Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 171354 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 954 AM EDT Thu Aug 17 2017 .UPDATE...
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Arcing line of elevated convection anchored along the lead push of mid level theta-e advection will clear the thumb corridor within the next couple hours. Attention now shifts upstream, as aggressive expansion of a very moist warm conveyor belt pivots toward the region. Focused increase in precipitation coverage ongoing as projected by recent CAMs guidance through this corridor /Lake Michigan into Indiana/, with a steady downstream propagation into southeast Michigan anticipated for the early-mid afternoon period. This timing certainly presents a murky picture in terms of possible convective vigor moving forward today. Ambient and inbound enviroment certainly characterized as tropical, defined by dewpoints in the lower 70s and PW in excess of 2 inches. This will contain lapse rate potential, while also suggesting that cloud cover will remain problematic. The aforementioned arrival of the upstream activity will only contribute to this issue, elevating concerns that a larger window for better diurnal heating and subsequent greater destabilization will never materialize. The underlying forcing - warm frontal, weak shortwave energy and continued moisture transport - presents higher confidence in witnessing additional convective development/maintenance through the afternoon/early evening period, but with low confidence in achieving a more robust convective response. A solid deep layer wind field and potential for a slightly backed near surface profile will continue to give pause for possible organized, rotating updrafts...particularly if convection can intersect the advancing warm front before the boundary lifts northeast of the region. In addition, a water loaded profile could also generate stronger wind gusts given the corresponding higher cape density. Update simply to refine precipitation chances going forward based on latest radar and model trends.
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&& .PREV DISCUSSION...
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Issued at 655 AM EDT Thu Aug 17 2017 AVIATION... Ongoing moisture advection along/ahead of an approaching warm front will sustain showers and few thunderstorms across the area this morning, possibly lasting into early afternoon. The degree of low level moisture advection will result in steadily lowering ceilings heights during the course of the morning, with some IFR possible in some of the heavier showers. Se Mi will be within the warm sector this afternoon. Ample low level moisture will likely sustain some MVFR or low end MVFR strato cu. Building daytime instability will result in convective redevelopment during the afternoon, likely to impact the terminals either late in the afternoon or evening. For DTW...Ongoing increasing elevated instability will pose a continued chance for some thunderstorms this morning. The passage of the warm front around 15Z will veer the winds toward the south. Timing for afternoon and evening thunderstorms carries some degree of uncertainty. Recent hi res guidance suggests convective development off a sfc trough preceding the actual cold front, then impacting metro sometime in the 21Z to 01Z time frame. //DTW THRESHOLD PROBABILITIES... * Moderate in thunderstorms today and this evening. * Moderate in ceilings at or below 5000 ft today. Low tonight. PREV DISCUSSION... Issued at 344 AM EDT Thu Aug 17 2017 DISCUSSION... A mature and stacked low pressure system over the upper MS Valley will lift northeast into the northern Great Lakes tonight and into Quebec by Friday night. This system will provide the focus for showers and thunderstorms over the next two days as well as the chance of severe weather this afternoon. A warm front will lift north through lower MI this morning putting SE MI solidly in the warm sector for the bulk of the afternoon and evening hours. A prefrontal trough will push through this afternoon/evening before the cold front sweeps through early tonight. Best chance of severe storms will occur from about 17-02Z. Current radar shows the eastern flank of the 850mb front lifting into southern MI at press time. This early convection should remain elevated but could produce some brief heavy downpours. Best frontal forcing this morning will be well to the NW closer to the low and the current 850mb jet is surging northward through WI and western portions of Lake MI. This leading front will usher in the first wave of deeper moisture with PWATs rising to nearly 2 inches. Models advertise a second jump as the surface warm front lifts through sending PWATs over 2.25 inches. The setup for the afternoon will garner a bit more attention as ingredients continue to come together for potential severe weather. As the upper low slides north and east, region of stronger upper level diffluence will slide overhead. This occuring as the left exit region of the jet noses into lower MI. Looking lower in the column, a resurgence of the low level jet will work into lower MI with speeds of 30-40 knots. MUCAPE is set to rise toward 1500 J/kg while ongoing convection tries to limit surfaced based CAPES more between 500-1000 J/kg. HIRES wind profile is keeping winds backed a bit more to the SE increasing the chances of rotating storms and perhaps a tornado. Low level helicity and model hodographs look pretty good so potential is there, especially taking into account such a moist environment and lower LCLs. Otherwise, the instability and shear present should be enough to produce strong to severe storms. Wind is the primary threat with heavy rainfall producing localized flooding a possibility as well. SPC has included SE MI in Slight Risk for severe weather this afternoon. Chances of showers will continue into Friday as a shortwave trough rotates around the low and through SE MI. Lingering low level moisture and increase low level lapse rates with cooler air moving in aloft behind the cold front should produce a scattered coverage of showers. Broad area of surface high pressure will begin building back north into the region Friday and looks to hold at least into Monday as it drifts toward the Atlantic coast. Before ridging follows suite aloft, a shortwave and strengthening vort max will dive down through the backside of the trough on Saturday. This will bring a chance of showers and thunderstorms during the day. The trend is for a stronger system which will dig deeper southward into the Ohio Valley. The result would be for decreasing precip chances across Mid MI, possibly becoming a dry forecast. As for temperatures, the surface ridge sliding to our southeast will open the door for southerly return flow and a warming trend early in the week. This will end Tuesday when the next cold front is slated to sweep through the region. MARINE... The combination of low pressure lifting into Upper Michigan today and departing high pressure to the east of the region will support increasing southeast winds during the course of the day. An associated warm front will lift into Lake Huron by evening, which will only act to enhance the S-Se pressure gradient. Gusts across the open waters of Lake Huron will top 25 knot at times today, while the warm front provides showers and thunderstorms. Rainfall from this convection will be quite heavy and will significantly reduce visibility. Winds will veer to the southwest tonight as the surface low passing across the northern Great Lakes drives an associated cold front across the lakes. Post frontal cold air advection will boost the over-lake instability profile, thus causing gusty winds and hazardous small craft conditions that will persist through the day Friday. Winds and waves will then decrease over the weekend as the gradient relaxes. HYDROLOGY... A low pressure system approaching the northern Great Lakes will drive a moisture rich atmosphere into Se Mi this morning. The initial arrival of this air mass will be marked by the passage of a warm front this morning, triggering showers and thunderstorms. A moist and unstable air mass will then persist across the area through late evening when the passage of an associated cold front. Multiple rounds of showers and thunderstorms will be possible between these two frontal features. With a tropical airmass in place, thunderstorms will be capable of producing very heavy rainfall. There is high probabilities that 24 hour rain totals, ending Friday morning, will range from one half inch to one inch. Locally higher amounts are expected, possibly greater than two inches. This raises the concerns for some localized flooding today.
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&& .DTX WATCHES/WARNINGS/ADVISORIES...
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MI...Beach Hazards Statement through this evening for MIZ049. Lake Huron...Small Craft Advisory from 10 AM to 8 PM EDT Friday for LHZ421-422. Small Craft Advisory until 8 PM EDT Friday for LHZ441-442. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE.
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&& $$ UPDATE.......MR AVIATION.....SC DISCUSSION...DRK MARINE.......SC HYDROLOGY....SC You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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