Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 212054

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
354 PM CDT Fri Jul 21 2017

.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 MN & west-central WI. Tonight`s severe
threat now turns to the eastern Dakotas & NW 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 MN. Hi-res models
agree on bringing storms down from the Dakotas across western MN
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 MN 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 CWA but this will bear watching as we get closer to
tomorrow afternoon.

.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 ESE
across far northern MN 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.  Northlery
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 18Z TAFS through 18Z Saturday afternoon)
Issued at 1220 PM CDT Fri Jul 21 2017

SHRA/TSRA that affected a large area of southern Minnesota since
earlier this morning has modified the forecast greatly this
afternoon, as instability, due to the clouds/rainfall, has lessen
the overall chance of precipitation in the next 6 hours.
Therefore, VFR and little chance of redevelopment through 00z,
with even a low chance of VCTS until 6z looks reasonable. I will
continue the VCTS after 6z for most of the TAFs but timing and
overall coverage remains low. Winds will gradually shift from the
south/southeast this afternoon, to a more west/north/northwest
tonight/Saturday morning.


As previously stated, kept nearly no chances of VCTS in the
forecast until after 6z. Only concern this TAF period is the
development of MVFR/IFR cigs in the eastern/northern forecast area
due to light and variable winds in the boundary layer and nothing
to move the moisture out of the area until after 18z Saturday.
Started to introduce SCT015 BKN040 after 12z which could be too
late and/or not low enough.


Sun...VFR. Wind NW 10 kts.
Mon...VFR. Wind ENE - ESE 5 kts.
Tue...VFR. Chc TSRA late. Wind S/SSE 5-10 kts.




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