Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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563 FXUS63 KDTX 090205 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 905 PM EST Thu Dec 8 2016 .UPDATE... There are a few lake effect flare-ups to monitor for the rest of tonight through Friday morning. The first is off central Lake Michigan which is getting low level support inland along and ahead of the larger scale surface trough preceded by weaker convective elements over central sections of our area during mid evening. The stronger part of the band has a chance to brush the western suburbs of Detroit to the Ohio border before northwest flow sets in overnight. The wind shift will open the door for clusters of lake effect snow showers to make it down from northern Lower Michigan overnight through Friday morning. As mentioned in the afternoon forecast discussion, equilibrium level will remain in the 7-8 kft range through the night which will make it possible for elements from a Lake Superior to Lake Michigan band to reach the Saginaw Valley. Surface analysis indicates a lake aggregate pressure trough setting up favorably to support that trajectory. About the only negative factors are the downslope from higher terrain in the north and 0-1 km theta-e lapse rate dropping to about 1 K/km by western Saginaw Bay. This actually may act to limit activity from reaching Flint more than the Tri Cities, so plan to maintain high chance/low likely POPs in that area. The other trouble spot is central Lake Huron where lake effect has been strong enough for a few lightning strikes near Goderich. Expect mid lake bands to remain nearby but offshore of the Thumb while the Lake Superior/Lake Michigan contribution to aggregate troughing remains strong. As soon as it weakens, then the mid Lake Huron surface trough will become more dominant and bands will migrate farther from the Thumb into mid lake. Interestingly, not much change in equilibrium level/convective depth is shown in model soundings suggesting mid level subsidence will be weak and not very effective at suppressing lake effect as the short wave ridge builds overhead by morning. About the only forecast adjustment being considered is an upward bump in POPs to likely/numerous in the Saginaw Valley depending on how strong the snow showers remain as the Lake Superior/Michigan band matures during the rest of the evening. && .PREV DISCUSSION... Issued at 653 PM EST Thu Dec 8 2016 AVIATION... A trough of low pressure will move in from northern Lower Michigan during the evening with a brief flare up of snow showers possible along and ahead of the boundary. There is also potential for bands of lake effect to remain active in the northwest flow during the night after the trough passage. This will be monitored for trends to set up west of the terminal corridor. For now, the forecast will lean toward low end VFR ceiling that usually accompanies northwest flow and closely monitor both cloud and snow shower trends for later updates. Assuming the favorable zone for lake effect does set up to our west overnight, then a slight backing of the low level flow will help shift activity eastward mid Friday morning through early afternoon. MVFR in a combination of snow showers/flurries and stratocu should then move quickly through SE Michigan by mid afternoon. This will allow a period of VFR before continued backing low level flow guides more lake clouds into the region Friday evening. FOR DTW... MVFR in patches of snow showers and stratocu remain possible through the evening as a surface front/trough moves through the region. There has been a flare up of lake effect between GRR and RQB over toward MBS that will be monitored which could make it into the JXN to DTW area during the evening. Broken VFR ceiling overnight is expected to remain below 5000 feet through mid Friday morning. //DTW Threshold Probabilities... * High for ceiling 5000 ft or less through tonight. * High for snow as precipitation type. PREV DISCUSSION... Issued at 325 PM EST Thu Dec 8 2016 DISCUSSION... Snow squall activity around this afternoon will be diminishing/decreasing in coverage as sharp shear axis/trough swings south through the Central Great Lakes this evening, allowing for wind shift to the northwest and general subsidence and a drying out process (some breaks in cloud cover). However, there looks to be a good chance for a dominate/narrow Lake Superior-Lake Michigan band to set up overnight into Friday, as average 925-850 mb temps looks to be lowering to favorable -10 to -12 C range. 12z Regional GEM most aggressive with the band Impacting Tri-Cities into Flint Vicinity, while the HRRR is farther west. 12z nam appears to be the farthest east, with northern Thumb region being impacted on Friday. Inversion heights look to be at or above 7 kft, which favors the case for band(s) reaching southeast Michigan. Narrow nature of the band and uncertainty with movement makes for difficult call on amounts, but potential for localized amounts greater than 1 inch is there as we get into the tail of the upper level pv lifting northeast, providing some support during Friday, with near saturated and cold 700 mb temps of -20 to -23 C. Backing flow in the afternoon will also likely bring some snow shower activity back into the I-94 corridor as well. Upper level northwest confluent flow Friday Night/Saturday, but westerly flow and 850 mb temps still hovering in the negative mid teens will continue to support flurries or light snow showers. Warm advection/Fgen Saturday Night will bring increasing chance of light snow by Sunday Morning. Favorable thermal profiles and 850 mb specific humidity approaching 2 G/KG suggest a few inches possible. A brief lull from snow will be possible throughout the late morning to early afternoon hours. Any periods or little to no snow will be short-lived as low pressure centered over northern Texas/Oklahoma early Sunday will begin to push northeast into the Ohio Valley into early Monday. The chance for snow will once again increase throughout the late afternoon to early evening hours on Sunday, with the chance for snow continuing into Monday morning. While we are starting to narrow down on timing of the system as models converge, uncertainty still exists regarding placement of the low. The GFS and ECMWF models have flip-flopped over the past 24 hours, with the GFS model bringing the low more north, clipping SE MI Monday morning and afternoon. This allows SE winds to pool in warmer air into the region, allowing 850 mb temperatures to straddle the 0 degree isotherm during the morning, which has the potential to influence precipitation type. Both the ECMWF and GEM models keep the low just south, tracking across Ohio throughout Monday morning, keeping conditions much cooler and allowing precipitation to remain as snow. We will likely be a day off before issuing snowfall estimates for Monday`s snow, as models continue to converge, however, it`s safe to assume accumulation totals will impact travel Sunday through Monday. Snow from this system will taper off late Monday, however, a shift in winds from northeast to east will bring additional chances for lake effect snow throughout the early part of the week. The second main story for the extended will be the advent of polar air, which looks to take daytime high from the lower 30s to upper 20s during the early part of the week, to the upper 20s to lower teens by mid- week. MARINE... Existing near gale conditions across the southern Lake Huron waters will gradually ease early tonight as the become more northwesterly with a frontal passage. This will maintain small craft advisory conditions for all near shore waters into tonight. This will shift to northwesterly will introduce a greater potential for snow squall development tonight into Friday. Building high pressure south of the Great Lakes will then allow moderate winds to take hold for the end of the week before the arrival of the next low Sunday into Monday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory until 10 PM EST this evening FOR LHZ422. Small Craft Advisory until 10 AM EST Friday FOR LHZ441>443. Small Craft Advisory until 6 AM EST Friday FOR LHZ421. Lake St Clair...Small Craft Advisory until 10 PM EST this evening FOR LCZ460. Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EST this evening FOR LEZ444. && $$ UPDATE.......BT AVIATION.....BT DISCUSSION...SF/AM MARINE.......MR You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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