Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 PM EST Sun Jan 14 2024 Valid 00Z Mon Jan 15 2024 - 00Z Thu Jan 18 2024 ...Prolonged Great Lake Effect Snow... Days 1-3... Expansive and lumbering upper low over James Bay will slowly move westward then southwestward on Tuesday before finally continuing its loop southward then southeastward and eastward just north of Lake Superior on Wednesday. The prolonged cyclonic flow across the Great Lakes will maintain a multi-day snow for nearly all typical lake belts, with variations in the wind favoring some areas over others with time. Over the Upper Great Lakes, NW to W flow suggests some single band streamers over Lake Superior into the U.P. and more multi-band over Lower Michigan. while downwind of Lakes Erie/Ontario WSW to SW flow would continue a rather robust single band into the BUF metro and into and on the northern side of the Tug Hill (i.e., Watertown/Fort Drum). The most substantial snowfall is expected on days 1 and 3, the former due to a stream of mid-level vorticity through the region and the latter as the parent upper low rotates closer to the region, with embedded vort maxes moving through. WPC probabilities for at least 8 inches of snow through the period are highest (>70%) downwind of Lakes Erie/Ontario with moderate probs for at least 12 inches (40-70%). Areas of the U.P. of Michigan, particularly along the Keweenaw Peninsula but also over eastern sections, show high (>70%) probabilities of at least 8 inches of snow as well. A secondary favored area is over northwestern Lower Michigan. ...Mid-South/TN Valley/Deep South/Appalachians through Northeast... Days 1-3... The dominant/expansive troughing over most of the CONUS will swing vort maxes out of the southern Plains and across the Mid-South D1, well north of an Arctic boundary that has brought sub-freezing air to near the Gulf Coast this evening, and will push a bit farther south on Monday. Multi-stream jet will continue across the region and increase to >130kts over OK to the Mid-Atlantic, placing the Mid-South/Lower MS Valley in the right entrance region, adding broad lift to the region. With southerly to southwesterly flow above the surface, milder air will override the cold boundary layer, setting the stage for a rather expansive mixed precip area over eastern TX eastward through southern AR, LA, MS and into northern AL. The models have really struggled with the depth of the cold air at the surface and the amount of warming (T>0C) aloft, resulting in continued wavering in the sleet/freezing rain transition zone. Northerly to northeasterly surface flow should help maintain the cold boundary layer, allowing freezing rain to fall all the way to the I-10 corridor in TX/LA (before a northeastward axis is favored along and north of I-20). WPC probabilities for at least 0.10" ice are at least 10% from around I-35 near Waco eastward through northern LA and into central MS (I-20/I-55). A much broader area has the chance for any measurable freezing rain from the TX Hill Country eastward to northern/central AL and north Georgia. There will likely be an area of sleet as well on the northern side of the freezing rain axis, within a deeper surface cold layer. To the north, the arctic air will be much deeper, supporting all snow from central/eastern AR eastward across northern MS and through much of TN. Relatively high SLRs (~12-16:1) would favor several inches of snow that may be enhanced by frontogenetical banding as the entire system shifts eastward tomorrow evening. WPC probabilities for at least 4 inches of snow on D1 are at least 40% from northern MS across Middle TN and into the southern Appalachians. Into D2, an area of light snow will stream northeastward across the central Appalachians and Mid-Atlantic into the Northeast, aided by the strong 150kt jet overhead and moistening lower levels. Trend has been up in QPF for the region, and have increased snow totals for some areas of the Mid-Atlantic including the I-95 corridor, though there remains some drier models in the mix. A weak area of low pressure is expected to form off the Mid-Atlantic coast and lift northeastward, coincident with an incoming vort max over the eastern Great Lakes, which will enhance snow over New England D2 where WPC probabilities of at least 4 inches are 20-50%. Into D3, the low will strengthen some more as the jet digs into the East, spreading even more snow across portions of Downeast Maine as the low steadily pulls into the Canadian Maritimes. WPC probabilities for at least 4 inches of snow into D3 are highest east of I-91 and north of I-90 in New England, especially into Maine. ...Wasatch and Colorado Rockies... Day 1... Shortwave on the southwest side of the broad troughing over the eastern 3/4ths of the CONUS will move out of the Four Corners region tonight, maintaining light to moderate snow over UT into CO as the Arctic boundary lingers through the region. This will be reinforced by additional shortwave vorticity diving southward along the Divide on Monday, with surface high pressure on its heels. Snow will gradually subside by the end of D1 into the first part of D2 as heights rise. WPC probabilities for an additional 6 inches of snow after 00Z tonight are highest (>70%) over the CO Rockies with lower probabilities over the Wasatch. ...Northwest... Day 3... Very strong closed high over the eastern Aleutians (570dm) will help drag a vort max southward across the Alaska Panhandle over the northeastern Pacific at the same time a another vort max splits off from a Pacific system west of 130W, yielding a one-two punch at the mid-levels behind a surface front that moves into the area late Tue into Wed. Lead moisture plume will mostly focus into OR and CA, with PW anomalies only slightly above normal as the front comes ashore. Trailing vort max will help bring some lingering Pacific moisture inland on Wed as an upper jet stream moves through the northern Great Basin. Cold temperatures in the next few days will be very slow to warm up, supporting low snow levels initially that rise as more milder Pacific air pushes in (at least west of the Cascades). East of the Cascades, deep cold air will remain entrenched which favors higher SLRs as moisture moves across the region into northern Idaho and northwestern MT. WPC probabilities for at least 4 inches of snow are high (>70%) over northeastern WA around Spokane and eastward into MT, with moderate (40-70%) probabilities for at least 8 inches of snow. Over the Cascades, WPC probabilities for at least 12 inches of snow are above 50% at and above 3000ft or so, suggesting significant impacts for the Cascade passes. Over northern OR and southern WA around Portland into the Columbia Gorge, the dense cold air mass will remain in place as precipitation and milder air aloft move into the region Tuesday. A wintry mix of snow/sleet to freezing rain is likely where sub-freezing temperatures hold on until the milder air reaches through the whole column. Models show a wide range of solutions, but arctic air is usually loathe to retreat, so favored the colder solutions at this point. WPC probabilities for at least 0.10" ice are at least 50% around and east of Portland. Fracasso ***Key Messages for Jan 13-16 Winter Storm*** -Southern Snow and Ice Tonight and Monday Areas of snow, sleet, and freezing rain will continue to develop and extend east from the southern Plains to the Tennessee Valley overnight into Monday. Several inches of snow are likely from Arkansas through the Tennessee Valley into the southern Appalachians. Accumulating ice is expected from portions of central and southern Texas through the lower Mississippi Valley into parts of the Tennessee Valley and southern Appalachians. Plan on dangerous road conditions. With arctic air in place, impacts from wintry weather may last several days, resulting in a prolonged period of hazardous travel across some areas. -Snow Reaching the Central Appalachians and Mid-Atlantic Monday and Tuesday Areas of light snow are expected to extend into the central Appalachians and Mid-Atlantic beginning early Monday before potentially heavier snow lifts across the region late Monday into early Tuesday. Plan on slippery road conditions. ***Key Messages for Mid-January Arctic Blast*** -Dangerously Cold Wind Chills Continue Sub-zero wind chills will continue to affect much of the U.S. through the early part of the week. Wind chills below minus 30 degrees will persist from the northern Rockies to the central Plains and mid Mississippi Valley, dipping as low as minus 50 across portions of Montana and North Dakota. These wind chills could cause frostbite on exposed skin in a few minutes and hypothermia shortly thereafter. Avoid outside activities if possible. If you must be outside, wear appropriate clothing. Keep pets indoors. Have a cold survival kit if you must travel. -Lake Effect Snow Persists Periods of lake effect snow will continue downwind of the Great Lakes into midweek, with additional heavy accumulations possible, especially across portions of northern Michigan, western and northern Upstate New York. -Reinforcing Cold Air Later this Week Temperatures are expected to moderate midweek. However, a new surge of colder air will drop south over the northern Plains and Midwest, reaching the Deep South by the end of the week.