Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 282313 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 713 PM EDT Fri Apr 28 2017 .AVIATION...
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A variable pattern of precipitation and frontal positioning will add complexity to the aviation forecast tonight through Saturday. Showers will expand over Lower Michigan from both the Midwest and Ohio Valley during the evening that will be VFR initially. There is also some coverage of MVFR ceiling that will expand eastward as the surface trough over the southern Great Lakes merges with the Ohio Valley front. This process will make timing, placement, and duration of MVFR difficult to pin down under the complex pattern of light rain and showers.As it stands now, a shorter duration of MVFR is expected at MBS compared to locations farther south toward DTW. This will be followed by improvement as the first wave of rain exits Lower Michigan before sunrise. The improving cloud trend will be aided by dry air arriving on wind becoming moderate northeast from high pressure building over the northern Great Lakes. Low pressure will concurrently move from Texas along the Ohio Valley front and bring the next round of rain to our region spreading from Ohio border northward mainly during Saturday afternoon. For DTW... Another batch of showers is expected to move in from northern Indiana during the evening that will help develop a period of MVFR ceiling overnight through about mid Saturday morning. A return of VFR after that will occur as the wind turns northeast by sunrise and becomes moderate in speed through Saturday. //DTW THRESHOLD PROBABILITIES... * High confidence for ceilings 5000 ft or less tonight through Saturday morning.
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&& .PREV DISCUSSION...
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Issued at 320 PM EDT Fri Apr 28 2017 DISCUSSION... 12Z DTX sounding revealed an 850 mb dew pt depression of 31 C, with PW value of 0.42 inches. None-the-less, looks like there will be just enough isentropic ascent (295 K) and moisture advection working through southeast Michigan (mainly south of M-59) this evening/overnight to support some light showers based on latest satellite/radar trends and hi-res solutions of HRRR and Rap13. Negative LI`s stay south of the border, and left thunder mention out. Surface cold front sinking south and exiting jet forcing up north will also support chance/scatter pops there. Latest water vapor imagery shows highly amplified upper level pattern over North America, with longwave trough over the Rockies and excellent upper level jet energy tracking into Four Corners region. A convergence of Pacific Moisture/Pineapple connection, coupled with the Gulf of Mexico/Caribbean moisture as large Bermuda High will remain in place through the weekend, drawing flooding concerns by Monday as large and powerful 500 MB low develops over the Central Conus over the weekend. The system will be lifting northeast into the Midwest (Iowa) by Monday Morning. Moisture parameters will be record/near record for end of April/start of May this far north, as 700 mb dew pts approach 5 C toward Monday morning, nearing tropical status, with 850 mb dew pts equally impressive checking in between 13-15 C. With sufficient destabilization/convective elements, showalter index going slightly negative and deep warm cloud layer, several inches of rainfall is certainly possible for the second half of the Weekend into Monday, which will create flood concerns. Unfortunately, placement of heavy rain axis remains difficult, but good low level FGEN/isentropic ascent will be established by Sunday, as strong high (1032-1034 MB) remains parked over/near James Bay. Do think the surface warm front will not clear much past the southern Michigan border during the day on Sunday (Lake Erie influence), which should help showers persist even in southern areas, but max mid level forcing does appear to be lining up across northern sections of the CWA into northern lower Michigan Sunday Night. As of right now, it appears the robust max 6HR height fall center will already be over Central Great Lakes Monday afternoon, thus strong cold frontal/occlusion passage with good upper level support will occur early in the day. Moisture/instability axis passing east by around NOON, helping to mitigate any severe threat. Still not out of the woods Sunday night with 65 knot jet at 850 MB, but meager lapse rates within the moist airmass should be hindrance. Bottom- line, Heavy rain remains the biggest concern, and flood watch will likely be needed for the second half of the weekend. Lingering precipitation and gusty conditions will be possible Monday evening into Tuesday afternoon as low pressure travels northeast from Wisconsin into the Upper Peninsula. Cooler air that is expected to wrap around the low and push into Michigan throughout Monday afternoon will help to increase stability, minimizing the chance for thunderstorms, as conditions remain breezy. The low pressure system will continue to push northeast through Quebec late Tuesday into Wednesday, as a surface high edges eastward across the Great Lakes region. This will act to keep us dry and will keep winds light through the middle of the week, as temperatures remain moderate with daytime highs capped in the mid-50s. The next chance for rain will return Thursday into Friday as a warm front slowly pushes north into the MI/OH border. The GEM model run suggests that the warm front will protrude into Michigan, allowing rain showers to fall over our southern counties that border Ohio, however, several other long-range models hold the rain south of Michigan through the end of the week. MARINE... Gusty southwesterly winds over the morning will slowly continue to diminish this afternoon. Relatively light winds are then expected by early tonight before strong high pressure builds into Ontario and low pressure begins to organize over the plains. This will bring an increase in north/northeast winds to the marine areas and potentially allowing waves to build high enough to warrant a Small Craft Advisory for nearshore waters of Lake Huron and Saginaw Bay. Monday will then see wave heights decreasing as southerly winds take over ahead of a cold front. However, winds will once again gust up to 30 knots. As the low approaches the Great Lakes region this weekend, an increasingly unsettled, wet pattern will set up. Thunderstorms will also be possible Sunday into Monday.
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&& .DTX WATCHES/WARNINGS/ADVISORIES...
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MI...NONE. Lake Huron...Small Craft Advisory from 3 AM Saturday to 6 AM EDT Monday for LHZ421-422. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE.
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&& $$ AVIATION.....BT DISCUSSION...SF/AM MARINE.......SS You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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