Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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193 FXUS63 KDTX 180449 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1149 PM EST Fri Nov 17 2017 .AVIATION... The heavier rain showers will diminish within the first couple hours after TAF issuance. This lead edge of showers is occurring within strong low level moisture transport. Although the heavier showers will end, light rain/drizzle will persist through daybreak under steady low level moisture advection. Upstream observations support ceilings dropping to IFR by daybreak under this moisture advection. Deepening low pressure will track across NW Ohio early Sat afternoon, then into southern Ontario by evening. This low will bring another round of heavier rain showers, especially Sat afternoon. Additional moisture lifting north of the associated sfc warm front will sustain IFR/LIFR cigs and vsby into the afternoon. As the low continues to deepen as it exits east Sat afternoon, north- northwest winds on the back side of this system will strengthen. A deepening mixed layer as a result of cold air advection will lead to increasing winds, with gusts likely topping 30 knots Sat evening. For DTW...The risk of early morning thunderstorms looks to be decreasing. The better instability Sat afternoon is expected to remain south of the Mi/Oh border given the forecast track of the sfc low. The influx of low level moisture ahead of the sfc low may however support periods of fog into the early afternoon. A wind shift to the northwest is expected between 22 and 00Z Sat evening. //DTW THRESHOLD PROBABILITIES... * High for ceiling 5000 ft or less overnight through Saturday night. * Low in ceilings and/or visibilities below 200 ft and/or 1/2 miles Saturday morning. * Moderate in crosswind thresholds being exceeded Saturday evening. && .PREV DISCUSSION... Issued at 816 PM EST Fri Nov 17 2017 UPDATE... Rain showers have already advanced into the Tri Cities of 8 PM and will advance into the remainder of the forecast area during the rest of the evening. Recent satellite data have been showing cooling cloud tops across Lake Mi and western Lower Mi within region of increasing moist isentropic ascent. Significant strengthening of the low level inflow into srn Lower Mi will occur tonight (850mb winds increasing to 60-70 knots across nrn Indiana/far srn Lower Mi). This will enhance the moisture transport along steepening isentropic surfaces, resulting in expansion of rain across the forecast area. The peak ascent looks to be focused around 06Z. The degree of elevated instability looks weak. Taking into consideration the amount of lightning that has occurred upstream and given the intensification of the ascent over the next 3 to 6 hours, the chances for thunderstorms will be increased tonight with the evening update. Otherwise, the going forecast looks reasonable. PREV DISCUSSION... Issued at 402 PM EST Fri Nov 17 2017 DISCUSSION... Main focus for this forecast will be the unsettled weather for the weekend in the form of increased rainfall and gusty winds. An upper trough exiting the Rockies and tracking across the central plains this evening. This will begin advecting moist air into Lower Michigan evidenced in the 700mb theta-e field as a cold front dips south across the western Great Lakes. This front will be help drive precipitation tonight as the LLJ kicks in and provides some deeper forcing. The front will eventually stall out across Lower Michigan as the surface low pressure system becomes more organized and gains strength on Saturday while the aforementioned upper trough begins to phase with another upper trough diving southeast across Ontario. The low pressure system will move along this stalled boundary from northern Illinois through southeast Michigan. All of Michigan will be under the influence of the left exit region of the 140 knot southern stream jet and the right entrance region of the departing upper jet. Forcing and deep ascent with this is system will lead to moderate rainfall through Saturday night with PWATs reaching over an inch across far southeast Michigan. A chance for thunderstorms exists with this system based on the track of the surface low. An area of convective instability within the warm sector will be enough to produce some thunderstorms. The track of the low has come into more alignment with the NAM being the only outlier keeping it further south. The others have it tracking a little north of the state line. Believe chances are still good enough to leave a slight mention of thunder in the forecast for the further southeast portions of the CWA. The low pressure system will continue to deepen as it moves northeastward into the eastern Great Lakes by tomorrow evening. The timing of how quickly the moisture exits and the cold air arrives in the wake of this system will cause a transition from rain to a period of a rain snow mix tomorrow night for areas mainly north of M- 59. Expect the coldest air aloft to lag behind just enough that it will be difficult to see any snowfall accumulate from this system. Temperatures near the surface look to remain warm enough as the bulk of the precipitation exits. The deepening low will also increase the pressure gradient resulting in strong wind gusts late Saturday afternoon through Sunday morning. Wind gusts around 30-40 mph will be possible as strong winds above the surface should be able to mix down. Temperatures at 850 mb will drop to around 10-11 degrees C by Sunday afternoon. High temperatures will only come up a few degrees from the morning lows and top out in the low to mid 30s under mostly cloudy conditions. The cold air advection under stout northwest flow will result in the potential for scattered snow showers throughout the day on Sunday aided by the instability across the warmer Great Lake waters. Ridging translating east across the region early next week will yield dry, milder conditions as highs top out in the mid 40s on Monday. Low pressure moving eastward across northern Ontario on Tuesday will pull a cold front through the region late Tuesday. Ahead of the front, increasing southerly winds will boost temps close to 50 Tuesday afternoon. A few showers will be possible across the Thumb with the frontal passage, but overall a dry frontal passage is expected with the bulk of forcing staying well north of the region. A colder air mass with 850-mb temps falling below -10 C will then overspread the region Tuesday night into Wednesday as upper troughing sets up through late next week. Highs will be noticeably colder with readings in the 30s for both Wednesday and Thursday. However high pressure building in from the west will keep dry, calm conditions in place through Thursday. MARINE... Wind will be the primary marine weather concern tonight through the weekend highlighted by high end gales on Lake Huron Saturday night. Moderate southerly wind will continue tonight ahead of the responsible low pressure system organizing over the Plains. Small craft advisories continue for the Lake Huron shoreline where waves will be elevated due to an onshore wind component there. The gales are expected to develop as the Plains low pressure moves into Lower Michigan Saturday on a track near the west end of Lake Erie. It will then strengthen considerably over Lake Ontario Saturday night and produce strong northerly gales over Lake Huron where gusts near 45 knots will be possible producing significant waves of 10-14 feet and max waves around 20 feet over the south third of the lake. Gales will be lower end intensity over marine areas closer to land such as inner Saginaw Bay, Lake St Clair, and the Michigan waters of Lake Erie. Gale warnings are now in effect for all marine areas through Sunday. The system will otherwise produce widespread rain showers tonight through Saturday and possibly a rain/snow mix before ending Saturday night. HYDROLOGY... Moderate rain tonight through Saturday will occur in two rounds late tonight and Saturday afternoon. Widespread rainfall in the 0.25 to 0.5 inch range tonight will be focused mainly in the 10 pm to 4 am time period. Totals could approach 0.75 inch from the Detroit area to the Ohio border where a thunderstorm will also be possible. The second round will again produce widespread 0.25 to 0.5 inch totals Saturday afternoon and totals around 1 inch from the Detroit area to the Ohio border before diminishing Saturday evening. Storm total rainfall in the 1 to 2 inch range will be capable of minor urban flooding and moderate rises in area rivers and streams, especially in the Metro Area and surrounding suburbs. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Gale Warning from 3 PM Saturday to 7 PM EST Sunday for LHZ361>363- 462>464. Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LHZ421- 441>443. Small Craft Advisory until 4 AM EST Saturday for LHZ441>443. Gale Warning from 7 PM Saturday to 7 AM EST Sunday for LHZ422. Lake St Clair...Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LCZ460. Michigan waters of Lake Erie...Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LEZ444. && $$ AVIATION.....SC UPDATE.......SC DISCUSSION...AA/JD MARINE.......BT HYDROLOGY....BT You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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