gradually progress westward Wednesday morning. Sounding profiles have been consistently signaling potential for freezing drizzle. Temperature profiles in the moist layer hold above -10 C limiting ice crystal growth along with a dry layer above. With below freezing surface temperatures, freezing drizzle is a possibility. By early Wednesday morning, freezing drizzle chances increase westward into the urban corridor in time for the morning commute. A light glaze is possible on roads, sidewalks, and elevated surfaces which may impact the AM commute. Model soundings show the NE plains to cool quickly through the morning, so development of flurries/light snow become more likely. Chances for freezing drizzle decrease from east to west Wednesday afternoon and chances for light snow increase as temperatures profiles further cool. Snow accumulations are expected to be minimal. .LONG TERM...(Wednesday night through Tuesday) Issued at 348 PM MST Tue Jan 18 2022 Models still show a subtle shortwave sliding across our area Wednesday night. It`s trending weaker and further southwest though. There`s always been a question of how much that would interact with the low level cold air, but there`s at least a chance it could reinvigorate some light snow for a while overnight. This is now looking more likely over the mountains and southern part of our area. Otherwise, the low level forcing for precip will be fading and dropping south of our area during the evening. We`ll hang on to the Winter Weather Advisory for the foothills and Palmer Divide areas until 8 PM in case there`s enough snow to keep things slick, but we may just have very light snow continuing. Temperatures Thursday are a challenge. There will be clearing, but there might still be some clouds in the morning. This will also depend on how much snow there is. We compromised between the various solutions and will keep temperatures in the 30s on the plains. Clouds will spread back in Thursday night ahead of the next shortwave. There`s getting to be pretty good agreement now on the Friday system. We`re on the edge of the QG lift as the system shears out to our west, and the low level wind and temperature fields aren`t too exciting. But there will be a zone of low level convergence, a little cooling aloft, and upslope into the foothills that will produce light snow over the mountains and there`s a pretty good threat of a few hours of light snow on the plains as well. This will mainly be during the day, though upslope areas west and south of Denver could hang on into the evening and these areas should have the greatest snowfall. Like Wednesday`s system, it doesn`t look like much precipitation, but the placement/timing could still mess up the Friday evening rush hour around Denver and into the mountains. Behind the trough, we`ll have NNW flow over the weekend with a bit more wind in the mountains. Saturday looks sunny, but we might not have displaced the cold air from the plains much yet. There should be more of this on Sunday, with warmer temperatures at least near the foothills and possible across the plains as well. The models are slowing with the development of the next upstream trough (lift arriving Tuesday instead of Monday now), but there is a weak front in northwest flow bringing just a little cooling and probably more clouds for Monday. There will be another chance of snow along with cooler temperatures when the next trough arrives. For now, this is slated for Tuesday into Wednesday, but it looks like another system that might shear off more to the southwest so some degree of additional cooling is likely but the threat of snow is more speculative. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday evening) Issued at 457 PM MST Tue Jan 18 2022 This is challenging forecast for Wednesday. The general ingredients are well understood, but trying to pin down details and the impacts to aviation operations will be very challenging. At a high level, a cold front moves through a little before midnight, with ENE winds shifting to E and going from 12-15 kt to 10 kts or so by 1-3 AM. IFR CIGs will be delayed until midnight or an hour or two after midnight given significant dry air to overcome. Eventually the lowest levels saturate but it remains dry aloft, a classic set up for freezing drizzle. The freezing drizzle will transition to snow at some point Wednesday afternoon or early evening. Amounts of FZDZ and snow will be light, but even a few hundredths of FZDZ can cause impacts to runways and aircraft, especially if VIS drops to under 1 mile and the CIG drops below 003, both of which could happen. The issue is it`s a very low confidence scenario in terms of pinning down when the FZDZ may occur along with the lowest CIG/VIS. Our best bet for now is some time between 3 AM and 11 AM. Light snow is possible throughout the afternoon but under an inch of accumulation is expected, and IFR/ILS conditions should continue well past midnight into Thursday morning though not as low as during the Wednesday morning hours. If there is any good news to this forecast it is that winds should not be much of an issue. Much of the day Wednesday they should be out of the east at 10 kts or so. VFR conditions should return around sunrise on Thursday morning and winds will be light out of the south. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 2 AM to 5 PM MST Wednesday for COZ042-044-048>051. Winter Weather Advisory from 5 AM to 8 PM MST Wednesday for COZ035-036-038>041-043-045>047. && $$ UPDATE...Schlatter SHORT TERM...Danielson/Mensch LONG TERM...Gimmestad AVIATION...Schlatter
Area Forecast Discussion
National Weather Service Memphis TN
906 PM CST Tue Jan 18 2022 .DISCUSSION...00Z upper air analysis this evening indicates a mid-level trough axis over portions of the Central and Southern Plains this evening. 02Z surface analysis places a developing area of low pressure in the Red River Valley of Texas while a ridge of high pressure is centered over the Carolinas. In addition, an occluded front is located from the Great Lakes region back through the central Plains. Southerly winds on the back side of high pressure is bringing some milder air up into the Lower Mississippi Valley. As of 8 PM CST, temperatures across the Mid- South range from the middle 40s to lower 50s. 00Z KLZK upper air sounding indicates a substantial amount of dry air present across the region. Short term models including Convective Allowing Models (CAMs) suggest rain chances should hold off until around daybreak as the atmosphere will need to sufficiently moisten up across the Mid-South. Forecast overall in good shape for the rest of the night. CJC && .PREV DISCUSSION... /issued 529 PM CST Tue Jan 18 2022/ UPDATE... See aviation discussion. PREV DISCUSSION... /issued 338 PM CST Tue Jan 18 2022/ DISCUSSION... One more mild (if rainy) day ahead for the Midsouth, before Arctic air arrives. Models have remained consistent with timing and strength of the Arctic front, which will pass through the Midsouth late Wednesday afternoon through the evening. Low level moisture will be sufficient for widespread showers, developing ahead of weak shortwave ejecting from the Red River valley. Marginal instability (MUCAPE around 600 J/KG) will support thunder south of I-40, but mainly across north MS Wednesday afternoon. As the shortwave exits by 03Z Thursday / 9PM CST Wednesday, precip coverage should diminish. NAM depicts isentropic downglide on the 290K surface by this time. But with frontal depth increasing, the 290K surface as a whole will be lifting at a good clip, likely enough to generate light snow or flurries, following a brief rain- freezing rain and sleet transition. In all, little to no accumulation of ice or snow is forecast. Main impact would be possible light glazing on elevated surfaces such as signs and mailboxes. A quick look at the 18Z HRRR depicts Memphis being dry- slotted, with no freezing or frozen precip after rain tapers off by early evening. NBM likely holds onto precip chances too long into the night Wednesday night, but have kept low PoPs for consistency and likelihood of sensible, if not measurable wintry precip into the early overnight. As mentioned previously, the biggest weather impact -by far- will be the much colder air flowing into the Midsouth. Models have locked onto the strength of the cold air and this afternoon forecast remains consistent with earlier forecasts. Dry weather appears likely into the weekend, as subtropical moisture remains along the gulf coast. A weak clipper-type northern branch will likely bring a brief shot of snow to the Ohio Valley Sunday night. Will need to watch this feature for possible track change if the overall longwave trof over the eastern CONUS is deeper than currently forecast. Winter is here for a while, with no significant warm ups on the 7 to 10 day horizon. PWB && .AVIATION... No major updates to the forecast. VFR conds will prevail through 08Z. Expect MVFR cigs to move into the region as a cold front moves into the area. Light showers will begin to develop around by sunrise and will increase in coverage through end of TAF period. Expect conditions to deteriorate by mid- morning to IFR by 15-18Z. In addition, a few TSRAs could develop near KMEM between 19-23Z tomorrow as the front moves into the vicinity. Southerly winds of 10-12 KTS with higher gusts at times will occur until the front moves through. Will continue with low level wind shear tonight as well. && .MEG WATCHES/WARNINGS/ADVISORIES... AR...None. MO...None. MS...None. TN...None. && $$
Area Forecast Discussion
National Weather Service Pocatello ID
233 PM MST Tue Jan 18 2022 .SHORT TERM...Tonight through Thursday night. Satellite imagery shows broad area of low stratus and/or fog throughout the Snake Plain and slowly creeping into adjacent valleys, with a secondary area over Bear Lake. Inversions remain robust overnight, and guidance has trended into not only holding on to larger areas of light fog, but returns the dense fog to a significant portion of the region 02z- 06z. Dense Fog Advisory from earlier was extended through tonight into Wednesday morning, and expanded further north into the Arco Desert, Upper Snake Plain, and Shoshone areas of the Snake Plain. There is strong evidence that the scenario could actually repeat Wednesday night into Thursday ahead of the incoming shortwave. Which may be problematic moving into Thursday afternoon. More on that momentarily. Guidance remains steadfast in bringing light snowfall into the central mountains Thursday morning, spreading southeast across the remainder of the forecast area during the afternoon. The bulk of the snowfall should occur from mid afternoon to late evening, then lingering in the southeast highlands ahead of the trough axis descending through East Idaho late Thursday night into Friday. Snow accumulations range from under an inch across shadowed portions of the Snake Plain to roughly 3-5 inches in some of the higher elevations of the eastern highlands for Thursday and Thursday night. Accumulations of 1-3" look very reasonable at this time for the main travel corridors, but some of the ensemble probabilities are pushing the 2-4" narrative. This is where the fog scenario ahead of the trough may come into play. If there is not enough available mixing ahead of the main trough axis, it is entirely plausible to see the stratus linger, and then advancing mid-cloud provide some additional seeder-feeder potential into the lower elevation areas, which would start the snowfall earlier and bump the totals. DMH .LONG TERM...FRIDAY THROUGH NEXT TUESDAY... Snow showers will continue Fri AM with the shortwave trough axis roughly overhead, along with breezy conditions at higher elevations across the Central Mntns as well as across the ern Magic Valley and srn highlands west of I-15. Have nudged these areas a bit higher than NBM wind guidance through Fri afternoon (closer to the experimental NBM 4.1 which does a much better job with our winds), although still well below advisory criteria. Snow showers should end by late AM for most areas, but may linger into the afternoon east of I-15, with additional daytime accumulations generally a dusting to an inch. As the shortwave splits, both pieces of energy progress south/east of our CWA allowing a Pacific ridge of high pressure to exert greater dominance Sat into next week. Our forecast focus will likely turn back to temp inversions and fog/low stratus potential. A shortwave riding the flow between the ridge and departing trough may brush Island Park and the WY border mntns with light snow showers Mon/Tue, but confidence is low as this will depend on its exact track. High temps remain stagnant each day ranging from the mid 20s to mid 30s. Early indications are that the nearby ridge may hold intact throughout next week, keeping the active storm track well north of our area. - KSmith && .AVIATION... Widespread IFR fog/low stratus continues to blanket the ern Magic Valley and Snake Plain corridor including KBYI, KPIH, and KIDA. We`re just not seeing anything that is going to significant break up/scour out this activity, so have nudged TAFs down into continued IFR conditions all day with only modest improvement to vsbys. After sunset, HRRR cig/vsby guidance, HREF RH and sky ensemble means, and NBM guidance all reinforce fog potential and slightly lower cigs once again as the sfc cools/saturates, and given trends today and a lack of any significant disruptive factor, have leaned on these with all 3 terminals potentially reaching airfield minimums yet again between 04-07z/9pm-midnight and staying there through late Wed AM. Overall confidence is moderate (lower on whether or not KDIJ will join the party as currently forecast...if anything, there are some hints that cigs/vsbys could deteriorate again an hour or two earlier than currently forecast this eve. We will need to continue to monitor closely and amend as needed. Winds remain fairly light throughout the period with no precip expected. At KSUN, the diurnal wind shift to SE is now advecting fringes of the fog/low stratus shield into the terminal. Covering the worst impacts with a TEMPO as satellite shows the edge right over the terminal with potential for it to drift in and out. A return to VFR is likely around 03z/8pm there as soon as winds shift to NW and both introduce slight downslope drying and also physically push nearby activity back out of the Wood River Valley. At least some cig/vis improvement is likely Wed afternoon, but another impactful descent into madness is possible Wed night. Our next chance of snow and associated impacts arrives Thu afternoon. - KSmith && .PIH WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 11 AM MST Wednesday for IDZ051>055-057. && $$