464 
fxus63 kmpx 240752 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
252 am CDT Thursday Jul 24 2008 


Discussion... 


The challenge with the forecast is convective chances later today 
as well as Friday. 


GOES WV loop showing a series of waves approaching upper 
Mississippi Valley around a central one moving east into southern 
Manitoba. Upstream middle and upper level ridging currently reside 
over the forecast area...though deamplification is expected today. 
This is not necessarily due to the impingement of these shortwaves 
but the slight retrograding of the eastern Great Lakes long wave 
trough. This will lead to an almost squishing/weakening of these 
shortwaves as they near Minnesota this afternoon and evening. So this should be a 
major caveat to sustained severe weather later today. 


For this morning...continued 5k-10kft warm air advection /waa/ will 
lead to light showers with maybe a few lightning strikes. 
Convective chances seem none for eastern Minnesota and western WI through at least 
through middle afternoon if not throughout the daylight hours...as 
convective indicies are weak and actually weaken further after 
initial warm air advection push this morning. They then begin to creep up again 
in the late afternoon. So through middle afternoon for much of the County Warning Area think 
thunder will be few and far between with any showers. 


Surface cold front will be entering Minnesota late this afternoon into early this 
evening. Ahead of this...primarily in western and ctrl Minnesota...a fair amount 
of sun is expected today. This combined with the aforementioned 
gradual height falls will help destabilization. Surface dew points are 
expected to be in the 66 to 70 degree range ahead of the front. 
This creates surface based cape between 1500 and 2500 j/kg in a 
tongue into western and ctrl Minnesota by late afternoon. Marginal upper level 
divergence is present at that time with incoming 80-90 knots 250mb 
jet in the Western Plains. Associated thickness difluence also seen 
on model solutions. So isolated to scattered convection is expected in at 
least western Minnesota late this afternoon into early this evening. These should be 
pulsy-type storms...possibly with some clustering in a fairly 
unidirectional lower tropopause profile. Middle-level lapse rates are 
marginal for strong updrafts. Predicted dcape values are 600-1000 
j/kg...though further examination of model soundings shows Delta 
Theta-E to be slight in the vertical...so wind threat thought to 
be marginal too. Some thought too of convection getting active in 
Nebraska/western Iowa absorbing some of our buoyancy and low level winds. 
This should come to fruition later in the evening as nocturnal low level jet 
will be focused in Iowa with expected ongoing convection. So all in 
all...probability of precipitation fairly small today and tonight and severe weather 
potential and possible areal coverage seem small as well. 


Front will gradually work southeast overnight into Friday morning before 
getting a kick Friday afternoon from a diving 500 mb wave in Saskatchewan. 
Instability expected to build ahead of the front in southern Minnesota and western 
WI with dew points in the upper 60s to lower 70s. Dynamics should be more 
impressive than today with a digging wave and an associated 
stronger jet. So severe potential does exist in southeast Minnesota and moreso in 
western WI on Friday afternoon and very early evening. After that point...front 
should be east of the forecast area. 


Despite these days being are warmest climatologically...we are 
not expecting any marked heat. In fact...temperatures should be 
slightly below normal most days. Friday may see middle to upper 
80s...as 800 mb thermal ridge gradually works southeast across the area with 
aforementioned front. Dry and cool air advection then will prevail 
Friday night and Sat. The weekend looks dry with mainly lower 80s. 
Elevated moist advection should remain to our south and west during 
that time. Next week may be slightly more active as this 
advection approaches...but primary low level baroclinic and dew point 
gradient along with any remnant moisture from once Hurricane Dolly 
looks to be to our south. 


&& 


Aviation.../06z taf issuance/ 


Warm advection pattern over forecast area strengthens later tonight 
as high pressure moves off. A couple of areas of showers moving 
across portions of Minnesota...largely missing most taf sites the next few 
hours. More substantial showers over SC Minnesota. By Thursday morning... 
ceilings close to MVFR could develop over western Minnesota and possibly as 
far east as kmsp. Surface cold front a bit delayed and should move 
into west central Minnesota late Thursday afternoon with a chance of 
thunder developing. 


&& 


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


$$ 


Mtf/tdk