Special Weather Statement, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
306 PM CDT Thu Aug 18 2022

.DISCUSSION...(This evening through Thursday)
Issued at 238 PM CDT Thu Aug 18 2022

KEY MESSAGES:

- Localized heavy rainfall and minor flooding tonight through Friday.

- A few strong thunderstorms are possible tonight.

Thunderstorms have become more widespread today with strong forcing
aloft arriving as a shortwave drops out of the Canadian prairies.
This shortwave deepens into a closed low overnight and sits over the
region through Saturday, which will generate multiple rounds of
scattered showers and thunderstorms. The most robust thunderstorms
and best chance for heavy rain comes this afternoon through the
overnight hours with MLCAPE values of 1000-1500 J/kg across the
region. Deep shear remains weak, with values only around 20-30 kts,
but this should be sufficient for a few strong thunderstorms capable
of producing small hail and brief gusty winds. Localized heavy rain
appears to be the main threat with thunderstorms this afternoon and
tonight, as slow storm motion vectors coupled with precipitable water
values near 1.5" result in rainfall rates approaching 2" per hour.
Dry antecedent moisture conditions and fairly scattered thunderstorms
should prevent any widespread flooding impacts, but localized
flooding is possible tonight - primarily in urban areas that may see
several hours of thunderstorms.

A lull in the showers and thunderstorms is expected late tonight
through Friday morning as the center of the circulation aloft sits
directly over the region, and thus little vorticity advection exists
for upward motion. Scattered showers and thunderstorms will redevelop
during the afternoon as vorticity advection increases aloft, and
surface heating leads to a few hundred J/kg of CAPE developing. The
thunderstorms Friday afternoon will likely be weaker than the ones
today, along with less of a chance for heavy rain. Precipitation
will wane overnight Friday into Saturday as the upper level low
finally begins to move east of the area. isolated showers are
possible through saturday afternoon across eastern Minnesota and
western Wisconsin as weak cyclonic flow aloft lingers over the Upper
Mississippi valley.

Weak surface high pressure develops over the region in the wake of
this weekend's system, leading to mostly dry conditions Sunday
through midweek. Cool temperatures are expected this weekend with
widespread cloud cover and precipitation keeping daytime high
temperatures in the upper 60s to mid 70s. Temperatures slightly warm
Sunday through midweek, but will remain seasonable with daytime highs
near 80

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1238 PM CDT Thu Aug 18 2022

Active weather will continue through the afternoon and evening hours
with slow moving showers and storms expected to blossom over the
next couple of hours. A few of these storms could be strong to severe
but isolated in nature. Showery activity will begin to wane
overnight leaving some low ceilings across the region. Look for
ceilings around MVFR/IFR. Winds should remain less than 10 kts with
the exception of storms passing over a terminal. Another dose of
showers and thunderstorms is on tap for tomorrow as well.

KMSP...SHowers and thunderstorms are still expected to breach the
terminal this afternoon and linger for a couple of hours. The main
line of storms are expected to push east after 02Z with some
residual showers in the area. Ceilings will lower to IFR through the
overnight period and may have some visibility reductions due to
drizzle. Another round of showers and storms is possible tomorrow
afternoon.

/OUTLOOK FOR KMSP/
Fri...Periods of -SHRA/-TSRA/MVFR. Wind S 5-10 kts.
Sat...VFR. -SHRA likely. Chc -TSRA/MVFR. Wind NE 5-10 kts.

&&

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

DISCUSSION...ETA
AVIATION...Britt

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