Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 210403

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1103 PM CDT Thu Jul 20 2017

.UPDATE...For 06z Aviation discussion below


.SHORT TERM...(This evening through Friday)
Issued at 351 PM CDT Thu Jul 20 2017

The main concerns in the short term period are for thunderstorm
chances overnight through tomorrow, and heat and humidity building
tomorrow as well.  The forecast is messy for this period, with
multiple rounds of showers and thunderstorms possible, that initiate

For tonight, the remainder of the evening will continue to be quiet
with mainly high clouds passing through.  Strong theta-e advection
ahead of a low pressure system in the high plains will likely begin
producing precipitation after midnight in extreme southern and
western Minnesota. Thunderstorms will be possible as MLCAPE values
around 1000 j/kg move through.

For Friday...the overnight activity should proceed northeastward
following the forcing of the theta-e advection max.  Behind this
activity, we expect a break in the acitivity and for the heat and
humidity to build.  Dew points will rise above 70 degrees during the
day as the warm front draped east from the high plains low inches
northward through Iowa.  MLCAPE will build during the day and could
easily see around 3,000 j/kg develop.  The SPC has included a slight
risk for severe weather for much of the area, with hail and damaging
winds the primary severe threats, in addition to heavy rainfall
potential given such high dew points and PWAT values around 2.0
inches.  Expect most of the activity tomorrow afternoon locally to
be in eastern MN to western WI.

.LONG TERM...(Friday night through Thursday)
Issued at 351 PM CDT Thu Jul 20 2017

Convection will continue to move east through the area Saturday
evening, clearing the area and moving off into central Wisconsin
after midnight. CAPE values greater than 2500 J/kg along with
effective deep layer shear over 40 kts will continue to support a
threat for severe weather through the evening, as highlighted in the
SPC Day 2 outlook. Damaging wind still looks to be the main threat
with this event given the shear vector orientation supporting a
linear storm mode. Additionally a flooding threat may develop as the
night goes on with training thunderstorms likely given the
orientation of the steering flow parallel to the warm front. Current
thinking is that the heaviest rainfall should stay off to the
southeast but will have to monitor this situation given rainfall
amounts of 3-5" from the Twin Cities metro and southeast on
Wednesday night.

The surface low departs to the east on Saturday with an occluded
front lingering from along the I-35 corridor to points east during
the afternoon. Current thinking is that some scattered thunderstorms
may develop along & east of I-35 with the most widespread convection
up across the MN Arrowhead where there is better upper-level support.

High pressure at the surface dominates the region Sunday & Monday
with a return to northerly winds bringing much drier air and a brief
return to below-normal temperatures. This relief won`t last for
long as the high slides off to the east and muggy summer weather
returns, along with another active and rainy period beginning
Tuesday evening. Models agree with southerly flow bringing plenty
of moisture at the surface and a number of disturbances rounding the
periphery of the upper-level ridge, but diverge on the placement of
the warm front at the surface. For now count on at least general
chances of precip across the southern half of MN for the second half
of the week


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1104 PM CDT Thu Jul 20 2017

High clouds will eventually stream in from the west/south
overnight. In the meantime clear skies, light winds, and low level
moisture will aid in fog development overnight, particularly from
central MN into west central WI. Expect occasional/patchy
visibility reductions to IFR, and possibly even LIFR at KRNH and
KEAU. The next area of showers and thunderstorms is expected to
enter southwest MN around daybreak, then spread toward central
MN/WI during the morning hours. MVFR conditions attendant with the
shra/ts are possible. More development will be possible in the
afternoon/eve. Variable winds under 10 knots are expected
overnight, then winds increase from the southeast on Friday.

Could see 4-5sm visibility reductions with patchy fog around the
area overnight through daybreak. Showers in the vicinity of the
field still look to be possible around 15z, with stronger activity
possible after 20z into the evening hours.

Sat...Mainly VFR. Chc MVFR in -SHRA/TSRA. Winds lgt and vrbl.
Sun...VFR. Wind NW 10 kts.
Mon...VFR. Wind ESE 5 kts.




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