Weather

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Area forecast discussion 
National Weather Service Grand Rapids Michigan 
149 PM EDT sun Sep 7 2008 


Latest update...aviation... 


Synopsis...(330 am EDT sun Sep 7 2008) 
an upper level disturbance will result in some showers this morning 
which will taper off this afternoon. A low pressure system will 
develop and track northeast and bring US our best chance of rain and 
some thunderstorms from late Monday through Monday night. Dry and 
cool weather is anticipated Tuesday behind the cold front after a 
few lingering morning showers. 


&& 


Short term...(1130 am EDT sun Sep 7 2008) 
(this afternoon through Tuesday afternoon) 


Have updated to include a mention of thunder in the forecast this 
afternoon... mainly north of Interstate 96. Dew points are lingering 
in the middle to upper 50s and chilly 500 mb temperatures around -20c have 
overspread the region. We are currently seeing a fair amount of 
insolation behind the departing wave which brought the morning 
rain... and moderate to towering cumulus is already developing on 
the lake breeze front. Convective available potential energy are currently near 1000 j/kg with 
lifted indices of -3 to -5. There is some positive vorticity advection at 500 mb to work with and 
a front approaching from the northwest. 


Believe some scattered storms will form along the lake breeze front 
by 18z and move inland. Additional scattered showers and storms may 
arrive from the northwest later this afternoon ahead of the approaching surface 
front. Some small hail cannot be ruled out considering the cold air 
aloft. 


Showers will taper off tonight with loss of diurnal component and 
passage of cold front. Our next big weather maker will move in late 
Monday and Monday night as low pressure tracks NE into our region. 


A time height x-section from WRF guidance shows strong omegas on the 
order of 10 to 20 microbars/second Monday night. Model guidance also 
shows strong fgen and vigorous middle level f-vector convergence 
oriented SW to NE across our County warning forecast area. We will also get into vigorous 
positive vorticity advection as the primary shortwave amplifies as it tracks east into our 
area with impressive potential vorticity as well. 


Therefore rain with some convection is likely Monday night. We 
raised probability of precipitation slightly (to 70 pct) and strongly considered going 
categorical. Future shifts may need to continue this trend of 
ramping up probability of precipitation a bit based on 12z guidance and model trends. 


Convective potential at first glance seems quite limited by several 
factors including (most notably) limited instability. However... a 
rather sharp baroclinic zone is noted with the front and a vigorous 
low level jet will develop Monday evening... with our County warning forecast area potentially right 
on the nose of it. Both speed and directional wind shear should 
become quite strong. 


Therefore... I would not be surprised if future shifts need to ramp 
up discussion of convective potential Monday evening. We feel that 
in spite of limited instability we may have to deal with some strong 
to potentially severe convection given strength of the front and 
wind fields with height... definitely a tricky forecast. 


Lingering showers across our eastern County warning forecast area Tuesday morning should 
taper off fairly quickly as brisk northwest winds behind the cold front 
advect much cooler and drier air in. Given the magnitude of cold air advection 
anticipated... we lowered the maximum temperature forecast for Tuesday just a bit. 


&& 


Long term...(330 am EDT sun Sep 7 2008) 
(tuesday night through saturday) 


The main challenge in the long term deals with the temperatures for 
the Tuesday to Wednesday timeframe...along with the potential for some 
rain Friday night into Saturday. Strong high pressure arrives for 
Tuesday night. This system has been trending stronger with each 
run. This high will dry our airmass out and also draw down cooler 
air from Canada. As a result I lowered temperatures several degrees 
for the Tuesday night into Wednesday night timeframe. The GFS shows a 
wave of low pressure bringing some rain to the area Friday night into 
Sat. However the European model (ecmwf) is slower with this system and still 
supports dry weather to start the weekend. As we currently have a 
dry forecast going for Friday night into Sat and the European model (ecmwf) supports 
this scenario...I will maintain the dry weather. 


&& 


Marine...(1130 am EDT sun Sep 7 2008) 
latest model guidance is increasing the winds with the passage of 
the cold front this evening... and it looks like waves may build to 
Small Craft Advisory criteria tonight. Have updated the nsh forecast to 
reflect this thinking. 


Lake Michigan will likely turn quite choppy Monday night and Tuesday as 
strong northwest flow cold air advection develops behind the front. Rain and thunderstorms 
will also pose a hazard to mariners Monday afternoon into Monday 
night. 


&& 


Aviation...(149 PM EDT sun Sep 6 2008) 
band of isolated showers that initially developed along the lake 
breeze earlier this afternoon is making quick progress across the 
area. In the wake of the line of isolated showers...stabilizing 
effects from Lake Michigan and the short wave moving east/NE will end 
the threat. The threat for shower activity has pretty much ended 
already at all of the terminals as of 18z except kjxn and klan. The 
threat will end there by 19z per satellite trends. Few-scattered cumulus will 
then be expected through sunset. 


Middle clouds will start advecting in late tonight/tomorrow morning 
ahead of the next system. Increasing winds aloft and some drier air 
moving in temporarily will keep the fog threat tonight rather low. 
Precipitation with the next system looks to spread across the terminals 
toward 00z Monday with MVFR conditions likely and IFR conditions 
possible. 


&& 


Hydrology...(330 am EDT sun Sep 7 2008) 
the system on Monday night has the potential of producing rainfall 
amounts of a half an inch to an inch. A few urban and small stream 
type Hydro issues could develop in that time frame and linger into 
Tuesday morning. 


&& 


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


&& 


$$ 


Synopsis: Laurens 
short term: Meade 
long term: mjs 
marine: Meade 
aviation: njj 
hydrology: Laurens 

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