Subj : HVYSNOW: NE US Storm To : All From : Dumas Walker Date : Wed Jan 17 2024 09:23 am FOUS11 KWBC 171052 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 551 AM EST Wed Jan 17 2024 Valid 12Z Wed Jan 17 2024 - 12Z Sat Jan 20 2024 ....Prolonged Great Lake Effect Snow... Days 1-3... Core of sprawling upper low centered south of Hudson Bay this morning will eject east today, but upper troughing will linger over Ontario/shift south across the Great Lakes through Friday night, maintaining LES from all five Lakes through this time. A cold front crosses the lakes from NW to SE on tonight into Thursday with continued Wly flow. The aforementioned troughing approaches Thursday night with surface low pressure to the south causing flow to back northerly, opening new LES snow belts by late Friday night - most notably single banding shifting down Lake Michigan into NW Indiana. Only the margins of the Great Lakes icing so far in this cold snap according to GLERL, with water temperatures generally +4C. The persistent CAA across these still warm lake temperatures will result in steepening lapse rates to drive inversion depths to potentially above 10,000 ft, highest east of Lake Ontario, which will support lake-induced instability of 500-750 J/kg. Additionally, forecast soundings still indicate a favorable cross section of ascent into the DGZ, further evidence of potential for additional heavy snowfall. With 850mb temperatures wavering at times between -10C and -25C, it is likely that any bands of lake effect snow will produce heavy rates of 1-2+"/hr, and although the multiple boundary passages will cause wavering to the Wly flow into Thursday morning before a lull when the northerly flow sweeps through Thursday night. Days 1 and 2 WPC snow probabilities for >6" are present in the Keweenaw Peninsula, around Buffalo and north from the Tug Hill with Day 2 a bit south of 1. heavy snow maximize in the favored N/NW LES belts across the U.P. of MI, and east of both Lakes Erie and Ontario. WPC probabilities for more than 6 inches reach 50-80+% both days in these areas, and 2-day snowfall could add up to several feet in some areas, especially in the Tug Hill Plateau. Day 3 probs are notable in NW Indiana for the single band off Lake Michigan and around Cleveland given the additional fetch from Huron/Georgian Bay (probs are very low off Lake Ontario which does not have the added fetch that Erie has in north flow). ....Pacific Northwest through the Central Rockies... Days 1-3... An active pattern will continue to bring rounds of heavy precipitation to the Pacific Northwest, with heavy icing and heavy snow both expected into or through Friday. A compact mid-level low moving on the OR Coast early this morning will weaken into an open wave over OR this morning and shift to the northern Rockies rest of today and provide considerable forcing for ascent as it races east in the left exit region of a potent 110+kt NWly Pacific jet streak. Moisture ahead of this low will continue to surge inland with snow levels around 5500ft over OR and around 5000ft in the western slopes of the northern Rockies today. Immediately behind this low is a northern stream trough that shifts down the southern BC coast this morning and surges ESE over northern WA/ID/MT this afternoon through tonight, providing lift along the remaining baroclinic zone with Arctic air to the northeast and producing a notable swath of heavy snow with 2"/hr rates common per 00Z HREF means across the WA Cascades this afternoon, and the ID/MT into northwest WY Rockies tonight where Day 1 snow probs for >12" are over 70%. Pacific moisture streaming over the northern to central Rockies also allows over 70% probs for more than 8" over the Sawtooths, Wasatch and northern CO Rockies. Furthermore, surface ridging extending from the lee of the Canadian Rockies through northeast MT make for a strong low level frontogenetical zone over north-central MT this afternoon which the northern stream wave tracks over late tonight. Bands of heavy snow develop this afternoon and shift southeast, weakening as they lose topographical lift by the time they reach SD overnight - Day 1 snow probs for more than 6" is over 70% on the north-central MT High Plains. Ongoing freezing rain will continue from the Portland metro up through the Gorge and into the Columbia basin today before precip diminishes (though briefly) from west to east as ridging builds in behind the two aforementioned waves. Day 1 WPC probabilities D3 for more than an additional 0.1" of ice after 12Z is 30-50% through these areas. The lull in activity over the Pacific Northwest is short as moisture ahead of the next system reaches the OR/WA coast on SWly flow late tonight. Snow levels surface up to 7000-8000ft over OR and western WA Thursday, though cold air remains entrenched over the Columbia Basin. Day 2 snow probs for over 8" are generally 40-70% for the WA Cascades. However, surface cold air lingers west of the Cascades and streams down the Columbia Gorge to reignite freezing rain chances midday Thursday into Friday. Day 2 ice probs for over 0.1" are 30-60% over portions of the Columbia Basin, the Gorge, the Portland metro, with lower probs north up through the Seattle metro. Probabilities decrease for Day 3, but linger near the Columbia Gorge. ....Northern/Central Plains... Days 2-3... The aforementioned northern stream wave that crosses north-central MT late tonight shifts southeast over the central Plains Thursday. While weakening of the related snow bands is expected as they come out of Montana late tonight, a boost from waves rounding the low over Ontario and the increasingly diffluent left exit of a NWly upper jet streak over the northern Rockies that strengthens to 130kt+ Thursday morning. This will yield a shallow wave of low pressure moving across WY, with modest downstream WAA along a leading warm front surging moist isentropic ascent for SD/NE into IA through the day Thursday. While available moisture is modest, there may be some enhancement due to the fgen response to the favorably placed upper jet streak above the low-level fgen along the sloped warm front. Given the in place Arctic-sourced air, deep DGZ is expected with a fast moving band of very high SLR snow which could accumulate to several inches as it advects east. Day 2 WPC snow probabilities for more than 2" are 40-60% over central SD to eastern Neb. ....Mid-South... Days 2-3... A weak shortwave and accompanying PVA will slide southeast from the Missouri Valley late Thursday into Friday bringing weak height falls and PVA as the shortwave fills with time. Modest moist advection on increasing isentropic ascent downstream of this feature will surge PWs favorable to support a stripe of light to moderate precipitation from the Mid-South east over the southern Appalachians. Due to shallow saturation, there is potential for freezing rain in this stripe with Day 2 ice probs for over 0.01" are 10-30% with 40-60% probs in western TN. ....Ohio Valley, Mid-Atlantic into Northeast... Days 2/3... A fast moving shortwave trough shifts east below the dislodging low core over Ontario and through the Midwest/OH Valley Thursday night and over the Mid-Atlantic Friday. The flow is fast, and the shortwave is of modest intensity, but it will be efficiently topped by a potent Wly 150+kt jet streak diving through the trough to enhance deep layer ascent. At the same time, a cold front will make its way eastward from the Great Lakes to the Mid-Atlantic states and the associated fgen along this front could be enhanced by the upper jet streak pivoting overhead. This will enhance ascent and will result in a coastal low developing east of the Carolinas Thursday night and then racing east-northeast Friday. PWs downstream of the best ascent are modest within the generally zonal and fast flow. The CMC remains the most bullish with this wave. Day 2 snow probs for over 2" are 30-60% from central IL through OH and over eastern KY into WV. Day 3 snow probs for over 4" are 40-70% over the central/WV Appalachians into the Laurels of PA where colder temperatures and some upslope flow should result in efficient snowfall accumulations and generally 10 to 30% over PA/MD/DE up through southern New England. Jackson ***Key Messages for Northwest Winter Storm*** --Active Pattern Continues An upper trough crosses Washington this afternoon and Montana tonight, while a second storm system directs another round of Pacific moisture across the Pacific Northwest Thursday into Friday. --Continued Ice Threat for Columbia Basin/Gorge Freezing rain continues today east from Portland with ample cold air draining from the Columbia Basin through the Gorge. Freezing rain is likely over portions of the Cascades and Columbia Basin with the next round Thursday and Friday. An additional inch of ice is possible around the Columbia Gorge. --Heavy Snow into Thursday Particularly heavy snow ahead of the upper trough crosses the Washington Cascades today with snowfall rates exceeding 2../hr expected above the 2500ft snow level. This impactful heavy snow then crosses the Idaho, Montana, and northwest Wyoming Rockies tonight with continued risk for 2../hr snowfall rates. Bands of locally heavy snow also develop over the north-central Montana High Plains tonight. $$ --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175) .