763 
fxus63 kgrr 211940 
afdgrr 


Area forecast discussion 
National Weather Service Grand Rapids Michigan 
330 PM EDT Thursday Aug 21 2008 


Latest update...all but aviation... 


Synopsis...(330 PM EDT Thursday Aug 21 2008) 
high pressure will move away from the region tonight which will 
allow for an area of low pressure to move up from the south. 
Clouds will be on the increase tonight and some showers and 
thunderstorms will arrive by Friday. The warm and unsettled pattern 
will persist until Saturday night when a cold front tracks through. 
Dry weather will return for Sunday. 


&& 


Short term...(330 PM EDT Thursday Aug 21 2008) 
(tonight through Saturday night) 


The deep moisture will advect in from the south tonight. Some 
diurnal showers or thunderstorms will try to move into the South 
Haven area early this evening. I added some probability of precipitation down there for 
that. A better area of elevated instability shows up around 
12z...and builds through Friday am. A low level jet is shown to lift 
northward through Lake Michigan Friday am and could enhance some of the rain 
here in SW lower Michigan for Friday am. I am not totally sure this will 
happen...but the wetter NAM initialized the upstream moisture the 
best. The sref probability of precipitation also support higher probability of precipitation on Friday here in SW 
lower Michigan. I will continue to feature higher probability of precipitation on Friday. If the 
NAM ends up right...stronger convection is certainly possible. Cape 
values exceed 2000 j/kg and middle level winds at this point are around 
35 kts. I would certainly monitor the situation closely...especially 
as the low level jet nears. 


It should end up rather warm Friday and Sat...although clouds may limit 
the temperatures somewhat. Will continue to feature well above 
normal values. The cold front tracks through most likely Sat 
night. Ahead of the front in the unstable airmass some showers and 
thunderstorms could develop on Saturday and persist until the front 
tracks through. 




&& 


Long term...(330 PM EDT Thursday Aug 21 2008) 
(sunday through thursday) 
forecast concerns during the extended period are minimal as we 
expect mostly dry weather. 


Sunday will start off on the cloudy side as the cold front will be 
exiting the southeast County Warning Area. Kept a slight chance of precipitation across the southeast but most 
locations will be dry. High pressure then builds south and lasts 
into middle week. Then the uncertainty begins. A strong wave moving 
across the northern tier of states and southern Canada will push a 
cold front east. Height falls ahead of the front may help to draw 
up the remnants of Fay. The European model (ecmwf) is a lot more aggressive with this 
scenario...but even the GFS moves the remnant low north. Not sure 
I/M ready to buy this yet but the cold front should generate thunderstorms and rain 
Thursday ad it encounters and increasingly warm and moist airmass. 
If the remnant low does indeed move northward toward lower Michigan...we may 
need to add probability of precipitation to the Wednesday night grids. 


&& 


Marine...(330 PM EDT Thursday Aug 21 2008) 
I did feature some higher waves in the nearshore mainly Friday night 
into Saturday as southwest winds will increase. Thunderstorms will 
continue to be mentioned in the forecasted for tonight into Saturday. 


&& 


Aviation...(130 PM EDT Thursday Aug 21 2008) 
low pressure was moving north through western Illinois/IA/MO at this time. Middle/hi clouds 
will slowly spread north into the County Warning Area this evening. With the northward 
movement of the low...rain will be slow to spread east. 18z tafs are 
dry with VFR conditions through the period. 


&& 


Hydrology...(330 PM EDT Thursday Aug 21 2008) 
locally in excess of an inch of rain could fall before the cold 
front comes through Saturday night. Any issues should be limited to 
poor drainage runoff. 


&& 


GRR watches/warnings/advisories... 
Michigan...none. 
Lm...none. 


&& 


$$ 


Synopsis: mjs 
short term: mjs 
long term: 93 
marine: mjs 
aviation: 93 
hydrology: mjs