Special Weather Statement, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1228 PM CDT Sat Aug 8 2020

.UPDATE...
Issued at 1227 PM CDT Sat Aug 8 2020

Updated to include 18Z aviation discussion below.

UPDATE
&&

.SHORT TERM...(Today through Sunday Night)
Issued at 300 AM CDT Sat Aug 8 2020

A potentially active weekend in terms of weather is on the
horizon. The chance for strong storms exists right out of the
gate this morning, as storms over northeast MN and northern WI
shift into the west central WI portion of the forecast area. This
activity could flare a bit after sunrise this morning as daytime
heating ensues. The primary threat would be damaging winds. Given
modest lapse rates around 7 C/km, large hail can't be ruled out,
but is a slightly lesser threat.

By mid to late morning we should see a break in any thunderstorm
activity as we await the next round. Heat and humidity will build
in the warm sector ahead of the front. Have increased temperatures
into the mid to upper 80s across most of the area (slightly
"cooler" in WI with low to mid 80s). Dewpoints will be pushing 75
degrees, which will mean heat indices will flirt with mid 90s.
Given my forecast temps and dewpoints are near the higher end of
numerical guidance, and considering the potential for
clouds/convection to hinder heating a tad, will not issue a Heat
Advisory, but highlight the threat in the HWO/graphical products.
It will certainly be an uncomfortable afternoon for anyone active
outside given the relatively light winds combined with the heat &
humidity, as illustrated by WBGT (Wet Bulb Globe Temperature)
values nearing 85 degrees this afternoon.

The August-like humidity will also be revealed in progged
MLCAPEs circa 4000 J/kg from southwest into central Minnesota this
afternoon. H7-H5 lapse rates are also forecast to be around 8.5-9
C/km. Therefore, if we can get any convection to pop, strong to
severe storms will be a decent possibility. The one big limiting
factor will be warm H7 temps of +13C to +14C. Therefore, any
afternoon activity would likely be isolated in nature, so a 20 POP
seems sufficient for now.

Tonight we expect to see a mid-level shortwave pass across the
area, which will act to override any capping aloft and bring more
widespread activity. Given the stalled boundary bisecting the area,
precipitable water values around 2 inches, and forcing supplied
by the shortwave, heavy rainfall is expected. Exactly where that
sets up is still of low confidence, but as of now west central MN
looks to have the greatest risk on the cool side of the boundary.
Given antecedent soil conditions, thirsty crops, etc., think this
area can withstand a fairly heavy bout of rainfall before we see
much in the way of impacts, so will hold off on any flash flood
headlines at this point.

Showers and thunderstorms spread east across the area on Sunday
attendant with the frontal passage. Steep lapse rates and low-
level shear look sufficient to bring a threat for damaging winds.
Sunday night differential divergence associated with the entrance
region of the upper level jet streak over northwest Ontario
supplies renewed forcing for showers and thunderstorms through
Sunday night.

.LONG TERM...(Monday through Friday)
Issued at 300 AM CDT Sat Aug 8 2020

The long term period will start off cooler and drier before
transitioning to a more typical August pattern.

At the start of the period the cold front will have exited the CWA
to the southeast and surface high pressure will move in behind it.
High temperatures on Monday will be in the upper 70s with a few 80s
possible. Drier air behind the cold front will drop the dewpoints
back into the comfortable 50s where they will remain through
Tuesday. Tuesday will be a few degrees warmer than Monday, but with
relatively lower dewpoints should still feel reasonably comfortable.

On Wednesday high pressure moves off to the east and warmer, more
humid air starts to return to the area. Highs temperatures through
the rest of the period will be seasonable in the low to mid 80s, and
overnight lows mainly in the 60s. The weather should also become
more active with shortwave energy bringing periodic chances for
showers and thunderstorms through the rest of the long term.
There is not much timing on the details of when and where any
precipitation will occur, so will stick with the low PoPs from
Wednesday on that were given by the NBM.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1227 PM CDT Sat Aug 8 2020

Anything but a high confidence forecast this go around with an
unstable airmass and various weak forcing mechanisms in the area.
Tried to time some convection this afternoon with the presence of
upstream activity and some of the CAMs bringing activity through
the central portion of the area. A better chance would seem to be
later tonight when the shorwave trough over western South Dakota
evident on water vapor imagery work into the area. Cloud cover
remains an issue, with widespread MVFR ceilings slow to leave
today. Satellite shows some mixing starting to occur, although
debris clouds streaming in from the west could work against that
for a time. However, continued with an improving trend in ceiling
heights this afternoon. But, generally kept VFR and MVFR ceilings
around at most sites through the period.

KMSP...There is significant uncertainty in the likelihood and
timing of any convection this afternoon and tonight, so it's
likely that later forecasts will make some adjustments. Should see
ceilings get into VFR fairly soon, but MVFR will return tonight,
then should lift again sunday.

/OUTLOOK FOR KMSP/
Sunday night...MVFR possible with a chance of showers and
thunderstorms. South wind 5 to 15 kt becoming northwest.
Monday...VFR. Northwest wind 5 to 10 kt.
Monday night...VFR. Variable wind less than 5 kt.
Tuesday...VFR. Southwest wind around 10 kt.
Tuesday night...VFR. South wind around 5 kt.
Wednesday...VFR. Southeast wind around 10 kt.

&&

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

UPDATE...
SHORT TERM...LS
LONG TERM...CEO
AVIATION...

Hazardous Weather Outlook
National Weather Service Twin Cities/Chanhassen MN
956 AM CDT Sat Aug 8 2020

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
091500-
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-
956 AM CDT Sat Aug 8 2020

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

.DAY ONE...Today and Tonight.

Thunderstorms are possible today and tonight. A few storms could
become severe this afternoon and tonight, with the primary
threats being large hail, damaging winds, and heavy rainfall.
Heat and humidity will build today, with afternoon heat indices
peaking in the low to mid 90s across the Minnesota portion of the
outlook area.

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

Additional thunderstorms are expected on Sunday and Sunday night.
A few storms could become severe with the primary threats being
large hail, damaging winds, and heavy rainfall.

Periodic thunderstorms are possible Wednesday through Friday.
Severe storms are not expected at this time.

.SPOTTER INFORMATION STATEMENT...

SKYWARN spotter activation may be needed this afternoon and
evening.

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