Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 329 PM EDT Sun Mar 31 2024 Valid 00Z Mon Apr 01 2024 - 00Z Thu Apr 04 2024 ...The West/Southwest... Days 1-3... Positively-tilted upper trough in the West will continue to progress eastward tonight and through Monday, with additional backside vorticity elongated/stretching the trough. This will help kick out the lead vort max in favor of the secondary vort max that will eventually reform another positively- tilted trough over the Southwest. The result will be a continuation of generally light snow for the Sierra (this evening) and across the mountains in the Four Corners. Broad SW flow will maintain a moisture influx into the region, though current PW anomalies around +1 will subside over the next 36 hours. By D2, the upper trough will finally progress/spilt eastward/southeastward and precipitation will largely come to an end, with lingering snow over the Sangre de Cristos early in the period. For the two-day period, WPC probabilities of at least 8 inches of additional snowfall are highest (>50%) over the White Mountains in east central AZ and parts of the San Juans and Sangre de Cristos in CO/NM. By D3, a new system will enter the PacNW with snow for the Olympics and Cascades. Snow levels will start high (6000ft) but fall quite quickly as the cold front moves through, down to about 3000ft. Much of the snow will be confined to the northern WA Cascades. ...Central High Plains into the Northern/Central Plains... Days 1-2... A surface low developing in the lee of the Rockies across the High Plains of CO will advect east into the Central Plains and Upper Midwest through Monday, while a secondary surface low then develops in its wake across the Southern High Plains. The primary synoptic driver of this surface low evolution is a broad positively tilted longwave trough draped NE to SW from the Northern Plains through the Desert Southwest D1, and this trough will pivot eastward through D2, placing strong height falls and PVA into the High Plains, aided by persistent upper level diffluence within the LFQ of an arcing upper jet streak to help drive cyclogenesis. At the same time, a stationary front and cold front draped across the area will provide a favorable baroclinic gradient on which these surface lows will develop, to additionally enhance ascent. The first low will track east from near the WY/CO border and advect rapidly eastward ahead of a modest shortwave lifting through the flow. This low will work into a region of increasing moisture on the mid-level isentropic lift, with low-level WAA into the elevated warm front providing additional moisture and lift. This should produce a corridor of heavy precipitation from eastern WY through IA, and although the column will be marginal for wintry precip, the northern edge of the precip shield should be all snow, with additional heavy snow likely near the SD/NE border as a translating fgen band driven by the WAA appears likely. The DGZ depth is modest (SREF probabilities for 50mb of depth 50-70%) but sufficient in the presence of the strong forcing to drive 1"/hr snowfall rates, and WPC probabilities for more than 4 inches of snow from this fast moving system reach 30-50% along the NE/SD border, and as high as 80% in the Black Hills. Behind the first low, the second low will develop rapidly and farther south along the baroclinic gradient, moving from near the NM/CO border into the Southern High Plains D2. Moisture associated with this secondary low appears more significant, and as the low pivots east there will be intensifying upslope flow on NE winds behind the low/front into the Front Range and Sangre de Cristos. Places into the high plains will likely be too warm for wintry precipitation, so the focus of snowfall D1 into D2 is expected to be confined to the higher terrain from the Front Range, into the San Juans, Sangre de Cristos, and along the Raton Mesa. Here, WPC probabilities are high for more than 6 inches on D1 in the San Juans and Sangre de Cristos, with similar high probabilities continuing in the Sangre de Cristos D2 leading to 1-2 feet of snow in some areas. Lower but still significant probabilities encompass the Front Range D1, and down into the Raton Mesa D2. ... Great Lakes into the Northeast... Day 3... ...The potential for a late season significant winter storm is increasing... Although guidance still features a wide variety of solutions across the Great Lakes and Northeast by Wednesday/Thursday, the consensus is for a significant nor'easter to develop as a Miller-B type event across the Northeast. This even stems from impressively phasing northern and southern stream energy over the Great Lakes early Wednesday, leading to an anomalously strong closed 500mb low digging into the Ohio Valley. Heights beneath this feature are progged by NAEFS to reach as low as -3 sigma, indicating the impressiveness of the developing pattern. As this low consolidates and digs southeast, reaching the Ohio Valley by the end of the forecast period, downstream ascent will intensify dramatically through height falls, mid-level divergence, and a coupled jet structure driving intense diffluence as the subtropical jet streak approaches 170 kts over the Southeast, which would approach early- April records according to the SPC sounding climatology. All of this together indicates a favorable pattern for a surface low moving into the Great Lakes, then translating to rapid secondary development off the northern Mid-Atlantic coast, resulting in an expansive area of precipitation. While the primary low will erode into Ontario, the secondary feature may explosively deepen as it moves off the New England coast, resulting in a significant April nor'easter. Moisture streaming into the system will drive PWs to +2 to +3 sigma according to NAEFS, and although some of this will get cut off my the occlusion/secondary development, the latter low will likely be accompanied by an impressive TROWAL pivoting into New England, enhancing both moisture and instability late D3 and into D4. It is also important to note that some of the guidance shows a robust dry slot lifting into eastern New England late D3. If this occurs it could dramatically cut down on both precip and snowfall as the DGZ dries out resulting in only a period of drizzle or freezing drizzle. The GFS is the most robust with this evolution, but is definitely another challenge with this forecast that needs to be monitored. The deterministic models have shown quite a bit of placement fluctuation the past few runs, but the ensembles, especially GEFS/ECENS, have been rather steady and consistent with each other. Using these as a guide, this suggests the first low will lift across the OH VLY and into the Great Lakes before weakening, but could result in a stripe of impressive deformation snow to its north and west. As the first low weakens and secondary development occurs off NJ, this second low is progged to translate towards Cape Cod during D3 while deepening rapidly, and then beyond this forecast period it may get captured by the digging closed low to stall near eastern New England. A long duration significant winter event is likely in this setup. WPC probabilities D3 for more than 4 inches of snow have increased to 30-60% across the U.P. and northern L.P. of MI, with lower probabilities extending even into northern IN and eastern WI. The more significant probabilities continue in the Northeast where they reach above 70% in the Adirondacks and across most of central New England and into parts of northern New England. The potential exists for much greater snow totals though, especially in higher terrain and inland from I-95. Additionally, depending on the exact track of this low and the accompanying thermal structure, there could be an axis of freezing rain accretion exceeding 0.1" south of the snow footprint. Uncertainty is quite high in this, but current WPC probabilities for 0.1" of ice are as high as 5% in the Catskills. Fracasso/Weiss *** Key Messages for Early April Nor'easter *** ---Long duration winter storm becoming likely Confidence is increasing that a large storm system will produce a swath of heavy snow and gusty winds across portions of the Great Lakes and into the Northeast Wednesday through Friday. ---Widespread heavy snow. Heavy snow may develop across portions of the Great Lakes early Wednesday through early Thursday. Secondary low pressure development along the coast is likely to bring heavy snow and mixed freezing rain/sleet to the Northeast late Wednesday through Friday. ---Significant impacts from heavy snow and wind The combination of heavy snow rates and gusty winds will likely result in hazardous travel due to low visibility and snow-covered roads. Additionally, the heavy and wet nature of the snow could produce impacts to infrastructure. ---Forecast changes anticipated Uncertainty remains with the timing and location of the storm track, which will affect where the most significant impacts will occur. Keep up to date with the latest forecasts as this storm evolves.