Weather

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fxus63 kdtx 280500 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
100 am EDT Thursday Aug 28 2008 


Aviation... 


Recent trends in surface observations over Northwest Ohio indicate a 
period of IFR ceiling expected in the Detroit area early this 
morning, along with some MVFR fog and light rain. The lower clouds 
will push toward the Flint area toward sunrise with some MVFR 
ceiling likely there while mbs remains under VFR. The clouds and 
light rain are from the remnants of "fay" wrapping into Southeast 
Michigan as the system slide through Ohio. Expect the clouds to 
linger through the morning with ceiling slowly rising toward 
afternoon. A return to solid VFR conditions are expected by middle 
afternoon into this evening. 


&& 


Previous discussion...issued 819 PM EDT Wednesday Aug 27 2008 


Update... 


18z NAM/GFS both suggest light rain will back northwest a bit 
further into Southeast Michigan than previously thought. Satellite 
and radar trends seem to support this to some degree. Will update 
to include a 40 percent chance of light rain over the far 
southeast County Warning Area...including Detroit. Will also extend the mention of 
sprinkles to include the northern/western suburbs of the metropolitan 
area as well as much of Washtenaw/Lenawee counties. Even with the 
tropical nature of the system...expect any rainfall totals to be 
light...generally one tenth of an inch or less...since much of the 
moisture surge remains well east of the area. Still expect dry 
conditions through midnight...with a few showers then beginning to 
encroach on the area between midnight and 2 am. 


Previous discussion...issued 317 PM EDT Wednesday Aug 27 2008 


Short term...tonight 


The 12z DTX sounding showed a very dry airmass in place (pw value of 
0.5 inches/50 percent of normal). However...moisture has been on the 
increase (as evident by the current surface dew points in the 50s 
to near 60). Moisture (specifically 925-700 mb) will continue to be 
on the increase tonight as low level southeast winds increase and 
the western fringe of the deeper moisture associated with the 
remnants of Fay settles over the eastern third of the County Warning Area. Based on 
the current extent of the low clouds over the eastern Ohio 
Valley...favor the 12z GFS over NAM...which results in a sharp 
west-east low level moisture gradient. The end result is increasing 
clouds tonight...with still an outside shot of a sprinkle or light 
shower across the far southeast. With the modified airmass and 
clouds...mins should be significantly warmer tonight...middle 50s to 
lower 60s. 


Long term...Thursday through Wednesday 


The weak upper low over the upper Ohio Valley...remnants from ts 
Fay...is forecast to shear apart as it lifts across the eastern 
Great Lakes on Thursday. The main impact to Southeast Michigan from this system 
will be an increase in low and middle level moisture...particularly 
across the eastern half of the forecast area. GFS and NAM do 
indicate some weak isentropic lift brushing the Lake St Clair/Port 
Huron areas Thursday morning. The lift however is really quite weak and 
supports nothing more than a slight chance of showers. Thicker 
clouds across the east should keep maximum temperatures suppressed in the low 
to middle 70s...while temperatures should reach upper 70s farther west where 
some peaks of sun are expected. 


The middle level wave currently over British Columbia is forecast to 
amplify across the northern plains on Thursday ahead of a strong 
upper jet over southern Canada. The wave is then forecast to pass 
through the upper Great Lakes Thursday night...while the associated 
cold front moves across lower Michigan. There continues to be some 
indications in the models of a surface wave developing along the 
front...although the GFS appears to suffer from convective feedback 
and is thus much too strong with this wave. An axis of higher low to 
middle level moisture will accompany the front through Southeast Michigan late Thursday 
night into Friday morning...with precipitable water forecast to reach 
1.8 inches. Despite this...the better large scale lift will pass to 
the north of the forecast area...while the best instability remains 
to the south. Weak low to middle level frontogenesis in the presence of 
ample moisture however will continue to warrant a chance of 
convection late Thursday night into Friday morning. 


High pressure will quickly build into the region in the wake of this 
front on Friday...bringing much drier air back to the region. 
Overall temperatures will remain seasonal through the weekend with 
the dry air allowing plenty of sunshine. 


The medium range model suite continue to show mean long wave trough 
amplification over the western US early next week...with heights 
building across the Great Lakes. This will allow the surface high 
pressure and dry conditions to maintain their grip on the Great 
Lakes through at least the middle of next week. Strong model 
agreement adds a high degree of confidence to this scenario. Temperatures 
will warm to above climatology by early next week...with the dry air 
allowing a large diurnal spread in temperatures. 


Marine... 


Southeasterly winds will increase on Thursday in advance of a cold 
front. The cold front will move across the region early Friday 
morning. Wind gusts around 25 knots are expected on the open waters 
of Lake Huron by Thursday afternoon as the gradient tightens. Winds 
will veer to the northwest on Friday in the wake of the cold front. 
High pressure will build in from the west late Friday through 
Saturday...allowing a steady decrease in winds and waves. 


&& 


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


&& 


$$ 


Aviation.....Bt 
update.......dg 
short term...sf 
long term....SC 
marine.......SC 




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