Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/15/20

Area Forecast Discussion
National Weather Service Charleston SC
923 PM EST Mon Dec 14 2020 .SYNOPSIS... High pressure will build across the region tonight, then pass to the north on Tuesday while a trough develops along the coast. A storm system will bring impacts to the area Tuesday night into Wednesday, followed by more high pressure during the later half of the week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... No meaningful changes were made for the late evening update. The cold front is now well offshore with high pressure starting to wedge south into the Southeast States. Dry conditions will prevail overnight, although isentropic assent will begin to increase early Tuesday as winds above the boundary layer begin to veer in response to low pressure organizing over the Texas Caprock. This will result in a slow increase in clouds overnight, first in the form of cirrus, followed by a 2500-4500 ft cloud deck forming by daybreak across mainly the coastal counties. Lows from the upper 30s well inland to the upper 40s/lower 50s at the beaches look on track. Breezy conditions will also redevelop along the beaches late in response to a tightening pressure gradient. Lake Winds: Winds at PNOS1 remain below Lake Wind Advisory thresholds. However, the latest RAP data suggest another weak surge will work south over the next few hours which could push gusts into the 25 kt range. Per coordination with WFO Columbia, the decision has been made to keep the advisory in place through its current 4 AM expiration time. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/... Tuesday: The area will remain situated between a ridge of high pressure to the southeast and a trough of low pressure digging across the Central United States. At the sfc, high pressure will prevail across the area, especially inland and north, while a coastal low develops off the Southeast coast late. The pattern favors mainly dry conditions across land, especially away from the coast for much of the day. Temps will be considerably cooler than the previous day within a northerly sfc wind. In general, highs will range in the mid 50s. Tuesday night into Wednesday: Temps should dip into the low/mid 40s Tuesday night as clouds spread across the area. A coastal low will deepen along the Southeast coast overnight, then shift north of the area early Wednesday as a low pressure system and associated cold front approach from the west and shift offshore Wednesday afternoon. Precip chances should initially develop along coastal locations Tuesday night as the coastal low deepens, before the onset of greater precip coverage associated with the arriving cold front late Wednesday morning into the afternoon. Rainfall accumulations will be limited somewhat given how quickly the front passes through the region. However, PWATs peaking in the 1.25-1.50 inch range suggest accumulations up to around 1/4 inch. High temps will remain below normal Wednesday, peaking in the low/mid 50s inland to upper 50s/lower 60s along the immediate coast (warmest in Southeast Georgia). Thursday: Dry high pressure will build across the region behind a departing cold front well offshore. Mostly sunny skies and a downslope wind should result in slightly warmer temps (especially inland), but temps should still remain a few degrees below normal for this time of year. In general, highs will range in the mid/upper 50s. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... High pressure will move over the Northeastern U.S. into the weekend, leading to dry weather and below normal temperatures into Saturday. Models then diverge early next week regarding rainfall potential, so we generally went with chance POPs as a placeholder until the models line up better. && .AVIATION /03Z TUESDAY THROUGH SATURDAY/... VFR will prevail overnight. Cigs are expected to lower to MVFR thresholds by mid-morning at KSAV and early afternoon at KCHS in response to increasing isentropic assent atop the inland wedge of high pressure. There are some indications that isolated- scattered light showers could develop across the coastal counties tomorrow in response to low pressure developing off the coast. How close the low tracks will determine how much shower coverage develops over land. Will keep any mention of shower activity out the terminals for now given low confidence on both timing and coverage. Gusty winds with gusts near 20 kt will impact the terminals, especially from daybreak on. Extended Aviation Outlook: Gusty winds near 20 kt are likely into Tuesday evening, especially at KCHS. A low pressure system and associated cold front should then bring flight restrictions Wednesday before dry high pressure prevails across the area late Wednesday night into Friday. && .MARINE... Tonight: High pressure will build in and winds will increasingly become northeasterly. A solid surge is expected due to the tightening pressure gradient and cold advection, mainly 20 to 25 kt with gusts to 30 kt. Seas will build back up to 3-7 feet, highest beyond 20 nm. Tuesday and Wednesday: Strong northerly winds (25-30 kt) and seas up to 6-8 ft will prevail across all waters Tuesday and Tuesday night while a coastal low develops/deepens along the Southeast coast. Small Craft Advisories will therefore remain in effect across all waters through Tuesday night. The coastal low is then expected to shift north of the local waters early Wednesday, resulting in a decreasing pressure gradient across the area and improving wind/sea conditions. Small Craft Advisories will begin to end across the CHS Harbor and nearshore waters south of Charleston County by late Wednesday morning, while lingering 6 ft seas result in Advisories across northern South Carolina waters and offshore Georgia waters into early Wednesday afternoon. Wednesday Night through Friday: High pressure will build across the area Wednesday night behind a departing cold front, then persist through Friday. Winds/seas are expected to remain below small craft advisory level conditions during the second half of the week. && .TIDES/COASTAL FLOODING... Tide levels will remain elevated through the middle of the week due to lingering astronomical influences and favorable winds. For the Charleston Harbor tide gauge, guidance continues to indicate at minor/moderate coastal flooding (7.0 ft MLLW/7.5 ft MLLW) with each morning high tide cycle through Wednesday. At the Fort Pulaski tide gauge, minor coastal flooding (9.5 ft MLLW) will be possible Tuesday and Wednesday. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...Lake Wind Advisory until 4 AM EST Tuesday for SCZ045. MARINE...Small Craft Advisory until 9 AM EST Wednesday for AMZ352. Small Craft Advisory until 1 PM EST Wednesday for AMZ350. Small Craft Advisory until 7 AM EST Wednesday for AMZ330-354. Small Craft Advisory until 4 PM EST Wednesday for AMZ374. && $$
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
556 PM EST Mon Dec 14 2020 .AVIATION... There has already been a downward trend in wind speeds. This will continue during the evening with the loss in diurnal mixing and a relaxing of the cold air advection. The moisture flux off Lake Mi is still sustaining a healthy VFR based strato cu field across Lower Mi. Area observations and recent RAP soundings suggest these clouds will prevail during the evening. Persistent mid level subsidence, the advection of dry air across the lake and a more northerly and divergent component to the low level flow will all contribute to the eventual erosion of these clouds overnight. For DTW...Based on the RAP and with clouds still extending all the way back to Lake Mi, the 4-5kt strato cu field may hold over metro until at least 06Z. Otherwise, wind directions will hold around 300 to 310 degrees but speeds will continue to drop markedly through the evening. .DTW THRESHOLD PROBABILITIES... * Moderate in ceilings at or below 5000 feet this evening. && .PREV DISCUSSION... Issued at 300 PM EST Mon Dec 14 2020 DISCUSSION... Low level cold advection under the northwest flow this afternoon is just about exhausted. With 850 MB temps around -13 C, steep low level lapse rates have been able to generate scattered flurries, but aggressive subsidence/drying is now beginning to taking place, as 850 MB dews pt depressions of 30-40 C take hold this evening through tomorrow. A ridge of high pressure will build in tonight, with winds likely decoupling by morning. Fortunately, lack of snow cover around should prevent temps from really cratering, but still a cold night with mins down into the teens, as there also may be some lingering low clouds but more likely some increase in high clouds to keep temps stabilized. The strong upper level northwest confluent flow in place over the Great Lakes Region will be important, as the upper level energy/wave ejects out of the Four Corners region. None-the-less, still enough model consensus of the lingering mid level trough/500 MB low reaching the southern Michigan border by 00z Thursday as the transfer of energy/height falls occur along the East Coast Wednesday night. Although the airmass is very dry initially over southern Lower Michigan Wednesday morning, the 12z NAM does show the remnant 850-700 MB circulation tracking along the southern Michigan border during the afternoon, with specific humidity rising to 2-2.25 g/kg in that layer. Thus, light snow is now appearing likely along and south of I-94, as the low level easterly winds off off Lake Erie/Lake St. Clair also help pre-condition/moisten up the levels. Thermal profiles are not bad, with 850 MB temps in the negative mid- high single numbers, with 700 MB temps in the negative lower teens. Certainly can see us grind out up to an inch of snow accumulation Wednesday afternoon-early evening along and south of the I-94 corridor, with snow chances quickly tapering off as one heads north. The forecast for Thursday is a bit murky for light snow/flurry chances, as the pace of the 500 MB shortwave/trough exiting east is in question, with another upper wave diving south through the Western Great Lakes during Thursday. Inverted surface trough arching through western Lake Erie and into southern Lake Huron, coupled with the favorable 850 mb temps of -7 to -10 C argues to maintain slight chance pops/flurries across far eastern areas with the low level convergence/northeast winds off the lakes. Good warm advection on Friday as amplified upper level ridge translates east through Lower Michigan. A strong upper level wave will track through Ontario Friday evening/night, but will be tracking off to the northeast. Thus, the cold front moving through the Great Lakes region will be weakening, and losing its identity on Saturday. Per 12z Euro, another series of fast moving Pacific waves to track across the northern Great Lakes late in the Weekend into early next week, but 850 MB temps look marginal enough not to illicit a whole lot of help from the Great Lakes to support snow showers. Thus, only slight chance pops appear warranted, especially when you factor in the timing uncertainty to the equation. MARINE... Cold advection continues across the region this afternoon in the wake of a cold front. West-northwest gusts to marginal gales will continue through the afternoon across northern Lake Huron where the Gale Warning remains in effect through this evening. The gradient and wind speed weaken tonight as arctic high pressure builds in from the north and west. This high will dominate the weather on Tuesday with lower impact marine conditions in store as waves diminish. Wind then reorients from the east on Wednesday as a low pressure system passes through the Tennessee Valley into the eastern seaboard. Some light snow will be possible across western Lake Erie and Lake St. Clair, otherwise the greater impacts will remain to the south and east of our area. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Gale Warning until 10 PM EST this evening for LHZ361-362. Small Craft Advisory until 4 AM EST Tuesday for LHZ421-441-442. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....SC DISCUSSION...SF MARINE.......TF You can obtain your latest National Weather Service forecasts online at
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
855 PM CST Mon Dec 14 2020 .UPDATE... 855 PM CST The forecast through the rest of tonight and into the day Tuesday is in fine shape. Also keeping an eye on 00Z guidance for Wednesday morning, which thus far in RAP and NAM solutions continue the trend of inching light snow northward with the increasing possibility of light accumulation for mainly, but maybe not entirely, south of I-80 and east of I-55. High clouds have been a little slow to thicken this evening over far northern Illinois and northwest Indiana, but immediate upstream satellite and model solutions support a greater coverage and thickness by after midnight. The day shift had already accounted for this in the low temperature forecasts. Without the clouds, single digits would be realized in the far north and west. Clouds may thin temporarily Tuesday morning before thickening again Tuesday afternoon. Water vapor imagery early this evening depicts the potent upper trough (mentioned in the prior issued discussion) directly over the Four Corners region. It is potent in that it is still digging yet moving eastward at a good clip. The 00Z Flagstaff AZ sounding indicated a 500 mb temperature of -27C, in the lowest percentile for this time of year, just an indicator of the anomalous nature. The 00Z RAP and NAM continue to trend a wee bit slower, deeper, and further north with this as it migrates into the Lower Ohio Valley region late Tuesday night into Wednesday morning. The 850-700 mb flow is now more southeasterly with better moisture transport and sufficient saturation into the southeast third of the forecast area to support light snow. The overnight shift will fully analyze the suite of guidance to address probable higher PoPs in this area, and with the timing near daybreak into the morning after a couple cold days, to potentially mention light snow accumulation. Conceivably this could even be over an inch in the far southeast CWA (Benton, Iroquois Counties area) looking at the incoming guidance and upper/mid-level pattern and vorticity maximum path. Further north into the heart of the Chicago metro, the atmosphere has less deep saturation persistence, but the northeast flow off the lake does look to possibly support lake- induced instability with the anticipated lake effect clouds for a regular flurry, maybe even at times light snow, on Wednesday. Again overnight shift will assess in full and update if needed. MTF && .SHORT TERM... 1224 PM CST Through Tuesday night... Currently temperatures are in the upper 20s to lower 30s with mostly sunny skies. A weak cold front passed through this morning, giving way to breezy northwest winds (the air does have a bite today). All things considered it`s a pleasant December day. Tranquil conditions will prevail through the next 48 hours. Tonight, clouds will increase in accordance with a developing storm system well southwest of our area. As a surface high pressure system slides over the northern Great lakes, winds will gradually become easterly. Lows overnight should fall into the upper teens to lower 20s areawide, with the coldest temperatures along the IL/WI stateline where a snowpack (seen nicely in visible satellite imagery today) resides. Tomorrow will be rather drab with mostly cloudy skies and highs similar to today - near the freezing mark. With flow off Lake Michigan, a few flurries will be possible along the lakeshore. Borchardt && .LONG TERM... 150 PM CST Wednesday through Monday... Overall quiescent weather conditions are expected through much of the extended portion of the forecast, although our tranquil periods may be punctuated by some occasional precipitation chances as an amplified pattern across the Pacific continues. The first such disturbance we`ll be keeping an eye on will be at the start of the extended very late Tuesday night and into Wednesday morning. This feature is currently dropping southeastward across the Great Basin within a positively-tilted trough axis which stretches northeast into the Upper Midwest. Deterministic guidance remains in pretty good agreement depicting this wave`s eastward translation across the Central Great Plains tomorrow and tomorrow night as it gradually acquires more of a neutral tilt with time. The ensemble envelope--which began a shift to the northwest yesterday--is now being reflected a bit more in the current iterations of the ECMWF, GFS, and UKMET which are now squeezing out some (light) precipitation south and east of the I-57 corridor into early Wednesday afternoon. Large scale forcing for ascent, at least for a brief window, is actually relatively robust as the combination of warm advection, DCVA, and some degree of upper jet divergence overlap before the trough axis pushes east of our longitude. This forcing is now showing up well on the 300-305 K theta surfaces in today`s guidance. Fairly cool temperatures aloft (-28 C at 500 mb) will support at least some steepening of the mid-upper lapse rate plume with this feature, so will be monitoring this system for any additional northwestward jogs given the potential for embedded convective elements. Also seeing some signs of a weak lake- enhancement and f-gen circulation in some of the guidance trying to establish farther northwest closer to the I-88 corridor, although prolonged dry easterly trajectories will reinforce a rather dry low-level airmass with north and westward extent across our CWA. For now, have continued to expand some low PoPs farther northwest than the offered blend today with some light snow accumulations for locales roughly south and east of I-57 with dry conditions elsewhere, although could envision some seeder-feeder type scenario developing across northern Illinois which could bring some low snow chances farther north than currently depicted in the forecast grids. We`ll get stuck in a col/saddle point in the low-level flow into Thursday afternoon. The surface pressure gradient begins to tighten as we head into Friday which should allow our temperatures to begin to climb back above the 40-degree mark with the return of breezy south to southwesterly winds. The associated upper vort/jet streak pass overhead then on Saturday. The main corridor of large scale forcing looks to remain just north of the region with a somewhat less amplified look to things than earlier runs today. Moisture is sufficient, however, to warrant continued low PoPs with thermal profiles quickly transitioning to one favoring rain as surface temperatures warm to either side of 40 degrees or so. The active upper pattern will persist into next week although the belt of faster flow is presently forecast to remain immediately to our north. Carlaw && .AVIATION... For the 00Z TAFs... 547 PM CST Minor concerns include: * MVFR CIGs likely developing on Tuesday over Chicago area * East winds of 10+ kt Tuesday afternoon-evening, gusts possible Another mostly quiet aviation period is in store. The only forecast items of note are the potential for MVFR CIGs moving west and southwest off the lake on Tuesday and the east winds. High pressure will provide light winds into Tuesday morning, with high VFR CIGS streaming overhead. Very marginal lake effect parameters are then expected to result in BKN 2000-3500 ft CIGs developing over Lake Michigan and then pushing inland as easterly winds increase. GYY being on the south tip of the lake would probably have temporary CIGs into early afternoon, with a medium confidence forecast of prevailing MVFR CIGs by mid afternoon for ORD, MDW, and DPA. RFD may see MVFR CIGs after the current TAF period. Winds will pick up out of the east as the pressure gradient strengthens Tuesday PM, with possibility of sporadic gusts in the mid to high teens kts at ORD and MDW. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...Gary to Michigan City IN until midnight Tuesday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at:
Area Forecast Discussion
National Weather Service Riverton WY
353 PM MST Mon Dec 14 2020 .SHORT TERM...(This evening through Tuesday night) Issued at 115 PM MST Mon Dec 14 2020 Elongated slightly positively-tilted trough is currently moving through the forecast area. Moderate snow fell overnight across parts of western WY; light snow showers will continue through midnight with little to no accumulation. As the trough moves eastward, snow showers have been occurring east of the divide and will also continue through about midnight, with areas around Casper Mountain seeing light showers through sunrise. Weak upslope is possibly this evening along the SE Absarokas and Lander Foothills as moist NNE flow strengthens somewhat. Southern Johnson county into Natrona county and over to east-central Fremont county will also see snow through midnight or so with about an inch accumulation. Another area to watch out for tonight according to the HRRR is Rock Springs-Green River area along the western edge of the northerly flow where weak convergence zones like to set up. Some clearing is already occurring across NW and western WY as the system progresses. Likely to see some very cold temperatures again overnight with the fresh snow and partly cloudy to mostly clear skies. Tuesday morning as mentioned will see the storm mostly cleared of the forecast area with moist northwest flow poised to push into NW WY as weak high pressure builds over the Great Basin. Thus, clouds will increase over NW WY by noon, with snow showers starting over the Tetons and SW YNP by mid afternoon. The snow should intensify by early evening from YNP south to the northern Salts/Wyoming Range with snow falling in Jackson and other western valleys. The snowfall rate does not appear to be very strong through, with the mountains perhaps picking up another 2-4 inches by Wednesday morning. The surface pressure gradient will keep breezy to windy flow with gusts over 25 mph from southern into central WY Tuesday afternoon and evening; the breezy flow is likely through the overnight hours as well. Have increased the western mountain winds with the NW flow pattern since models tend to weaken them as compared with the Summit and Mt Coffin observations. .LONG TERM...(Wednesday through Monday) Issued at 1234 PM MST Mon Dec 14 2020 Model agreement has northwest flow set up across the entire area allowing for upslope snow showers to occur across the western mountains through Thursday. As the next trough approaches, brief southwest flow ahead of the trough is possible for Thursday morning. Snow looks to start around early Thursday morning for the western mountains and slowly spread west throughout the day Thursday and into Friday. The bulk of the snow will remain in the west but some accumulating snow looks possible East of the Divide through late Friday morning. Will start to begin looking at snow totals for this event in the next coming shifts. Winds will increase across the Wind Corridor with this system especially from Jeffrey City to Casper. Northwest flow regime sticks around after the departing low; keeping snow in the mountains through the weekend. Brief zonal flow sets up late Sunday night ahead of the next potential mountain snow for Tuesday. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 348 PM MST Mon Dec 14 2020 East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Snow will continue to impact KRIW and KLND through 07Z Tuesday which could cause some IFR/MVFR visibilities at those two sites. KCPR will begin to see snow after 04Z which could create some IFR/MVFR visibilities at that terminal through 09Z. KCOD and KWRL will stay out of the snow but winds will begin to impact KCOD and KCPR after 10Z where winds will gust to 25 knots. Mountaintop obscuration may be possible with this most recent snow event. West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals Snow will continue to taper off from west to east for the first part of the TAF period. Isolated snow showers could develop over KRKS after 02Z and cause IFR/MVFR visibilities before diminishing after 07Z. Winds will pick up at KRKS after 08Z with wind gusts up to 30 knots by the end of the TAF period. KPNA will also see increased winds but will decrease around 03Z and become light and variable along with KBPI and KJAC. Mountaintop obscuration may be possible. 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 130 PM MST Mon Dec 14 2020 Storm system currently moving through western and central Wyoming today, with some clearing already occurring across western WY. Isolated to scattered snow showers will continue through midnight east of the divide and into SW WY. Lingering snow is likely around Casper Mountain through sunrise Tuesday. Tuesday morning will again see temps below zero in the western valleys. Due to moist northwest flow, light snow showers will return to northwest WY Tuesday afternoon and spread across western WY Tuesday night, bringing another 2-4 inches to the mountains and up to an inch in the western valleys. Tuesday will also see west-southwest wind increase from SW into central WY with gusts over 25-30 mph. Breezy flow will continue Tuesday night. Weak high pressure moves over the area Wednesday, with a strong storm on tap for Thursday afternoon into Friday. Smoke dispersal through the week will only be poor to fair with low mixing heights, though a bit better in areas where the winds increase. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...McDonald LONG TERM...Vorst AVIATION...Vorst FIRE WEATHER...McDonald