Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 225 PM EST Mon Jan 27 2025 Valid 00Z Tue Jan 28 2025 - 00Z Fri Jan 31 2025 ...Great Lakes, Northeast, Mid-Atlantic... Days 1-3... Current mid-level WV satellite imagery indicates a strong mid- latitude gyre focused over the southern tip of the Hudson Bay with a strong surface cold front analyzed just north of the Canadian border with sights on the Upper Great Lakes and areas downstream. The expectation is for this potent cold front to drive southward through the U.P, pivoting south and southeastward into the L.P. of MI this evening eventually racing eastward through the interior Northeast and New England overnight into Tuesday morning. Impressive CAA along the front will drive an intense baroclinic boundary, leading to a stripe of impressive low- level fgen, especially in the 925-850mb layer. This will drive strong mesoscale lift into a saturating column reflected by high low-level RH, and as lapse rates steepen, some weak to modest instability will develop. High-res guidance has maintained an aggressive posture with regards to a band/corridor of convective snow showers and snow squalls with the strongest low-level FGEN indication within the confines of the primary Arctic front. Time frame of interest is between 06-12z Tuesday with a forward propagation from northwest to southeast beginning in the NY state North Country with motions downstream into the adjacent Adirondack through Western NY. Recent HREF probabilities have increased from the previous forecast with a more robust 30-50% chance for at least 1"hr snowfall rates within strongest low-level forcing regime aligned with the Arctic front. Other lower end probs exist behind the primary Arctic push with an extension through parts of Upstate NY before the heavier snow threat fades downstream as we lose the accompanying mid-level forcing as it scoots off to the northeast. The accompanying steep low to mid level lapse rates will help mix down strong winds, encouraging the development of low visibility during any heavier snow showers/squalls. The limiting factor to more robust snow squalls may be a relative dearth of 0-2km CAPE, and it is beneficial that the most significant impacts will occur at night, but still, convective snow showers and isolated snow squalls are expected Monday night into Tuesday morning from the U.P. of Michigan through much of the interior Northeast and New England, and potentially as far southeast as the Mid-Atlantic. Accumulations during this time will generally be minor, but some LES and some upslope flow with the front across the Adirondacks has resulted in WPC probabilities reaching 70-90+% for 4 inches on D1 with some mid-range probabilities (40-60%) located downwind of Lake Erie within NY state. Some modest LES will follow in the wake of this front, but more significant and widespread snow will occur D2 into D3, as yet another fast moving impulse dives through the broad cyclonic flow across the east. This third impulse will track along the residual baroclinic gradient left by the potent cold front, and will be overlapped by a strengthening jet streak downstream of the trough axis to produce more robust deep layer ascent. This will support modest cyclogenesis as a clipper-type low swings southeast from Minnesota Tuesday afternoon, to Upstate New York Wednesday morning, and then across New England, exiting into Atlantic Canada by Wednesday evening. Brief but impressive WAA downstream of this low will result in expanding precipitation falling as moderate to heavy snow across many areas. Then, in the wake of this system, additional LES will develop as NW flow causes additional accumulations in the favored NW snow belts, and with strong winds, some of this could push as far SE as the coastal Mid-Atlantic states by the end of the forecast period. WPC probabilities D2 are high (70-90+%) for 4+ inches across the U.P., the NW L.P. near Traverse City, and east of Lakes Erie and Ontario, with a swath of moderate probabilities for 2+ inches surrounding those areas for the more synoptically forced snowfall. For D3, the greatest risk for 4+ inches of snow shifts into the Adirondacks and Greens where WPC probabilities feature a high risk (>70%) while additional modest LES occurs east of Lakes Erie and Ontario. Locally 1-2 feet is possible in some areas through the period. Finally, a second round of convective snow showers or snow squalls is becoming more likely on Wednesday, this time a little south of the event Tuesday morning. These convective snow showers will occur during a more favorable time of day to support better CAPE behind the secondary front, and the Snow Squall parameter is suggesting a risk across areas from eastern OH through southern Upstate NY and PA. This secondary set of squalls, if they occur, will again be accompanied by strong winds and heavy snow rates leading to the potential for significant travel impacts despite modest snowfall accumulations. The two rounds of potential snow squalls continue the issuance of Key Messages linked below. ...Southwest through the Four Corners... Days 1-3... Mid-level water vapor satellite imagery continues to highlight the anomalous upper low pivoting slowly across CA with distinct horizontal elongation within the confines of the disturbance. Increased upper forcing directly under and downstream of the primary circulation will bring increasing snowfall concerns across the Southwest US through Thursday as the disturbance wanders eastward, generally cut off from the mean upper flow. This disturbance begins the period quite amplified, with 500-700mb heights as low as -2 sigma over CA according to the NAEFS ensemble tables. Two distinct spokes of vorticity will dance around the core of this upper low, maintaining the elongated structure over time as it stretches NE to SW into the Four Corners by Tuesday night. The interaction of these vorticity impulses rotating around the closed center will help keep the low amplified, but also very slow moving, reaching NM/CO by the end of the forecast period. During this synoptic evolution, the upper pattern will alter considerably as well. Initially, a strong 250mb jet streak will be arced downstream of the upper low, lifting northeast into the Southern Plains. However, this feature will weaken D1 as the upper low elongates, leaving less substantial diffluent-caused ascent into D2. However, a secondary surge of jet level energy will occur Wednesday into Thursday as the jet streak re-amplifies and arcs meridionally from the Gulf of California into the Southern Plains, placing intense LFQ diffluent ascent over the Four Corners. This jet evolution, combined with the mid-level closed low, and at least subtle low-level baroclinicity in the vicinity of a weakening front will cause waves of low pressure to move eastward from CA to the Four Corners D1-D2. This will cause periods of snowfall from the Transverse and Peninsular Ranges of CA eastward through the Mogollon Rim and Kaibab Plateau where WPC probabilities are moderate to high (40-70%) D1 for 4 inches of snow. However, the most significant snowfall is likely to begin late-D2 into D3 as a low pressure develops near the TX Big Bend and moisture advection surges on isentropic ascent across TX. The accompanying theta-e ridge is progged to lift cyclonically into a TROWAL, with the most intense theta-e advection being directed into the DGZ over NM/CO. There is still quite a bit of uncertainty regarding the placement and evolution of this system, but snowfall chances are increasing, especially in the higher terrain above 5000 ft in the Sangre de Cristos and San Juans. Here, WPC probabilities for D2 and D3 are as high as 70% for 6+ inches, and impactful snow is becoming more likely for parts of this area Wednesday into Thursday. For the Days 1-3 period, the probability of significant icing greater than 0.10" is less than 10 percent. Weiss/Kleebauer ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png