Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 04 2024 19:32:16 FOUS11 KWBC 041931 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 331 PM EDT Thu Apr 4 2024 Valid 12Z Thu Apr 04 2024 - 12Z Sun Apr 07 2024 ....Northeast... Days 1-2... A potent early April nor'easter will continue tonight into Friday as a classic pattern for a major low pressure affecting New England persists. The primary driver of this event is an anomalous closed mid-level low centered over the Northeast which will virtually spin in place through Saturday before slowly ejecting to the east and into Canada by Sunday. This feature will move little both due to its amplitude, reflected by 500mb heights reaching the bottom 0.5 percentile of the CFSR climatology tonight, but also in response to lobes of vorticity swinging around it which may actually result in a slight retrograde tonight before pivoting back to the east into Sunday. At the same time, at least periphery LFQ diffluence will persist as the strong jet streak arcing over the Mid-Atlantic shifts slowly to the east. While this will allow the surface low to slowly fill, it will remain an impressive low as it rotates in place through Saturday before finally opening to a trough and kicking east on Sunday. The intensity of this low and the accompanying moisture advection will continue to result in widespread heavy precipitation D1, with lighter wrap-around precipitation continuing D2, and maybe even slightly into D3 in favored northerly-flow upslope regions. D1 will feature the most significant precipitation as impressive 290-295K isentropic ascent continues to wrap cyclonically around the low, supporting at least a residual piece of the TROWAL before it pivots away during Friday, and this will combine with weakening but still present 850-700mb fgen driving ascent into the DGZ. PW anomalies wane quickly as the TROWAL gets cut off Friday, but there will still be sufficient moisture to support waves of heavy precipitation Friday before things wane Friday night and Saturday. Low-level thermals will be marginal across southern New England tonight, but with the occlusion occurring, more cold air will wrap back southward, combining with persisting moisture and periods of PVA as spokes of vorticity rotate around the primary mid-level low to drive waves of snowfall with higher late D1 into D2 than earlier on D1. Once the heavy snow ends D1, accumulations may be more confined to nighttime and higher elevations. WPC probabilities D1 are high for more than 6 additional inches of snow in the highest terrain of the Adirondacks, Whites, and into northern Maine. Locally an additional 12 inches is possible. By D2 any meaningful probabilities for more than 6 inches are confined to just a few peaks around Mt Washington and Mt Katahdin. ....Central Appalachians... Days 1-2... Parent mid-level low spinning almost in place over New England will shed spokes of vorticity cyclonically around it and southward across the Central Appalachians. There appear to be two primary time periods where ascent through PVA downstream of these vorticity lobes will occur, one tonight and another Friday evening. As these impulses will be embedded within otherwise intensifying NW flow and CAA in the wake of the low as it slowly departs, this will likely result in pronounced upslope flow driving heavy snowfall into the Central Appalachians, especially over WV. Regional soundings indicate steep lapse rates within this CAA driving ascent into the DGZ, but a lack of moisture in the upper half of this snow growth layer may somewhat inhibit heavy snow. Still, a long duration of upslope flow and impressive ascent should offset some of this lack of moisture, leading to rounds of accumulating snowfall into Saturday. WPC probabilities D1 for more than 6 inches of snow reach as high as 50-70% across the highest terrain of WV. ....Western U.S.... Days 1-3... The calendar may read April, but much of the West will be entrenched with a mid-winter feel as an exceptionally amplified 500mb pattern across the CONUS manifests as a deep closed low drifting across the West. Although this closed low and accompanying full latitude trough will gradually move east through the weekend, NAEFS height anomalies are progged to remain around -2 to -3 sigma beneath it, supporting cold air and widespread heavy snow above snow levels that will hover generally around 2500-3500 ft across much of the West, maybe rising to around 4000 ft D3 as the upper low pivots into the Central Plains. Either way, the next few days will be quite active across the West with heavy snow across most of the terrain, and some lighter accumulations possible into the valleys. The primary upper trough driving this active weather will be aligned along the Pacific coast to start D1 /00Z Friday/ before beginning to advect east and reaching the Great Basin as a closed low to start D2. Impressive height falls, periods of PVA, and downstream divergence will all contribute to robust deep layer ascent, with additional contribution provided via the LFQ of a subtropical jet streak. This evolution will help drive dual surface lows D1 across the West, one reaching the CA coast and dissipating, while a secondary low strengthens across the Great Basin and lifts slowly northeast. Both of these will add to local ascent, with low-to-mid level confluence increasing moisture across the region as well despite modest IVT progged by NAEFS ensemble tables. The overlap of the broad but impressive synoptic ascent combined with locally enhanced mesoscale lift through upslope flow, especially into the Sierra, and some enhanced 700-600mb fgen pivoting across the northern Great Basin and into ID/OR will result in axes of heavy snowfall D1. Across the Sierra, WPC probabilities for more than 6 inches are above 80%, with locally 1-2 feet possible in the higher terrain of the southern Sierra. With snow levels low and heavy rates likely, some lighter accumulations are likely even down into the Santa Lucia mountains, with heavy snow also likely in the Transverse and Peninsular Ranges. Farther northeast, bands of heavy snow will likely (>60%) result in moderate to heavy accumulations from northeast CA, into the northern Great Basin, and towards the Blue Mountains of OR. D2 into D3, the most intense synoptic forcing will be due a potent closed low reaching the central High Plains by Saturday night and then briefly stalling before ejecting into the Upper Midwest by Sunday night. Most of the impressive lift will consolidate around this feature, which will cause the two aforementioned surface lows to weaken as a more substantial area of cyclogenesis occurs in the lee of the northern Front Range across eastern WY. This new development will intensify rapidly into a potent surface low by Sunday morning. Around this low, intense moist advection will pivot cyclonically into the High Plains, transporting anomalous PWs northwest into the low and as far NW as the Northern Rockies, supporting a developing TROWAL within the accompanying theta-e ridge. At the same time, the strength of this low and associated upper pattern will likely result in a strong deformation axis across the central/northern High Plains to enhance ascent and produce intense snowfall rates, potentially dynamically cooling the column to allow for snow accumulations even below the progged snow levels. While the individual ensemble clusters are well aligned, there is still quite a bit of temporal and spatial spread between the models, so confidence in the exact development is low. However, it is becoming more likely that an impactful winter storm will affect portions of the High Plains and central/northern Rockies this weekend, with WSSI-P already showing a 30-60% chance of moderate impacts due to snow and wind. WPC probabilities for snow D2 and D3 peak from the northern Great Basin into the Northern Rockies and eastward into the central High Plains, where a lot of the terrain has a higher than 50% chance for 6+ inches each day, and a local max exists along the Big Horn Range. However, the impressive deformation and intensifying moisture advection could result in locally heavier accumulations anywhere across MT/WY, and this will need to be monitored closely with future updates. Weiss ***Key Messages for Early April Nor'easter*** ---Additional heavy snow through Friday While the heaviest snowfall rates will begin to subside this evening, periods of snow will continue tonight and through Friday. Additional snowfall totals of 6-12 inches are forecast in parts o the Adirondacks and both the Green and White Mountains, while additional amounts over 12 inches are expected in portions of central and northern Maine. ---Lingering wind impacts Wind gusts will gradually decrease this evening and through Friday, but occasional gusts of 20-30 mph across New England will still be capable of causing reduced visibilities and hazardous travel through Saturday morning. ---Localized minor coastal flooding possible Prolonged onshore winds may cause additional areas of localized coastal flooding along Downeast Maine. Splashover is possible in the most susceptible spots located on east facing coastlines. $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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