Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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288 FXUS63 KDTX 201808 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 208 PM EDT Thu Jun 20 2024 .KEY MESSAGES... - Hot and humid conditions will exist again Thursday, particularly across areas along and south of the M-59 corridor where a Heat Advisory remains in effect. To the north, a weak cold frontal boundary will briefly bring slightly cooler and less humid conditions. - The heat and humidity builds back into the entire region Friday and Saturday. - 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. && .AVIATION... Convergence along various shallow boundaries will bring scattered showers and thunderstorms across Southeast Michigan this afternoon. Storm motion is expected to be slow with cell movement perhaps at 20 knots or less. Main aviation concern will be obstructed visibilities due to brief heavy rainfall rates. The potential does exist for wind gusts to reach 50 knots in thunderstorm activity, but the likely of occurrence is too low to include in the TEMPO. A main synoptic dewpoint gradient augmented by outflow should push south of the Southeast Michigan between 00-06Z. Very weak flow and moist surface conditions will support br/hz at daybreak Friday morning. For DTW/D21 Convection...Scattered thunderstorm activity is expected to develop near DTW in the 18-21Z timeframe. Low confidence in when thunderstorms will push southward and storm motion may become variable and slow moving. DTW THRESHOLD PROBABILITIES... * Moderate in thunderstorms this afternoon and evening. && .PREV DISCUSSION... Issued at 1102 AM EDT Thu Jun 20 2024 UPDATE... Update this morning is focused on convective trends for the afternoon, as the SWODY1 Marginal Risk has been expanded westward from the northern metro Detroit suburbs to the Ohio border. Overall, the message varies little from the previous several days of convection, although an added forcing mechanism exists owing to a weak front that has penetrated inland through the morning. This boundary arcs from roughly GRR to TOL, and may be an initiation point for thunderstorms as early as noon when surface destabilization is expected to occur. Added forcing from differential heating/lake breeze boundaries arrive later in the afternoon, adding more opportunities for updraft initiation and increasing coverage. Once thunderstorms do initiate, they will have potential to become strong to severe as hot/humid conditions once again support loaded precipitation cores and downburst potential with gusts of 60+ mph. Torrential downpours will be another primary concern, with more details in the Hydrology section. Main timing window for thunderstorms is from noon to 9pm. PREV DISCUSSION... Issued at 410 AM EDT Thu Jun 20 2024 DISCUSSION... As expected, the anomalous ridge parked over the Mid-Atlantic and occupying most of eastern CONUS has broadened longitudinally over the past 24 hours. This is evidenced by 20.00Z RAOB H5 geopotential heights of 599 dam at KPIT (WFO Pittsburgh), KOKX (WFO Upton/NYC), and KLWX (WFO Sterling). Although none of the coarser 00Z models resolved a 600 dam center, it`s certainly possible that such magnitude existed between the three observation sites prior to the westward migration of the ridge centroid into Appalachia. Initial GOES and ceilometer data affirm healthy coverage of nocturnal cloud cover which should, once again, hamper nighttime cooling rates. Made very minor upward revisions to MinTs this morning, outside the lakeshores. As discussed throughout the past several days, a low-level anticyclonic gyre builds over western Lake Huron this morning. This eventually leads to the development of a 1030 mb surface high which then drives a backdoor cold front into The Thumb area and points further southwest. The northeast flow draws down slightly cooler Canadian air and will be partially augmented by sub-60F water temperatures over much of Lake Huron. Main question with this boundary remains the extent of inland penetration and the resultant cooling effect for post-frontal locations. Opted to go slightly more bullish with the frontal progression in the outgoing forecast by bringing the thermal gradient closer to the M-59 corridor. Prior evening update to the Heat Advisory will stand with the more marginal counties being Livingston, Oakland, and Macomb. Best opportunity to approach 100F heat indices for those northern three counties should be along the extreme southern extent of the boundaries where the advection of cooler air comes up short (or is more muted). Today will be the fourth consecutive day of heat indices in the upper 90s to near 100F for the remaining locations within the Heat Advisory as dewpoints linger in the lower 70s. PoP forecast continues to be a major challenge with numerous periods of convection potential. Made first-period changes to account for ongoing line of sub-severe storms between I-94 and I-96 while another cluster tracks toward/into the Tri-Cities. Outside of the FGEN forcing along the inbound frontal slope, additional surrounding convection will be tied to what has become a rather familiar high- CAPE/low-shear quasi-uncapped environment. Further contributions to ascent offered by the reconfiguration of the ridge and a southward push of height falls across the UP and northern Lower MI creates a more favorable corridor for CVA during the afternoon hours with passing shortwave perturbations aloft. Conditions and coverage are less certain with any outflow boundary interactions and differential heating boundaries. Main threat with any strong to severe storms will be short-lived damaging gusts, especially with well-loaded core releases in the form of wet microbursts. The system drifts into The Southeast by Saturday and could interact with another ridge over The Southwest. The deterministic NAM, GFS, and ECMWF remain in decent agreement with a zonal upper level pattern which moderates height changes. Meanwhile, another warm advection/frontal surge lifts back into Lower Michigan marked by H9 temps in the mid 20C range (potentially higher). Expect further refinements to highs Saturday, dependent on the northern edge of the realized thermal gradient. Regardless, most areas should see highs back into the lower 90s with increasing humidity. A cold front driven by a progressive shortwave swinging through the Upper Midwest provides some relief Sunday with showers and storms. This precedes somewhat more seasonable conditions and dry weather Monday before the heat (sans humidity) returns Tuesday. MARINE... Some breakup in the weather pattern occurs today as high pressure builds across Ontario which forces a cold front south through Lake Huron. This will cause winds to turn northerly but gusts remain largely below 20 knots. Warm temperatures remain across region which adds stability over the waters helping keep wave heights minimized as well. 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 will remain in place through the end of the week. Isolated to scattered thunderstorm development is expected across portions of SE MI today. The most intense thunderstorms will have the potential to produce highly localized rainfall amounts of 1- 2 inches in a 30-60 min period. Thunderstorm training (repeated thunderstorms) can also provide the chance see highly localized totals of 1-2+ inches. Uncertainty in specifics of location and exact timing for any flooding precludes the issuance of an flood watches. Additional storms with highly localized flooding potential will again be possible on Thursday during the afternoon and evening. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until 10 PM EDT this evening for MIZ068>070-075-076- 082-083. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....CB UPDATE.......MV DISCUSSION...KGK MARINE.......DRK HYDROLOGY....AM/KGK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.