Subj : HVYSNOW: Probabilistic He To : All From : Mike Powell Date : Wed Dec 13 2023 14:30:00 FOUS11 KWBC 130828 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 AM EST Wed Dec 13 2023 Valid 12Z Wed Dec 13 2023 - 12Z Sat Dec 16 2023 ....Southern Rockies into the High Plains... Days 1-2... A closed low over the Southwest will track eastward today and will be centered over NM on Thursday before ejecting into the Plains on Friday as an open trough. Forcing associated with this feature and an attendant upper jet will support a heavy snow threat over portions of the southern Rockies and adjacent portions of the High Plains. An impressive amount of moisture will be advected northward ahead of this system, with PW values exceeding the 99th percentile for mid December over portions of northeast NM into southeast CO into the High Plains. This ample moisture combined with the strong mid/upper forcing associated with the closed low and areas of orographic enhancement will result in a significant snow event for portions of the area today into Thursday. The heaviest snowfall is expected across the favored terrain of the Sangre de Cristos, where upwards of one to two feet is expected. Across the rest of northeast NM north and east of the Sangre de Cristos, the probability of exceeding 8" of snow generally remains over 60%, with these higher probabilities spilling into adjacent areas of far southern CO as well. It should be noted that the precipitation over this region has a very convective look to it in simulated model reflectivity, likely due to the impressive forcing and steep lapse rates associated with the closed low. Thus not surprising that 1"/hr snowfall probabilities from the HREF periodically get into the 50-90% range, both with the initial WAA snow today and with the approach of the closed low later tonight into Thursday. Overall this remains a high confidence event for significant and impactful snowfall over portions of northeast NM and adjacent areas of southern CO today into Thursday...with the caveat that elevation will play a role (lower elevations will see less snow) and the eventual snowfall totals will probably not be uniformly distributed given the showery/convective nature of the snow. Confidence in snowfall amounts and impacts decrease as you head eastward into far northeast NM, the TX/OK Panhandle and southwest KS as thermal profiles remain quite marginal. Thus will be a close call whether the majority of the precipitation falls as rain or wet snow. With that said, if you ignore the warmer GFS/GEFS solutions the guidance is actually in decent agreement. Again this does not necessarily mean the forecast won't change as the event nears, but at the moment there is a pretty good clustering among the HREF/GEM/ECMWF/NAM for ptype. The rain/snow line will be rather sharp over far northeast NM into the OK/TX Panhandle today into early Thursday, with the general consensus supporting a bit more snow compared to previous forecasts. Totals over KS should still generally stay below 4", but more significant totals are possible over far northeast NM into immediate adjacent areas of the OK/TX Panhandle. Again this will be a close call, but the current guidance supports increased 5"+ probabilities into the 40-70% range. Just like over the aforementioned areas of CO/NM the snow will be convective in nature here, so where it is snowing it will be a heavy and wet snow with 1"/hr snowfall possible at times. Lower totals of 1-3" are still expected over most of the rest of the TX Panhandle...where rain will be the predominate ptype until a transition to snow on the back end of the precip shield Thursday afternoon/evening. The shorter duration of snow will limit totals...but snowfall intensity could be briefly quite high given the convective nature of the precip. Another facet of this storm will be the potential of light freezing rain accretion across portions of western KS, far eastern CO and far southwest NE today into this evening. Precipitation moving northward will overrun a cold low level airmass setting up the potential for freezing rain. Temperatures are borderline, but there was pretty good consensus with HREF members (which we'd expect to have the best handle of low level thermals) in a light icing event. WPC probabilities of at least .01" ice have increased to over 70%, with probabilities of 0.1" as high as 20-40% centered over western KS. These probabilities could even be a tad low given the influence of the warmer GEFS members. Nonetheless, still not expecting a significant freezing rain event, but confidence is increasing in some lower end accretion amounts, especially on elevated surfaces. ....Interior Northeast and New England... Day 1... A strong cold front is currently moving across the Northeast, with snow showers expected to increase in coverage through the day behind it. The main area of snow showers will be off of Lake Ontario across portions of central NY. The snow squall parameter is elevated across the Northeast, which makes sense given the steep lapse rates seen in model soundings. Snow shower activity should be enhanced as a mid level trough and shortwave energy dives into the Northeast this afternoon...and would expect some snow shower activity to make it across New England, potentially all the way to the coast. The high-res simulated reflectivity also supports convective snow showers, and while not expecting an organized/widespread snow squall threat given the cellular nature of cells and modest CAA...do think a few localized snow squall warnings are a possibility. This isolated squall potential is maximized where the NAM snow squall parameter maximizes above +4 across central/northern NY, but scattered convective snow showers are still expected elsewhere across much of the Northeast. Chenard $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .