Subj : HVYSNOW: Probabilistic He To : All From : Mike Powell Date : Wed Apr 05 2023 17:41:00 FOUS11 KWBC 051835 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 235 PM EDT Wed Apr 05 2023 Valid 00Z Thu Apr 06 2023 - 00Z Sun Apr 09 2023 ....Pacific Northwest... Days 1-3... A series of shortwaves dropping out of the Gulf of Alaska will move towards the Pacific Northwest coast bringing enhanced ascent and moisture through late week and into the weekend. There are likely to be two primary impulses to enhance ascent and precipitation during the forecast period, one Thursday night and then a trailing impulse on Saturday. The lead shortwave will initially be a closed feature over the Pacific but should weaken with time becoming an open trough before it advects onshore by Friday morning. While this feature is weakening with time, downstream divergence and increasingly SW flow will produce enhanced WAA, while also driving some more pronounced upslope flow into the terrain where flow becomes more orthogonal. Subtle diffluence within the LFQ of an approaching Pacific jet streak will additionally enhance ascent, while also driving more pronounced moisture onshore adding to the impressive moisture plume driven by the mid-level evolution. CW3E probabilities from the GEFS and ECENS suggest the accompanying atmospheric river (AR) will exceed 500 kg/m-s on Thursday, resulting in an expanding area of precipitation along and ahead of a surface cold front, first along the coast, but eventually spreading east of the Cascades and into the Northern Rockies by Friday morning. Thanks to the enhanced WAA, snow levels will climb to 4000-5000 ft during the heaviest precipitation, so heavy snow accumulations should generally be confined to above pass level. WPC probabilities for 6+ inches both D1 and D2 feature small areas of high risk, generally in the higher terrain of the Olympics, Cascades, Shasta/Trinity region, and northern Sierra. Locally more than 12 inches is likely in the highest terrain. After the first impulse moves eastward, a brief respite will occur before the subsequent shortwave drops out of the Gulf of Alaska and brings renewed ascent and moisture into the Pacific Northwest. This impulse is currently progged to be a bit weaker than the first, and also displaced north of the leading shortwave. This indicates that heavy precipitation will be confined more to the OR/WA area even by the end of D3, with snow levels remaining around 4000 ft. High probabilities for more than 6 inches are confined to the Olympics on that D3. ....New England... Day 1... Impressive closed 700-500mb low over the Upper Midwest will pivot eastward through Ontario on D1, while a potent 170kt jet streak arcs across the Great Lakes and into Quebec. The overlap of these features will continue to drive a moderately strong surface low pressure northeast into Canada, but as features become vertically stacked tonight, the primary low will occlude to a triple point over the Great Lakes. This will drive secondary low pressure development, albeit modest, along this triple point, and this wave will shift eastward along a warm front into Thursday. Downstream, impressive meridional moisture transport out of the Gulf of Mexico noted by robust isentropic upglide will drive PW anomalies to as high as +3 standard deviations over New England early on D1 /Wednesday night/, resulting in an expanding area of precipitation where the ascent through WAA overlaps with the greatest moisture anomalies. This precipitation will expand northeastward ahead of the secondary wave of low pressure, rising atop a slowly retreating Canadian high pressure which will initially be entrenched as a CAD signature develops. This overrunning precipitation will fall as snow only briefly across far northern New England, but will otherwise be a mixture of sleet, freezing rain, and eventually rain from the St. Lawrence Valley eastward into the White Mountains of NH and much of central/northern ME. The regional forecast soundings indicate an extremely deep sub-freezing layer beneath the warm nose, which indicate a high potential for accumulating sleet, some of which could be significant, across primarily central/northern ME, with freezing rain likely the predominant p-type farther SW. The intense WAA should eventually overwhelm the low-level cold air as winds become more SE than E, eroding any dry bulb affects. With some instability noted aloft, this will likely manifest as periods of heavy mixed precip, which may not efficiently accrete during the freezing-rain portion, but could result in significant sleet accumulations in some areas. WPC probabilities for more than 0.1" of freezing rain are above 80% for far northeast NH and much of west-central ME, where locally 0.2-0.3" of ice accretion is likely. Additionally, heavy sleet accumulations of more than 0.5" are expected for portions of northern ME. ....Upper Midwest... Day 3... A narrow band of heavy snow may develop Friday night into Saturday in response to some increased 850-700mb fgen overlapped with the favorable LFQ of a strengthening but departing jet streak ahead of a warm front. The guidance is quite variable in its evolution of this band of precipitation, both with respect to placement and intensity, but it is likely that at least a narrow corridor of moderate to heavy snow will develop near WI/MI. With the event generally expected to occur at night, the low-level thermal fields will be more hospitable to accumulating snow, but still a modestly favorable column and narrow region of ascent should be limiting. WSE plumes and NBM probabilities suggest snow will be light overall, but there is potential for some over-performance in the core of the band should everything line up correctly. Despite high uncertainty, WPC probabilities currently indicate a 30-50% chance for more than 2 inches in a narrow stripe from near Green Bay, WI eastward towards Traverse City, MI, but locally more than 4 inches is possible in a few isolated locations. Weiss $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .