599 
fxus63 kmpx 061741 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
1241 PM CDT Sat Sep 6 2008 


Update...for 18z aviation discussion below 


&& 


Discussion.../issued at 333 am CDT Sat Sep 6 2008/ 


Fall like weather will continue for a few days yet but there is 
moderation on the horizon. Models in pretty decent agreement on the 
overall evolution of the upper air pattern through the middle of 
next week. Early morning water vapor imagery shows broad troughing 
still in place over central noam with ridging over the western 
Atlantic and over the eastern Pacific. A series of shortwaves move 
through the trough over the next few days bringing some chances of 
showers and isolated thunderstorms and cool temperatures. The ridge 
over the Atlantic flattens Monday with weak ridging building in over 
the County Warning Area and upper Great Lakes for Tuesday. A sharp trough enters the 
Pacific northwest as this occurs with surface cyclogenesis spawning in the 
Lee of The Rockies. The upper trough and associated surface low will 
pass the area allowing upper ridging to become established over the 
west. 


Some details... 
one well defined shortwave which passed last evening continues to 
move to the east of the area. Skies have generally cleared with some 
lower level stratus/stratocu remaining in the eastern County Warning Area and fog 
developing in spots. Many areas will see sunshine this morning 
before clouds increase by afternoon. GOES water vapor imagery shows 
a weak circulation over eastern North Dakota which will cross 
northern Minnesota this morning with the tail of this vorticity maximum 
and associated trough cross the northern County Warning Area this afternoon. 
Features are pretty weak so it will be tough to get any precipitation 
going in the dry atmosphere but with coldest 500 mb air positioned 
across the north this afternoon with lapse rates steepening there 
may be enough support for some isolated showers. Therefore have 
added slight probability of precipitation across the far north this afternoon but not 
terribly confident in these developing. With some sun through at 
least the first part of the day highs should reach the upper 60s 
to around 70. 


A better organized middle level trough swings eastward towards southern 
Minnesota this evening. A weak surface low accompanies the low with a 
cold front coming along with it. Isentropic ascent increases over 
southwestern Minnesota this evening as the trough axis advances east. The 
bulk of the rain tonight looks to fall over the southern County Warning Area but the 
entire County Warning Area has a good chance for precipitation Sunday as the upper trough 
axis and cold front pass...especially the east half by Sunday 
afternoon. Chilly air will return with highs only around 60 thanks 
to the clouds and rain. High pressure noses in from the west Sunday 
night in the wake of the upper trough/cold front. Clearing conditions are 
expected and it looks as though strong radiational cooling 
conditions will set up across west central Minnesota with a better opportunity 
for it across the entire County Warning Area Monday night. 850 mb temperatures drop to around 
+2 by Monday afternoon leaving highs only in the lower to middle 60s. 
Upper troughing will still be in place Monday and a reinforcing 
shortwave dives southeastward into the Central Plains with a surface 
inverted trough tracking across the Missouri and middle Mississippi 
River valleys. Isentropic lift and band of frontogenetical forcing 
will be focused south of the area with middle level baroclinic zone set 
up across central Nebraska and Iowa so am not expecting probability of precipitation at this 
time in the local County Warning Area. Will need to watch for this system to creep 
further north and affect areas along Interstate 90. Ridging will be 
overhead for Tuesday morning as mentioned previously allowing for a 
chilly night and perhaps some patchy frost in the northeast. 


Weak upper ridging will build in briefly for Wednesday. The surface high 
will shift to the east allowing southerly flow to return. A strong 
upper trough will dig into the northern U.S. Rockies at that time 
with surface cyclogenesis developing in the Lee of The Rockies. 850 mb 
temperatures warm back into the double digits as warm advection commences 
late Tuesday night bringing at least some middle level clouds and 
potentially some sprinkles/light showers. A better surge of warm 
advection moves northward Wednesday afternoon with the GFS showing a 
lead shortwave passing ahead of the main trough axis. Precipitable 
water values increase markedly to 1.5 inches or better Wednesday so 
decent rain amounts look favorable at this time from Wednesday into 
Thursday. ..mdb.. 


&& 


Aviation.../18z taf issuance/ 
scattered-broken cumulus clouds and west/southwest winds will prevail at 
the taf sites this afternoon. Most of the clouds will be of the 
VFR nature...with the exception of west central WI...where 
occasional broken ceilings circa 2500 feet will be possible. A 
shortwave was generating rain showers over eastern South Dakota at taf 
issuance time. While the general trend is to diminish this area of 
rain with eastward progression...a few stray showers may hold 
together across west/south central Minnesota this afternoon/evening. Have 
included a vcsh at krwf. Another impulse tracks across west/north 
central Minnesota overnight...and may clip kaxn toward daybreak...where a 
prob30 was maintained. Condensational pressure deficits also 
indicate that MVFR or low-end VFR ceilings may accompany this wave. 
Another shortwave...accompanied by a surface trough...pushes 
across the area on Sunday. Model solutions tend to favor east 
central Minnesota and west central WI for higher rain chances. Expect 
scattered rain showers to begin affecting kmsp/krnh/Keau after 15z 
Sunday. 




&& 


Mpx watches/warnings/advisories... 
Minnesota...none. 
WI...none. 
&& 


$$ 


Ls/mdb