186 
fxus63 kgrr 182023 
afdgrr 


Area forecast discussion 
National Weather Service Grand Rapids Michigan 
400 PM EDT Friday Jul 18 2008 


Latest update...all sections but aviation... 


Synopsis...(400 PM EDT Friday Jul 18 2008) 
a frontal boundary will be stalled from lower Michigan to the 
plains states over the weekend. Areas of low pressure moving along 
this front will result in considerable cloudiness and periodic 
chances for showers and thunderstorms. 


The front and associated risk of showers and storms is expected to 
exit the area to the south early next week. 


&& 


Short term...(400 PM EDT Friday Jul 18 2008) 
(tonight through Sunday night) 
scattered diurnal convection across northern lower Michigan in the vicinity of 
the frontal boundary may clip the NE corner of the County warning forecast area early this 
evening. Otherwise only very small probability of precipitation warranted for the County warning forecast area until 
perhaps after midnight as weak shortwave seen over Iowa/MO tracks 
this way along southward sinking frontal boundary. It appears the 
southern and western portions of the County warning forecast area have the best potential of being 
affected by this activity. Have toned down the thunder threat a bit 
for tonight... but still left a mention of it in the forecast since 
sufficient instability will be present. 


The most significant threat of showers and thunderstorms exists on Saturday 
and Saturday night when the surface frontal boundary is overhead and a 
decent shortwave moves east from the South Dakota/Nebraska area. 
Severe weather threat appears low at this time due to expected considerable 
cloudiness and limited instability... but if we somehow do manage to 
see a few hours of good insolation that could be a concern. 


On Sunday a rather significant 500 mb trough is prognosticated to dig into the 
region from the northwest. Big discrepancies exist between the NAM and the 
GFS as to where the associated surface wave will track. The NAM has it 
passing well to our north and creates sbcapes over 3000 j/kg while 
the GFS tracks the low across far Southern Lower Michigan and has only a few 
hundred joules of convective available potential energy. Have leaned closer to the GFS solution... 
but should the NAM be on the right track there could be a healthy 
severe weather threat. That/S a pretty potent looking 500 mb wave for this time 
of year. 




&& 


Long term...(400 PM EDT Friday Jul 18 2008) 
(monday through friday) 
we/ll be following the European model (ecmwf) for guidance during the extended 
period. The upper flow will be trending from zonal flow to a broad 
ridge in the western US and a trough in the east. That places the 
Midwest in a dry northwest flow aloft which in turn results in a quiet 
weather pattern. Expect seasonable high temperatures in the lower 80s and 
lows in the upper 50s to lower 60s. Surface high pressure over Hudson 
Bay will provide a NE wind flow through Thursday. This will likely 
cause some upwelling conditions on the lake which will make for 
cooler lake temperatures near the shore. We have a chance thunderstorms and rain in the forecast for 
Friday when the high moves east and warm front moves back through 
the County Warning Area. 


&& 


Marine...(400 PM EDT Friday Jul 18 2008) 
fairly light wind/wave regime expected over the weekend with frontal 
boundary stalled nearby. Winds should pick up out of the north on 
Sunday night behind exiting final/stronger surface wave. 


&& 


Aviation...(200 PM EDT Friday Jul 18 2008) 
radar shows some scattered rain showers developing across the region. At this time believe 
ceilings will remain VFR. Expect some fog to redevelop overnight 
due to high dewpoints. Higher chance of rain will arrive at the end 
of the taf period as a frontal boundary sags south through lower Michigan. 


&& 


Hydrology...(400 PM EDT Friday Jul 18 2008) 
limited Hydro concerns at this time since basin average quantitative precipitation forecast should be mostly 
below one inch. However until this humid air mass and front exits 
the region we could easily get some localized heavy rainfall events 
in excess of 2 inches... possibly producing urban and small stream 
flooding. 


&& 


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


&& 


$$ 


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