552 
fxus63 kdtx 082008 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
408 PM EDT Friday Aug 8 2008 


Short term...tonight 


Scattered clouds over Southeast Michigan will begin to dissipate as 
we pass the peak heating period into early evening. A nice 
evening is in store with temperatures dropping into the 50s 
overnight...and have gone just a degree or two below guidance which 
is showing dewpoints running a few degrees higher than they actually 
are. Cloud cover will begin to increase from north to south late 
tonight ahead of the strong upper low seen over northern Ontario. 
Winds through 700mb will back around to the west allowing for weak 
Theta-E advection preceding this wave. Little in the way of showers 
is expected until after 12z but will leave the going slight chance 
of rain in the forecast across the Saginaw Valley/northern thumb as 
moisture increases in an area of increasing cyclonic flow. 


&& 


Long term...Saturday through next Friday 


The first concern in the forecast for Saturday will be thunderstorm 
intensity. Indications for potentially severe storms remain in the 
12z model runs with the main question being how much boundary layer 
moisture can recover during the afternoon hours. There is certainly 
support for strong forcing given the appearance of the upper low 
on satellite today, leaving the degree of instability as the main 
uncertainty. Even that looks adequate with a few lower 60 surface 
dewpoint observations in the Midwest this afternoon. Model 
forecast soundings suggest upper 50s will be a more likely surface 
parcel due to mixing of the shallow boundary layer moisture, but 
even a 78/58 parcel generates nearly 1200 j/kg mixed layer cape. 
That might be enough to hold up against the increasingly long 
hodograph that will develop during the afternoon and could result 
in some linear modes of organized convection to go along with 
numerous shower coverage spreading from north to south over the 
area. 


After some lingering showers Saturday evening, the most notable 
feature of the upper low will be the chilly boundary layer air mass 
that is pulled down over Southeast Michigan by Sunday morning. Daytime 
heating will then result in a broken cloud deck by afternoon with a 
few instability showers possible during peak heating. Given that 
model 850 mb temperatures look similar to this afternoon, plan to 
keep high temperatures in the lower 70s at most locations, 
although some upper 60s will be possible in The Thumb where cloud 
coverage will likely be greater. 


After a break in the rain action on Monday, and some moderating 
temperatures, a return to a more challenging pattern will take place 
during Tuesday and Wednesday. The persistent large upper level 
trough over the North Pacific is expected to deepen further during 
the middle week period. This will ensure that the long wave ridge 
remains in place over western North America and will force the 
smaller scale but energetic low pressure features to find their way 
into the corresponding long wave trough over the eastern states. 
Looking at the large scale water vapor imagery today removes all 
doubt regarding the strength of these smaller circulations. That 
being the case, the latest global models seem to have a reasonable 
handle on the situation by allowing the Pacific northwest upper low to move 
inland under the ridge during the middle week period while keeping the 
Great Lakes under the influence of northwest flow aloft and energy coming 
down from northern Canada. There will be day to day timing 
differences in the pace of the wave moving under the ridge and into 
the Great Lakes but the predictability of the northwest flow aloft and any 
associated northern Canadian energy will be a more stable part of 
the forecast. This will be associated with the position of the long 
wave trough over the eastern U.S., Which will be pretty stable in 
the models. That being the case, we can expect a chance of rain 
both Tuesday and Wednesday, along with temperatures lingering on 
the low side of normal. After that, we will look for dry 
conditions around the Thursday and Friday period followed by the 
central U.S. Energy making an appearance in the Great Lakes by 
the weekend. 


&& 


Marine... 


Cool and unstable northwest winds over the marine areas will 
diminish tonight and allow wave action to subside for a few hours. 
Wind coming up from the west again during Saturday morning will then 
build some waves ahead of the next cold front with small craft 
advisories likely needed again by Saturday night. Another surge of 
colder air will produce a longer period of rough marine conditions 
lasting through Sunday night. The air will also be cold enough to 
make waterspout formation possible over the open waters of Lake 
Huron. 


&& 


Previous discussion...issued 148 PM EDT Friday Aug 8 2008 


Aviation... 


VFR conditions are expected through the taf cycle with scattered clouds 
around 5000 feet this afternoon...clearing out after sunset tonight. 
North to northwesterly winds will be breezy this afternoon with 
winds gusting to around 20 knots. Winds will decrease this evening 
and be light and variable overnight as high pressure builds into the 
upper Ohio Valley. Scattered showers and thunderstorms will approach 
from the north towards the end of the taf cycle on Saturday. 


&& 


DTX watches/warnings/advisories... 


Michigan...none. 


Lake Huron... 
Small Craft Advisory...nearshore waters from Port Austin to Port 
Huron including Saginaw Bay...until 10 PM Friday. 


Lake St Clair...none. 


Michigan waters of Lake Erie...none. 


&& 


$$ 


Short term...kec 
long term....bt 
marine.......bt 
aviation.....Kec 




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