Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 211011
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
511 AM CDT WED SEP 21 2016

.SHORT TERM...(Today and tonight)
Issued at 354 AM CDT Wed Sep 21 2016

The main threat continues to be for excessive rainfall this
afternoon through tonight. This morning will be relatively quiet
across the forecast area.

Overnight, strong to severe thunderstorms initiated along the low
level convergence zone along the warm front that extends from
approximately western Nebraska through far southern Minnesota. There
were some training thunderstorms initially and Waseca and Steele
counties were the hardest hit.  There, widespread 2 to 4 inches of
rain has fallen, possibly higher isolated amounts.  That cluster of
storms eventually became forward-propagating, and just in time or
the situation could have been worse in Waseca and Steele counties.

The forecast for this evening has remained pretty consistent.  We`ve
been very concerned about the excessive rainfall aspect and
rightfully so given the near record PWATs, and similarity of this
pattern to that of the September 22-23, 2010 heavy rain event.

However, there is a severe weather threat today and tonight.  The
SPC has included most of southern MN and a portion of west central
WI in a `slight` risk of severe weather.  Instability will build
throughout the day across southern MN as the warm front lifts north
of Mankato. CAPE of 2,000-3,000 j/kg will build south of the front.
The front will essentially become stationary across southern MN by
this afternoon, with elevated thunderstorms likely to fire just to
its north initially. PWATs increase to near 2 inches as the upper
wave which originated off the coast of California surges into the
area by this evening. This will aid in the development of
thunderstorms with the increase in moisture and vorticity
advection. CAMs agree that storms will be oriented east to west
just north of the front (north of the metro) late this afternoon
and will begin sinking southward this evening. This would indicate
that once the storms congeal, they may become forward-
propagating/outflow dominated. Still, these will be very
efficient rainfall producers given the ingredients at hand and
could produce flash-flooding, large hail, and damaging downburst
winds. The tornado threat is low, but if storms become surface
based south of the front, can`t completely rule out the
possibility.

.LONG TERM...(Thursday through Tuesday)
Issued at 354 AM CDT Wed Sep 21 2016

Thursday through Sunday...The aforementioned surface boundary
approaching from the northwest today will become stalled out
across the region starting Thursday, lasting through Friday. The
front will slowly sag south into Iowa during this period. This
motion combined with a sharp incoming upper level ridge axis will
help to subdue the coverage and intensity of rainfall by late day
Thursday on through early Saturday. However, as the upper level
ridge departs, a deep upper level low over the central Rockies
will open up and swing to the north, helping drag the stalled
front back north as a warm front. This warm front will become
part of a more organized low pressure system developing over the
lee of the Rockies in CO/NE. The upper low will then drive the
deepening surface low into the Dakotas Friday night through
Saturday, which will shift the cold front of this system across
the central Plains Saturday and across the mid-and upper-
Mississippi River Valley regions Saturday night into Sunday. This
will then bring an increase to pops from the slight-low end
chance categories Thursday and Friday to the likely
category Saturday and Saturday night. Still a bit early to
determine any severe and/or flooding potential but the
possibility for either cannot be ruled out given the orientation
and support of the upper level flow plus the resurgence of deep
moisture into the region with these features.

Monday and Tuesday...Difficult to pin down even large-scale
features for early next week due to model differences but the
general theme is to have the upper level low drop back down over
the western Great Lakes region. The idea of having sharp ridge
axes over the eastern and western CONUS coasts seems to be more
consistent between the GFS and ECWMF but will need to see how
this idea evolves over the next few days before increasing pops
beyond chance category.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1126 PM CDT Tue Sep 20 2016

Scattered SHRA/TSRA will continue to push eastward early this
morning across southern MN and adjoining areas of western WI. The
northern edge of the activity will scrape KMSP while KEAU is more
in line for a few hours of off and on TSRA. There is some concern
on the MVFR ceilings developing late tonight across these same
areas. Unified guidance from earlier this evening is beginning to
diverge. However, given the low level moisture increase along the
stationary front across southern MN, decided to hold on to the
MVFR ceilings and visibilities for now. The ceilings may be hard
to lift on Wednesday with the boundary holding steady across the
south. In addition, more TSRA is likely to begin developing by
late afternoon and continuing through the evening. This activity
should be farther north then the current activity with KSTC, the
Twin Cities, KRNH and KEAU having pretty good chances for
SHRA/TSRA Wednesday evening.

KMSP...Much of the ongoing SHRA/TSRA will stay south of the
airfield early this morning, but an isolated SHRA/TSRA cannot be
ruled out especially prior to 21/06z. Still holding on to MVFR
ceilings and visibilities developing toward daybreak and lasting
through the day. Better chance for scattered to numerous SHRA/TSRA
by Wednesday evening.

/OUTLOOK FOR KMSP/
Thu...MVFR. IFR likely with SHRA/TS. Wind SE 10-20 kts.
Fri...MVFR possible. Wind E 5-15kts.
Sat...VFR with MVFR/SHRA possible. Wind SE at 10g15kt.

&&

.HYDROLOGY...
Issued at 354 AM CDT Wed Sep 21 2016

Flash flooding is obviously a threat into Thursday. Depending on
where and how much rain does fall, we could certainly see river
flooding become an issue heading into this weekend. A very wet
last few months has maintained high soil moisture and being late
September, the crops won`t be very helpful in using up the
moisture as compared to mid summer. Hence, the runoff rate could
be very high across the area which will end up flowing quickly
into the river systems. In particular, our areas of concern are
all of southern MN and into west central WI where we have
widespread 1.5 to 3 inches of QPF in the forecast. Isolated
amounts will likely exceed 3 inches.

&&

.MPX Watches/Warnings/Advisories...
WI...Flash Flood Watch through Thursday evening for WIZ024-026-028.

     Flash Flood Watch from 1 PM CDT this afternoon through Thursday
     morning for WIZ014>016-023-025-027.

MN...Flash Flood Watch through Thursday evening for MNZ067-069-070-
     074>078-082>085-091>093.

     Flash Flood Watch from 1 PM CDT this afternoon through Thursday
     morning for MNZ051>053-059>063-066-068.

&&

$$

SHORT TERM...SPD
LONG TERM...JPC
AVIATION...RAH
HYDROLOGY...SPD



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