939 
fxus63 karx 042007 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
307 PM CDT Friday Jul 4 2008 


Short term...tonight through Monday 


Main forecast concern in the short term will be rain showers/ts chances 
Sunday through Monday. 


A pleasant Fourth of July day in progress across the forecast area 
and the rest of the upper Midwest thanks to high pressure centered 
over lower Michigan. Visible satellite showing fair weather cumulus with 
bases in the 5-6kft range widespread across the area with 
temperatures in the 70s. Water vapor imagery showing strong ridge in 
place across the Front Range of The Rockies while a vigorous 
shortwave was making landfall on the Pacific northwest coast. This shortwave 
will be our weather-maker for Sunday through Monday. More on this 
later. 


04.12z GFS and NAM in good agreement through Sunday night...then 
diverge on Monday with strength of shortwave crossing the High 
Plains. GFS seems a bit overblown with surface cyclogenesis with 
perhaps convective feedback coming into play. Generally took a blend 
of the two through Sunday night...then more of a sref/NAM blend 
through Monday. 


For tonight...look for the Fourth of July firework displays to go 
unhampered by weather this evening as high pressure remains centered 
over lower Michigan. Diurnally driven cumulus will quickly dissipate 
by sunset as temperatures cool into the 60s with light south winds 
under 10 miles per hour by the time the fireworks begin. Rest of the overnight 
hours will be clear with lows falling into the low to middle 50s. 


On Saturday...that aforementioned vigorous middle-level shortwave and 
attendant surface cold front over the Pacific northwest will push into the 
Dakotas by the afternoon hours. Models continuing to show eastern 
fringes of 850-700mb moisture transport nosing into eastern 
Minnesota/northwest WI through the afternoon hours. This could produce some 
middle-level instability altocumulus castellanus across western portions of the area. 
Best axis of moisture advection will remain well well of the area...so 
as a result...some increased cloud cover expected across western 
forecast area. Otherwise...with south winds/warm air advection and 
850mb temperatures increasing into the 15-17c range...afternoon 
highs should top off no problem reaching Lower/Middle 80s. 


The cold front will continue to push slowly into northwest Minnesota 
Saturday night with the forecast area remaining on the eastern 
periphery of 850-700mb moisture transport. Again...just expecting 
some scattered altocumulus castellanus clouds over northwestern portions of the 
forecast area with the rest of the area remaining mostly clear. Was 
tempted to include a small chance mention of rain showers/ts across the 
northwest fringes of the forecast area but will keep dry for now and 
pass concern onto subsequent shifts for possible inclusion of small 
probability of precipitation. 


Models in good agreement in bringing main axis of moisture advection 
into the forecast area by Sunday afternoon during peak heating as 
the cold front sinks into northwest WI. GFS/NAM producing 0-3km 
MUCAPES in the 2500-3500j/kg range with 0-3km bulk shear around 
25kt. Cape was produced with modeled dewpoints in the upper 
60s/lower 70s. This seems feasible with moisture 
convergence/trajectory ahead of the front. So...looks like 
thunderstorms a good bet through Sunday afternoon/evening...possibly 
becoming severe with favorable cape/shear indicated. 


A good chance of rain showers/ts will continue Sunday night through Monday as 
the front sags and becomes east/west oriented across 
central/southern WI...paralleling the high zonal flow aloft. 
Shortwave moving through the Central Plains will produce weak to 
moderate moisture transport into the frontal boundary to fuel 
convection. 


Long term...Monday night through Friday 


Main concern with the extended period will be active flow and rain 
chances. 


04.00z ECMWF/04.06z GFS both show a couple fairly vigorous middle-level 
shortwave troughs progressing across the upper Midwest Monday night 
through Wednesday for a chance of shra/ts. Meanwhile...at the surface 
a quasi-stationary east to west frontal boundary looks to be laid up 
somewhere from lower Michigan through Iowa. The European model (ecmwf) and more so the GFS 
show a substantial amount of moisture transport ahead of the 
shortwave Monday night up and into this frontal boundary with 
precipitable water values increasing to around 2 inches. Will have to 
watch for heavy rain potential Monday night. Have opted to hold off 
on esf product for now due to uncertainty of where frontal boundary 
will set up and where/if heavy rain potential will occur. Will 
consider if subsequent model runs show a more consistent signal. For 
Wednesday through Thursday...Main Ridge appears to set up over western 
Continental U.S. With our area dominated by shallow northwest flow. European model (ecmwf)/GFS 
show several impulses rippling through the flow for on/off rain showers/ts 
chances. As a result of more northwest flow...heat will be held off 
to the west. Therefore lowered high temperatures a few degrees for 
Wednesday/Thursday. Both models show a stronger system coming across 
the plains on Friday with warm air/moist advection into our area. 
With this signal...have included a small chance of rain showers/ts along 
with temperatures in the middle/upper 80s. 


&& 


Aviation... 


Period of VFR conditions will continue into Saturday afternoon. High 
pressure over Wisconsin will shift east into the eastern Great Lakes 
tonight and Saturday. This will allow for a south flow to set up 
over the area and help some low level moisture slowly return to the 
region. The diurnal cumulus clouds will dissipate early this 
evening. 04.12z models continue to bring a pocket of moisture 
currently over Nebraska and South Dakota across the area tonight on 
the back side of the high ahead of a weak short wave trough. This 
will result in at least scattered clouds with heights around 7000 
feet and possibly some short lived ceilings as well. These clouds 
should move out around 12z Saturday only to be replaced by more 
diurnal cumulus by middle to late morning. 


&& 


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


&& 


$$ 


Short/long term...das 
aviation..........04