Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 06 2024 19:50:36 FOUS11 KWBC 061950 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 249 PM EST Tue Feb 06 2024 Valid 00Z Wed Feb 07 2024 - 00Z Sat Feb 10 2024 ....The West... Days 1-3... The main upper trough will move through the CA deserts tonight with the cold front progression steadily through the Intermountain West. Upper jet will, in turn, move eastward with its LFQ over much of the Great Basin/Four Corners into Wednesday, providing broad lift to the region. Southerly to southwesterly flow through the column will maximize upslope enhancement on SW-facing ranges from the Mogollon Rim to the Uintas and San Juans D1, where more than a foot of snow is likely. Impressive height falls and PVA will combine with strong lift through the DGZ and plentiful moisture (PW anomalies +1 to +2.5 sigma) to yield 1-3"/hr rates especially in the higher mountains tonight into early Wednesday. Additional snowfall will spread as far north as the central Idaho ranges into western MT/WY as weaker areas of low pressure lumber through the terrain. Into D2, as height falls ease along and east of the Divide, another shortwave or closed low will dive into NorCal Wednesday afternoon, tracking into the Sierra overnight. This will continue through southern NV into the Four Corners with another round of snow, but with much less moisture to work with. However, southwesterly flow will still help to maximize snowfall into the Mogollon Rim, where WPC probabilities for at least 6 inches of snow on D2 are high (>70%). This extends to the San Juans and parts of the Wasatch as well. By D3, yet another northeast Pacific shortwave will move into the West, focused on WA/OR as building ridging into northern BC forces it farther inland. Limited moisture will preclude much heavy snow, mainly confined to the Oregon Cascades and into the southern WA Cascades where WPC probabilities for at least 4 inches of snow are moderate (40-70%), generally above 3000ft as snow levels remain on the lower side (1500-3000ft from north to south). Lighter snow on the order of a few inches is likely across much of the rest of the Great Basin and Four Corners region as the trough axis slowly moves eastward past 115W. ....Northern High Plains and Northern Plains... Days 1-3... Longwave troughing moving through the West today will begin to reach the Rockies and Plains by mid-week, resulting in the development of a surface low in the lee of the Colorado Rockies late Wednesday afternoon and intensified by the strengthening subtropical jet reaching 150+ kt providing favorable left exit quadrant diffluence. By Thursday, there is consensus that the shortwave energy coming out of the Rockies will close off at 500 mb over North Dakota while the surface low deepens and tracks toward the western shores of Lake Superior. However, deep layer ascent will be a bit less impressive due to the fact that the upper diffluence from the jet streak will become separated from the mid-level ascent. Regardless, widespread precipitation is likely to break out across portions of MT through ND later Wednesday and continue through late Thursday with a pivoting deformation band likely across portions of eastern MT, western ND. While moisture should be more than sufficient for heavy precipitation, characterized by PW anomalies reaching +3 at times during the event, however the best forcing/lift is a bit offset from the DGZ, which may end up making this a more widespread light/moderate snowfall event compared to heavy snow. The WPC snow probabilities for 4 inches have lowered some this cycle and generally peak between 30-40% across eastern Montana and western North Dakota. A look at the latest WSSI-P shows that probabilities for moderate level impacts is quite low as well, though reaches above 60% for the minor level, driven by the combination of snow amounts and blowing snow, especially over the Dakotas where the strengthening pressure gradient should bring a strong wind that may result in areas of blowing snow and reduced visibilities as well as travel disruptions. The probability of significant icing greater than 0.10" is less than 10% for days 1-3. Fracasso/Taylor Key Messages for the Feb 4-7 Atmospheric River are below: --Continued California Rainfall and Flash Flooding Occasional heavy rain will continue today across Southern California and the Transverse Ranges. Additional rainfall of 1-3" atop fully saturated grounds will create new, or enhance ongoing, flash flooding. Locally significant impacts will persist today, with river flooding, debris flows, and mudslides expected. --Flash Flooding Risk into the Southwest Heavy rain will (1-2" with locally higher amounts near three inches) is forecast for parts of the Calidornia deserts into Arizona today. This will likely produce instances of flash flooding along creeks and washes, potentially resulting in dangerous travel and road closures. --Heavy Mountain Snow and Gusty Winds Heavy snow will expand from the Transverse Ranges and Great Basin across the Four Corners today. Snowfall rates of 2+"/hr are possible (30-50%) above 6000 ft along the Mogollon Rim and into the Wasatch and San Juans. This snowfall will combine with winds gusting above 40 mph to produce significantly reduced visibility and very difficult travel. $$ = = = 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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .