471 
fxus63 kmpx 182018 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
318 PM CDT Friday Jul 18 2008 


Discussion... 


Frontal boundary just south of the forecast area this afternoon...meandering 
from southern WI through northern Iowa to eastern NE. Small band of 
convergence from Winona to Albert Lea to Fairmont has yielded a 
few showers. These should be out of the County Warning Area by the start of the 
tonight period. 


The overnight period should be dry. A short wave and strengthen 
low pressure over South Dakota/NE will induce thunderstorms over central South Dakota 
during the early morning hours...with storms expected just west 
of the County Warning Area by daybreak Saturday. The NAM and GFS are quite 
different on the movement of the low on Saturday. The NAM passes 
the low across central Minnesota while the GFS passes the low to the 
south of US. The timing is the same. Jet structure aloft is 
similar. We are in the right entrance region of the jet with 
rather strong differential layer divergence moving across the 
region on Saturday. The tie breaker GOES to the 12z European model (ecmwf) which 
moves the surface wave across northern Iowa. This seems logical with 
the aforementioned boundary to our south. This place the County Warning Area in a 
favorable low/middle level up Glide pattern tomorrow. GFS lapse rates 
are not very impressive with the best instability over far 
southern Minnesota and points southward. The 15z sref precipitation 
probabilities keep US dry through 12z then increase into the 40 to 
50 percent range during the morning over west central Minnesota and then 
into the 50 to 60 percent range during the afternoon and evening 
over eastern and southern Minnesota and west central WI. Have indicated 
high chance probability of precipitation developing tomorrow and continuing for a time 
Saturday night. Seems the best threat for severe weather Saturday would 
be over southern Minnesota...which agrees with the GFS differential 
temperature advection scheme. How strong the low level jet remains 
in the morning as it enters our area will be a key. 


A dry start to the day on Sunday in the wake of the surface wave 
but another short wave and surface low is indicated to follow 
this one Sunday night and Monday with conditions much like what we 
are expecting for Saturday and Saturday night. Hence...probability of precipitation back 
in again for these periods. 


Great collaboration today with surrounding offices about the 
Tuesday through Friday period. Preepd sparked the discussion and 
led US to put more emphasis on the GFS ensemble...with ridging 
over the region...over the 00z European model (ecmwf) which was trying to drive a 
closed upper low and surface system through. The new 12z European model (ecmwf) is 
now backing away and much more supportive of dry weather. 
Agreement was to leave the Tuesday through Friday period dry 
instead of indicating below climatological probability of precipitation. 


&& 


Aviation... /18z taf issuance/ 


IFR clouds have finally scattered out at Keau while broken conditions 
around 2000-2500 feet have quickly filled in again with heating. 
Similar conditions extend into SC Minnesota but krwf and kmsp are north 
of this area...with only some scattered cumulus elsewhere. Winds will 
gradually shift around from north to east by Sat morning. Light 
winds and residual moisture could mean more fog at Keau late 
tonight and have included this in the taf. Significant difference 
between NAM and GFS re storm potential for Saturday. More on this 
in the afternoon discussion. For now have brought in middle clouds 
during the middle morning at most sites...and chance of precipitation at 
krwf. ... 


&& 


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


$$ 


Rah/tdk