Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 212308 CCA AFDDTX Area Forecast Discussion...Corrected National Weather Service Detroit/Pontiac MI 606 PM EST Sat Jan 21 2017 .AVIATION...
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Leading edge of low level drying has stalled from near KMBS southeast to just northeast of KFNT/KPTK and 11u-3.9u imagery suggests lower stratus is just beginning to build back to the south/southwest. Additional stratus will most likely form as this mild/moist airmass rides over ice over Lake Erie as light winds back to the southeast and east. Timing may be a bit difficult this evening, but end result will be another long period of generally LIFR/VLIFR conditions late this evening into Sunday morning. Did adjust forecast for a quicker return of this conditions given HRRRX fields and recent trends, but still "stair step" visibilities down during the course of the evening. For DTW...VLFIR cigs/vsbys should build back over area with time this evening as light southeast flow off of Lake Erie provides an excellent source for saturating low levels, especially once the boundary layer fully decouples. These conditions, in general, should hold most of the forecast through 15z or so Sunday morning. //DTW Threshold Probabilities... * High for cigs aob 5kft after 02z. * High for cig/vsbys aob 200ft and 1/4SM tonight 02z-15z.
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&& .PREV DISCUSSION... Issued at 415 PM EST Sat Jan 21 2017 DISCUSSION... Today really was a page taken from the early Spring chapter as a stationary front hung up along the terrain. This led to a stable lake aggregate effect/persistent dense fog over the northern cwa, and a legitimate southwesterly flow warm sector over the southern cwa. As it stands here at press time, temperatures have made a run at 60 degrees in Detroit, with low 50s at Flint. These readings are only 5 degrees off of record values. The warmth has been a direct result of an equivalent potential temperature ridge that was able to opportunistically bubble into the central Great Lakes region with the rapid demise of the the broader wavelength midlevel trough/Potential Vorticity anomaly that had been in place. Really is surprising this warmth happened with the time of year, clouds, and preceding lowered geopotential height anomaly. This added theta e advection has only added to the moisture in the lower troposphere. Tonight...Lack of upper level Jet overhead and slight drifting northward of the WI/MN PV anomaly will allow for slight geopotential height rebound over Lower Michigan. This will allow for a weak positive surface pressure tendency preserving a lack of gradient flow over southeastern Michigan throughout the overnight. The lack of any true dry air advection and no pattern turnover screams once again for a persistence forecast of higher impact fog development. There is a big difference today which has been the development of some better near surface mixing across the southern cwa with downright warm temperatures and actual surface T/Td depressions of 5 to 8C. However, consensus of the model data including suite of 4km hi resolution output and MET/MAV MOS is again very bullish on fog. Forecast soundings are again supporting a strong signal of near surface saturation below a very low subsidence inversion. With the lack of improvement across the north, the easy call was to issue a dense fog advisory beginning immediately over the Tri Cities and Thumb. More difficult across the south with the clearing, but did begin the dense fog advisory at 00Z tied to sunset. Sunday...Extremely dynamic upper level jet structure and Potential Vorticity Anomaly to scream across the southern US tonight. The anomaly will track east of the Mississippi River Sunday afternoon. This will allow a sprawling, initial surface reflection/low pressure center to lift into the Tennessee River. An impressive midlatitude cyclone that far south, with result in an increased easterly pressure gradient to develop over the central Great Lakes region. Models may have backed off on timing of better east winds a smidge, but expecting the east gradient winds increasingly notably Sunday afternoon. Not a whole lot of impact on the tangible weather with mainly dry conditions expected. The one exception may be for areas immediately downwind of Lake Erie where enhanced near surface lakeshore convergence may yield some heavy drizzle or light rain showers. Sunday night and Monday...Precipitation chances will then increase significantly beginning 03Z or 06Z Sunday, depending on which model runs one favors. Forcing for much of this event will come in the form of midlevel deformation that will shed off and emanate northward away from the upper level low. It is also suggest that few bands of near surface convergence may develop under increasingly cyclonic flow. The big negative early on until Monday morning will likely be a high amount of dry air that need to be overcome or saturate. Forecast models show this to be a tangible struggle. However, a northward drift of the main upper level low is forecasted on Monday which should allow some contraction of midlevel theta e axis. As a result, rainfall is likely for Monday afternoon. Conditions will be noticeably cooler Monday afternoon with wind chill values in the upper 30s. Unsettled weather pattern continues next week, as a series of surface and upper level waves migrate through the region. Chances of rain or rain/snow mixed will generally persist, with a potential for a brief break in conditions Tuesday. Temperatures will remain above average into Thursday, with the warmest temps occurring during the overnight hours. A decent cold front begins pushing through the area during the day on Thursday, which will start the downward trend on temperatures. Temperatures, more in line with the seasonal norm, are expected by the weekend and will persist into the following workweek. MARINE... Light east to southeasterly winds will remain in place through the overnight hours... with the exception of a more southerly component predominant over wesern lake erie south of a slow northward moving warm front. Widespread fog should also continue over most marine areas of Lake Huron overnight. Interestingly, not much fog has occurred over Lake Erie, Lake St. Clair or even Lake Micigan despite dewpoints climbing into the 50s. Likely there is an extremely shallow cold marine layer keeping moisture off the surface. Weak to moderate surface low expected to pass south of the Great Lakes Sunday will gradually back winds to a more easterly direction and perhaps briefly northeast. Wind speeds likely to increase to around 25-30 knots or so by the end of the day Sunday and continue through Monday afternoon. Next low progged to cross Lower Micigan and Lake Huron between 12z and 18z Monday turning wind westerly in the southern lakes Wednesday and northwesterly in the northen lakes by Thursday morning. Speeds should remain just sub-gale for the most part. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Dense Fog Advisory until 10 AM EST Sunday FOR MIZ060>063-068>070- 075-076-082-083. Dense Fog Advisory until 10 AM EST Sunday FOR MIZ047>049-053>055. Lake Huron...Dense Fog Advisory until 10 AM EST Sunday FOR LHZ361>363-421-422- 441>443-462>464. Lake St Clair...Dense Fog Advisory until 10 AM EST Sunday FOR LCZ460. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....DG DISCUSSION...CB/DE MARINE.......CB You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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