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fxus63 karx 050800 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
300 am CDT Sat Jul 5 2008 


Short term...through Monday night 


Primary forecast concerns remain focused on rain chances during 
the latter half of the weekend and into early part of next 
week...along with potential for locally heavy quantitative precipitation forecast amounts. 


Upper Mississippi Valley continues to be under influence of surface 
high pressure extending from lower Michigan to Missouri. Further 
west...middle level ridge over northern rockies beginning to flatten... 
with short waves advancing out of eastern Pacific trough. Water 
vapor and infrared satellite imagery indicated large convective 
cluster moving along international border vicinity of North 
Dakota and Montana ahead of one of aforementioned short waves. 


05.00z NCEP and European deterministic models continued to share 
overall good agreement in synoptic scale features through 08.12z and 
were corroborated by latest sref data. This supports a scenario of 
increasing rain probabilities late in the weekend and into the early 
part of next week. Therefore...forecast confidence was high that 
forecast area should receive rainfall between Sunday and Tuesday. In 
fact...potential for locally heavy rain appears to still be viable... 
with highest probability Monday night into early Tuesday morning. 


Tonight...question was whether to include rain chances into parts of 
southeast Minnesota and northeast Iowa between 06.06z-06.12z. Obviously... 
primary focus for rainfall appears to be along approaching cold front 
in north and west sections of Minnesota in this time frame. However...Theta- 
east convergence along eastern periphery of 850mb moisture transport 
suggests rain chance warranted in aforementioned parts of local area. 
Thus...opted to include low-end rain chance in those locations. 


Sunday into Sunday night...cold front expected to advance further 
eastward...as upper level and surface wave track across southern 
Canada. Front expected to encounter building instability...with 
MLCAPE of 1500-2500j/kg and plume of ml dew points in middle to upper 
60s. Highest probabilities of convection and thus rainfall should 
occur Sunday afternoon and evening...especially from 06.21z-07.03z. 
After 07... instability and lack of low level convergence 
in north and west sections of forecast area supported trending rain 
probabilities downward in those locations. 


Potential for a few severe storms should peak 06.21z-07.03z time 
frame too...although threat appears to be low-end based on weak wind 
fields. Wind shear of 25kts expected to be mainly confined to lowest 
3km and unidirectional according to BUFKIT forecast soundings. As a 
result...multicell storm Mode anticipated...which should be outflow 
dominated. In addition...wet bulb zero and freezing levels quite high 
Sunday afternoon and evening...about 10k-13k feet respectively. 


Monday into Monday night...front takes on an east-west orientation 
as it parallels middle and upper level flow. Deterministic and 
probabilistic data have been supportive of another upper level and 
surface wave to advance across the upper Mississippi Valley Monday 
night. Due to model consistency...forecast confidence was high 
enough to increase rain probabilities in the time frame of 08.00z- 
08... further increase probably warranted with time. 


Dynamics associated with upper level and surface wave...in 
combination with precipitable water values of nearly 2 inches... 
strong 850mb moisture transport impinging upon front...and warm cloud 
depth up to around 4km...suggests efficient rain producing storms 
could occur Monday night. This could be enhanced by back building 
and therefore potential for convection to repeat over same areas. 


Therefore...appears locally heavy rainfall has highest probability 
of occurrence Monday night and may need highlighting in subsequent 
forecasts...once details become more clear. In addition...a 
hydrologic statement may be prudent before the weekend is over to 
heighten awareness of heavy rain potential early in the week. Should 
a heavy rain threat materialize...consideration should also be given 
to dry antecedent conditions...resulting in 1hr and 3hr flash flood 
guidance values of 2.5-4.0 inches. 


Long term...Tuesday through Friday 


Forecast concerns were primarily focused on rain chances during the 
rest of the week. Deterministic and probabilistic data suite suggests 
a relative minimum in rain chances at midweek...before next surface 
and upper level wave advance into the northern plains toward the end 
of the week. In fact...deterministic 05.00z European model (ecmwf) and GFS suggest 
surface high pressure lingers through midweek and thus would support 
a drier period than current data base indicated. No changes were 
made...but day shift may need to include Wednesday in a dry period. 


Signal in mref data supported relative rain chance minimum during 
the middle and latter part of the week...with systems generally south 
and north of the forecast area. Current data base utilizes broad- 
brush low-end rain chances through the extended outlook. Subsequent 
forecasts should be able to add detail as higher confidence is 
gained...thus incorporating some dry periods in the extended outlook. 
For now...no changes were made in this time frame. 


&& 


Aviation... 
05.00z models continue to show that southerly winds on the back side 
of a departing high pressure system will bring some middle moisture 
northward into parts of northeast Iowa and southeast Minnesota this 
morning. There may be a brief period of broken ceilings between 
6000 and 8000 feet early this morning. Otherwise skies should be 
mainly mostly sunny today. Like the past few days...there will be 
likely be some diurnal cumulus clouds. 


As the low level jet gradually turns toward our area late tonight 
/likely after 06... may be a few showers and thunderstorms 
move into southeast Minnesota and northeast Iowa. In addition... 
these same areas will likely see a broken deck of clouds between 
6000 and 8000 feet during this time period. 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 


&& 


$$ 


Short/long term...Thompson 
aviation..........boyne