Area Forecast Discussion
Issued by NWS Twin Cities, MN

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433
FXUS63 KMPX 201830 AAB
AFDMPX

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
130 PM CDT WED JUL 20 2016

.SHORT TERM...(Today and tonight)
Issued at 356 AM CDT Wed Jul 20 2016

Scattered showers and thunderstorms have popped in west central MN
overnight and the general expectation is for that activity to evolve
east from 09Z to 12Z this morning, with the highest pops across
south central MN.  The activity is at odds with the GFS and NAM
which likely hold back on precip development due to warmer air aloft
(700mb temps of +12C).  Many of the CAMs came in to a little bit of
agreement overnight, following the lead of the operational HRRR. The
ECMWF also hinted at precip across southern MN into western WI.
Decided to cancel the Flash Flood Watch though, as development as
been subdued and the low level jet will be weakening through the
remainder of the morning.

Expecting the precip and cloud cover to move east of I-35 by about
noon today, but could linger through a good portion of the afternoon
in western Wisconsin.  Some of the hi-res guidance wants to produce
more showers and thunderstorms this afternoon, but we are not buying
it at this time with the thinking that the mid level warming will be
too much for any vertical growth to overcome.

The previous forecast for todays highs looked really good, so just a
few minor tweaks were made to highs across the area.  Still
expecting low 90s with dewpoints well into the 70s across south
central MN, possibly approaching 80s especially for locations that
see measurable precipitation this morning.  This combination of heat
and humidity brings our heat index values in the 100 to 110 degree
range across all of southern and central MN.  In western WI, lower
temperatures due to the thermal ridge being off to the west and the
lingering cloud cover.  For this reason, continued the Excessive
Heat Watch for Thursday in the Wisconsin counties.  Today will be
hot and humid there as well, and heat indices will be in the upper
90s.

Tonight is going to be very warm and humid as advertised.  Again,
only minor tweaks needed to the overnight low forecast. The thermal
ridge aloft will really push through our area overnight tonight,
which will make tonight the warmest nighttime during this heat wave.
This is a dangerous situation for those with no air conditioning as
lows of 75 to 80 degrees and dew points around 70 to 75 will provide
little relief from the heat.

.LONG TERM...(Thursday through Tuesday)
Issued at 356 AM CDT Wed Jul 20 2016

There are two concerns in the long term. The first is the heat
wave peaking on Thursday but lingering through Friday. The second
concern is severe weather potential from Friday night through
Saturday night.

Confidence remains high across the MN CWA for the excessive heat
warning on Thursday as heat and humidity peak. Afternoon heat
indices from 105 to 112 degrees cover much of the MN CWA. The
exception is north of the Twin Cities, around Onamia, Mora and
Cambridge, where we issued a heat advisory for heat indices in
the 100 to 105 degree range. We held on the excessive heat watch
for the WI CWA for Thursday, instead of going with a warning,
primarily due to convective concerns. Some deterministic
solutions, as well as CAMS, are trying to generate convection
along the frontal boundary that slips slowly southeast across the
FA on Thursday. Although 700 mb temperatures of +14 deg C are
progged across the MN CWA on Thursday, temperatures drop off
heading east across WI with convection certainly possible. If
later model/CAM trends negate the convection, then it`s a green
light for an excessive heat warning with heat indices similar to
those in the MN CWA.

The boundary that moves through on Thursday will switch the
surface flow around to the north Thursday night and early Friday.
This will result in much lower dew points (middle 60`s) working
into area. An exception is along I-90 where lower to middle 70 dew
points may remain. High temperatures on Friday will still be hot,
with 90-95 degrees FA wide. However, with the lower dew points we
are failing to meet heat advisory criteria across the FA except
along the IA border. Collaboration with surrounding offices on
adjusting headlines for Friday, either to drop down to an advisory
or cancel altogether was met with a wait and see on how the
boundary situation works out. This was another reason for the
watch continuing in our WI CWA through Friday due to the
uncertainty.

Switching gears into the weekend, a more organized and rather
strong low pressure system will move through. First, the boundary
that moves through on Thursday will move back north Friday night
and early Saturday as a warm front. Convection will likely develop
in ND and slide southeast along the front late Friday night and
Saturday. This will be followed by a cold frontal passage late
Saturday into Sunday with more showers and thunderstorms
developing. There is certainly potential for severe weather and
heavy rainfall as this system moves through during the weekend.

We may salvage a dry day on Monday but then another low pressure
system will likely be right on our western doorstep by Tuesday
bringing more showers and thunderstorms to close the long term.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 130 PM CDT Wed Jul 20 2016

MVFR cigs continue to linger near AXN, but bases are beginning to
rise and clouds should scatter this afternoon. Elsewhere, VFR
conditions are expected through the period.

KMSP...No additional concerns.

/OUTLOOK FOR KMSP/
Thu...VFR. Wind SSW at 5-10kts.
Fri...VFR. Chc -TSRA late. Wind SE 5 kt.
Sat...VFR. Chc -TSRA late. Wind S-SE 5-10 kt.

&&

.MPX Watches/Warnings/Advisories...
WI...Excessive Heat Warning until 7 PM CDT Friday for WIZ024-026.

     Excessive Heat Warning from noon to 10 PM CDT Thursday for
     WIZ014>016-023-025-027-028.

MN...Excessive Heat Warning until 7 PM CDT Friday for MNZ054-056-
     060>070-073>078-082>085-091>093.

     Excessive Heat Warning until 10 PM CDT Thursday for MNZ041>045-
     047>053-055-057>059.

&&

$$

SHORT TERM...SPD
LONG TERM...RAH
AVIATION...BORGHOFF



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