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fxus63 kdtx 211701 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
1201 PM EST Sat Nov 21 2009 


Aviation... 


Remnant low level moisture with ill defined low level inversion will 
make for a difficult taf forecast...as the debate will be low 
stratus vs fog as calm/light winds promote good radational cooling 
tonight. Slow drying of the column this afternoon should allow for 
partial clearing...but boundary layer cooling when the sun sets 
will allow for either MVFR vsby's or ceilings quickly 
redeveloping...with either IFR vsby's or ceilings expected 
overnight with at least a strong surface based inversion setting 
up as general subsidence will be the rule with anti-cyclonic flow 
building in. Southeast flow off Lake Erie will make dtw especially 
susceptible for possible dense fog or low stratus tomorrow 
morning. As typical for this time of year...fog and stratus will 
likely take all morning to burn off/dissipate with the light winds 
and limited mixing. 


&& 


Previous discussion...issued 340 am EST Sat Nov 21 2009 


Short term...today 


Very little change today as cloudy skies persist most if not all of 
the day. The airmass has been able to modify...so temperatures will 
top out in the low/middle 50s -vs- Friday/S around 50. There will be a 
better chance at a few breaks in the clouds during the afternoon but 
with sunset just after 5 PM...the window will close pretty quickly. 
No significant changes were needed to the going forecast. 


Long term...tonight through next Friday 


The concerns in the extended forecast revolve around the potential 
for fog development Sunday and Monday mornings and then the 
development of the low pressure system that will head for the area 
around Thanksgiving. 


A weak area of high pressure will slide east of the region tonight 
giving our wind field a slight easterly component. This moist flow 
off Lake Erie will act to reinforce the already moist low levels. 
Fairly high surface dewpoints in the upper 30s to low 40s will be in 
place during Sunday and Monday mornings. Nambufr soundings show a 
very strong inversion forming between 925 and 950mb which will trap 
this moisture at the surface. See no reason not to continue 
inherited forecast mention of fog for both these mornings. 


Temperatures look to remain slightly above normal this weekend 
through the early part of the week...44/29 fnt and 46/32 
dtw...though there will be a gradual decrease each day along the 
way. Temperatures will then fall to at or below normal values as we feel 
the effects of the next low pressure system around Thanksgiving. 


The 00z model suite continue to show disagreement with the 
development of the middle week low as their prior runs have done all 
along as well. The GFS develops a low over the northern plains early 
in the week and shows hints of cutting off the low but a fast moving 
shortwave behind the initial low acts to reinforce the longwave 
trough and never let it get fully cutoff. This solution keeps things 
quite progressive meaning precipitation moves in faster along with a cold 
Canadian airmass. On the other hand you have the European model (ecmwf)/NGM/dgex all 
have the low becoming more cutoff and located further south. This 
leads to a slower progression and holds off precipitation an extra day for 
the region. One things the models have in common is the cold air 
this system will bring down to Michigan. 850mb temperatures will fall from 
their current value of around +4c down to -5c...and if the GFS pans 
out possibly -10c. This equates to maximum temperatures only in the low 40s and 
lows in the middle 20s. This cold airmass will lead to a rain/snow mix 
Thursday and possibly all snow on Friday on the backside of the 
passing low. 


Marine... 


Winds will be light today as high pressure builds over the area. 
This high will remain in place into Sunday and then only slowly 
progress to the east early next week...allowing a very gradual 
increase in southeast flow from Monday into Tuesday. The pattern 
becomes active again middle/late next week as a strong shortwave and 
associated jet streak dig into the upper Midwest and lead to the 
development of a large upper low over/near the region. This time 
frame will bring the next period of headline worthy winds/waves. 


&& 


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


&& 


$$ 


Aviation.....Sf 
short term...dg 
long term....drk 
marine.......dg 




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