641 FXUS63 KDTX 131722 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 122 PM EDT Mon Sep 13 2021 .AVIATION... A line of showers and thunderstorms is quickly moving across the region early this afternoon. Given its speed, this convection will depart the northern terminals by 19Z at the latest. A chaotic ceiling pattern follows this line with obs showing a mix of VFR and MVFR cigs (even an isolated IFR) over west MI. Will err towards the the lower side given the lingering frontal boundary and low level moisture through this afternoon. Ceilings are then expected to gradually lift from MVFR to VFR with more breaks likely overnight tonight as shortwave energy pushes toward the Northeast. For DTW...MVFR ceilings persist through the afternoon as the boundary lingers over the area. The terminal looks to miss the thunderstorm activity today as convection stays to the north. Low VFR ceilings are possible by this evening into tonight. .DTW THRESHOLD PROBABILITIES... * High for ceilings at or below 5 kft daytime today. * Low for thunderstorms through today and tonight. && .PREV DISCUSSION... Issued at 1012 AM EDT Mon Sep 13 2021 UPDATE... Surface/low level front and corresponding moisture axis near the southern Michigan border, with good instability (showalter index of -5 to -6 C) and steep mid level lapse rates. Fortunately, moisture content quickly drops off as one heads north, with 12z APX sounding indicating a 40 C dew pt depression at 850 MB. Strong belt of westerlies in place, with jet streak exiting southern Wisconsin and tracking through Lower Michigan this afternoon. It looks like this mid/upper level wave will remain detached/just north of the low level instability gradient, and then as the wave exits east, the low level front will accelerate northward late today/this evening with 850-700 MB Theta-e minimum then settling over southeast Michigan tonight. Will have to keep our guard up for isolated severe due to the impressive wind shear and possible interaction with the better low level instability this afternoon, but bulk of CAMS suggest muted response. On the flip side, as thunderstorm activity moves into central Lake Michigan, noted surface obs gusting to 30+ knots in/around Milwaukee, so can't totally bank on the low level stable layer holding on. Will watch to see how well activity holds together for possible adjustments to pops. If higher coverage materializes, max temps will come up a little short. PREV DISCUSSION... Issued at 351 AM EDT Mon Sep 13 2021 DISCUSSION... Elevated baroclinic zone featured within a stout lower amplitude mid level westerly gradient will define conditions locally today. Surface boundary will anchor near the Ohio border during much of the daylight period, solidifying a stable low level environment across the lowest 1 km within a high coverage of low and mid cloud in east/northeast flow. Thermodynamic profile remains characterized by a steep lapse rate environment above 700 mb, but definitively lacking in low level instability with non-existent diurnal mixed layer cape and pathetic lapse rates. Large scale ascent derived by an ill-defined signal for some degree of mid level fgen to emerge along the frontal slope as shreds of pva accompany a mid level speed max /60 knots at 500 mb/. Most favorable corridor for at least a chaotic coverage of elevated convective showers to develop aligned generally from the M-59 corridor northward. A more organized convective response again reliant on realizing deeper forced ascent to capitalize on those higher 700-500 mb lapse rates and allow updrafts to attain some upscale growth within a high magnitude deep layer shear environment /0-6 km bulk shear above 60 knots/. No real signal yet within latest model output and confidence in occurrence will remain quite low, but the underlying environment gives pause given the degree of speed shear through the column and some directional shear in the lowest 3 km. A very conditional setup, but one that could yield an isolated elevated robust cell or two late this afternoon/evening, favoring a discrete/supercellular mode with an accompanying large hail threat. Temperatures today governed by degree/longevity of cloud cover - forecast leans toward a smaller diurnal response with a general humid feel as dewpoints still hover in the 60s. Strengthening southwest flow and a corresponding uptick in low level warm air advection downstream of heights falls ongoing across the plains prompt a steady northeast migration of the sfc-925 mb frontal zone across southeast Michigan tonight. A moistening nocturnal advective process maintains a low end precip chance through the night, particularly with northward extent. Brief window of dry conditions early Tuesday within a diurnally destabilizing, but capped and minimally forced warm sector environment. Inbound low level thermal ridge anchors across eastern sections by 18z, characterized by 850 mb temperatures near 17c. Window of solid insolation potential within a well mixed boundary layer lends to highs arriving solidly in the mid to upper 80s. Gusty southwest winds 30 to 35 mph during this time. Eastward propagating cold frontal boundary tied to the height falls arrives favorably during the back half of peak heating. Axis of moderate pre-frontal instability in the presence of modest, yet sufficient deep layer shear should encourage development and steady linear organization of convection along the length of the frontal axis late afternoon through the evening hours. Improving severe wind probability, as highlighted by recent upgrade in the Tue SPC outlook, appropriate given the underlying setup. Lingering moisture within the column as the elevated frontal zone gradually vacates the region and mid level flow becomes weakly cyclonic may leave a higher coverage of cloud for Wednesday. Moisture depth appears lacking to support a meaningful mention of precip during this period, but worthy of monitoring as favorable positioning of the upper jet working through the base of low amplitude trough could eventually intersect the boundary. Otherwise, post-cold frontal thermal profile relatively seasonable - meaning highs in the 70s. Dry and stable conditions with a high degree of insolation potential for Thursday as surface ridging lingers beneath benign low amplitude mid level westerly flow. MARINE... Active weather pattern continues today as the frontal boundary remains stalled over the southern Great Lakes, with shower and thunderstorm chances persisting throughout the day. Gusty winds, lightning, and briefly heightened waves will be possible within any thunderstorms. Advancing low pressure then lifts the stalled boundary northward as a warm front, shifting the overall wind field to southwesterly. In the wake of the tightened pressure gradient, winds may gust to headline criteria around 25 kts on Tuesday. Cold frontal passage to follow late Tuesday into Wednesday, bringing better coverage of precipitation and a gradual shift back to northwesterly flow by Wednesday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....KDK UPDATE.......SF DISCUSSION...MR MARINE.......MV You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.