Scientific Forecaster Discussion
fxus63 kmpx 201653
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1153 am CDT Wed Sep 20 2017
Updated for 18z taf discussion below...
Short term...(today and tonight)
issued at 443 am CDT Wed Sep 20 2017
The line of severe thunderstorms has weakened considerably early
this morning into just a line of showers and scattered
thunderstorms. Some redevelopment may occur along the cold front later
this morning and early afternoon mostly south and east of Eau
Claire before drier air reaches the entire County Warning Area by mid afternoon.
Severe weather is not anticipated.
A surface ridge will pass overhead early this evening. Clear
skies and calm winds into the early overnight hours should allow
for some fog development over western Wisconsin. Believe return
flow will develop around midnight further west and keep fog from
Long term...(thursday through tuesday)
issued at 443 am CDT Wed Sep 20 2017
Southeast flow will strengthen Thursday. Low level moisture will
return rather quickly with the cold front washing out over
southern Iowa and southern Wisconsin tonight. The daylight hours
should be dry with mid level height rises and the lack of a
trigger, but by Thursday night with a strengthening low level jet and
deeper low level moisture establishing itself locally, some
elevated storms may fire. Capping should limit the overall areal
coverage of activity. Believe the best chance for convection will
be north of a line from Willmar Minnesota to Rice Lake WI. GFS continues
to be too widespread with development, although I suppose some
altocumulus castellanus isn't out of the realm of possibility. Given MUCAPE in
excess of 2500-3000 j/kg and enough wind shear, could see a few
severe storms capable of large hail across central Minnesota and
northwest WI. The warm front lifts north early Friday morning
with any convection focusing more across northern Minnesota and Lake
Friday still looks quite hot and humid. Thicknesses reaching 580
dm, and thermal ridging characterized by 850 mb temps between
21-24c and 925 mb temps of 25-28c with deep mixing should send
temperatures into the lower 90s over much of the area (possibly
middle 90s in spots) if we mix as deeply as forecast soundings
indicate. Given unusually high pwats, this deep mixing will likely
not result in a decline of afternoon dew points and if anything
they may continue to rise with continued moisture advection. Some
dew points may rise into the lower 70s by peak heating with heat
indices reaching the mid to upper 90s. We may need to eventually
issue a heat advisory. Highs were inched upward again given latest
guidance, but are still a couple degrees shy of records. Lows
Friday will likely set new records at msp, stc, and eau in the low
Capping will hang on over much of the area into Friday night,
except it should break over western Minnesota Friday afternoon or early
evening. Scattered to numerous thunderstorms are likely to develop
as the front approaches. A very unstable atmosphere in conjunction
with impressive wind shear will lead to a heightened risk of
severe weather, with all modes possible. As the Storm Prediction Center day 3 notes,
shear parallel to the boundary may limit the tornado threat, but
will likely increase coverage of convection on the boundary.
This front has slowed on much of the guidance for the weekend,
resulting in a slight shift westward with the heavy rain threat
and an increase in temperatures further east. This isn't too
surprising given the ridge to the east and possible interactions
with Hurricane Maria. Nonetheless, it still appears to be a very
good setup for excessive rainfall from west central to northeast
Minnesota with widespread totals of 3 to 6 inches possible. By Monday
the front will finally begin making decent progress eastward.
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1153 am CDT Wed Sep 20 2017
Some MVFR cigs to the east early then becoming skc by 00z. Expect
W-NW winds through the afternoon some gusts to 18kts possible
mainly east after fropa. Could still see a -shra isold -tsra near
Keau through about 21z but coverage should be low enough to
exclude in taf for now. Increasing se winds into Thu afternoon
with some stratocu developing ahead of warm front to the south.
Kmsp...becoming sky clear with NW-W winds becoming light se into
tonight. Some cu developing out ahead of incoming warm front to
the south Thu afternoon.
/Outlook for kmsp/
Thu night...VFR. Winds southeast 10 kts.
Fri...VFR. Winds S 10-15 kts.
Sat...mainly VFR. Chance MVFR/tsra. Winds SW 5-10 kts.