Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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544 FXUS63 KDTX 051816 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 216 PM EDT Fri Jul 5 2024 .KEY MESSAGES... - A Low pressure system arrives over lower Michigan this afternoon and evening. This will bring scattered to numerous showers with embedded thunderstorms, with highest coverage favored over the Tri- Cities and Thumb. - There is a Marginal Risk of severe thunderstorms this afternoon and evening for all of SE MI. - The low pressure system lingers over the Great Lakes tomorrow keeping a low chance (25%) of showers and thunderstorms in the forecast. - The holiday weekend closes out with dry weather Sunday under partly sunny sky. && .AVIATION... Ample amount of shear and CAPE exist to sustain scattered thunderstorms this afternoon and late evening. Several storms have already developed INVOF MBS along a convergence boundary while thunderless showers crop up over the Metro Detroit terminals. Expectation is that most thunderstorm activity will hold off until late this evening for FNT south, thus the later TEMPO group adjustments. Clouds are largely expected to remain high-based, but a brief BKN high-MVFR ceiling could arise early this afternoon. Light to modest southwest winds drop-off overnight with some HZ reintroduced Saturday morning. Potential exists for low-MVFR ceilings/visibilities overnight, but not confident enough for inclusion for the 18Z TAF cycle. Borderline MVFR ceilings should settle in Saturday morning with stratus favored amidst the cooler post-frontal airmass. For DTW/D21 Convection...Scattered showers are expected this afternoon along a pre-frontal trough with storms more likely to develop this evening ahead of an inbound cold front. Will highlight the earlier period with -SHRA then target the late evening (01-04Z) period for thunder. Storms will generally be single cellular or multicellular with SW-NE storm motion. DTW THRESHOLD PROBABILITIES... * Low for thunderstorms between 18Z and 22Z today, then medium between 01Z and 04Z this evening. * Low for ceilings at or below 5000 ft this afternoon then medium late tonight into Saturday. && .PREV DISCUSSION... Issued at 931 AM EDT Fri Jul 5 2024 UPDATE... Ongoing cluster of showers and scattered thunderstorms extending from central Lower Mi into northern Lake Mi is tied to a mid level short wave. An axis of positive mid level vorticity extends from this short wave south across southern Lake Mi. It is this region of positive mid level vorticity that will initiate and/or sustain convection as it moves east across Se Mi this afternoon. Diurnal destabilization will have some degree of limitation as a result of areas of mid/high clouds. Current hi res solutions suggest ML CAPE values ranging anywhere from a a few hundred J/kg across the south to around 1k J/kg across the Saginaw Valley and thumb region with afternoon temps in the low 80s and sfc dewpoints in the mid/upper 60s. The 12Z DTX sounding and latest NAM/RAP soundings have some indication of a shallow mid level stable layer (strongest in the south). The combination of large scale ascent and diurnal destabilization should erode this weak cap later today. Steep mid level lapse rates this afternoon and effective shear values around 35 knots will support some stronger updrafts, capable of producing locally strong wind gusts (up to 60 MPH) and large hail (up to one inch in diameter). These factors will continue to warrant an isolated (marginal) risk of severe thunderstorms today. While this initial region of ascent will advance across Se Mi mid to late afternoon, a secondary short wave pivoting around the longer wavelength trough will rotate across central Lower Mi this evening. This may allow some convective redevelopment across the Saginaw Valley and thumb region during the course of the evening, predicated on the amount of residual instability. PREV DISCUSSION... Issued at 337 AM EDT Fri Jul 5 2024 DISCUSSION... 1008 mb low pressure system over southern Wisconsin will expand and move into lower Michigan through this afternoon and evening. Prior to the arrival of low pressure, moisture transport magnitude will increase this morning and afternoon under favorable southwest flow, increasing PW values to around 1.50 inches and h850 dew points aoa 12C. Isolated to scattered rain showers will initially be possible through the late morning and early afternoon hours once daytime heating starts and instability builds. Expansion to numerous showers with embedded thunderstorms then turns increasingly likely through the late afternoon and evening hours as the low and associated triple point arrival over Michigan, coinciding with peak diurnal heating, where modest CAPE values of 500-800 J/kg will be in place. Latest model guidance places the low center around the Tri- Cities or just north with the warm front extending into the Thumb by the afternoon/early evening. This is where the bulk of the activity will likely commence and thus where PoPs are greatest (70%). Expansion of a cold front moves through after 21Z through late tonight, where increasing shower and storm activity will then be possible for the remainder of SE MI. There will be plenty upstream mid and upper-level clouds filtering in through the day which will not be a favorable factor for instability. 0-6/1-6km shear around 30-35 knots will help support some organized convection. The hodographs are quite impressive by 21Z and are nearly straight through northern lower and portions of the Tri-Cities and Thumb, with a bulk of strong shear noted in the 6-12 km layer. EL in modeled soundings fall within the 6-12 km layer so a portion of this shear will not be realized with convection, but overall it shows strong speed shear through the mid to upper-levels. Pulse, multicell clusters and/or mini-lines with possible bowing segments will all be in the realm of potential storm modes noting the above. Hail to an inch (lower than average freezing levels, straight hodographs) and/or gusts to 60 mph (wet microbursts) will be the primary severe threats. SWODY1 highlights a marginal risk for severe weather across SE MI today, with 2PM - 10 PM as the primary threat window, noting the modest instability as an inhibiting factor. Some isolated residual showers will be possible during the overnight period as a trailing upper-level trough axis swings over the state. Otherwise, passage of the low pressure system will have pushed the cold front over SE MI, which will bring a little relief from the heat as highs peak in the upper 70s tomorrow. A lower end chance (25%) for a shower or storm will hold through the afternoon and evening as a weaker shortwave pulses overhead while very modest instability builds (mainly north of M59). A major inhibiting factor to organized shower and storms activity will be a building mid-level cap around h600 that increases in strength through the afternoon. An brief shortwave ridge with an expanding surface high pressure system will fill in over Michigan on Sunday limiting precipitation as highs rise back into the low to mid 80s. A series of upper-level waves will expand through the Plains through the early week period with trough amplification commencing into the western portion of the Midwest. This will draw in warm air downstream across SE MI, with h850 temperatures peaking around 18C by Monday evening supporting highs into the mid to upper 80s. Additionally this will mark increasing chances for showers and thunderstorms by Monday, initially along a leading shortwave, with increasing potential for showers and storms on Tuesday as the trough axis moves over the state. MARINE... Relatively quiet conditions to start the day as a weak ridge passes over the region this morning. This is followed by the arrival of a low pressure system which sends a warm/occluded front eastward across the central Great Lakes during the daylight hours. Scattered showers and storms will accompany this system, and isolated storms may become severe with gusts in excess of 35 knots and hail possible during the afternoon and evening. The cold front moves through this evening as the low slowly departs to the east which will bring an end to much of the convection tonight. Lingering showers may continue into parts of Saturday as weak troughing holds over the region. A broad ridge of high pressure then fills in on Sunday with mainly dry conditions and continued light winds. The next frontal boundary moves into the Great Lakes on Monday, bringing another period of unsettled weather to start the week. HYDROLOGY... Low pressure moving into the Midwest swings a frontal system through Lower Michigan during Friday while supporting scattered to numerous showers and scattered thunderstorms across the area. Locally heavy rainfall could briefly reach rates of 1 inch per hour while basin average totals come in closer to 0.25 to 0.75 inch until activity diminishes and exits eastward toward evening. Highly localized totals between 1- 2 inches will be possible with repeated thunderstorms, which will be most likely across the Tri-Cities and/or Thumb. Steady motion of the system location and location of the higher end rainfall totals mitigate any serious considerations of flooding, with threats limited to ponding of water on roads and in areas prone to collecting standing water. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....KGK UPDATE.......SC DISCUSSION...AM MARINE.......TF HYDROLOGY....AM You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.