Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 450 AM EST Sun Jan 14 2024 Valid 12Z Sun Jan 14 2024 - 12Z Wed Jan 17 2024 ...Prolonged Great Lake Effect and Snow Squalls in Northeast today... Days 1-3... A slow moving and sprawling low pressure gyre centered over northern Ontario develops today and likely persists there into midweek as a powerful trough axis swings over the Northeast today. This means a prolonged WSW to W wind lake effect snow event downwind of all five Great Lakes that have a few ebbs and flows through this time. Cold air through the column (850mb temps <-18C) over the still nearly unfrozen lakes will lead to more than steep enough lapse rates for snow, but the cold air will generally favor lower than maximum SLRs (past the DGZ on the cold end) also from dendritic fracturing in the gusty winds today. Snow squalls (a brief burst of snow accompanied by winds and a real threat to overland travel) are expected from the upper trough axis moving over much of PA/NY this morning through midday, shifting across the northern the northern Mid-Atlantic and southern/central New England all the way to the coast this afternoon/evening. Snow squalls are most dangerous when temperatures fall below freezing with the squall, causing a flash freeze on roadways and this looks to be the case this afternoon over eastern PA/northern NJ/southern NY and southern New England. LES snow probs: Day 1 for at least 6 inches of snow are high (>70%) over the eastern U.P. of Michigan and especially in the single band setups east of Lakes Erie/Ontario in WSW flow (favoring near and just south of BUF) where double-digit totals are quite likely. Tonight into Monday, flow becomes westerly across the western Lakes, supporting multi-bands into much of Lower Michigan but still some single bands into the Keweenaw Peninsula on the U.P. where WPC probabilities for at least another 4 inches are around 50%. Intense single bands off Erie/Ontario will waver a bit and lift northward but weaken, still allowing for several more inches of snow with high probs for >6" in narrow bands north of Buffalo and the Tug Hill. Westerly flow prevails Tuesday into Wednesday with Day 3 probs moderate for additional >6" in the Keweenaw and back over Buffalo/the Tug Hill ...Mid-South/TN Valley/Deep South/Appalachians through Northeast... Days 1-3... A series of impulses is lined up from the Four Corners to the OR/CA coast early this morning. These will ride a WNWly jet that rounds the longwave trough spanning most of the CONUS. That jet turns Wly and increases to 130-kt jet over OK to the Mid-Atlantic tonight with the Mid-South in the right entrance region aiding lift as these impulses/vort maxes push through. Arctic front will reach the upper TX coast this morning and the central Gulf Coast by this evening, bringing sub-freezing temperatures in its wake from eastern TX through the Lower MS Valley. However, low level ridging over the Gulf will allow return flow north over the Arctic-sourced airmass in place, setting up a broad area of a wintry mix of sleet and freezing rain tonight through Monday night from east Texas across much of LA and central MS/AL. Farther north in the deeper cold air, light to locally moderate snow will expand eastward intermittently with the impulse arrivals from central OK and through AR this morning through this evening before settling on a low level frontogenetic zone from southern AR east-northeast over northern MS/most of TN to the southern Appalachians tonight through Monday. Along this zone, expect moderate to possibly heavy snowfall with relatively high SLRs (~12-16:1). This focus has shifted south a bit with the 00Z consensus tonight with the stripe of several inches of snow now more over south-central AR and northern MS, but still extending ENE across TN and far northern AL. How far north the warm air aloft reaches will determine the north area of the sleet area progresses, current thinking is the NAMnest remains too far north into with the warm nose, especially with the rest of 00Z guidance shifting south. Snow should be heaviest just north of the sleet area making the warm nose progression key to a decent forecast. Day 1.5/2 WPC probabilities of at least 4 inches of snow are moderate (40-70%) over a stripe from southeast AR through northern MS and most of TN and then up the southern Appalachians into WV. Farther south, cold surface temperatures beneath a warm nose aloft supports an area of freezing rain/sleet starting this evening from the TX Hill Country/South-central TX eastward across much of LA through central MS and into AL. It will be a battle between the arctic air beneath the advancing warmer air aloft, and the latest guidance with potential for freezing rain accumulations to or near the Gulf Coast given how cold the surface air is. In general, the northern portion of the wintry mix area is more confidently going to be sleet given the depth and magnitude of surface cold air. The Day 1.5 WPC probabilities of at least 0.10" ice are moderate (30-60%) from around I-35 between San Antonio and DFW eastward into central MS (I-55) with little to no chances for 0.25" at this time. The Day 1/2 chance of any (>0.01") icing (>10% chance) extends from south TX across east TX nearly all of LA (outside NOLA) through much of MS, central AL, and northwest GA into far eastern TN. A shortwave trough rounding the longwave pushes down the northern Rockies tonight before shifting east over the Ohio Valley into Tuesday and the Northeast Tuesday night. This would promote coastal low development off the northern Mid-Atlantic late Tuesday and past New England Tuesday night. Day 3 snow probs for >4" are low over the northern Mid-Atlantic, but become moderate (40-60%) over Down East Maine. Uncertainty remains with the strength of this low, but probabilities are increasing for the first inch of snow of the winter to some of the I-95 corridor (DC-PHL-NYC) where the snow drought continues. ...Great Basin and Colorado Rockies... Days 1-2... The last impulse in a series of waves moving into OR starting Saturday cross the coast at the OR/CA border around 12Z today and tracks inland along/south of a stationary front marking the south border of the Arctic sourced air that invaded the Northwest. Mountain snows continue over the Great Basin ranges, the Wasatch and Colorado Rockies as noted by the Day 1 snow probs which are high for >6" for the Wasatch and most CO ranges. Ridging shifts inland behind this last impulse, cutting off precip quickly today for the West Coast and by Tuesday for the Rockies. However, a northern stream shortwave trough rounding the deep low gyre developing over Ontario will shift down the northern Rockies tonight into Monday providing lift and extending snow over the central Rockies (with light snow extending east onto the Plains across KS) with some low to moderate Day 2 snow probs for an additional >6" for the central CO Rockies. ...Northwest... Day 3... The next compact low to cut east through the Northeast Pacific/Alaska ridge likely reaches the OR/WA coast late Tuesday and may be joined by northern stream troughing shifting south from BC Tuesday night into Wednesday allowing a quick shift inland to the northern Rockies Tuesday night. With anomalously cold surface air likely lingering over the Pacific Northwest including Seattle and Portland metros, a wintry mix to sea level is in the cards. Day 3 PWPF are 10-50% for >0.1" ice along the western OR/WA border including the Portland metro and 20-60% for >6" over the WA Cascades as well as eastern WA/northern ID and the Sawtooths toward the Boise metro in west-central ID. This low/wave combo looks to be the first part of a rather active winter weather pattern for the Northwest continuing the rest of the week. Jackson ***Key Messages for Mid-January Arctic Blast, Lake Effect, and Snow Squalls*** --Dangerously Cold Wind Chills Expected Sub-zero wind chills will affect much of the U.S. and reach into portions of the South. Wind chills into early this week will fall below minus 30 degrees from the northern Rockies to the central Plains and mid Mississippi Valley. Values will drop as low as minus 70 degrees from Montana to North Dakota. These wind chills will pose a risk of frostbite on exposed skin and hypothermia. Have a cold survival kit if you must travel. --Lake Effect Snow Persists This Weekend Heavy snow will continue downwind of the Great Lakes into midweek. Whiteout conditions in the lake effect snow bands are expected. An additional 12-24 inches of snow will occur through Monday across portions of western and northern New York. --Snow Squalls Expected over the Northern Mid-Atlantic and Northeast Today Snow squalls will impact portions of the northern Mid-Atlantic and Northeast today. Gusty winds, rapidly reduced visibility, and flash freezing on roads will result in dangerous travel where squalls occur. --Hazardous Cold Weather to Persist and Redevelop A new surge of Arctic air will drop south over the northern Plains and Midwest midweek, reaching the Deep South by the end of the week. This will reinforce dangerously low temperatures and wind chills. ***Key Messages for Western U.S. to Mid-South Winter Storm*** --Heavy Snow Impacting the Intermountain West Heavy snow will taper down along the West Coast this morning and focus over the Wasatch of Utah and the Colorado Rockies today where continued significant impacts are expected. --High Avalanche Danger Continues for Portions of the Sierra Nevada and the Rockies Heavy snowfall and strong winds have created dangerous avalanche conditions across portions of the Sierra Nevada and the northern to central Rockies. Travel in, near, or below avalanche terrain is not recommended. --Snow and Ice to Reach the Southern Plains Later Today and the Mid-South Tennessee Valley Tonight and Monday Areas of snow, sleet, and freezing rain are likely to develop and spread across portions of the southern Plains, through the ArkLaTex, and into the Tennessee Valley today through Monday. Several inches of snow are likely. Accumulating ice is expected from portions of central and southern Texas through the Lower Mississippi Valley into portions of the Tennessee Valley. With Arctic air continuing to filter south this week, impacts from wintry weather may last for several days, resulting in a prolonged period of hazardous travel.