Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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222 FXUS63 KDTX 300125 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 925 PM EDT Sat Apr 29 2017 .UPDATE... An increasing trend looks solid in both coverage and intensity of showers and scattered storms during the evening and overnight hours, matching up nicely with the flood watch valid time. Model analysis fields indicate a strong warm front over southern Lower Michigan in the 850-700 mb layer by measures of theta-e and thickness, supported by the 00Z DTX sounding that shows the top of a sharp inversion around 750 mb. Deep and strong southwest flow under the broad 160 kt anticyclonic upper jet is driving strong moisture transport/theta-e advection and frontogenesis through the elevated frontal zone. Recent radar composite also indicates an MCV organizing within the mature convection over Illinois. This will only enhance the low level jet that is already projected to be in the 40-50 kt range at 850 mb through the night. Coverage in SE Michigan will be more widespread toward the Tri Cities and a bit less in the Detroit area. The widespread/categorical POPs in the north to likely/numerous toward the Ohio border in the going forecast look good with no adjustments needed. Plan to just add a few short term forecast products to advertise coverage and strength trends during the night that will likely include heavy downpours and possibly some small hail. && .PREV DISCUSSION... Issued at 720 PM EDT Sat Apr 29 2017 AVIATION... Light to moderate northeast wind will continue to provide enough dry air in the low levels to prevent MVFR ceiling from moving much farther north than the Ohio border during early evening. At the same time, elevated moisture is proving adequate to penetrate the surface based dry layer north of PTK which will persist as pockets of showers are shed from the main precipitation shield over Illinois. This activity will continue to expand over Lower Michigan during the night with highest concentration from FNT to MBS. Plan to use short term updates for addition of thunder as observations provide a more solid indication of timing and coverage. Low pressure over Oklahoma will then bring the warm front closer to the Ohio border by sunrise which will carry MVFR ceiling into the region with IFR becoming more likely along the DTW corridor. Northward progress of the warm front will be disrupted by easterly flow over the colder Great Lakes, especially while the primary surface low is to the southwest. It may take until the occluded frontal passage Sunday night before MVFR/IFR is swept out of SE Michigan. In the meantime, low ceiling and reduced visibility in showers/storms will be widespread over the region through Sunday. For DTW... Thunderstorm timing and coverage will need refinement before inclusion in the forecast. A loose window for tonight would be about 08-12Z and then again during Sunday afternoon. Light to moderate northeast to east wind will persist through the forecast. //DTW THRESHOLD PROBABILITIES... * High for ceiling 5000 ft or less tonight through Sunday. * Low chance for thunderstorms late tonight and Sunday. PREV DISCUSSION... Issued at 300 PM EDT Sat Apr 29 2017 DISCUSSION... Cool/dry low level northeast flow mostly in control today as high pressure translates through the northern Great Lakes, with a more dominate center (1032-1033 MB) taking over Tonight over just south of James Bay. Deep surface based convection fired up along the Ohio River this morning and was the focus of heavy rainfall during the morning hours, cutting off moisture transport into the southern Great Lakes. However, transport vectors quickly ramping up this afternoon/evening, as wave ejects out of northern Missouri this afternoon, and and still looking at ribbon of up to 5 C dew pts at 700 MB lifting through southern Lower Michigan Tonight. Depending on the strength of this wave/ripple, with have to watch locations close to the southern Michigan border as showalter index reaches between -2 to -5 C, depending on your model of choice, with HRRR showing stronger convective reflectivity tracking along the border as well. Fortunately, no concerns for surface based instability, and would expect a sufficient stable surface-925 mb layer to prevent any stronger wind gusts. Small hail will be possible however. Rapid development/quicker spin up of the upper level low/circulation over the Four Corners region earlier Today. Thus with the deeper system and low level jet displaced a bit farther to the west, likely assuring the better/prolonged 850-700 MB FGEN will be farther northwest Tonight into Sunday, but still likely clipping Tri-Cities region, where we have elected to issue a long duration flood watch (added a few surrounding counties as a buffer as well). Local Probabilistic SREF weighted guidance still projecting around 2 inches of total rainfall up that way by Monday Morning, with potential for 3+ inches. Good chance of Flooding along Saginaw River based on those amounts. PW values of 1.5 to 1.75 inches in place Tomorrow will allow promote localized heavy rainfall farther south with any vigorous convection/thunderstorms, but confidence is lower in coverage/duration/locations. Still thinking the surface warm front will not get much past the Michigan border, but could clear Lenawee county, setting up very tight temp gradient along I-94 corridor and points south, with high degree of 0-1 km bulk shear to work with if any thunderstorm is able to latch onto this boundary. Outside of this area, MUcapes of 500-1000 J/kg would just support small hail. For high temperatures on Sunday, looks like a 30-35 degree temperature difference from north to south, with highs in the upper 40s across northern Bay/Midland counties, to around 80 degrees late in the day along the southern Michigan border. The highest confidence in the most widespread rain showers is late Sunday Night with the main upper level forcing/moisture axis and cold front finally moving through, still looking to clear southeast Michigan by Noon Monday. A very wet Monday morning commute, and normally flood prone roads in Detroit Metro area could experience problems with the high rainfall rates. Mid level dry slot taking hold, with good low level cold advection from the southwest, as 850 mb temps tumble toward zero by day`s end. Steep low level lapse rates and increased mixing depths, as well as local probabilistic guidance suggest wind gusts of 40+ MPH by Sunset Monday into Monday evening. Wind potential will obviously hinder on how fast the low fills/weakens Monday afternoon-night as it lifts through the Western Great Lakes. Lingering isolated light rain showers will be possible throughout Tuesday morning and afternoon as low pressure pushes northeast from the Upper Peninsula into southern Quebec. As the low moves out of the Great Lakes, a surface high pressure system is expected to build in from the west, bringing relief from the rain late Tuesday into Wednesday, as temperature daytime highs cap in the mid-50s for both Tuesday and Wednesday. Low to moderate confidence regarding rain potential starting Thursday morning and continuing into the weekend. The latest 12Z ECMWF model suite shows a low pressure system strengthening over Kentucky/Indiana early Thursday, and pushes the low northeast over Ohio by Friday morning, bringing the chance for rain showers across Southeast Michigan as it moves northeast. The GEM model tries to derive a similar low, however, the low pressure system remains weak and keeps all precipitation just south of Michigan, similar to what the GFS is resolving, albeit some slight timing and track differences. As a result, PoP values will remain sub-30 until convergences is seen exhibited future runs. For the weekend, the GFS solution shows a nice omega blocking pattern in the mid-levels which would be mild and dry conditions, however, if the GEM and ECMWF solutions pan out, the potential for dry conditions could be spoiled with isolated showers as both models keep the 500 mb low over PA/NY, well northeast of the GFS run. MARINE... Unsettled conditions through early next week as a strong low pressure system slowly tracks through the area. The approach of this system will establish a long duration of moderate northeasterly winds lasting through the latter half of the weekend. The combination of gusty conditions and corresponding increase in wave heights across the western lake Huron basin will lead to extended small craft advisory conditions. Winds turn southerly and strengthen throughout Monday in advance of a cold front. Potential for gusts up to 30 knots from western Lake Erie to southern Lake Huron during this time. Winds then become westerly with the frontal passage on Tuesday. A wet and active pattern tonight through early Monday, with periods of heavy rain and thunderstorms possible. HYDROLOGY... Several rounds of rain and possible thunderstorms are forecast to lift across southeast Michigan between tonight and early Monday. This rainfall could become heavy at times. Total rainfall through Monday afternoon is expected to range between 1 and 3 inches, with the highest amounts focused across the Saginaw Valley. Locally higher totals will be possible for areas that experience thunderstorms Sunday into Monday, as storms will bring brief periods of torrential rainfall. Rivers and streams will see significant rises through the middle of the week, and ponding of water may develop in low-lying areas and roadways. Flooding could become a concern by Sunday and Monday, particularly over the Saginaw Valley. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch from midnight EDT tonight through Monday morning for MIZ047>049-053-054-060-061. Lake Huron...Small Craft Advisory until 4 PM EDT Sunday for LHZ443. Small Craft Advisory until 6 AM EDT Monday for LHZ421-422-441. Small Craft Advisory from 4 AM to 4 PM EDT Sunday for LHZ442. Lake St Clair...NONE. Michigan waters of Lake Erie...Small Craft Advisory until 4 PM EDT Sunday for LEZ444. && $$ UPDATE.......BT AVIATION.....BT DISCUSSION...SF/AM MARINE.......MR HYDROLOGY....MR You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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