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fxus63 kdtx 212321 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
721 PM EDT Thursday Aug 21 2008 


Aviation... 


Expansive middle to high level cloud cover will persist through the 
forecast period. Low-level moisture will slowly increase late 
tonight and during the day Friday as winds finally veer around to a 
more southerly direction. It will likely take the balance of the 
day to moisten the column to support any appreciable shower activity 
near the terminal sites. Therefore...ceilings will remain VFR and 
any visibility restrictions in the neighborhood of showers will be 
very brief. 


&& 


Previous discussion...issued 324 PM EDT Thursday Aug 21 2008 


Short term...tonight 


Return flow around large surface high pressure over New England will 
bring a gradual increase in low level moisture. A band of deeper 
moisture with precipitable waters  of 1.5 to 2 inches along the Mississippi River 
valley will nudge its way to the western counties of the County Warning Area by 
morning as a middle level wave near the Illinois-Iowa boarder shears 
apart as it moves into the western Great Lakes bringing an increase 
in middle to high level clouds overnight. Current radar trends 
shows shower activity across western Kentucky and southern Indiana 
associated with ill defined warm front lifting north and may brush 
our western counties toward day break. Have added a slight chance 
for showers for those counties. Else where...there should be enough 
middle level dry air across the rest of the counties to keep any light 
rain from upper level forcing to evaporate before reaching the 
ground. 


Will lean toward the slightly cooler mav as NAM is bringing too high 
of dewpoints (upper 60s) too quick into the area as flow will remain 
southeasterly most of the night. 


Long term...Friday through Thursday 


Warm...moist southwest flow will drive 850 mb temperatures into the upper 
teens Friday and Saturday ahead of an approaching cold front. With 
the warm air...surface temperatures will warm into the upper 
80s...with a few 90 degree readings possible. 


Precipitation chances will remain in the Chancy category Friday and 
Friday night as convective available potential energy build to around 1000 j/kg in the northern and 
western portions of the County Warning Area. However the best instability and 
forcing from a shortwave will be to the west of the County Warning Area and mainly 
be confined to the afternoon and evening hours with diurnal heating. 
Therefore will taper precipitation chances to reflect this temporal 
and spatial trend. 


The passage of the cold front itself occurs Saturday and Saturday 
night around the time of peak heating. Despite the favorable timing 
and dynamics associated with the front...the GFS/NAM BUFKIT 
soundings show dry air in the low levels and an inverted v from 850 
mb downward. In addition the NAM model shows precipitation drying up as it 
approaches southeastern Michigan while the GFS maintains one quarter 
to half an inch of quantitative precipitation forecast. Given this conflicting information will 
leave the previous forecast as is until a clearer model solution 
emerges. 


High pressure quickly builds in on Sunday...then dominates the 
central Great Lakes into much of next week. Temperatures behind the 
cold front will be below normal to start the week...then moderate to 
just above normal by the middle of the week. 


Precipitation chances on Thursday become an issue as the decaying 
tropical system Fay enters the region. While the GFS takes the 
system into the northeastern US...the European model (ecmwf) takes the system into 
the central Great Lakes with abundant quantitative precipitation forecast. Given the mainly dry 
solution with the GFS ensemble model...will opt to leave Thursday 
dry given the model variability and the movement this tropical 
system will take across the region. 


Marine... 


Southeasterly flow will become southerly Friday ahead of an 
approaching cold front that will sweep through the area Saturday 
night. Large high pressure will follow and keep in control for most 
of next week. Unseasonably cold air during the first part of next 
week will create instability over the lake. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 
Lake Huron...none. 
Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Aviation.....Mann 
short term...drc 
long term....kurimski 
marine.......drc 




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at www.Weather.Gov/Detroit (all lower case).