Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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873 FXUS63 KDTX 190833 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 433 AM EDT Wed Jun 19 2024 .KEY MESSAGES... - Hot and humid conditions will persist across most of the area today as a Heat Advisory is in-place for all of Southeast Michigan. - Chances of thunderstorms continue each day of the week, some of which may be strong to severe with locally heavy downpours and isolated damaging wind gusts. && .DISCUSSION... Resident eastern CONUS ridge sampled by KLWX 19.00Z RAOB (Sterling, VA) revealed a H5 geopotential height center of 597 dam. The anticyclone is expected compress along the y-axis and broaden significantly along the x-axis today in response to the slight retrograde of a coastal California trough, a deepening progressive longwave over central Canada and a developing tropical cyclone over The Gulf. Early morning GOES vapor imagery highlights confluent southwesterly flow aloft driving a cloud street up into Lower Michigan, hampering nocturnal cooling. Deterministic H9 and H8 progs suggest minimal thermal advection processes today as significant level temps hover near 25C and 20C, respectively. Mixing capacity lends highs only near 90F (a 3-5F downward revision), but given the very warm starting point this morning (08Z readings in the mid 70s to near 80F) and the summer solstice being only a day away, insolation should sufficiently filter through the mid-high cloud deck offering MaxTs in the low 90s (up to 93F for Detroit heat island). As for heat indices, the slightly cooler shift with dewpoints generally AOB 70F holds peak index values AOB 100F today. Opted to downgrade the Excessive Heat Warning to a Heat Advisory given the lower expected HIs and a shorter expected duration to the cumulative high heat days. Other controlling factor that could further cut high temperatures (and max heat indices) is the potential for diurnal convection. Consensus of CAMs, ensembles, and deterministic solutions suggest a more robust thunderstorm response today. Instability pool build across Southeast Michigan with HREF SBCAPEs in the 1250-1750 J/kg range. The bulk of the activity should develop during the afternoon timeframe with a mix of pulse storms and multicell clusters. Given only 25 knots of surface to H5 shear, severe storms are unlikely, but the threat of damaging wind gusts from isolated wet microbursts exists. SPC`s SWODY1 did include most of the region in a Marginal Risk. Pattern unlock begins Thursday as a backdoor cold front washes across Lake Huron providing cooler air with northeast flow. Did shorten the Heat Advisory by a day for the northeast segment as heat indices should remain well below criteria Thursday behind the FROPA. The main question becomes, how far south(west) will the front get before stalling out. Latest forecast foreshadows some potential adjustments for the Tri-Cities and possibly further south. Additionally, yet another shortwave feature moving along the northern periphery of the ridge is expected to force more showers/storms Thursday afternoon with a better defined baroclinic zone. Some revisions to PoPs should be expected as the ridge adjusts over the next 24 hours. Thermal gradient stays entrenched across Southeast Michigan Friday with northeast flow holding on over The Thumb while the stalled front retreats northward as a reinvigorated warm front. Familiar dewpoints near 70F and southwest winds creep northeast with a return to highs in the lower 90s for the roughly the southern half of the forecast area. This scenario unfolds all while the ridge axis rolls southeastward across Appalachia and into the Mid-Mississippi Valley. By Saturday the system centers over The Southeast and could interact with another ridge over The Southwest. While the NAM, GFS, and ECMWF are in decent agreement with a more zonal upper level pattern and a draw-down in heights, another warm advection push out of The Plains could bring a more widespread return of heat/humidity. Note that a lot of spread still exists, even amongst the different ensemble groups. For reference, KDTW`s mean high temperature per the 19.00Z GEFS is 87F while the EPS has a high of 94F. NBM/extended highs currently reflect values more analogous to the latter ensemble. && .MARINE... Southwest winds will maintain the hot and humid condition today. A high degree of surface stability with the cooler waters should keep wind speeds under 25 knots. The exception will be in and near and strong thunderstorms which develop. Timing and location will be highly uncertain, but the typical peak heating of the day into early evening hours will be favored. A cold front is now progged to move south tonight. This will allow for northerly winds over Lake Huron on Thursday and Friday as seasonably strong high pressure tracks through Ontario. Even so, airmass remains warm, which should help limit wind speeds to around 15 knots. Thus, any wave build up with the longer fetch is expected to remain below 4 feet. A stronger cold front looks to be on track for the second half of the weekend however, and wind speeds then could reach or exceed 25 knots with waves building to around 4 feet. Small craft advisories may be needed. && .HYDROLOGY... A moist and unstable airmass remains in place today. Isolated showers and thunderstorms are possible south of I-96/696 this morning followed by broader coverage (with a northward expansion) this afternoon and evening. The most intense thunderstorms and repeated rounds have the potential to produce rainfall amounts in excess of an inch over a short period of time which could lead to possible flooding for urban and low lying areas. Uncertainty in specifics of location and exact timing for any flooding precludes the issuance of an flood watches. Additional storms are possible Thursday with low confidence in flood potential. && .PREV DISCUSSION... Issued at 1241 AM EDT Wed Jun 19 2024 AVIATION... VFR ceilings and vsbys are expected tonight and into Wednesday. Mid/high clouds will gradually fill in overnight, introducing some uncertainty in regards to how thunderstorms chances will play out on Wednesday. Thunderstorms remain possible, especially after 21Z on Wednesday, with both individual storms and clusters of storms possible. Winds will maintain a southerly component tonight, becoming southwesterly by Wednesday morning. A backdoor cold front begins to move in from the north late Wednesday night, with north to northeast winds overspreading southeast Michigan. For DTW/D21 Convection...Attention turns to Wednesday, mainly during the afternoon and evening hours for our next chance for thunderstorm activity. There will be a bit more deep layer wind shear to work with tomorrow, so we may see both individual pulse-type thunderstorms and clusters of storms move through the area. While there are a few high-rest models that bring thunderstorms in during the late morning hours, the best time period for convective activity will take shape after 21Z. The potential for wet microbursts will be a concern once again, should stronger storms develop over/near D21 airspace. DTW THRESHOLD PROBABILITIES... * Low to Moderate risk for thunderstorms Wednesday between 21Z and 03Z. * Low risk for ceiling at or below 5000 ft, conditional with thunderstorms. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until 10 PM EDT Thursday for MIZ047-053-060-061- 068>070-075-076-082-083. Heat Advisory until 10 PM EDT this evening for MIZ048-049-054-055- 062-063. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ DISCUSSION...KGK MARINE.......SF HYDROLOGY....KGK AVIATION.....JA You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.