Tornadoes and Flash Flooding in Minnesota, July 25 to 26, 2020

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
646 AM CDT Sat Jul 25 2020

.UPDATE...
Issued at 636 AM CDT Sat Jul 25 2020

Updated to include 12z aviation discussion below.

&&

.SHORT TERM...(Today through Sunday)
Issued at 455 AM CDT Sat Jul 25 2020

Heat/humidity plus scattered thunderstorms will be the main issues
for the short-term portion of the forecast.

Early this morning, a few showers are drifting northeast in
western MN ahead of a weak north-south oriented surface trough
over the eastern Dakotas. While some of these storms have grown to
strong-severe levels over northwestern MN, the activity over the
WFO MPX portion of MN are running into a relative instability
minima along with the back edge of ridging aloft so the activity
for our coverage area has been waning as it shifts east. Still,
some scattered showers and a few thunderstorms can be expected as
the day progresses over much of the area due to the eroding of the
upper level ridge, shifting into the Great Lakes and Ohio Valley,
and the surface trough moves eastward. As H5 heights fall and a
strong low-to-mid level jet shifts across the area, within a
highly moist environment, residual mesoscale boundaries left
behind from the morning activity may act as foci for re-
development of convection later this afternoon and evening,
especially for eastern and southern portions of the WFO MPX
coverage area. There remains the Marginal Risk for severe
thunderstorms today, especially later this afternoon and evening,
when CINH will be at its lowest and instability at its highest.
Mitigating better coverage/strength will be multiple cloud layers
blocking optimal insolation but a few strong-severe storms cannot
be ruled out. Main hazards would be large hail and damaging winds.
In addition, there is some potential for training storms to
produce heavy rain but the problem is that CAMs have been backing
off more and more on this idea, to the point that multiple
solutions display very little confidence in the idea of not only
where but how strong storms may develop. It is in this vein that
PoPs have been reduced to the chance category (and that the
Excessive Rain Outlook has been scaled back to Marginal) because
uncertainty is quite high and model confidence is rather low.
Ironically, there is actually decent agreement among the
operational models (GFS/EC/UKMET) of NOT having heavy rain over
much of the area. Nevertheless, the idea cannot be discounted. As
the cold front upstream from the surface trough slides east across
the Dakotas tonight then across MN/WI tomorrow, chances for rain
increase mainly due to the frontal lift influences within the
continued moist environment. Not really expecting severe weather
nor heavy rain tomorrow but the precipitation may have some better
organization because of the low level forcing factor of the front.
Highest chances for precip will be over western WI into southern
MN. There is also a sliver of a Marginal Risk for severe weather
into our coverage area for Sunday, mainly owing to the unstable
environment and more appreciable forcing aloft with the frontal
passage.

As far as the heat, this also looks a bit more uncertain due to
the behavior of the convection and how it produces cloud cover
across the region. Dewpoints are still likely to hit the mid 70s
with pockets of 77-79 degree dewpoints at times today. There is
good confidence there due to the strong moisture advection into
the region. Where the uncertainty comes in is from the
temperatures and cloud cover. Should there be more cloud cover
than advertised, temperatures may not reach the upper 80s to lower
90s as forecast. Overall, if realized as forecast, this
combination of temps/dewpoints will push heat index values to the
100-105 degree range for not only all over central-southern MN but
also for portions of western WI. Therefore, have opted to expand
the Heat Advisory into western WI for today. Heat concerns will
abate this evening for all portions of the coverage area except
the Twin Cities metro which will have low temperatures early
Sunday morning still into the mid-70s. This will be followed up by
cooler highs in the upper 70s to mid 80s for much of the area, the
lower temperatures caused by the frontal passage and cloud/rain
complications. Still, heat index values for the Twin Cities metro
still look to reach the mid-90s on Sunday so there is the
possibility of having a Heat Advisory in the 7-county metro for
Sunday while headlines would not be necessary elsewhere.

.LONG TERM...(Sunday night through Friday)
Issued at 455 AM CDT Sat Jul 25 2020

By Sunday evening, the last of the precipitation potential will
have pushed to the east alongside the departing cold front, with
the forcing pushing over the eastern Great Lakes. A ridge
building in behind the departing system will bring us cooling
temperatures and less humid conditions for the coming week, with
the consensus being temperatures generally around 80 for the first
few days until the ridge breaks down. Once the ridge breaks down
by Thursday or Friday, things get a bit more interesting as
temperatures will begin to rise towards the mid 80s for highs,
which also comes with better chances for shortwave energy and
synoptic forcing for continued showers and potential
thunderstorms.

Right now, models are not entirely enthusiastic about any sensible
weather potential this week with the first mention of anything
being Thursday or Friday after the ridge weakens. However, this is
also driven by shortwave energy, which aren't likely to be
resolved until we get a bit closer, so exact timing and details
are not apparent at the moment. There are a few solutions beyond
this heading into next weekend, with the GFS favoring split flow
with the main jet stream to the north which would favor scattered
severe weather chances and no major systems in our area. The ECMWF
meanwhile, favors continued northwesterly flow which would keep
us a bit cooler with less of a chance for storms. Hopefully a
better consensus becomes apparent as we move a bit closer.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 636 AM CDT Sat Jul 25 2020

Complicated TAF set due to high uncertainty in SHRA/TSRA timing
throughout this duration. Showery activity in western MN slowly
shifting east is expected to continually weaken but not avoid the
MN TAF sites entirely. Even if only VFR, most (if not all) the MN
TAF sites may well receive some precipitation. Chances increase a
bit this afternoon into this evening, but best chances for most
consistent precipitation looks to be tonight into early Sunday
morning. Difficult to foresee conditions any worse than MVFR, with
VFR more likely. Will monitor radar trends and amend as needed.
Winds to generally remain SSW to SW with 10-15kt sustained winds
and gusts to near 25kt.

KMSP...VFR -SHRA possible later this morning through about noon
then a shot at some TSRA late this afternoon into this evening.
Better chances for TSRA come late this evening. But overall, the
timing of this precipitation can change. In addition, multiple
cloud layers are expected throughout the day, owing to the
unstable atmosphere expected today.

/OUTLOOK FOR KMSP/
Mon-Wed...VFR. Wind NW 5-10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Heat Advisory until 8 PM CDT this evening for Chippewa-Dunn-Eau
     Claire-Pepin-Pierce-St. Croix.

MN...Heat Advisory until 8 PM CDT this evening for Blue Earth-Brown-
     Chippewa-Faribault-Freeborn-Goodhue-Kandiyohi-Lac Qui Parle-
     Le Sueur-Martin-McLeod-Meeker-Nicollet-Pope-Redwood-
     Renville-Rice-Sibley-Steele-Stevens-Swift-Waseca-Watonwan-
     Wright-Yellow Medicine.

     Excessive Heat Warning until 8 PM CDT this evening for Anoka-
     Carver-Dakota-Hennepin-Ramsey-Scott-Washington.

&&

UPDATE...JPC
SHORT TERM...JPC
LONG TERM...TDH
AVIATION...JPC

Hazardous Weather Outlook...CORRECTED
National Weather Service Twin Cities/Chanhassen MN
400 AM CDT Sat Jul 25 2020

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
260900-
Douglas-Todd-Morrison-Mille Lacs-Kanabec-Stevens-Pope-Stearns-
Benton-Sherburne-Isanti-Chisago-Lac Qui Parle-Swift-Chippewa-
Kandiyohi-Meeker-Wright-Hennepin-Anoka-Ramsey-Washington-
Yellow Medicine-Renville-McLeod-Sibley-Carver-Scott-Dakota-
Redwood-Brown-Nicollet-Le Sueur-Rice-Goodhue-Watonwan-Blue Earth-
Waseca-Steele-Martin-Faribault-Freeborn-Polk-Barron-Rusk-
St. Croix-Pierce-Dunn-Pepin-Eau Claire-
400 AM CDT Sat Jul 25 2020

This Hazardous Weather Outlook is for portions of central and
southern Minnesota...and west central Wisconsin.

.DAY ONE...Today and Tonight.

Excessive Heat Warnings and Heat Advisories are in effect today
across all of central and southern Minnesota into portions of
western Wisconsin. Heat index values are expected to reach 100 to
105 degrees.

Scattered thunderstorms can be expected across the entire coverage
area at times through tonight. There is a Marginal Risk of severe
thunderstorms with the main hazards being large hail and damaging
winds. In addition, some thunderstorms may produce heavy rain.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Scattered to numerous thunderstorms are expected on Sunday. There
is a Marginal Risk for severe thunderstorms for southeastern
Minnesota into western Wisconsin. The main hazards are large hail
and damaging winds.

.SPOTTER INFORMATION STATEMENT...

SKYWARN spotter activation may be needed at times today through
Sunday.