074 FXUS63 KDTX 052034 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 334 PM EST Fri Feb 5 2021 .DISCUSSION... A lobe of high planetary vorticity is becoming established over Far Northwest Ontario this afternoon and is leading to the development of deep upper level low pressure. Cyclonic flow around this upper level low is piping arctic air into the state on west southwest flow. The west southwest component has led to a mesoscale sfc-925mb convergence axis from roughly Saugatuck to Flint to Port Sanilac. This includes near and along the I 69 corridor through central sections of the cwa. Model data shows a relative steady state to this convergence axis through the evening. Sfc asos observations of reduced visibilities up north continue to suggest some combination of overshooting by the radar of the shallow lake effect convection or a blowing snow component. Will maintain the Winter Weather Advisory for the counties along and north of I 69 for intermittent snow showers and blowing snow potential. Farther south, specifically the Livingston and Washtenaw tiers including Metro Detroit there has been little to no evidence of potential travel impacts at this point. Traffic data shows good travel times throughout the early afternoon with little to flurries now at this point. Models show no trend toward a west low tropospheric mean wind until after 03Z this evening. Will cancel the advisory ahead of the evening commute. Still expecting some slick spots due to cold temperatures in the teens and possible refreezing. Focused lower tropospheric convergence axis is expected to drift southward after 03Z tonight allowing best lake effect banding potential to sag into the M59 and I96 corridors. Best of the cyclonic vorticity advection will pass the area by this evening, however,some additional cold advection rooted at 850mb will bring temperatures at that level to around -18C. Model do suggest gaining an extra 2000 ft in convective depths which is progged to supersaturated wrt ice. Did go somewhat aggressive with categorical/high likely PoPs through central cwa down into northern Metro Detroit. Accumulations are tough, but could definitely see some minor lake effect snow accumulations of an inch or so in some locations. Cold windchill Saturday morning of -5 to -10F. Geopotential height rises and Southeast Michigan's proximity to mid jet streak is expected to lead to subtle ridging on Saturday. Progged absolute vorticity structure is very interesting with legitimate anticyclonic vorticity right through the area. Models are showing extreme static stability in the 3.0 to 6.0 kft agl layer during the day. Supersaturation wrt ice exists through -10C so its possible for some flurries, but that is it. Models then show primary lobe of planetary vorticity swinging into the western Great Lakes Saturday evening. Absolute vorticity charts do qualitatively show a very good setup with shortwave energy digging right into Lower Michigan. The most uncertain aspect of the Saturday evening forecast centers on the integrity of a low to midlevel thetae axis as its forecasted to be directed right into southeast Michigan. If this feature is in fact coherent, moisture and saturation will be very good for system relative isentropic ascent to act upon to generate snow. One can go hunting and find some solutions that are quite bullish on QPF with perhaps a 0.25 inch or more. In contrast, the ECMWF (including the d(prog)/dt of ECMWF), virtual all of the 5.00Z EPS, and the RGEM are extremely lean on QPF amounts. There are both good things and bad things about the setup. The good...1. Potential for an outstanding frontal surface at 7.5 kft agl (750mb) with very low static stability above it. 2. Very good microphysics potential with crosshairs through the DGZ and cold near surface profile. Liquid to snow ratios could be north of 15:1. The not-so good...1. Less than spectacular, broad and diffuse upper level jet dynamics. 2. An attendant shortwave that passes through Arkansas/Tennessee. What this does to the aforementioned thetae ribbon of interest is uncertain. 3. System relative flow trajectories do remain veered. Bottom line is the potential exists for an accumulating snow event Saturday evening. A lot of uncertainty from the authors perspective on how this will go. If the moisture verifies could see a quick hitting overachiever, if the moisture does not make it here then could see lackluster snow. For now, toeing the line of less than a .10 of liquid equivalent with snow ratios of 15:1, forecasted snow amounts of 1 to 2 inches. Surface ridge axis is then progged to expand into Lower Michigan Sunday. A secondary surge of colder arctic air is expected to reach the state. Temperatures are expected to be some 5 degrees colder than Saturday. Windchills will have the opportunity to drop to -10F or colder Sunday morning. Even colder windchill conditions may be possible Monday morning with a long night of sub 20F 850mb temperatures. Windchills of -15F appear possible from this vantage point. The potential for another accumulating snow event continues to fall in the Monday night/Tuesday morning timeframe. Much too early to really offer much discussion but orientation of baroclinic zone and proximity of jet core overhead supports many different outcomes. The most recent data continues to support that well below normal temperatures will be around for the entirety of next week. The trend also continues to push the coldest of the airmass out in time. && .MARINE... Strong southwesterly winds are underway in the wake of a strong arctic cold front. Gale Warning remains on track across the entire central Great Lakes as gusts approach peak speeds during the next couple of hours. Expect gusts to maximize near 45 knots across central Lake Huron with peak sustained winds up to 35 knots. Gusts in the 35-40 knot range are expected across the rest of the basin through at least Saturday morning. Low-end gusts to gales are likely over Lake St. Clair and western Lake Erie until twilight. Due to the combination of arctic air and strong winds, moderate to heavy freezing spray is ongoing across the open waters of Lake Huron. Activity will continue through the overnight period, thus maintained the Heavy Freezing Spray Warning until Saturday afternoon. The associated surface low will move deeper into northern Ontario and Hudson Bay on Saturday allowing for a gradual weakening of the pressure gradient. Gusts diminish to around 25 knots out of the SW/WSW. && .PREV DISCUSSION... Issued at 1247 PM EST Fri Feb 5 2021 AVIATION... Scattered to numerous lake effect snow showers will continue to impact much of the airspace through at least this evening as the moisture connection and instability remains strong off of Lake Michigan. Outside of these snow showers, which will feature brief IFR to LIFR conditions at times, MVFR conditions will predominate with plentiful stratus locked in around 3 kft. Gusty southwest winds up to 30 knots at times will gradually subside heading into the night as the pressure gradient slackens. Lake effect snow showers will continue to linger into Saturday, although coverage should be minimal as drier air works into the region. For DTW...Best window for noticeable snow shower activity will be through 23z...and cannot rule out brief windows of IFR visibility, although will carry MVFR in the TEMPO group for now as better moisture should remain north of the TAF site. Southwest wind gusts decrease below 25 knots after 00z. DTW THRESHOLD PROBABILITIES... * High for ceiling height below 5000 ft through tonight. Moderate after 12z Saturday. * High for ptype as snow through the period. * Very low for visibility 1/2 SM and/or ceiling 200 ft through this evening. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Winter Weather Advisory until 7 PM EST this evening for MIZ047>049- 053>055-060>062. Lake Huron...Heavy Freezing Spray Warning until 4 PM EST Saturday for LHZ361>363- 462-463. Gale Warning until 4 PM EST Saturday for LHZ361>363-462. Gale Warning until 10 AM EST Saturday for LHZ463-464. Gale Warning until 4 AM EST Saturday for LHZ421-422-441>443. Lake St Clair...Gale Warning until 10 PM EST this evening for LCZ460. Michigan waters of Lake Erie...Low Water Advisory until 4 AM EST Saturday for LEZ444. Gale Warning until 10 PM EST this evening for LEZ444. && $$ DISCUSSION...99 MARINE.......KK AVIATION.....IRL You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.