796 
fxus63 kgrb 200837 
afdgrb 


Area forecast discussion 
National Weather Service Green Bay WI 
337 am CDT Wednesday Aug 20 2008 


Short term...today through Thursday...small precipitation chances and temperatures 
are the main forecast concerns. 


Expect high pressure centered over the eastern Great Lakes to gradually 
shift east over the next few days as a cold front edges into the 
northern plains. Increasing return flow will result in a gradual 
increase in Gulf moisture. 


Isolated thunderstorms developed yesterday after/evg from msp into SW WI...within an 
instability axis. This instability axis will set up across c/NC WI 
this afternoon...where modified forecast soundings show convective available potential energy 
approaching 1000 j/kg...with little or no convective inhibition. 
There is a warm layer from 650-600 mb...and low level convergence is 
expected to be weak...so have opted to leave the dry forecast intact. 
That being said...would not be surprised to see isolated thunderstorms over 
our far wstrn counties during the late afternoon/early evg hours. 
Mixing through 850-800 mb supports high temperatures in the lower to middle 
80s...expect 70s near the lake...due to southeast winds. 


Tonight...expect most of the clouds to dissipate after sunset. 
Despite clear to partly cloudy skies and light winds...don't expect 
much fog to develop due to recent dry conditions and 925 mb winds 
increasing to around 20 kts. Stayed close to mav guidance for 
min temperatures. 


On Thursday...models agree that the deeper Gulf moisture will 
advect into our SW counties during the afternoon. Some weak 
forcing associated with a short wave trough and the rrq of a 50 knots jet streak 
may provide enough forcing to trigger a few storms over central 
WI...so have added a small pop for the afternoon. Little change 
in the low level thermal profile is expected...so expect maximum temperatures to 
remain in the lower half of the 80s away from the Lake. 


Long term...Thursday night through next Tuesday. Main forecast focus remains on 
precipitation/convective chances during the Thursday night through Sat time frame. 


The combination of a surface hi pressure over the western Atlantic and a deep 
upper trough moving into the northern rockies will bring a deep southerly flow 
from the Gulf Coast to the western Great Lakes Thursday night. An old middle level 
wave will be brought north into the Midwest while Gulf moisture streams 
north into WI. Precipitable water values are prognosticated to rise into the 1.5 to 2.O 
inch range by 12z Friday. There is some lift indicated...mainly from 
a jetlet passing through Ontario...but overall the forcing aprs 
rather weak. Prefer to raise probability of precipitation into the low end chance category 
over southern sections of the forecast area Thursday evening adn over northern WI after 
midngt as deeper moisture spreads north. Thickening clouds/precipitation chances will 
prevent temperatures from falling much Thursday night with readings remaining in 
the 60s. 


As the upper trough shifts east into the northern plains...remnants of the 
middle level wave reaches WI with a persistent SW flow bringing more 
moisture from the Gulf. While instability aprs marginal (capes around 
500 j/kg...lifted indice's around zero) and lift is weak...there should be 
enough forcing present to warrant chance probability of precipitation across the entire forecast 
area. Temperatures may be Dicey depending on the extent of precipitation. May need 
to temperature readings a degree or two from the previous forecast. 


Some of the models (sref/ecwmf/gfs) are hinting at a possible break in 
the precipitation for a time Friday afternoon into Friday evening between the now 
defunct middle level wave and the approaching upper trough/associated surface 
cold front. Since timing is the Big Key to the forecast...prefer to 
diminish probability of precipitation to only slight chance for now and if trends continue to point 
in this direction...later forecasts can drop the precipitation. That being said... 
expect the next round of showers and thunderstorms to move into most of WI 
later Friday night as the cold front pushes into WI. 


The progressive upper trough moves into the western Great Lakes on Sat and 
helps sweep the cold front through the rest of WI. Consistent signal from 
the models is that shower/thunderstorm activity will diminish over central/far NE 
WI by Sat afternoon as drier...more stable airmass builds into the 
region. Have tried to indicate this in the zn/grids. In fact...expect 
precipitation to be on the wane over east-central WI as the afternoon wears on as 
well. Have lowered maximum temperatures a bit on Sat as modest cold air advection takes over. 
Canadian hi pressure builds southeast into the western Great Lakes by Sunday and 
should provide for mostly sunny skies and comfortable conditions 
throughout NE WI. Even with somewhat cool temperatures at 800 mb (+8c)...maximum 
temperatures on Sunday should remain at or sligtly above normal. 


Upper heights build across the plains/Midwest early next week as a new 
upper trough digs into the northwest Continental U.S.. the hi pressure is forecast to remain in 
our general vicinity through Thursday...so have kept the Monday/Tuesday time frame 
dry. Caveat to all this is that moisture from Tropical Storm Fay 
remains to our east. Temperatures to remain at or above normal through the 
first part of next week. 
&& 


Aviation...expect MVFR/IFR ceilings over c/NC WI to improve to VFR 
by middle-late morning as daytime mixing causes cloud bases to rise. 
Patchy fog will dissipate shortly after sunrise. Expect mainly 
scattered cumulus this afternoon...especially over c/NC WI. Tonight...VFR 
conditions should prevail in most locales. Boundary layer winds 
should increase enough to prevent fog from developing...however 
patchy MVFR ceilings cannot be ruled out. 
&& 


Grb watches/warnings/advisories... 
none. 
&& 


$$ 
Kieckbusch/kallas