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fxus63 kdtx 172342 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
742 PM EDT Wednesday Mar 17 2010 




Aviation... 


VFR conditions will continue through the taf period with a very 
dry air mass in place across the region. Winds will be generally 
light and westerly through the overnight period. However...they 
will begin to increase tomorrow afternoon as the pressure gradient 
tightens and stronger winds mix down to the surface. 


&& 


Previous discussion...issued 402 PM EDT Wednesday Mar 17 2010 


Short term...tonight 


Surface ridge over area will drop south tonight. Very dry airmass 
will remain in place overnight with the sky remaining clear. With 
dewpoints generally in the teens along with clear skies and light 
winds...temperatures should drop sharply after sunset. Guidance has 
been too high on the lows the past couple of nights. Even with 
slight warm air advection aloft...radiational cooling should still 
be great enough to drop lows a few degrees below guidance. Will 
undercut guidance numbers a couple of degrees and see lows range 
anywhere from the upper 20s mainly in sheltered areas to the middle 30s 
in the city. 


Long term...Thursday through next Wednesday 


Upper shortwave currently over Alberta will track east through the 
northern Great Lakes Thursday and Thursday night. Main impact to 
Southeast Michigan will be an increase in west-southwest winds 
tomorrow. With good mixing expected...have continued to forecast 
above MOS...with forecast highs for most locations in the middle 60s. 
Should also mix into a good wind core of 25 knots aloft...with these 
gusts expected to tumble down to the surface in the afternoon. 
Humidities will also fall quite low tomorrow afternoon...with mins 
in the low to middle 20s. 


Upper air pattern over the Great Lakes will start to change on 
Friday as shortwave energy originating over the Gulf of Alaska dives 
through the central rockies and then tracks east across the eastern 
U.S. Meanwhile...shortwave energy currently over the northwest 
territories/Nunavut will dive south and form a fairly deep trough 
extending from the Great Lakes to the Gulf Coast. Models have shown 
a wide range of variability over the past few days with timing and 
placement of both these features...and also vary with how much 
interaction these two systems have. This unfortunately leads to a 
good deal of uncertainty with the forecast by Saturday and Sunday. 


Current idea is for a baroclinic zone to stretch from the departing 
low over the northern Great Lakes back into the Southern Plains by 
Friday. This boundary will slowly push south and east through the 
weekend. Models remain in strong agreement with excellent 
convergence developing along the front over Michigan during the 
weekend...with very cold air pushing down on the north side and warm 
air advection occurring on the south side. This will bring a very 
tight temperatures gradient across central lower Michigan over the 
weekend. Would also expect to see a good push of moisture 
Saturday/Sunday as this system taps into Gulf moisture. 
Precipitation is likely...however how timing...amount...and type are 
still very much in question. 


Model consensus still matches going forecast fairly well...with best 
chances for precipitation starting on Saturday and continuing into 
Sunday. There is a potential for accumulating snowfall...however how 
far south the rain/snow line drops will be extremely dependent on 
where the front settles over the region...with amounts more 
determined by how quickly this feature exits to our southeast. Will 
likely see some changes to the forecast as this system draws closer 
and better upper air sampling of the various shortwaves and jet 
streaks occurs. 


After a prolonged stretch of long dry weather...will see 
temperatures fall for the weekend...with locations on the north side 
of the front (currently thinking the Tri-Cities and thumb) only 
peaking into the upper 30s to low 40s for highs by Sunday. 


Models all want to push a clipper through late Sunday into 
Monday...though some runs are phasing this with the larger system 
ahead of it. Either way...will see gusty winds and additional cold 
air spill in for Monday. This should drop everyone into the 30s for 
highs. 


High pressure is advertised to build over the central Great Lakes 
for the middle-week period. This should warm highs back up to near 
normal by Tuesday. 


Marine... 


Surface gradient will tighten as surface ridge slides to the south. 
This will bring a slight increase in southwesterly flow over the 
lakes. Winds will shift to the north as a weak cold front drops 
through on Friday. A storm system will effect the area Saturday and 
Sunday bringing the possibility of snow over Lake Huron. As this 
system departs...a strong northwesterly flow will develop Sunday 
night and Monday bringing small craft conditions to the nearshores 
and gusts to 30 knots possible over the open waters of Lake Huron. 


&& 


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


&& 


$$ 


Aviation.....Agd 
short term...drx 
long term....hlo 
marine.......drc 




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