Weather

606 
fxus63 kmpx 071109 aaa 
afdmpx 


Area forecast discussion...updated 
National Weather Service Twin Cities/Chanhassen Minnesota 
609 am CDT sun Sep 7 2008 


Update... 


Updated for 12z aviation discussion below. 


&& 


Discussion... 
/issued at 314 am CDT sun Sep 7 2008/ 


Rather active period ahead with a series of shortwave troughs and 
surface boundaries providing focus for showers and isolated 
thunderstorms. Water vapor imagery shows broad longwave trough axis 
just about centered over Minnesota and extending north and south. 
One shortwave is seen departing the WI County Warning Area with another shortwave 
trough axis pivoting across the eastern Dakotas with pronounced 
drying evident behind it. Regional radar mosaic shows scattered 
bands of showers and isolated thunderstorms along the trough axis 
and ahead of an associated surface front indicated by surface analysis. The 
trough will make it across the County Warning Area by early afternoon with shower 
threat mainly confined to the morning hours in the Minnesota counties and 
until middle/late afternoon in the WI County Warning Area. Weak surface ridging builds in 
across central Minnesota and northern WI tonight with a brief period of dry 
air aloft so expect some clearing across the northern County Warning Area. Highs 
will only be in the middle 60s. Low temperature forecast tonight will be very 
dependent on the cloud cover but expect least coverage in the 
northern County Warning Area and have trimmed lows back a few degrees in those 
areas. Clouds will will increase across the south as the next system 
approaches so warmer mins will occur. 


Another and better organized shortwave currently dropping southward 
across western Montana will push across the Central High plains this 
evening. Surface low pressure center meanders eastward across Kansas 
with an inverted trough/frontal boundary northeastward across the 
middle Mississippi River valley by Monday morning. Trough positioning 
is problematic with the GFS wanting to take keep the boundary 
further north than last night at this time and than the latest 
NAM...sref...and European model (ecmwf). As a result middle level baroclinic zone is 
shunted northward by the GFS with frontogenetical forcing shifted 
northward accordingly. At this point there is much more support for 
the NAM/sref/ECMWF positioning and will adjust the forecast for 
this. The result is isentropic ascent increasing over far southern 
Minnesota early Monday morning and expanding northeastward. The 
upper trough axis crosses the area Monday afternoon. Therefore 
expect a southwest to northeast oriented are of showers/isolated 
thunder from the Fairmont/St. James area to Owatonna and perhaps Red 
Wing and then up through the eastern portions of the WI County Warning Area from Eau 
Claire north to Rice Lake and Ladysmith as the surface low turns a bit 
more northerly. With the upper trough axis crossing the area 
isolated showers will be possible across much of the area but the 
main focus will be the far south and east. Strong 1024 mb high will 
settle along the Kansas/Nebraska border Monday night with ridge axis 
extending across Minnesota. This will setup good radiational cooling 
conditions later Monday night through daybreak Tuesday as much drier 
air moves in scouring out cloud cover. Middle/upper 30s lows will be 
possible across the north along with patchy frost. 


The high will shift past the County Warning Area Tuesday afternoon with southerly 
flow returning Tuesday evening into Wednesday morning as low 
amplitude upper ridging crests overhead. Meanwhile another sharp 
upper trough digs into the Pacific northwest inducing surface cyclogenesis 
across the High Plains Wednesday. The upper trough cuts off over the 
west with a lower amplitude shortwave continuing eastward in the 
northern stream giving the surface low continued forward momentum. One 
surge of middle level warm advection moves over the area early 
Wednesday but the atmosphere is still pretty dry at that point so it 
will be difficult to get more than middle clouds. The surface low will pass 
Thursday giving chances for showers and thunderstorms from later 
Wednesday through Thursday afternoon. High pressure builds in behind 
the low for a short time Friday but the previously cutoff upper low 
gets kicked eastward late Friday bringing another chance for 
showers. ..mdb.. 


&& 


Aviation.../12z taf issuance/ 


Fairly active first 6 hours or so of the valid taf period...with region 
of rain showers in the western forecast area...and some very patchy MVFR visibilities/IFR ceilings. 
Region of rain showers has not affect flight category really at all...and do not 
expect this to change. 06z NAM 300k isentropic surface currently depicting 
area quite well so used this to time approach of rain showers at each taf 
site. Axn will remain dry...with the rain showers already having passed 
over them...and stc should be affected by/shortly after 12z...rwf by 
13z...msp by 16z...rnh by 17z...and eau by 19z. Conditions pressure deficits 
at 300k indicate that currently mostly solid line should begin to 
weaken/break up as it moves ewrd...and begins to affect 
msp/rnh/eau. IFR ceilings affecting msp and rnh at present look to be 
induced somewhat by the river valleys...according to presence on 
Sat. Expect that this will not last long after sunrise...especially 
with signs at surrounding airports that ceilings are already beginning 
to rise slightly...and recent tamdars showing a fairly shallow 
layer of saturation. The latter part of the taf period for all 6 
sites should be largely uneventful...with decr cloud cover and light 
nearly winds. Rwf should see a late increase in middle level clouds with the 
approach of 125 knots upper jet and associated precipitation south of the Iowa 
brdr. This precipitation should stay south of the taf sites after the 12z Monday 
period. 


&& 


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


$$ 


Mdb/kat