Scientific Forecaster Discussion
fxus63 kmpx 212338
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
638 PM CDT Fri Jul 21 2017
Update...for 00z aviation discussion below
Short term...(this evening through saturday)
issued at 354 PM CDT Fri Jul 21 2017
Convection across far-southern-MN this morning & earlier this
afternoon has complicated the setup for potential severe weather
this evening. Residual cloud cover from this convection as well as a
blocking of the richest moisture return has greatly limited
instability across central Minnesota & west-central WI. Tonight's severe
threat now turns to the eastern Dakotas & northwest Minnesota which has
remained unaffected from this mornings convection. Storms should fire
in a healthy environment with 3000 j/kg of MLCAPE & over 40 kts of
effective deep shear supporting severe storms, with large hail &
damaging winds both possible. The question then becomes how far
south & east these storms make it into central Minnesota. Hi-res models
agree on bringing storms down from the Dakotas across western Minnesota
after sunset. These storms then run into the more stable air over
central Minnesota and quickly dissipate, generally along & west of a
Brainerd-Fairmont line. Thus we are expecting the severe threat
tonight to be limited to western Minnesota and for the rest of the
area, including the Twin Cities Metro, to have a relatively quiet
night. If storms can overcome the more stable air & penetrate
farther into central & eastern Minnesota, these areas could also see
a conditional severe threat materialize but confidence in this
scenario is low.
The severe weather threat don't get any clearer tomorrow as a weak
surface trough lingers across the area and provides a chance of
thunderstorms along & east of I-35. Forcing is nebulous across our
area with the main upper-level support from the approaching
shortwave remaining further north across the Minnesota arrowhead, so expect
any storms that can develop to be scattered at best. However
dewpoints remaining near 70 across the area will lead to instability
increasing with daytime heating. 3000 j/kg cape to go along with 30-
40 kts of effective deep layer shear will provide a supportive
atmosphere for any storms that can develop. Current thinking is that
the issues with forcing will keep the main severe threat north &
east of the County Warning Area but this will bear watching as we get closer to
Long term...(saturday night through friday)
issued at 354 PM CDT Fri Jul 21 2017
By tomorrow evening, expect any daytime activity across the area to
be departing to the east, with the upper level shortwave moving east-southeast
across far northern Minnesota and Lake Superior through Saturday night.
We'll really feel the affects of the frontal passage by Sunday
morning as dew points drop to more comfortable levels. Northerly
flow will keep temperature slightly below normal as high pressure
works in from the north.
Meanwhile, our next system of interest will be pushing into southern
British Columbia and will set to work eastward through southern
Canada. The 500h closed low will stay well to the north of our
area, but the frontal boundary extending southward from the system
will work across the Dakotas Monday-Tuesday, with the front in our
region most likely on Tuesday. This could begin an active period
for US depending on where the front stalls out. As it looks
currently, the front will work through our area and get hung up from
South Dakota through northern Iowa and into southern Wisconsin.
Being that this is 6-7 days out, confidence in placing a front is
quite low. This will continue to be the main feature to watch in
the coming days for the long term period, and how the models handle
the evolution of the system and overall pattern.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 638 PM CDT Fri Jul 21 2017
First question this taf period is whether convection will affect
sites tonight. Showers and storms over northwest Minnesota are expected
to decrease in intensity with south/southeastward progression, and
decay as they enter central Minnesota. Kaxn and kstc may be affected,
but said activity would likely be showers and most by the time it
approaches kmsp. Another area of storms may develop farther west
and move into the area between 08z-12z. Confidence is low however.
The other threat overnight is fog development again with little
in the setup having changed. Expect MVFR/IFR visibilities, with
LIFR possible at krnh and Keau. This all will depend on how the
convection pans out and impacts cloud cover/low level moisture.
Sct-bkn mid level clouds will persist on Saturday, although Minnesota
sites should become VFR for most of the day. Variable winds
overnight shift to the northwest on Saturday and increase above 10
knots at most sites.
although confidence is low, the time frame for thunder/shower
potential between 09z and 12z appears to be the most possible
window for activity. Patchy fog with 4-5sm visibilities is also
expected to develop overnight, with a potential for sct-bkn 1500
ft clouds as well.
/Outlook for kmsp/
sun...VFR. Wind northwest 10 kts.
Mon...VFR. Wind east-northeast - east-southeast 5 kts.
Tue...VFR. Chc thunderstorms and rain late. Wind S/south-southeast 5-10 kts.