Hail and Damaging Winds in Minnesota, September 5 to 6, 2020

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
638 PM CDT Sat Sep 5 2020

.SHORT TERM...(This evening through Sunday night)
Issued at 350 PM CDT Sat Sep 5 2020

Confidence is now pretty high that focus of thunderstorm and severe
threat this evening will be centered on the Minnesota River Valley
from west central down into south central MN. The primary threat
continues to be for very large hail, as stable boundary layer
profiles look deep enough to for sure preclude any tornado threat,
but likely any wind threat as well.

If you head out today with mostly sunny skies, temperatures mainly
in the 70s, and dewpoints in the 40s/50s, the furthest thing from
your mind is that we have an atmosphere that will be primed for
producing supercells with very large hail. However, we are already
seeing the first clues from the atmosphere that things are going to
get rather bumpy tonight. Visible/IR satellite has this clue with
the band of rather healthy accas that has developed across southwest
MN. This accas is forming to the northeast of a strong EML and
building instability that reaches up into southeast SODAK. This area
of accas is where storms will eventually develop this evening.
Current thinking is that thunderstorms will initiate between 8pm and
10pm out by Madison and Canby and dive southeast along the MN
River toward Albert Lea from there. Thermodynamic profiles will
favor the development of vigorous updrafts, with upwards of 50 kts
of bulk shear to work with. Looking closer at forecast soundings
though, it does appear all of this activity will be rooted above
the boundary, likely around h9, or 2000 to 2500 ft AGL. Below
this, a rather strong inversion will be in place. This inversion
will keep the tornado threat at nearly zero, with even strong
surface winds being tough to achieve, even if we get good bowing
segments, similar to what we saw with the null severe event back
in the early morning hours of August 28th. Of course this
inversion won't stop large hail stones and the Enhanced risk area
on the Day 1 Convective Outlook from the SPC continues to look
good for where the highest severe threat is.

Sunday continues to look dry, with any thunderstorm development
along the cold front in the afternoon happening well east of the MPX
area. Temperatures Sunday will be dependent on how long we can hold
onto the early morning thermal ridge, and how persistent cloud cover
ends up being in our northern and eastern sections of the CWA. It
continues to look likely that we'll see a few 90s down along the I-
90 corridor, but those clouds are starting to look like they'll be
pretty persistent out toward Ladysmith and Eau Claire, where low 70s
may be about it for highs.

No weather issues expected Sunday night with dry high pressure
coming in from Canada, resulting in temperatures dropping into the
40s and low 50s as we start getting into the abnormally cold airmass
that will be with us through the middle of next week.

.LONG TERM...(Monday through Saturday)
Issued at 350 PM CDT Sat Sep 5 2020

The transition to much cooler weather will be underway at start of
the period effectively putting an end to our summer. This pattern
change is due to a building upper-level ridge over the Gulf of
Alaska and a strong northerly downstream jet max over British
Columbia and Alberta. These two features will eventually amplify the
very broad cyclonic flow over the northern US and favor troughing
over the western US prior to mid-week. Strong, northerly flow over
the northern US is forecast to continually advect a polar,
continental airmass from northern Canada into our region. For the
MPX forecast area, Labor Day does not look pleasant as highs will
struggle to make 60 and skies will be overcast. Rain is also
possible as Monday progresses (especially in southern MN) as low-
level diffluence within a zonally-oriented, strong 850 hPa thermal
gradient will foster frontogenesis. The aforementioned western US
trough is forecast to continue to amplify and shift eastward into
the central High Plains, dragging more cold air southward into the
northern US. In turn, temperatures across the CWA will cool to 15-20
degrees below normal from Monday night into Wednesday night. The
possibility of frost exists for western and northern portions of our
CWA Tuesday and Wednesday night as temperatures will creep close to
32F. The aforementioned 850 hPa thermal gradient draped through
the Midwest will also mean that Tuesday and Wednesday could be on
the wet side across southeastern MN and western WI as any
dynamical lift would generate precipitation. Clouds should also
plague much of the region.

Large questions surround the evolution of the aforementioned western
US trough. Forecast models eventually want to form a cutoff low from
this feature, but timing and placement differences exist. One thing
the models do agree on is that the low will move northeast into the
Midwest forming an open wave system by late week. However, a more
western track would bring a greater possibility of precipitation for
our region while an eastern track would keep us drier. Regardless,
temperatures do appear to rebound into the upper-60s and low-70s by
Friday continuing to warm into next weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 638 PM CDT Sat Sep 5 2020

Accas continues to develop along a warm front across southwestern
MN early this evening. Thunderstorm development will likely occur
rapidly between 03-04Z along the MN river and spread eastward
through the overnight, likely impacting all the TAF sites in some
capacity. The heaviest rain will occur near MKT and RWF, with
large hail is also possible. MVFR cigs could develop across
central MN and western WI late tonight and early Sunday, with cigs
scattering out by late morning. Southerly wind will increase late
and could gust through the mid morning before easing a bit with
the approach of the front. Gusty northwest winds to 30-35 kt
possible across western and central MN Sunday afternoon.

KMSP...TSRA likely in the 07-09Z time range. MVFR cigs are
possible following the convective activity, but the best chance
will be to the north and east, so left a SCT mention in for now.

/OUTLOOK FOR KMSP/
Mon...VFR with -RA/MVFR possible. Wind NW 10-15 kts.
Tue...VFR with -RA/MVFR possible. Wind NE 10-15 kts.
Wed...MVFR with -RA/IFR possible. Wind NE 10-15 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.
&&

SHORT TERM...MPG
LONG TERM...CTG
AVIATION...Borghoff

Hazardous Weather Outlook...UPDATED
National Weather Service Twin Cities/Chanhassen MN
757 PM CDT Sat Sep 5 2020

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
070100-
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-
757 PM CDT Sat Sep 5 2020

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

.DAY ONE...Tonight.

Thunderstorms are expected to develop by 11 PM in western
Minnesota and track eastward across southern Minnesota through
the night. Thunderstorms will be capable of producing very large
hail, possibly to baseball size, and isolated wind damage. The
greatest risk of severe weather is expected to be along the
Minnesota River Valley from west central into south central
Minnesota. In addition, some training of thunderstorms is
possible with a low risk of flash flooding.

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

Frost is possible over west central and central Minnesota
midweek.

.SPOTTER INFORMATION STATEMENT...

SKYWARN spotter activation will be needed late this evening and
overnight across western and southern Minnesota.

Borghoff