Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
Issued by NWS Detroit/Pontiac, MI
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336 FXUS63 KDTX 132002 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 402 PM EDT Sat Jul 13 2024 .KEY MESSAGES... - Two rounds of isolated strong to severe thunderstorms possible on Sunday, the first wave during the morning and the second in the late evening and overnight. - The main hazards are large hail to one inch, damaging wind gusts to 60 mph, and heavy rainfall with localized flooding. - Warm and humid on Sunday as well with heat indices reaching the low to mid 90s. - Shower and thunderstorm chances continue Monday and Tuesday. There is a chance for isolated strong to severe thunderstorms both days. && .DISCUSSION... Resident high pressure has led to a quiet Saturday for SE Michigan, although temperature readings in the upper 80s still sit a few degrees above normal even as we approach the mid-July climatological maximum. Dry and subsident conditions highlighted in the 12z KDTX sounding hold through the evening, followed by a shift to an increasingly humid and active pattern by Sunday morning. Leading edge of the warm front currently extends NW-SE through Chicago, followed by an enhanced theta-e plume boasting MLCAPE values of 2000-3000 J/kg over southern WI and IL. This instability plume will eventually pivot into SE MI by early Sunday morning on the nose of a 20-25 knot low level jet. There are still details to work out depending on upstream convective evolution, but the general trend to the 12z model suite has been to bring a complex of decaying thunderstorms to the Tri Cities/Thumb along the ensuing instability gradient early Sunday morning. For most of the cwa, showers/storms will occur during the diurnal minimum with questionable instability prospects, so confidence is low in any severe potential prior to 15z (11am local time). That said, the majority of CAMs do suggest some updraft reinvigoration across the easternmost counties by mid-day, presenting a highly conditional strong to severe thunderstorm threat of large hail and/or damaging wind gusts to 60 mph between about 11 AM and 3 PM local time east of I-75 with potential for storms to grow upscale. There is increasing confidence in a period of post-convective subsidence and drier conditions late Sunday afternoon, although upper 60s-low 70s dewpoints will keep conditions muggy. Heat indices Sunday afternoon may approach the low to mid 90s, though will depend on positioning of any outflow boundaries and cloud cover as noted in MOS guidance where max T differs by some 10 degrees. There is an emerging signal in guidance for a second round of convection to develop later Sunday evening-overnight along a reinforced outflow/theta-e boundary that will gradually lift back to the north through the late afternoon. How far north this boundary reaches is still uncertain, with some guidance keeping it south of the state line and other guidance bringing it as far north as I-94. Should the northern solution play out, the urban Detroit corridor will be at risk for both isolated strong-severe thunderstorms and heavy rainfall amidst training thunderstorms. The west-east orientation of the boundary and ~20-25 knot flow aloft will result in slow-moving and likely training storms wherever the boundary sets up, capitalizing on efficient warm rain processes. See the Hydrology section for more details. Showers and thunderstorms likely to linger through early Monday morning with a continuation of unsettled weather through the early half of the week. The strong ridge over the Four Corners region maintains a storm track favorable for embedded shortwaves to impact southeast Michigan each day, introducing thunderstorm chances throughout the week amidst prolonged instability. Otherwise warm and humid conditions will continue through mid-week ahead of a stronger synoptic disturbance that will impact the region on Wednesday. This will usher in a cooler/drier Canadian airmass, but the system itself will be one to watch for more organized convection potential. && .MARINE... Diffuse high pressure congeals around the central/eastern Great Lakes today offering a rather unimpressive surface gradient which generally keeps prevailing winds AOB 10 knots through the rest of today and overnight. The resident humid airmass could interact with lake breeze and/or differential heating boundaries this evening triggering some isolated convective activity. Should showers/storms develop (more probable over land) a weakening trend will likely ensure shortly after crossing into the waterways. Sunday features the arrival of a warm front bringing with it a shift to southwest flow and expanding coverage of thunderstorm activity. Previous considerations regarding marginal afternoon Small Craft Advisory conditions for Saginaw Bay remain similar, therefore decided to forego a headline for isolated/occasional peak gusts near 25 knots. The warm and muggy airmass holds across the region Monday and early Tuesday offering several additional opportunities for thunderstorms as packets of upper level energy arrive in staggered succession. Periods of stronger storms are possible Sunday through Tuesday which could pose a heightened risk for locally higher winds/waves and some hail. && .HYDROLOGY... There is potential for two rounds of heavy rainfall as a warm and humid airmass tracks into SE Michigan on Sunday. Thunderstorms will be capable of producing torrential downpours with a quick inch or more of rainfall possible Sunday morning and again Sunday evening in a single storm. There is a chance for training thunderstorms to impact the urban Detroit corridor Sunday evening and overnight as an instability axis may settle west-east over the area. In this case, rainfall amounts may exceed two inches although there is low confidence in exactly where this boundary may set up. Heavy rainfall will occur amongst already saturated soils and elevated rivers from the previous week`s rain. Localized flooding, especially low-lying and urban areas, will be possible. There are several additional chances for showers and thunderstorms through Wednesday. && .PREV DISCUSSION... Issued at 100 PM EDT Sat Jul 13 2024 AVIATION... The center of the near surface anti-cyclone is positioned near Lake St Clair and the diurnal cumulus field is pinwheeling about this feature. Cumulus response has been greatest where the soil moisture content is higher from recent rains. Expect the axis of cloud formation to drift eastward into the Detroit airspace though the course of the afternoon. There is a low chance that an isolated shower/storm will emerge from the region of better cloud coverage as the afternoon wears on. Upstream convection is expected to become quite active this evening and overnight. Expectations are for remnant activity to approach Southeast Michigan around daybreak and persist through the balance of Sunday morning with an opportunity for thunderstorms to rejuvenate in the Detroit region around midday before exiting to the east. Details are difficult to come by regarding the character of the activity - but the timing window has been rather consistent for quite some time. For DTW/D21 Convection... An isolated shower/storm possible late this afternoon. Expanding storms upstream of Lake Michigan have a chance to survive into SE MI/D21/DTW late tonight through Sunday morning. DTW THRESHOLD PROBABILITIES... * Low for ceiling 5000 ft or less this afternoon. * Medium for thunderstorms Sunday morning. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ DISCUSSION...MV MARINE.......KGK HYDROLOGY....MV AVIATION.....Mann You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.