Weather

670 
fxus63 kmpx 080826 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
326 am CDT Monday Sep 8 2008 


Discussion... 


Broad upper troughing continues to be in place over central North 
America this morning with several embedded troughs running through 
it. Upper ridging is in place upstream to the northwest over western 
British Columbia extending southwest into the Pacific offshore of the 
U.S. Pacific northwest. A deep upper low continues to circulate over 
southern Alaska and the Aleutians with water vapor imagery showing 
a potent trough axis rounding its base. The evolution of this trough 
will be affecting our weather by middle week. At the surface high pressure 
across the Dakotas was nosing into Minnesota. Late evening infrared 
satellite imagery showed high cirrus over much of far southern 
Minnesota and Wisconsin with mostly clear skies to its north...or 
across the northern County Warning Area where temperatures have dropped off sharply 
as expected. Higher clouds have now moved over most of that area. 
Water vapor imagery shows a trough entering the western Dakotas 
while surface low pressure over southeast Colorado with a surface boundary 
extending northeastward across northern Kansas and into Iowa. Radar 
mosaic shows a band of increasing reflectivity across northern 
Nebraska and northwest Iowa with weaker returns extending all the 
way into western Wisconsin in conjunction of well defined 700 mb-h6 fn 
vector divergence and entrance region of 120+ knots upper jet being 
pulled along the Iowa/Minnesota border. 


The initial forecast concern is the approaching Dakotas trough and 
the surface inverted trough/surface boundary that gets kicked northeastward 
ahead of it. NAM and GFS are both indicating that the middle level 
frontogenesis will stay just to the south of the County Warning Area in Iowa this 
morning. Latest radar trends support this. Potential vorticity anomaly 
associated with the approaching trough will continue to push 
southward and eastward enough so to suggest that the middle level 
baroclinic zone and frontogenetical band will stay just south of the 
County Warning Area as the GFS and NAM indicate. 300k isentropic surface forecasts keep 
condensation pressure deficits high across the southern County Warning Area as would 
be expected and isentropic adiabatic Omega is focused over central 
and north central Iowa this morning. With all of that will go ahead 
and trim probability of precipitation back in the south this morning and confine them to the 
southern tier of counties along the Iowa border. The afternoon becomes 
a bit more tricky with the upper trough axis passing the area and 
entrance region of the upper jet moving overhead as well. Height 
falls increase with the trough while pool of -21 to -23 c 500 mb air 
settles in by late afternoon with middle level moisture increasing as 
well per time Hight sections in conjunction with forecast 500 mb-300 mb 
q-vector convergence. Sunday afternoon saw some showers across 
Montana ahead of the wave as well. Thus anticipate isolated to 
scattered showers and perhaps a rumble of thunder /given a small 
amount of elevated cape in the forecast soundings/ by middle/late 
afternoon. Best rain chances will remain in proximity to the 
activity across the southern south which may shift a bit 
northeastward during the afternoon supporting slightly higher probability of precipitation 
from Red Wing to Eau Claire. Cirrus shield should be pretty 
expansive through the middle afternoon with cumulus/stratocu also 
developing there for temperatures will struggle to break 
60...especially south where middle/upper 50s should do it for highs. 


High pressure ridge will be squarely overhead with clearing skies 
tonight. Good radiational cooling conditions will set up resulting 
in overnight lows in the upper 30s to around 40 with patchy frost 
possible. The high shifts east Tuesday with pressure gradient 
tightening in the west by Tuesday evening. Upper flow will be fairly 
flat during this time while a sharp longwave trough pushes into the 
western Continental U.S. Spawning surface cyclogenesis in the Lee of the central 
and northern U.S. Rockies by Wednesday morning. Strong southerly 
winds will be in place locally for Wednesday as the pressure 
gradient tightens. Warm advection commences into Wednesday with 850 mb 
temperatures being boosted to around +12c by Wednesday evening. The main 
longwave axis /which will be positively tilted/ remains to the west 
but a series of lead shortwaves will propagate east-northeast in the 
fast upper flow reaching the County Warning Area Wednesday evening. Surface low pressure 
will track across the Dakotas with frontal boundary/inverted trough 
moving eastward across the County Warning Area into Thursday morning. Previous 
forecast of low chance probability of precipitation Wednesday and high chance probability of precipitation for 
Wednesday night looks on track. High pressure looks on track for 
Friday behind the low. Low confidence beyond that with the models 
disagreeing on the track of tropical cyclone Ike but it does appear 
that another upper trough will be approaching Saturday so will 
continue with continuity and keep low chance probability of precipitation for that time. ..mdb.. 


&& 


Aviation.../06z taf issuance/ 
weak upper ridge had build over the area this evening. Clouds had 
scoured out over the northern two thirds of the Minnesota and northern 
Wisconsin. Mainly clear skies overnight with some patchy fog 
developing in fog prone areas and river/small streams bogs. 
Meanwhile...a shortwave trough over the northern plains...will move 
east over the upper Midwest by daybreak Monday and move across the 
upper Midwest. Most of the dynamics associated with this shortwave 
trough...will stay well south of main taf sites. Clouds are expected 
to thicken over the taf sites during the day on Monday. Ceilings and 
cloud heights will lower to 4-5k feet above ground level. Surface winds will be 
northwest at 5 to 10 miles per hour then becoming light and variable or under 6 
knots by sunset Monday. 






&& 


Mpx watches/warnings/advisories... 
Minnesota...none. 
WI...none. 
&& 


$$ 


Mdb/jvm