Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 10 2025 20:48:52 FOUS11 KWBC 102048 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 PM EST Mon Feb 10 2025 Valid 12Z Mon Feb 10 2025 - 12Z Thu Feb 13 2025 ....Great Lakes... Day 1... A strong cold front will exit into New England Tuesday morning,=20 leaving strong CAA across the Great Lakes in its wake. 850mb temps=20 plummeting to -15C to -20C, coldest over Lake Superior, will=20 support a renewed period of lake effect snow (LES) in the favored=20 W/NW snow belts across the U.P. of MI and into the Tug Hill=20 Plateau. A very cold column over Lake Superior will drive the DGZ=20 down to the surface which will allow for a maximum intersection of=20 lake-induced ascent and the snow growth region. Farther east across Lake Ontario, the overlap won't be quite as efficient, but some=20 higher moisture due to an upstream connection will additionally=20 enhance the LES potential. Snowfall rates of 1"/hr or more are=20 likely Monday night and Tuesday before subtle height rises bring an end to LES before D2. WPC probabilities for 6+ inches are above=20 70% in the Tug Hill Plateau and portions of the eastern U.P. near=20 Whitefish Pt on D1. ....Mid-Mississippi Valley to the Mid-Atlantic...=20 Days 1-3... Active pattern continues with two significant winter weather=20 events progged to impact the area with different hazards through=20 the week. The first begins early Tuesday morning as precipitation begins to=20 expand across the Ozarks, Mid-Mississippi Valley, and Tennessee=20 Valley. This precipitation will be the result of an overlap of=20 forcing and moisture expanding into the region from the west and=20 south. A shortwave trough and accompanying vorticity maxima will=20 eject from the Four Corners this evening and race eastward into the Southern Plains by Tuesday morning as it weakens in response to=20 confluent/flat flow across the eastern CONUS. Despite the modest=20 amplitude of this feature, ascent will intensify through modest PVA and mid-level divergence overlapped with the right entrance region to a jet streak amplifying to the northeast. A weak surface wave=20 may develop and skirt almost due east accompanying this system, but in general precipitation will be driven by the overlap of the=20 synoptic ascent and increasingly impressive moist isentropic=20 upglide/WAA emerging from the Gulf of Mexico. The moisture surging northward will be significant as reflected by mixing ratios within the robust 290-295K isentropic ascent=20 reaching as high as 6-8 g/kg, drawing PWs as high as the 90th=20 climatological percentile as far north as VA/KY. This will an=20 expanding swath of precipitation, with rounds of heavier rates=20 likely embedded. The guidance has generally started to stabilize=20 the footprint and thermal structure, but some wavering is still=20 possible especially the upstream shortwave intensity and speed=20 continues to fluctuate. However, there is high confidence that a=20 stripe of heavy snow will spread east from Kentucky through=20 Delaware. Snowfall rates of 1+"/hr are likely at times, especially=20 within a west-to-east oriented fgen band progged to setup from=20 eastern KY into DE, where 850-700mb fgen maximizes ascent and=20 occurs in conjunction with CSS (EPV* < 0.25) to support CSI. Where=20 this band develops, locally higher amounts of snow are possible,=20 but in general WPC probabilities spanning D1 and D2 are high (>70%) for more than 4" from eastern KY though the Central Appalachians=20 eastward near Washington, D.C. and into southern DE. Locally more=20 than 8 inches is possible where the most intense band tracks. Farther to the south, a transition zone is still expected where a=20 mixture of snow, sleet, and freezing rain will occur. This is most=20 likely from the Ozarks, through KY and into the Central=20 Appalachians, especially along the Blue Ridge of NC/VA. While some=20 sleet is possible, the predominant mixed p-type is likely to be=20 freezing rain within this axis, and will almost certainly be=20 significant (to locally damaging) in the vicinity of the Blue=20 Ridge. WPC probabilities for more than 0.25" are above 70% in the=20 vicinity of the Blue Ridge where locally more than 0.5" is possible (30% chance). This will cause severe impacts including power=20 outages, scattered tree damage, and dangerous travel. Surrounding=20 this, WPC probabilities for more than 0.1" of 10-30% encompass a=20 much larger area including the Ozarks, parts of Kentucky, and the=20 Central Appalachians near the MD Panhandle and Laurel Highlands. This first event winds down late Wednesday morning through the=20 afternoon /D2/, but just as the column begins to dry aloft, renewed WAA ahead of the next system approaches within this progressive=20 pattern. While the DGZ dries out across the Mid-=20 Atlantic/Appalachians, the low levels re-saturate, and it is=20 possible in some areas there is never a break in wintry=20 precipitation due to very light snow/snow grains/freezing drizzle=20 Wednesday. Eventually the column re-saturates though as a more=20 intense low pressure developing over the Southern Plains lifts=20 northeast into the Ohio Valley with another round of mixed=20 precipitation spread into the region. This second wave will be stronger, but also farther inland, so=20 many areas will begin with snow or freezing rain and eventually=20 transition to all rain. The heaviest precipitation will likely=20 begin right around the end of D2 in the Central Appalachians and=20 then expand northeast into the Mid-Atlantic around 00Z/Thursday or=20 slightly afterwards. High pressure wedging into the region will=20 again force impressive isentropic overrunning, and locations from=20 northern VA through PA will likely begin as a burst of moderate=20 snowfall before quickly transitioning to sleet, freezing rain, and=20 eventually all rain before waning Thursday evening. The heaviest=20 snowfall is likely in the Poconos where WPC probabilities for 2+=20 inches are as high as 50%, with icing exceeding 0.1" possible=20 (>30%) in the Laurel Highlands and parts of the Poconos. ....Central Rockies/Plains/Midwest/Great Lakes...=20 Days 1-3... Troughing over the PacNW/BC this evening will dip into the Great=20 Basin and into the central Rockies tomorrow evening, with broad WSW flow across CO. This will favor an expansion of snowfall over the=20 Medicine Bow mountains and into the San Juans/Sangre de Cristos=20 where upslope enhancement will wring out several inches of snow=20 area-wide. For the D1.5 period, WPC probabilities for at least 6=20 inches of snow are >50% above 10,000ft or so.=20 By Wednesday morning, troughing will pass the spine of the Rockies as low-level moisture increases over the foothills to the Plains=20 via SE to E flow. This will combine with NE flow at the surface to=20 increase snow into the Front Range, though snow should generally be light overall. WPC probabilities for at least 4 inches are near=20 30% over parts of the I-25 corridor.=20 The more intense snowfall will manifest farther east over Kansas=20 starting late Tuesday into Wednesday. There, the potential exists=20 for moderate to heavy snow across much of Kansas as the upper jet=20 extends from TX northeastward to the Midwest. Combination of WAA,=20 tightening thermal gradient (increasing FGEN), and favorable/deep=20 DGZ/isothermal layer will promote locally heavy snow with >1"/hr=20 rates (per WPC snowband tool). A large high pressure to the north=20 will supply NE winds to the region but the gradient is not expected to be strong enough to produce much blowing snow even as low=20 pressure deepens over the ArkLaTex. SLRs may eclipse 20:1 in the=20 most favorable bands but otherwise hover close to 15:1 along the=20 I-70 corridor from GLD to MCI. Trend in the recent guidance has=20 been a bit to the N/NW especially east of the Plains, which may=20 bring more ice into southeastern KS than what was seen 24 hours=20 ago. There, overrunning will favor sleet and freezing rain from SW=20 to NE from OKC to southeastern KS into southern MO. WPC=20 probabilities for at least 0.1" ice are low-moderate in this region (10-50%). For snowfall, PVA and FGEN will force much of the=20 precipitation in advance of the trough with the axis of heavier=20 snowfall along I-70 toward/into Kansas City, then extending=20 northeastward. WPC probabilities for at least 4 inches of snow are=20 high (>70%) over most of central/northern KS into southern NE with=20 some probs (30-50%) for at least 8 inches. By Wednesday night, the system begins to further organize and pick up forward speed as the surface low lifts through the Ohio and=20 Tennessee valleys. Snow is expected to expand from southern IA and=20 northern/central MO through much of IL, southern WI, northern IN,=20 and the L.P. of Michigan. A narrow corridor of mixed ptype is also=20 forecast in a southwest to northeast band from southern MO through=20 central IN and northwestern OH. Here, the NW trend in the 12Z=20 guidance was more pronounced (esp the 12Z NAM), and WPC made a=20 modest adjustment to the NW given other signals from other=20 models/ensembles (which were not as dramatic or not shown at all).=20 SLRs should increase to the NW deeper into the colder column, but=20 pared back the NBM which could be a bit too high. Nevertheless,=20 combination of strong PVA and lower level FGEN into a deep DGZ=20 should provide for an axis of heavier snow, but where that=20 materializes is bit fuzzy (including any lake enhancement off Lake=20 Michigan on NE winds).=20 Right now, the highest probabilities of at least 4 inches of snow=20 maximize from northern MO through the Chicago metro and into=20 central Lower MI, with >50% probs extending to the NW and SE by=20 about 100 miles. This includes cities such as Kansas City, Des=20 Moines, Chicago, Milwaukee, and Detroit, just to name a few. Within this region, probabilities of at least 8 inches are moderate=20 (>40%) from northern MO to northern IL and across central Lower=20 Michigan. Like areas farther upstream (southern Plains), a mix or sleet and=20 freezing rain is likely closer to the track of the surface low due=20 to the overrunning of the cold surface with warmer air aloft over=20 portions of central IN into northwestern OH. There, WPC=20 probabilities of at least 0.10" icing are low (10-40%). The positive-tilt to the pattern will help move the system along,=20 with precipitation ending Thursday morning over Michigan, though NW flow on the backside will stir up some lake effect snow for U.P.=20 and L.P. of MI. Amounts should be light at the end of this forecast period (12Z Thu - 00Z Fri). ....Pacific Coast... Days 2-3... A large trough will amplify across the Intermountain West,=20 upstream of which shortwaves will begin to dig along the Pacific=20 coast as vorticity impulses shed east from an amplifying closed low over the Pacific. While this low won't really become organized=20 until late D3, energy pivoting onshore beginning D2 will spread an=20 axis of moisture into central and southern CA, with snow in the=20 terrain above generally 3000-4000 ft. This impulse is weak and will be directed almost due SW by the downstream trough, but WPC=20 probabilities are moderate (30-50%) for more than 6 inches across=20 primarily the southern Sierra D2. After this first impulse, the closed low offshore begins to deepen and pivot eastward. Height anomalies drop steadily offshore,=20 reaching as low as 1st percentile within the CFSR climatology by=20 00Z Friday near the CA/OR border, and the accompanying and=20 impressive downstream divergence will begin to displace the trough=20 with a brief period of shortwave ridging. Increasing moisture=20 advection onshore, driven by confluent mid-level flow south of this closed low topped by a collocated strengthening jet streak will=20 surge IVT as high as 500 kg/m/s into the coast late Thursday. This=20 will manifest as expanding and intensifying precipitation, with the accompanying warm front/WAA lifting snow levels rapidly to as much as 6000 ft in the Sierra. However, farther north into the=20 Shasta/Trinity region and into Oregon, pronounced cold air will be=20 slow to retreat, leading to some lowland snow potential as well as=20 overrunning/freezing rain. There continues to be a lot of=20 uncertainty in timing and placement, but there is high confidence=20 that this event will eventually result in heavy wintry=20 precipitation across the West. Current WPC probabilities are high=20 (>90%) for 12+ inches across much of the Sierra, generally above=20 4000 ft, and above 70% (above 3000 ft) in the=20 Shasta/Trinity/Klamath region. This could be quite impactful to=20 many area passes. ....Northeast... Days 1 & 3... On Tuesday, a strong cold front will race from the Great Lakes=20 through New England, bringing with it an increased risk for=20 convective snow showers and snow squalls. The high-res CAMs have=20 become more aggressive with their depiction of simulated=20 reflectivity along this front Tuesday aftn, suggesting a greater=20 threat for snow squalls. The greatest risk appears to be from=20 Upstate NY across northern New England which is where the best=20 overlap of 0-2km fgen and RH overlap, and although instability is=20 modest, the SnSq parameter does reach +2 across this region. This=20 may end up more as convective snow showers than true squalls, but=20 briefly intense snow rates and restricted visibility could cause=20 dangerous travel Tuesday afternoon/evening. Then during D3 /00Z Thursday to 00Z Friday/ more significant=20 precipitation will overspread the region from SW to NE. This=20 precipitation will be associated with a robust low pressure lifting across the Ohio Valley, driven by a modest shortwave on the=20 downstream edge of a deepening trough over the middle of the CONUS, overlapped with the RRQ of a 150 kt poleward arcing jet streak=20 moving across the eastern Great Lakes. Downstream of this system,=20 moist advection will maximize in response to low-level WAA on S/SE=20 flow overrunning a retreating high pressure that will try to wedge=20 back to the west into New England. This will result in an expansion of wintry precipitation, likely starting as snow everywhere before gradually transitioning to sleet and freezing rain, especially=20 south of I-90, before winding down late D3 everywhere but ME/NH. There is still considerable spread in the speed of this system=20 forecast by the deterministic global members, but at least moderate snowfall accumulations are likely, especially in higher terrain,=20 as reflected by WPC probabilities that are above 50% for 4+ inches=20 from the Adirondacks across much of northern New England.=20 Additionally, light to moderate icing is possible as reflected by=20 WPC probabilities of 10-30% for 0.1" in the Catskills, Berkshires,=20 and Litchfield Hills. Weiss/Fracasso/Snell ....Winter Storm Key Messages are in effect. Please see current=20 Key Messages below... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!924NT_I-3Zl98BzXxxOT32R60yVgs-zliGxJOIussbPLs= zH6XP1sOUErxvXjklcPTVUan4DqII6uhbTzZFV8Y2CuX1E$=20 https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!924NT_I-3Zl98BzXxxOT32R60yVgs-zliGxJOIussbPLs= zH6XP1sOUErxvXjklcPTVUan4DqII6uhbTzZFV8kZDSEIc$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .