003 FXUS63 KDTX 101540 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1140 AM EDT Wed Oct 10 2018 .UPDATE... Only minor tweaks made to the previous forecast... A subtropical airmass continues to hold across the Great Lakes region as a low pressure system pushes northeastward from western WI into the U.P. Low-level winds from the southwest have continued to pump additional moisture across the CWA and have brought in low-level stratus alto- stratus/cumulus that has filled in across most of SE MI. The cloud deck did move in a little earlier than forecasted and as a result have diminished forecasted highs by a degree or two, however, WAA and lack of late morning precipitation will still allow temperatures to warm up nicely into the upper-70s for a daytime high. The second minor change made was pushed back rain chances by a few hours to enter by the early afternoon onwards... This timing correlates nicely with axis of greatest moisture transport, allowing PW values to peak close to 2.00. Lack of diurnal heating due to expansive cloud cover will only allow MLCAPEs to range between 100 - 200 J/kg through daylight hours as mid-level lapse rates remain sub par. Lack of instability will continue to limit severe weather threat despite bulk shear values ranging between 40 - 50 knots. Overall, rain will fill in this afternoon which includes periods of heavy downpours and isolated thunderstorms, however, the severe weather threat has decreased due to lack of instability. && .PREV DISCUSSION... Issued at 658 AM EDT Wed Oct 10 2018 AVIATION... Overall VFR conditions with light south winds will prevail through the morning hours within the warm prefrontal environment now entrenched across Se Mi. An influx of moisture will lift into the area from the southwest this afternoon, leading to lowering ceiling heights and the development of widespread showers. There is likely to be some thunderstorms embedded within these showers. The more likely time period for thunderstorms will be after 23Z through 06Z in advance of the sfc cold front when large scale forcing will combine with a weakly unstable environment. For DTW...A steady lowering in ceilings heights will occur during the later half of the afternoon with the arrival of moisture. The highest probabilities for thunderstorms to impact metro Detroit look to be late evening, sometime in the 00Z to 05Z time frame. The cold front will move across metro around 08Z tonight, marked by a veering of the winds to the southwest and increase in the gustiness with the onset of post frontal cold air advection. DTW THRESHOLD PROBABILITIES... * Low in ceilings at or below 5000 ft Wednesday afternoon, increasing to high by evening. * Moderate in thunderstorms impacting the area this evening. PREV DISCUSSION... Issued at 321 AM EDT Wed Oct 10 2018 DISCUSSION... NEAR TERM / TODAY THROUGH TONIGHT As of 320 AM EDT...Very muggy subtropical airmass continues to remain firmly in place across the region early this morning as east coast upper-level ridging gradually decays with the northward track of Hurricane Michael in the northeastern Gulf of Mexico. Impressive to note that as of 3 AM, several locations across southeast Michigan are still sitting around 70 degrees, which is simply remarkable for late in an October night. Correspondingly, dewpoints are well into the 60s and a signal of the extremely anomalous moisture in place. Today will be the last day of the warm and muggy conditions as a strong cold front is still set to sweep across the region tonight. The cold front will be associated with a gradually deepening low pressure system that will lift northeast from the upper Mississippi River Valley to the northern Great Lakes. Sharp, negatively-tilted shortwave accompanying the surface low, noted by dry slot in water vapor imagery, will aid in enhanced dynamic lift as the front approaches today. Before the front approaches though, high temperatures will spike into the upper 70s to around 80, roughly 5 degrees colder than yesterday as cloud cover will rapidly be on the increase from the south and west as the day progresses. Hi-res guidance has come in a little more wet for today as warm air advection showers overspread much of lower Michigan late morning into the early afternoon. These showers, coupled with cloud cover, look to severely limit destabilization today, and thus expecting only around 500 J/kg of MLCAPE to work with as the prefrontal trough approaches western areas mid/late afternoon. With the lack of instability, coupled with very warm midlevel temperatures (500 hPa warming to -6 C), the severe threat will be limited despite an increasing wind field characterized by 0-6 km bulk shear of 40-50 knots and a coupled low/midlevel jet. Still, any taller showers or storms will be capable of mixing down 50-60 mph winds, and would not be surprised if there are a few bowing segments near severe limits. Best chance for strong storms will be in the 21z-03z timeframe (especially for the Flint and Tri-Cities areas) before the cold front and midlevel dry slot quickly sweep east across the region for the second half of the night. Strong cold air advection will ensue behind the front with temperatures falling into the 50s by daybreak Friday. SHORT TERM / THURSDAY THROUGH FRIDAY NIGHT Fall returns in earnest with strong cold air advection continuing during the day Thursday with little of a temperature response with highs only in the mid to upper 50s for most areas as 850 hPa temps crater to less than 0 C by 00z Friday. Continued strong drying (surface dewpoints falling into the upper 30s) will allow for efficient boundary layer mixing as the low departs to the northeast and continues to deepen. The result will be a gusty day with west/northwest winds gusting 25-35 mph at times. Despite the drying, a sharpening low-level inversion will likely allow for a fair amount of cloud cover throughout the course of the day. Lake-enhanced spotty showers may be possible as well, but should be isolated enough in nature. Winds begin to decrease Thursday night with continued scattered to broken cloud cover as low temperatures plummet into the mid and upper 30s. Some patchy frost will be possible, but will be negated to an extent by cloud cover and enough of a breeze through the night. Similar day expected Friday with the low-level thermal trough firmly overhead (850 hPa temps -2 to -4 C) keeping high temperatures only in the upper 40s north to mid 50s south. Generally dry weather with brisk west/northwest winds and good coverage of cloud cover aided by enhanced lake aggregate moisture response. High pressure building southeast from the northern Plains will result in increased subsidence, but will be battling shortwave energy skirting east across the northern Ohio Valley. Expecting northern areas to be dry, but cannot rule out a period of possible showers Friday night closer to the Ohio border. Where conditions are drier, frost concerns will be more prevalent with low temperatures solidly in the mid 30s for most areas. LONG TERM / SATURDAY THROUGH TUESDAY Any rain chances associated with the passing shortwave quickly move east by Saturday morning resulting in a dry start to the weekend as high pressure passes by. Long range guidance has now slowed the arrival of southern stream energy and increased rain chances until Monday, so Sunday may end up being another dry day before rain chances increase for the early work week. Temperatures will continue to be cool and below normal for mid-October. MARINE... Southerly winds, generally on the order of 10 to 20 knots, will prevail through the day within the warm prefrontal environment. The cold front is forecast to advance across Lake Huron overnight before tracking across the eastern Great Lakes Thursday morning. The passage of the front will be marked by a veering of the winds to the southwest. There is a low chance for a short period of gale force wind gusts within the post frontal cold air advection early Thursday morning. Cold air advection will then persist through the day Thursday, deepening the over lake mixing depths. There will be a slight veering of the winds toward the west during the course of the day. There is moderate to high probability of wind gusts around 30 knots. The chances for gale force wind gusts are still rather low over most areas, but a little higher downwind of Saginaw Bay. This has prompted a gale watch for a section of Lake Huron in the vicinity and downwind of Saginaw Bay. Conditions will however be hazardous to small craft. Westerly winds will actually remain rather gusty into Friday under persistent cold air advection. A weakening of the gradient will then lead to decreasing winds and waves late Friday into Saturday. HYDROLOGY... Moisture advection this afternoon prededing an approaching cold front will support widespread showers and a few thunderstorms. Convection will then impact the area off an on until the passage of the actual cold front overnight. Widespread rainfall amounts between a quarter and three quarters of an inch are expected. Locally higher rainfall totals are certainly possible if thunderstorm coverage is a little greater than currently expected. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Gale Watch from late tonight through Thursday afternoon for LHZ363- 421-441-462. Small Craft Advisory from 4 AM Thursday to 4 AM EDT Friday for LHZ422-442-443. Lake St Clair...Small Craft Advisory from 11 AM Thursday to 4 AM EDT Friday for LCZ460. Michigan waters of Lake Erie...Small Craft Advisory from 11 AM Thursday to 4 AM EDT Friday for LEZ444. && $$ UPDATE.......AM AVIATION.....SC DISCUSSION...IRL MARINE.......SC HYDROLOGY....SC You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.