Weather

647 
fxus63 kmkx 272023 
afdmkx 


Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI 
325 PM CDT Wednesday Aug 27 2008 


Discussion...forecast centers on increasing threat of precipitation 
later tonight into Thursday evening. Morning activity across Minnesota/Iowa 
diminished as planned as it pushed into large area of subsidence and 
drier air over central MS valley. Mesoscale convective vortex showing up in southeast Iowa 
per latest vsbl imagery. Prefer GFS solution for the short term due 
to better agreement in synoptic scale features between successive 
model runs. Low level jet beginning to increase across plains as 
winds beginning to veer ahead of strong short wave trough in Montana/southern 
Canada. 700h profilers showing veering winds in the High Plains 
indicating precursor short wave that will affect WI later tonight and 
Thursday morning. Ahead of this wave...nose of weaker low level jet of 20 to 30kts 
pushes across southern WI with a rapid increase in precipitable water of 
one half inch. Elevated lifted index drops to around zero by 12z in 
the west but middle level lapse rates still on the weak side. Thinking 
between activity to the west spilling southward along thicknesses 
and low level moisture flux convergence...scattered to numerous 
showers and thunderstorms are expected. 


Lingering cloud cover will act to retard heating somewhat in the 
afternoon as best low level convergence moves through...limiting 
more organized convection. Never the less...right entrance region 
of upper jet passes across region during the afternoon and 
evening...and fn response to increasing low to middle level 2d full 
wind frontogenesis Worth noting. Hence will go with scattered redevelopment 
in the afternoon south central increasing to numerous activity in the 
southeast during the evening. Short range ensemble probability of precipitation increases to 
70 percent during the evening in the southeast. 


Northwest breezes carry less humid air across area late Thursday night 
and Friday with light boundary layer winds setting up for Friday 
night. Weak frontal boundary to the north with patchy middle level 
clouds but with light winds and increase in low level moisture from 
rain...thinking fog a good bet for early Saturday. Pending cloud 
cover...may be some quarter mile visibilities. 


More uncertainty creeping into the extended forecast beyond Monday 
night. Uncertainty mainly due to timing and track of Gustav and its 
affect on approaching short wave trough for Tuesday and Wednesday. Latest 
extended guidance taking a more northeastly track and carrying 
remains of hurricane into western Gulf during the period...with a 
second tropical disturbance in the western Atlantic. GFS trending 
slower with weakening short wave trough into the upper Midwest 
during this period...while hi res European model (ecmwf) much weaker and even slower 
with surface trough...as high pressure and subsidence reinforces 
itself across eastern Continental U.S.. little run to run consistency in any 
extended guidance at this point. Moisture remains low...however low 
level increasing srly winds does result in a bit of elevated instability 
Tuesday into Wednesday. For now...will go with schc probability of precipitation most areas Tuesday into Wednesday 
night period. If current trends continue...will probably be able to 
pull probability of precipitation out from at least Tuesday...and possibly into Wednesday. GFS 
ensembles showing lots of variability in the later periods...so 
little stock can be put in dgex either. HPC leaning on European model (ecmwf) mean 
which has best probability of precipitation in last periods. 


GFS 500mb 5 day means show zonal flow with near normal heights 
across northern Continental U.S. Transitioning to increasing heights over the 
gtlakes due to upstream troffing across west. Hence 
warmer...daytime temperatures likely...and more southeasterly breezes should 
result in slightly warmer readings along Lakeshore as well. Dry 
pattern to continue into early September...pending track of Gustav. 


&& 


Aviation...weak surface ridging and light winds in place...while 
scattered cumulus this afternoon gives way to increasing middle to high 
clouds ahead of low pressure area to the northwest. Small chances 
for thunderstorms late tonight and Thursday morning for southwest and 
south-central WI..but opted to leave out of tafs for now. 
Increasing clouds overnight should hinder any fog development. 


&& 
Mkx watches/warnings/advisories... 
WI...none. 
Lm...none. 


$$ 
Short term/long term....mbk 
aviation................cjk