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Area forecast discussion 
National Weather Service Grand Rapids Michigan 
400 am EDT Wednesday Aug 20 2008 


Latest update...all except aviation... 


Synopsis...(400 am EDT Wednesday Aug 20 2008) 
dry weather with seasonable temperatures will continue through 
Thursday. There is a chance of rain Friday followed by a better 
chance for rain Saturday as a cold front moves in. Dry weather will 
return late this weekend through the majority of next week. 


&& 


Short term...(400 am EDT Wednesday Aug 20 2008) 
(today through Friday afternoon) 


Surface and upper ridging will result in fair and dry weather today through 
Thursday. Time height relative humidity forecasts and the NAM cumulus scheme show very 
little if any cumulus potential today with the dry airmass in place. Maximum 
temperatures will reach the upper 70s to lower 80s and should be a degree 
or two milder Thursday as low level wind fields veer more to the southeast 
to south-southeast. 


Southerly flow will advect a warmer and more humid airmass into our 
County warning forecast area Friday as dew points climb through the 60s. Due to the increasing 
moisture and several shortwaves which will move in from the south-southwest... 
we will stick with low probability of precipitation for a shower or tstorm Friday. Maximum temperatures 
Friday should easily reach the middle 80s at many locations and could 
reach the upper 80s to near ninety degrees at some spots. 


&& 


Long term...(400 am EDT Wednesday Aug 20 2008) 
(friday night through tuesday) 


Our primary focus remains on the potential for meaningful rainfall 
Saturday afternoon into early Sunday morning just ahead of the cold 
front that comes through Sunday morning. Confidence is building on 
the our current solution with this scenario. The short story is the 
best chance of showers and thunderstorms is Saturday evening... just 
ahead of the cold front. The entrance region of the polar jet 
crossing WI at that time helps the cause for convection over SW 
Michigan at that time (sat 21z through 06z sunday). Once that comes 
through another Canadian high builds in Sunday through Tuesday. 
Unlike this current push of cold air... it now seems the polar jet 
does get into the GRR County Warning Area. That will result in a stronger push of 
cold air than we are currently seeing. Thus I lowered highs and 
lows a touch Sunday night through Tuesday. 


As for the convection potential Saturday into Saturday night...good 
and bad news there. The precipitable water values push to near 1.9 
inches. That value is around 175% of normal. Clearly wet enough of a 
significant rain event. The ensemble pop for at least a tenth of an 
inch from 12z Sat through 00z sun... over most of the County Warning Area... shows 
92 percent over the southeast County Warning Area to near 100 percent over most of the northern and 
western County Warning Area. That very much says it will rain. Given the jet 
entrance region will help push the front through there should be 
decent convergence on the front too. Lake temperatures are on the 
warm side...mostly near 21c at Middle Lake. That will help the cause 
some too. The 1000 to 850 mb mixed layer cape is over 1000 j/kg T 
00z Sunday over the western County Warning Area. Another good thing for convection. 
On the other hand... the front comes through around midnight over 
the western County Warning Area and closer to 3 am over the eastern County Warning Area... sort of 
late for afternoon heating to help the cause of convection. Also 
the moisture transport vector convergence is for the most part north 
and east of the GRR County Warning Area (over Northern Lake Huron actually). With that 
the low level jet speed maximum is north and east of the GRR County Warning Area after 
18z Saturday. That means wind speed divergence near the cloud base... 
not so good for convection. Given all of this... I remain on the 
fence with this one. The precipitable water values are high and 
there is jet entrance region lift to support convection.... but 
timing is not so good nor is the low level jet. Thus I will live 
with the 40 to 50 percent pop and call that good for now. 


Now a though on our current dry spell over SW lower Michigan. I just 
computed the percent of days... for each day of the year... over the 
past 115 years...for at least .01 inches of precipitation to 
fall at GRR for each day of the year. As it turned out... from 
the 11th of August through 09th of September is the least likely 
time of the year for measurable precipitation on any given day in 
Grand Rapids. In fact the mean daily pop is around 27% for that 
entire time! Most of June and July it is closer to 35% and the last 
2 weeks of September see it again rising to around 35%. Thus it 
should be of no surprise we are not seeing much rain now in western 
lower Michigan. Over the past 115 years this is the time of year 
that at GRR... measurable precipitation on any given day is least 
likely. Put another way... it is the mostly likely time for it to be 
dry. 


&& 


Marine...(400 am EDT Wednesday Aug 20 2008) 
minimal wave action is anticipated today through Thursday with high 
pressure in control. Wave heights should begin to increase Friday as 
southerly winds increase. 


&& 


Aviation...(1206 am EDT Wednesday Aug 20 2008) 
VFR conditions are expected to persist through the next 24 
hours...from 06z-06z. Skies will remain clear. Winds through the 
period will be easterly (from 070-110 deg) between 5-10 miles per hour. 


&& 


Hydrology...(400 am EDT Wednesday Aug 20 2008) 
we anticipate steady to slowly falling river stages through Friday 
with no significant rain expected until Saturday. Rain on Saturday 
should not be significant enough to result in any Hydro issues. 


&& 


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


&& 


$$ 


Synopsis: Laurens 
short term: Laurens 
long term: wdm 
marine: Laurens 
aviation: Duke 
hydrology: Laurens