013 
fxus63 kfsd 160156 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
855 PM CDT Wednesday Oct 15 2008 




Discussion... 
the bottom is falling out of the temperatures this evening. Light 
winds coupled with a dry airmass is allowing for fast drop in 
temperatures. Considering the fast drop...dry air in place...and 
very light winds later tonight...lower lows a few degrees...but 
otherwise everything on track. Freeze warning continues for tonight 
along and east of the James Valley. Cloud deck will move into 
the SW County Warning Area later this evening and overnight keeping temperatures 
above freezing with some rising temperatures possible as clouds move 
in. 




&& 


Aviation... 
VFR conditions will prevail through 18z Thursday. 


&& 


Previous discussion... 
concern for the initial period is primarily temperatures and the 
associated frost/freeze threat. Ridge of surface high pressure across the 
Western Plains will gradually work ewrd into the i29 corridor by late 
tonight. Airmass is quite dessicated this afternoon...with mixed layer 
dewpoints in the lower to middle 20s over eastern South Dakota and southwestern Minnesota...and 
working down toward the upper 20s across northwestern Iowa and northestern Nebraska. 
Little moisture to interfere with efficient radiative conditions 
with incipient profiles...once rapid decoupling commences toward 
sunset. However...eyes on enhancing area of middle clouds within 
warm air advection/frontogenetic zone across southwestern South Dakota which begin to work gradually 
ewrd overnight. Clouds should start to increase during the evening 
hours through scntrl areas...building slowly net ewrd as some of the 
earlier clouds erode on the drier side. Clouds will be in early 
enough to prevent temperatures across the southwestern portions of the County Warning Area to fall below 
freezing. Elsewhere...esply khon and kbkx areas down the i29 
corridor...will be in the prime balance of gradient/clearing to take 
temperatures down into the upper 20s. Farther east...just enough weak gradient 
for much of the night before falling off late. Will issue a freeze 
warning for areas along and east of the James Valley for tonight. 


On Thursday and Friday...this area is marked by a slow progression eastward of 
a large surface ridge. This will give light winds. We still have the short wave 
energy moving through this area Thursday night and early Friday. The 12z NAM is 
much stronger with this system as compared to the GFS with a fair 
amount of middle level pv. Both models have a bulk of the isentropic lift 
near 300k in our southern forecast area...however the upper dynamics of the model 
moves through the NE half of our forecast area. I trended the higher probability of precipitation in our 
north where both models have deeper moisture. So despite the better 
isentropic lift near the MO River Valley...the low levels are drier 
in those locations. At this time...kept probability of precipitation only slight chance or chance as the GFS 
is absolutely dry with the passage...and sometimes the NAM GOES 
overboard on the quantitative precipitation forecast that far out. But I bet there will be some -ra 
around. For temperatures...there is a big discrepancy in 850 mb temperatures between 
the GFS and NAM...where the NAM is much cooler. With abundant middle 
level clouds around on Thursday...cannot see going real warm so met values 
looked better. And with dead air around on Friday...cool temperatures look 
prudent also. But as a Stout southerly flow returns on Saturday...temperatures 
should warm up to above normal. 


In the extended Sat night through Wednesday...the Gem continues to be an outlier 
in comparison with the European model (ecmwf)/GFS solutions. Therefore followed the 
latter two medr solutions for the basis of this forecast. The models continue to 
show a cold front passage in this County warning forecast area Sat night and early Sunday with 
varied degrees of timing. But the front looks to be definitely 
there. This front is in response to some height falls in S central 
Canada caused by a short wave passsage in that area. The European model (ecmwf) is slower 
then the last several GFS solutions with this front. So at this time...decided 
to blend the solutions as both models are very stubborn in deviating from 
their previous model runs timing regarding this front. At any rate...it 
looks to be a dry front...and a cool surface anticyclone will dominate our 
weather on Sunday. The short wave in S central Canada then deepens as it moves 
eastward into the Great Lakes area early next week...where it turns into 
bonafide upper troffing. The European model (ecmwf) is also slower with the 
progression of this troffing eastward. But other than a possible sprinkle 
in this forecast area Sunday night with the height falls to our east...the timing 
differences have little impact on this forecast area. Since I am not expecting 
measurable rain Sunday night...I left it dry. The next major player 
for this area is large upper troffing moving eastward through the 
intermountain west and into the plains Tuesday and Wednesday. The 12z ecwmf 
did a total flip flop with this feature...in that previous runs were 
closing the upper low off in the western Continental U.S.. the New Run keeps the 
trough open and rapidly progresses it eastward to the I 29 corridor by 00z 
Thursday. I prefer the last two GFS solutions which continue to close the upper 
low off...and move it slowly eastward Tuesday and Wednesday. This sets up what could 
be a rainy couple of days...and we will probably have to keep 
increasing probability of precipitation as time GOES on for Tuesday and Wednesday. 


For temperatures in the extended...kept things seasonably cool sun and Monday 
due to the aforementioned cool high pressure system. Then on Tuesday and 
Wednesday...we are likely to experience major cloud cover which could hold 
the temperatures down even more than what is now forecast despite the south 
flow. Instability parameters were too stable to mention any ts at this time. 


&& 


Fsd watches/warnings/advisories... 
Iowa...freeze warning late tonight and early Thursday for 
iaz001>003-012>014-020>022-031-032. 
Minnesota...freeze warning late tonight and early Thursday for 
mnz071-072-080-081-089-090-097-098. 
NE...freeze warning late tonight and early Thursday for 
nez013-014. 
South Dakota...freeze warning late tonight and early Thursday for 
sdz038>040-053>056-059>062-065>067-069>071. 


&& 


$$ 


Chapman/mjf