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fxus63 kdtx 200755 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
355 am EDT Wednesday Aug 20 2008 


Short term...today and tonight 


A middle level ridge axis is forecast to build across the state 
today...allowing the large surface high now centered over Lake Huron 
to slowly move toward the northeast US. The 00z radiosonde observations showed a very 
dry airmass in the lower to middle troposphere across the eastern Great 
Lakes. This dry air will actually advect westward across lower Michigan 
today as the anticyclone slides to the east...maintaining clear 
skies through the short term. After a very cold start to the 
day...temperatures should rebound a good 25 degrees as the airmass 
begins to modify under full daytime sunshine. Current forecast highs 
of upper 70s look reasonable...with cooler readings near Lake Huron 
due to light easterly flow. Temperatures will again drop off quite 
quickly after sunset. A tighter southeasterly gradient over Southeast Michigan 
tonight will however prevent temperatures from dropping as low as this 
morning. Thus plan to just undercut guidance by a degree or two 
tonight...resulting in forecast mins of upper 40s/lower 50s outside 
of metropolitan Detroit and middle/upper 50s within the metropolitan area. 


&& 


Long term...Thursday through Tuesday 


Upper ridge axis will shift east on Thursday allowing a southerly 
flow regime to begin setting up over Southeast Michigan. Upper 
Cutoff low spinning over northeast Texas will phase back into the 
northern stream...helping to push a very weak warm front up through 
Southeast Michigan Thursday night as it lifts up through the corn 
belt. Moist isentropic ascent does not look overly impressive as 
this feature lifts northward and low level jet will remain well to 
our west over Wisconsin. Looks like it will just be a stable warm 
air advection pattern...but strong Theta-E advection is a little 
concerning. May see some virga as showers dry to develop...but dry 
low-levels should overcome this. Will increase cloud cover but keep 
the forecast dry. 


Models continue to indicate weak shortwave features moving through 
the southwest flow over Michigan on Friday and Friday night...but 
keep the main shortwave trough (remnants of cutoff) well to our west 
over MN/WI. Southeast Michigan will remain within the warm and moist 
sector...with only low precipitation chances as no strong focusing 
mechanism looks to be present and instability looks weak as forecast 
soundings show a layer of warmer air present in the middle layers. 
Upper level low still on track to push across Ontario/Quebec over 
the weekend. Best chances for precipitation will be Saturday 
afternoon and evening ahead of the associated surface trough axis as 
it pushes through Southeast Michigan. Instability continues to look 
meager per model soundings...and will keep probability of precipitation at chance with no 
real concern for severe weather. Main frontal passage will occur on Sunday per 
GFS/Euro...with fairly strong cold air advection expected late 
Sunday into Monday. Canadian high pressure will then build once 
again over the region behind the front on Monday...and possibly hold 
through Tuesday as well...to bring dry but cooler weather. 


&& 


Marine... 


Strong surface high pressure will move from Lake Huron this morning 
to the northeast US by Thursday morning. The high will then remain 
anchored off the middle Atlantic coast through the end of the 
weak...maintaining overall light southeasterly winds across the 
Great Lakes. Winds will turn more southerly and increase in speed 
this weekend as a cold front approaches from the west. 


&& 


Previous discussion...issued 104 am EDT Wednesday Aug 20 2008 


Aviation... 


VFR conditions will predominate throughout the forecast period. Dry 
air will help to counter the excellent radiational cooling overnight 
to limit any fog formation. Do not expect visibilities to drop below 
6sm or so. Was tempted to remove mention of fog altogether...T-TD 
spreads have already dropped to around 5f north of I 69. Winds will 
remain insignificant. 


&& 


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


&& 


$$ 


Short term...SC 
long term....hlo 
marine.......SC 
aviation.....Dg 




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