160 
fxus63 kdlh 200820 
afddlh 


Area forecast discussion 
National Weather Service Duluth Minnesota 
320 am CDT Wednesday Aug 20 2008 


Discussion... 
southerly flow around high pressure to the east will allow 
moisture and temperatures to creep upward over the next couple 
days. 


There was some lower clouds over portions of our southern 
zones...and around Lake Superior. NAM forecasts these clouds to 
diminish this morning...and that will likely occur as it did 
yesterday with some mixing. A cumulus field should then form...especially 
over our eastern zones...but it should remain mostly scattered. 
There was also some patchy fog this morning...and that should lift 
shortly after sunrise. 


We went with a dry forecast today and tonight...but we will need 
to closely monitor our southern zones...especially in northwest 
Wisconsin for a possible afternoon shower or storm. There is a 
weak surface boundary south of this area...and an area of 
vorticity forecast to lift north this afternoon. There were a few 
thunderstorms that occurred in ec and southeast Minnesota yesterday afternoon. 
4km WRF simulated radar reference forecasts some weak returns over 
northwest Wisconsin...and far eastern Minnesota...to near kdlh this 
afternoon. 


Tonight...NAM forecast low level relative humidity to increase...and we may see 
more low level cloudiness form late tonight...especially around 
Lake Superior. 


We added some small probability of precipitation for Thursday afternoon over northwest 
Wisconsin. NAM forecasts low level winds around 25kt...and 
increasing moisture. 


&& 


Long term...Thursday night through Tuesday 


The latest model guidance appears to have come into better agreement 
regarding some of the details for the Thursday night to Saturday time 
period. 


The upper low currently centered near the Dallas/feet Worth 
metroplex is prognosticated to open up and weaken while moving northeastward into 
the western Great Lakes region Thursday/Thursday night as it is gradually 
absorbed into broader and increasingly strong west-southwest flow aloft in 
advance of progressive upper trough translating eastward from the Pacific 
northwest. This will likely result in some showers/thunderstorms across our southeast 
zones Thursday night and early Friday..so we have retained low probability of precipitation in 
these areas for now. 


However..it appears that any tstorm development Thursday 
afternoon/evening associated with the approaching impulse from the 
west will likely be confined to locations in proximity to the low 
level mass convergence axis and on the northern edge of a strengthening 
cap from the central Dakotas northward into Manitoba. So we have removed 
probability of precipitation in the western zones until after midnite Thursday night. 


There is better agreement between the various guidance packages now 
regarding the timing of the front on Friday/Friday night and also the 
handling of low level moisture magnitude and instability/shear 
distributions. As we suspected yesterday..the models now seem to be 
converging to a solution with slightly higher boundary layer 
moisture content than previous runs. Our best guess at the moment 
is that an axis of upper 60s/lower 70s dewpoints will develop along 
and just ahead of the front Friday across central Minnesota..resulting in 
cape values from 1000-2000 j/kg. Rather warm temperatures aloft should be 
eroded during the day by persistent mesoscale lift associated with 
an increasing frontal circulation..but weak middle level lapse rates 
may still act to inhibit severe potential somewhat. While the 
strongest deep layer shear is expected to lag behind the front and 
storm initiation axis..sufficient deep layer shear from 30-40 kts 
should still be enough to support at least a few severe storms. Areal 
coverage is expected to be greatest 00-09z Sat across central and 
eastern sections of the dlh County Warning Area when low level jet will increase and enhance 
low level warm air advection/fgen within the frontal zone as it translates eastward 
across Western Lake Superior/northwest WI. 


Surge of cooler and much drier air is expected to rapidly punch to 
the east in wake of strong surface cyclogenesis in Canada..so we have 
ended probability of precipitation from west to east a but sooner from the previous forecast 
Friday nite/Sat. Tranquil weather is expected Sat-Monday under the 
influence of low level ridging. We did introduce low probability of precipitation across 
our western zones late Monday/Tuesday as return flow commences on western periphery 
of ridge and low level jet interacts with low level baroclinic ribbon trying 
to move back to the NE. 


Aviation...warm front located roughly along the Interstate 94 
corridor in central Minnesota early this morning will continue to lift 
slowly northward today/tonight. Areas of stratus and fog will persist to 
the north of the front this morning..and are expected to be most 
widespread near the southwest tip of Lake Superior and NE 
Minnesota..affecting kdlh/khib/khyr. As moisture gradually increases today 
and tonite..there should be increasing coverage of middle level 
altocumulus field and scattered-broken 4-6 thousand foot clouds this 
afternoon. While not in the point taf forecasts..it should be noted 
that low probabilities for showers or thunderstorms cannot be entirely 
ruled out at all taf sites today as the warm front lifts northward. 
A strong southerly low level jet tonight is expected to result in 
low level wind shear conditions from roughly 03-12z Thursday. 


Point temps/pops... 
dlh 79 61 80 64 / 0 10 10 10 
inl 85 62 86 64 / 0 10 10 20 
brd 84 64 85 65 / 10 10 10 20 
hyr 83 60 84 62 / 10 10 20 20 
asx 83 63 83 64 / 0 10 10 20 


&& 


Dlh watches/warnings/advisories... 


Minnesota...none. 
WI...none. 
Ls...none. 


$$ 


Melde/Miller