Scientific Forecaster Discussion
fxus63 kmpx 230230
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
930 PM CDT Sat Jul 22 2017
Update...for 06z aviation discussion below
Short term...(this evening through sunday)
issued at 340 PM CDT Sat Jul 22 2017
Storms are beginning to develop along a convergent line where the
best instability exists in west central Wisconsin. This area is
also prime for isolated severe storms as wind shear values are
modest with instability parameters in the 2000-3000 j/kg range.
This area is likely the only place where a concentration of storms
will develop through the early evening. Based on the current trend
of mid-level winds and general movement of the storms, locations
from the southern Twin Cities Metro area, east toward Eau Claire
will have the best chance of storms through 6 PM. After 6 PM, most
of the activity should drift into southeast Minnesota, and toward
After this evenings storms, cooler air and much lower dew points
will push southward across the upper Midwest. This will be in the
wake of the upper storm system which is currently north of the
international border. There is enough moisture and instability
Sunday afternoon to generate a few showers in east central
Minnesota, and west central Wisconsin as moisture begins to wrap
around the upper low as it moves southeast across the Great Lakes.
Temperatures tomorrow will be 10 to 15 degrees cooler than today,
plus dew points will fall back into the 50s and 60s.
Long term...(sunday night through saturday)
issued at 330 PM CDT Sat Jul 22 2017
The main concern in the long term period is Tuesday and Tuesday
night when severe weather could be possible along and ahead of an
incoming cold front tied to a strong cyclone that will be
progressing eastward across southern Canada.
Sunday night and Monday...with high pressure in place Sunday night,
temperatures could be quite cool for mid-July with low 50s fairly
common and even a few 40s possible in prone areas from central Minnesota
through western WI. Southerly return flow will begin Monday in
western Minnesota and spread eastward with time. Hence, highs will be a
few degrees warmer than Sunday, especially across western Minnesota. Dew
points will be rise slowly across the west, likely into the mid 60s
by evening. Meanwhile, a strong cyclone will be pushing eastward
across Alberta and into Saskatchewan with an extensive surface cold
front draped to the south and emerging from the intermountain
west, reaching the High Plains later in the day on Monday.
Monday night into Tuesday...the strong cyclone in Canada will
continue essentially due east through Saskatchewan and eventually
through Manitoba from Monday night through Tuesday. On Monday
night, models are in decent agreement with the evolution of the
system and front; expect moist unstable air will build along the low
level convergence zone out ahead of the front, triggering
thunderstorms across the Dakotas by Monday evening. As the low
level jet in creases to around 40-50 knots overnight, could
foresee storms growing upscale into an MCS, which would primarily
impact our area Tuesday morning. A decaying mesoscale convective system reaching western
Minnesota by sunrise Tuesday seems like a plausible situation but is
highly uncertain. The NAM/GFS do indicate this potential, but the
European model (ecmwf) keeps any activity well to our north. So, chance pops
remain in the forecast for this period.
Tuesday through Wednesday...if we do have a decaying mesoscale convective system reach our
area Tuesday morning, the concern of course becomes whether or
not the atmosphere will recover for another round of potentially
stronger to severe storms later in the afternoon and evening. It's
difficult to tell 3 days out what is most likely in terms of a
morning mesoscale convective system either completely dying or holding together long
enough to keep US cloudy and mostly in check in terms of severe
weather potential. With a cold front coming in from the northwest,
expect that the odds of clearing it our and re-destabilizing
would be possible. Hence, Tuesday afternoon and evening is our
greatest risk of severe weather. It's a bit early to talk specific
values but with the front coming in from the west and the warm
sector destabilizing, and modest mid level flow aloft, could see a
chance for severe thunderstorms to develop. Hence, the Storm
Prediction Center has highlighted this region in the day 4
outlook. The timing of the frontal passage will drive the timing
of when storms leave the area, possibly another decaying mesoscale convective system by
Wednesday morning departing to our southeast. The models continue
to agree that the frontal boundary will be then shunted to our
south by mid-day Wednesday.
Wednesday night through Friday...the frontal boundary will be
oriented nearly east to west across southern Iowa by Wednesday night
and we expect it to sag even farther south with time as Canadian
high pressure fills in behind it as we will be in anti-cyclonic
northwesterly flow, downstream of the upper ridge axis. At this
time, it appears this could be a pleasant weather period with the
high pressure brining dew points down to more comfortable values
and temperatures running near normal for late July.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 930 PM CDT Sat Jul 22 2017
Mostly clear skies overnight with the only potential concern being
patchy fog over west central Wisconsin where winds are a bit
lighter. Have included MVFR reductions at Keau, but do not expect
the extent/impact of fog to be as big as the last several nights.
Sunday brings an increase in cu/strato-cu clouds. Kaxn and kstc
are expected to have MVFR cigs filter in during the morning, but
the remainder of sites should see a low-end VFR deck (circa 5000
ft). Prevalent northwest winds increase to around 9-12 knots
Sunday morning, with gusts into the upper teens. Skies clear
Sunday eve and winds decrease to around or below 9 knots and turn
no concerns other than mentioned above. Low-end VFR cigs develop
between 15-18z Sunday and persist through the afternoon.
/Outlook for kmsp/
Mon...VFR. Wind southeast 5 kts.
Tue...VFR. Chc thunderstorms and rain. MVFR/IFR in +shra. Wind S-SW 10-15 kts.
Wed...VFR. Chc thunderstorms and rain/rain showers early. MVFR/IFR in +shra. Wind northwest 5 kts.