527 
fxus63 kfsd 040158 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
858 PM CDT Thursday Jul 3 2008 


Discussion... 
weak wave producing area of middle level clouds extending southwest to 
northeast across the area this evening. A few light echoes showing 
up on 88d...but with dry surface based layer expect nothing more 
than virga...with surface observation hard-pressed to tap much below 12kft. 
Have adjusted some sky cover for the overnight...pushing this area 
of middle clouds southeast through the area through 06z-08z...though 
additional middle level miosture hangs tough across the southwest 1/3 
of the forecast area much of the night. 


Light east-southeast winds look good for the overnight as surface 
high pressure pulls east into the Great Lakes. Clouds will hold 
temperatures up a bit in some areas this evening and tweaked hourly temperatures 
a bit to reflect this...but overall lows looked reasonable with few 
changes made there. Updated grids/pfm/zfp have been sent. 


&& 


Aviation... 
VFR conditions will continue through 00z Sat...though some scattered MVFR level 
cumulus should develop by midday Friday. 


&& 


Previous discussion... 
surface high pressure will continue to have an influence across the 
region tonight...though moving slowly eastward through the overnight 
hours. With this...winds will remain very light...though having a 
more southerly component to them on the backside of the high. Will 
see an increase in midlevel clouds over the western portions of the 
County Warning Area through the night...with weak warm air advection/moisture transport on a 
developing low level jet...along with low level convergence in a 
tightening 850 mb thermal gradient through the southwestern and 
south central parts of South Dakota. European model (ecmwf) actually outputs some 
precipitation with this process...with the GFS also hinting at 
it...but kept dry for now with dynamics to our west. Will be another 
cool night across the area...though not as cold as last night under 
a moderating airmass. Still...with light winds and primarily 
scattered clouds...readings will drop to below seasonal 
norms...especially in the east where there will be less cloud cover. 


Does not appear to be a lot going on on Friday. Southerly flow begins to 
increase...especially west of the James River Valley...which will 
usher in warmer temperatures but still a bit below normal for this time of 
year. Then on Friday night and Sat morning...both the NAM and GFS show 
a weak shortwave moving through this forecast area over the upper ridge. This is 
coupled with strong warm air advection and a Stout low level jet which should help 
to keep the lows mild Friday night. In addition...a stripe of altocumulus castellanus may 
develop with instability aloft...moving from west to east during the overnight 
hours Friday night and early Sat. But for now...left the mention of ts 
out of the forecast but put probability of precipitation in the grids just below slight chance. 
Saturday will be a warm day...downright breezy to windy through the forecast area 
with decent mixing through 850 mb. 


Saturday night is also marked by a strong low level jet and warm air advection 
keeping lows warm. Height falls are also notable with very subtle 
shortwave energy in place. The degree of capping is quite different 
between the NAM and GFS by 12z Sunday. The GFS shows +10c to +11c 
through the forecast area while the NAM is closer to +14c. Therefore looked at the 
thermodynamic fields on the NAM and it is certainly capped off... 
which is why it keeps its quantitative precipitation forecast north and northwest of the fsd forecast area. Decided to 
keep probability of precipitation going however as there looks to be another repeat of altocumulus castellanus 
building up in the forecast area...and the height falls bother ME which is 
something we really do not have on Friday night. Placed the highest 
probability of precipitation in the S central portion of the fsd forecast area per the Gem...which is just 
ahead of a weak 700 mb shortwave shown by the various models. 


Sunday is a tricky forecast. The NAM barrels a surface front through the 
daytime which is not backed up by other solutions. So ignored that 
wind shift for now. However similar to the NAM solution...placed 
some of the highest probability of precipitation through our eastern forecast area Sunday afternoon where 
cooler 700 mb temperatures of +9 to +11c exists. This is normally a favored 
spot for a breaking of the cap for ts. But still only low probability of precipitation are 
warranted as the low level jet looks to be weak as well as the low 
level warm air advection. At this time...have Sunday pretty warm but the warming could be 
tempered when the time comes by abundant middle and high level cloud 
cover. 


In the extended Sunday night through Thursday...the various models 
still show a vigorous shortwave genning up in the Western Plains Sunday 
evening...then moving eastward overnight Sunday night and Monday. Where the 
exact mesoscale convective system placement will be is still a question mark. But there will be 
one...exiting with a boundary Monday evening in our southeastern forecast area. After 
that...things are quite a question mark on shortwave energy and 
temperatures. The 06z GFS and 12z European model (ecmwf) are much sharper with the ridge over 
The Rockies from Tuesday Onward...and this with the northwest flow aloft 
providing cooler temperatures in our area. The 00z/12z GFS and 00z European model (ecmwf) 
have a much flatter flow with the polar jet through the northern 
Continental U.S....which would be a much warmer solution. For now...went with 
the warmer solution but will leave an open mind when generating 
fridays forecast. In that if GFS runs begin to show the sharper northwest flow 
aloft...will cool things off next week. For now...left probability of precipitation dry after 
Monday evening with no agreement between the various medr models 
concerning shortwave placement. 




&& 


Fsd watches/warnings/advisories... 


Iowa...none. 
Minnesota...none. 
NE...none. 
South Dakota...none. 


&& 


$$ 
Jh/Chapman