Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 290413 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1213 AM EDT THU SEP 29 2016 .AVIATION... Lightning strikes have diminished considerably since late evening over areas upstream of the terminal corridor while light to moderate rain continue to expand south of FNT. The rain, along with increasing easterly flow off Lake Erie will combine to produce a downward trend through MVFR into IFR during the night. This will also be supported by the proximity of the low near the Ohio border for an extended period of time. That also being the case, FNT to MBS stand the best chance for low end VFR to return during the day being farther north into drier air. For now, plan to bring ceiling back into MVFR from PTK to DTW during the afternoon assuming modest daytime heating will help with some improvement. For DTW... Late evening radar trends suggest increasing frequency of moderate showers producing MVFR restriction but little or no coverage of thunderstorms over the D21 air space. The main concern will be increasing easterly wind combining with the rain to produce IFR ceiling during the night through the morning. //DTW Threshold Probabilities... * High for ceiling 5000 ft or less through the day. * Low for thunderstorms during the night. && .PREV DISCUSSION... Issued at 1007 PM EDT Wed SEP 28 2016 UPDATE... The large cut-off low pressure system over Indiana and Ohio is producing a classic pattern of isentropic ascent into mid level deformation over lower Michigan tonight. This is an efficient rainfall producer on its own but even more so when the moisture supply is so rich and with a perfect trajectory for parcels to borrow some instability from Lake Erie. The 00Z DTX sounding indicated PW at 0.92 inch which matches up pretty well with model soundings at 00Z and are forecast to rise near 1.25 inch by 12Z. Hourly mesoanalysis also indicates Lake Erie producing CAPE around 1000 J/kg and giving rise to bands of showers and thunderstorms. The storms have not held together very far inland over southern Ontario, instead broadening into bands of moderate showers. Synoptic scale models show average mid level lapse rates and just about no MU/MLCAPE inland from Lake Erie through the night. That will produce minimal thunderstorm potential but which will not really be needed for locally heavy rainfall. The persistence of isentropic ascent, occlusion dynamics, and increasing moisture will all combine to maintain bands of showers mainly south of M-59 to the Ohio border tonight through Thursday morning. It is possible that 12 hour rainfall, midnight to noon Thursday, in the 1-2 inch range will be within reach in this pattern. This will be closely monitored for urban and small stream headline needs, especially if multiple bands set up to affect the Detroit metro area through the morning commute. PREV DISCUSSION... Issued at 303 PM EDT Wed SEP 28 2016 First major vorticity spoke has pivoted north to northwest into Southeast Lower Michigan around parent upper level low pressure system now dropping gradually south through the Ohio Valley. Rain shield associated with this wave has featured light to occasionally moderate rain for the most part with isolated thunderstorms also noted during the day along the northeast periphery of the main area of rain where solar insolation from the southern tip of Lake Huron on into Thumb region has led to a pocket of better instability. While the higher resolution model solutions vary considerably in the mesoscale details of the anticipated bands of moderate to locally heavy rainfall, there is a strong consensus of maintaining a rather persistent arc of rain from western Lake Erie/far southeast lower Michigan northwestward through a bulk of the 1/2 to 2/3 of the forecast area. Will increase precipitation chances into the 80-100 percent range along this axis of most persistent rain and taper off from that, particularly over the northern forecast area. That said, even locations from parts of the Saginaw Valley back to the Thumb will have a good chance of showers along with a few thunderstorms, especially into evening before daytime heating and instability subside. Forecast thinking has not changed much through the rest of the week as the upper low settles south to Kentucky and the feed of moisture back west to northwest into the area remains steady. In fact, as an increasing levels of moisture from the Atlantic is drawn into the area within this flow and precipitable water values climb to 1.50 inches or so, periods of rainfall should become more prolific from tonight into Friday. The most persistent rain will focused roughly along/south of M-59 during this time frame, but a gradual expansion north through the rest of the area should also occur as the overall abundance of moisture expands. While not all locations will receive heavy rainfall amounts, several inches of rain can be expected where the this most persistent banding becomes established. With the increase in moisture and a more or less persistent shield of clouds over the area, temperatures will remain rather steady with highs in the mid/upper 60s for the rest of the week and lows in the middle 50s. Upper low projected to drift back northward across Lower Michigan this weekend with unsettled weather continuing. Upper ridging builds in early next week ahead of deep troughing over the western CONUS, leaving Southeast Michigan dry with a modest warming trend into midweek. MARINE... Strong northeast winds will develop over the Central Great Lakes tonight as high pressure settles in over Ontario and as low pressure drops into the Ohio Valley and then Kentucky. This pattern will remain fairly stagnant into Friday, providing a prolonged period of northeast winds with sustained speeds in the 15 to 25 knot range. The persistent northeast wind and large fetch over both Lake Huron and Western Lake Erie will allow large waves to build from tonight into Friday night. Small craft advisories remain in effect for all marine areas into late Friday night due to large waves and gusty winds. The area of low pressure will lift back northward into Michigan late Friday night into Saturday. This will decrease the surface pressure gradient and turn winds more to the east, allowing wind speeds and wave heights to decrease overnight. Moisture will meanwhile continue to wrap around the area of low pressure into the weekend, providing numerous showers but only a low chance for thunderstorms. The cool airmass will provide a chance for waterspouts tonight and Thursday as the colder air resides over the warm lake waters. HYDROLOGY... A large area of upper level low pressure will remain positioned between Kentucky and Michigan into the weekend, keeping a cool and showery pattern in place through Saturday. Total rainfall this evening through Friday night is expected to generally range between 1 and 2 inches, although some locations near the Ohio border may receive higher amounts. Periods of steadier and slightly heavier rain are expected to occur over the next few days. The first is expected to impact areas along and south of M-59 late tonight and Thursday, and another is expected to impact just about all of Southeast Michigan late Thursday night through Friday. The rain is expected to cause rises on area creeks and streams, and may allow some ponding to develop on roadways and low-lying areas. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory until midnight EDT Friday night FOR LHZ421-422- 441>443. Lake St Clair...Small Craft Advisory from 8 AM this morning to midnight EDT Friday night FOR LCZ460. Michigan waters of Lake Erie...Small Craft Advisory from 8 AM this morning to midnight EDT Friday night FOR LEZ444. && $$ AVIATION.....BT UPDATE.......BT DISCUSSION...DG/DT MARINE.......HLO HYDROLOGY....HLO You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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