Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 260121 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 921 PM EDT SUN SEP 25 2016 .UPDATE... The 00Z DTX sounding showed a respectable mid level warm and dry layer, a result of the persistent mid level subsidence which was overhead today. It is therefore no surprise that the showers/thunderstorms moving into wrn Lower Mi this evening have been diminishing as they advance to the east. The mid level ridge will be forced east of the region tonight with the approach of the upper level trough to the northwest. This will invoke some mid level height falls across Se Mi overnight (helping to erode the mid level warm layer), which will make showers more probable later in the night as the deep layer moist axis (now rotating into wrn Lower Mi) slides into Se Mi. More importantly, a mid level PV filament now sliding into wrn Minnesota will move across Wisconsin early Mon morning, causing the upper trough to take on a negative tilt. This will invoke a deep layer frontogenetical response along the back edge of the departing deep layer moist axis, thereby sustaining some showers behind the sfc cold front (tracking through between 09z and 12Z). While there is some concern that the larger scale forcing will be too far removed from the deep moist axis, most locals still seem to have a high likelihood of showers sometime between 2 AM and 11 AM Monday morning. The main adjustments to the forecast will be to nudge back the timing of the showers a couple hours. Very weak elevated instability will support a slight chance for a thunderstorm, but nothing more. && .PREV DISCUSSION... Issued at 702 PM EDT Sun SEP 25 2016 AVIATION... The high pressure across the eastern Great Lakes will maintain its influence across Se Mi through the rest of the evening, sustaining VFR conditions with Southeast winds generally at 6 to 10 knots. The high will exit the region to the east overnight, allowing an axis of moisture and instability to traverse the area associated with a cold front. The cold front will move across Se Mi between 09Z and 12Z. The ascent ahead of the front will lead to numerous showers late tonight, while the influx of low level moisture supports steadily lowering ceiling heights. The amount of instability along the front will be considerably less overnight as it crosses the area, so despite the amount of convection upstream, overnight thunderstorm chances remain quite low at the terminals. The passage of the front will be marked be a veering to the wind to the southwest, a gradual rise in ceiling heights and a few hours of post frontal showers. A strengthening west-southwest gradient and deepening mixed layer will support wind gusts up to or in excess of 25 knots Mon afternoon. For DTW...Fropa is expected to occur around 12Z at metro. Again weakening instability aloft will reduce the chances of overnight TSRA at metro. Showers, both ahead of the front and behind should however be fairly widespread. Ceiling heights will likely show considerable variability throughout Mon morning. In the 16 to 17Z time frame, there is expected to be a slight veering of the winds toward the W-SW (generally around 260 deg), with a notable uptick in gusts as the boundary layer deepens. //DTW Threshold Probabilities... * High in ceilings below 5000 ft overnight through Monday morning. * Low in thunderstorms overnight. * Low in wind gusts exceeding crosswind thresholds Monday afternoon. PREV DISCUSSION... Issued at 328 PM EDT Sun SEP 25 2016 DISCUSSION... Hi-res and NWP Models in better agreement with tonight`s and Monday morning`s rain event. Increase in coverage across WI and IL noted this afternoon, right on schedule. This is in response to a strong frontal system with good right rear entrance region jet support. There is also good and deep deformation and Fgen forcing. Instability is elevated and marginal, around 500 J/kg, by the time it moves over Southeast MI. Will keep the flavor of the current forecast with showers moving in after midnight with a chance of thunderstorms. The surface front passes through the forecast area by 12z. However the 925 and 850 mb front take a little longer. These frontal systems are expected to be somewhat active hanging onto some sort of decent and deep Fgen and deformation. Instability is gone by then so only expect showers for the morning hours. Things dry out considerable in the dry slot of the synoptic system during the afternoon with some peaks of sunshine. The dry slot, cold air advection and that sunshine will help to mix down gusty winds up to 30 mph. Not much more in the low level wind profile than around 30 kts so do not expect the winds to get too much higher than that. Upper low starts to migrate southward across the western Great Lakes as the cool air continues to filter over the region. By late Monday afternoon the Lakes will be activated with 850 mb delta t`s in excess of 15C and up to 20C. The combination of the upper low, surface low and lake effect will keep the region unsettled from late Monday afternoon through Tuesday. Showers will be first limited to the Saginaw Valley and Thumb for Monday night with southwest flow off of Lake Michigan and closer to the upper low forcing. Then for Tuesday, will bring the chance of showers as far south as the M 59 corridor. With plenty of clouds around, went toward the higher end of guidance for the overnight lows each night. MOS guidance for highs were close and did not stray from that. That upper-level low will bring the chance for rain showers on Wednesday, with a continued chance for precipitation as we head into the weekend. The low will move from Michigan into the Ohio Valley Wednesday into Thursday, taking on a south-southeast track. Uncertainty lies within the tracking of this low as we head into the weekend. GFS and GEM models show decent convergence, placing the low over the Carolinas on Thursday and veering it northeastward along the east coast. Central Atlantic ridging over the east coast exhibited in the ECMWF model gives the low a more northward track into Michigan throughout the weekend. As a result, cloud coverage has been upped for the later part of the week. Due to low uncertainty, chance of precipitation will remain on the lower side, but will have to be increased if future model runs exhibit convergence and show a more northern path with the upper- level low. MARINE... Small craft advisories remain in effect from tonight into Tuesday evening as southeast flow increases in advance of approaching cold front and leads to steadily building waves into tonight with strong southwest flow then becoming established in the wake of this front from Monday into Tuesday. Average wind gusts over the open waters of Lake Huron appear to top out around 30 to 32 knots, both in southeast flow over northern Lake Huron tonight and west to southwest flow from Monday into Tuesday. While gusts may approach gale force at times, will forgo a Gale Warning given the expectation of most gusts remaining closer to 30 knots. A large upper level low pressure system will gradually build into the central Great Lakes in the wake of the vigorous lead shortwave driving the cold front. This will maintain very unsettled conditions through the middle of the upcoming week. The cold airmass associated with this pattern will be conducive to thunderstorm development and possible waterspouts as instability increase above the relatively mild waters of the Great Lakes. HYDROLOGY... Low pressure will slowly deepen as it shift along the north shore of Lake Superior. This low will force a cold front through the region Monday morning. Showers are likely late tonight in advance of this front with activity continuing into Monday morning as the cold front crosses lower Michigan. Rainfall totals of a quarter to half an inch will be possible. The progressive nature of the front and lack of heavy rain producing thunderstorms will limit any potential for localized flooding. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory until 10 PM EDT Tuesday FOR LHZ421-422-441>443. Lake St Clair...Small Craft Advisory from 2 PM Monday to 10 PM EDT Tuesday FOR LCZ460. Michigan waters of Lake Erie...Small Craft Advisory from 2 PM Monday to 10 PM EDT Tuesday FOR LEZ444. && $$ UPDATE.......SC AVIATION.....SC DISCUSSION...RBP/AM MARINE.......DG HYDROLOGY....DG You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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