Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 09 2025 08:39:22 FOUS11 KWBC 090837 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 AM EST Thu Jan 9 2025 Valid 12Z Thu Jan 09 2025 - 12Z Sun Jan 12 2025 ....Great Lakes and Northeast... Day 1... Cyclonic flow on the backside of a large upper low in Atlantic=20 Canada will support another ~day of mainly mountain-based snow over the northern Green and White Mountains and northern Maine. A few=20 inches are possible in the higher elevations, with WPC=20 probabilities for at least another 4 inches of snow around 50%.=20 ....Northwest to the Northern Rockies/Plains... Days 2-3... Fairly sharp mid-level shortwave and sfc front will enter the=20 PacNW with mostly higher elevation snow but lowering snow levels=20 (from ~4500 to ~3000ft) as the front moves past the Cascades. The=20 shortwave will move southeastward beneath a weakening upper jet,=20 but combination of height falls and upslope enhancement will=20 maximize snow over the mountains, from the Bitterroots to the=20 Tetons and western MT D2 then into the Big/Little Belts into the=20 Bighorns and eventually the Black Hills D3 as the stronger height=20 falls move through. WPC probabilities for at least 12 inches of=20 snow are are >50% above ~5000-6000ft or so (west to east) where=20 two-day totals could exceed 18-24 inches above 6000-7000ft (40-70%=20 chance). Additionally, by D3, Canadian system will move into the northern=20 Plains/Upper Midwest with a swath of generally light snow via WAA=20 and FGEN on the northern side of an area of low pressure. Amounts=20 should generally be on the lighter side of a couple/few inches, but there are low-end probabilities (10-20%) of at least 4 inches of=20 snow over eastern ND. ....Southern Plains, Southeast, southern Mid-Atlantic... Days 1-2... ....Major winter storm is forecast to span from west Texas and=20 southeast Oklahoma starting today before crossing through much of=20 the Mid-South and into portions of the Mid-Atlantic by the end of=20 the week...=20 A potent positively-tilted longwave trough containing plentiful=20 upper level energy from an upper level low will support a very=20 strong upper level jet that stretches from West Texas east through=20 the Carolinas. Surface cyclogenesis will initiate this afternoon=20 along the Texas Coast in the RER of that jet, also helping to draw=20 in Gulf moisture with maximum PWATs along the Texas Gulf Coast=20 rising to around 1.75 inches. While that level of moisture doesn't=20 move too far inland, a large fraction of it will, providing ample=20 moisture for the developing surface low. The associated low level=20 jet will advect much of that moisture up the Lower Mississippi=20 Valley. Meanwhile, an Arctic air mass will be in place due to a retreating high pressure system over the Central Plains and Mid-South,=20 resulting in a strong temperature gradient which will both increase the forcing as well as lift that moisture and effectively wring it out, resulting in widespread wintry precipitation starting in=20 TX/OK and spreading eastward to the Mid-Atlantic by late=20 Friday/early Saturday as multiple jet streaks ride northeastward.=20 The tight thermal gradient on the north and west side of the low=20 will promote a similarly tight gradient in snowfall amounts. With a broad area of WAA off the deck, and in the areas where the cold=20 air at the surface is shallow but stubborn, an area of freezing=20 rain/sleet will develop from the Big Bend of Texas eastward over=20 northern Louisiana/southern AR into central Mississippi, Alabama,=20 and Georgia, then northeastward through the Carolinas. To the=20 north, where the colder air is deeper through the column, snow will be the dominant or sole ptype across Northwest TX,=20 central/northern AR, eastward through the Mid-South/Tennessee=20 Valley and into the central/southern Appalachians to the VA=20 Beaches. Ptypes will be transitioning from frozen to liquid in some southern areas where the cold air is shallower and overtaken by=20 WAA at all levels. The models still show a fair amount of spread in the strength of the warm nose and time residence of sub-freezing=20 air, resulting in lower confidence in these transition zones.=20 Trended a bit warmer overall but will still have a few model cycles to hopefully gain more certainty.=20 For many this will be a very impactful snowfall event and the=20 first winter storm of the season for areas such as Dallas-Fort=20 Worth north and east through the Ozarks and into the Memphis and=20 Nashville metro areas. Additional forecasting challenges include=20 banding potential on the northern and northwest side of the low=20 increased by strong mid-level FGEN and isentropic ascent through=20 the DGZ. This may lead to mesoscale banding that will become more=20 notable once inside the full suite of CAMs, but even the 00Z runs=20 still showed disagreement. This is particularly a concern for=20 north-central TX and OK. From Texas through the Lower Mississippi=20 Valley, forecast snow amounts have come down nominally along the=20 ptype gradient, but have consequently increased for ice=20 accumulations -- again, dependent on the strength of any warm air=20 and how long colder 2m temps can hang on. WPC probabilities for at=20 least 4 inches of snow are >50% near/north of D-FW across=20 southeastern OK eastward across northern AR, the Mid-South, into=20 Middle TN in a mostly continuous fashion. Orographic lift will also help the central/southern Appalachians see several inches of snow. Over AL/GA, initial WAA-driven snow should lead to a changeover to sleet/freezing rain and then rain for more southern areas, with=20 the highest probabilities for at least 2 inches of snow closer to=20 the TN border but non-zero probs to I-20/I-85. More uncertainty in=20 snow amounts lies east of the Appalachians across southeastern VA=20 where the guidance has wavered on the amount of QPF. Low pressure=20 should strengthen as it leaves the coast, perhaps allowing some=20 enhancement over SE VA early Saturday. For ice probabilities, there is a >40% chance of at least a tenth=20 of an inch of ice around the Arklatex region through southern AR.=20 In addition, areas from northern GA through parts of=20 Upstate/Midlands SC into eastern NC show >30% probabilities of at=20 least a tenth of an inch of ice.=20 Fracasso ....Winter Storm Key Messages are in effect. Please see current=20 Key Messages below... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!6hPeCUOYI6lqI1KOCsG0p9wSjwtDLoVc9kEmGA7_RB1os= lQPTZ1sRZbWyw46ALFDmB-H0tihyTyTRsQ-DRPjG9o8VbM$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .