Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 122342

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
642 PM CDT Sat Aug 12 2017

.SHORT TERM...(This evening through Sunday)
Issued at 409 PM CDT Sat Aug 12 2017

Upper low near the ND/MT/SK border has been slow to move, and
there has been dry air and confluent upper flow over our area for
a couple days. The upper low is moving a little faster and also showing
evidence of significant forcing. An intensifying jet core has
also been moving into SD, to the south of the upper low. This
setup is such that low level features will strengthen, and warm
and moist air will be advecting into MN tomorrow and then WI
tomorrow night, courtesy of 30 knot winds at the 850 level.
Abundant mid and upper level moisture has been streaming in from
the Pacific. Precipitable water values should reach at least 1.4
inches tomorrow afternoon. All in all, a good setup for plenty of
rain moving in late tonight and over most of our area on Sunday.
Indeed, multiple models are pointing plenty of rain, and the wpc
amounts look reasonable, with max amounts over one inch. Will
obviously have to watch for higher amounts in local areas.

Until then, surface ridge will slowly depart tonight, but not
before one more night of calm winds and nearly cloudless skies in
Wisconsin and eastern MN. It`s entirely possible some locales may
be back down into the 40s tomorrow morning in our eastern
counties. Incoming clouds from the west will help keep lows mostly
in the upper 50s over our western counties.

.LONG TERM...(Sunday night through Saturday)
Issued at 409 PM CDT Sat Aug 12 2017

Forecast models are starting to show the potential for heavy rain
mid-week. Its not a glaringly obvious scenario, but a few
ingredients are in place that raise awareness to the chance for
perhaps enough rain to cause flooding issues across the region for
Tuesday night and Wednesday. There is also a chance for severe
weather, but that is contingent on developing instability during
the day on Wednesday, which may not occur if the area is
encompassed in clouds from widespread convection. At this point
the highest threat would be severe wind, then a few tornadoes,
with severe hail as a distant third. It is important to note that
the heavy rain and severe weather threat are only marginal at this
time, but it does appear that the threat exists for late Tuesday
and Wednesday. Increased pops well above guidance to 70-80 percent
for Tuesday night.

A longwave trough currently located off the British Columbia coast
will move onshore and amplify. As of now, a few areas of enhanced
positive vorticity will develop and move over the High Plains, while
further south a weak shortwave trough with ties to the subtropical
jet will lift up over the Upper Midwest. A very moist atmosphere
will develop ahead of these PV anomalies with PWAT of 1.75 inches
lifting across the Dakotas and leaning eastward across the Upper
Midwest early next week. Forecast soundings show tropopause heights
above 150mb, which give additional supporting evidence to a tropical
airmass capable of producing heavy rain. This environment together
with the negative tilt of the approaching longwave trough indicates
that the sensible weather has the potential to over-achieve, meaning
could see much heavier rainfall than the model QPF currently
indicates. Indeed the soundings off the GFS 12.12 show MBE
velocities less than 10 kts for a period of time Tuesday night, with
PWAT values near 2 inches.

There isn`t much CAPE since the atmosphere is nearly moist
adiabatic.  Some of this is due to convective feedback given that
the models are generating precip, but given the limited CAPE profile
do not see much in the way of a severe weather threat unless a few
storms become forward propagating with a severe wind threat.
Hodographs show strong veering in the lower levels with +200 m2/s2
of 3km helicity so can`t rule out a tornado threat as well if the
atmosphere manages destabilize and generate some low level CAPE.

Looking ahead, high pressure will build in for Thursday, but there
is disagreement in whether or not a shortwave trough will move
across the region for Saturday and Sunday.  Continued with a blend
of the guidance given the uncertainty.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 640 PM CDT Sat Aug 12 2017

Only chance of SHRA/TSRA in the next 6 hours will occur at KAXN or
KRWF based on the amount of instability and the ongoing convection
in the Dakotas. Otherwise, between 6-12z, scattered SHRA, and
isolated TSRA will develop in western Minnesota and spread
north/northeast and mainly affect west central and central
Minnesota during the pre dawn hours. How far east these SHRA/TSRA
maintain remains uncertain. Will continue with either VCSH or SHRA
with VFR cigs through 12z. After 12z, conditions will likely
deteriorate in west central/central Minnesota as LCLs lower and
abundant SHRA are in the area. Kept VCTS in the TAF for most areas
during the afternoon as instability increases. Winds will be
generally from the southeast/south around 5 to 10 kts overnight,
and 10-15 kts Sunday.


No concerns through 9z as the bulk of the SHRA/TSRA are west of
the airport. Between 9-12z, SHRA should begin to affect the Twin
Cities area, but uncertain on the coverage, so will continue with
only VCSH or -SHRA. Will hold off on VCTS until late in the day
due to uncertain in the amount of instability in eastern Minnesota
after morning SHRA.


Mon...VFR. Isolated SHRA early/Possible MVFR cigs. Wind SE at 5 kts.
Tue...VFR. MVFR/IFR possible late with SHRA/TSRA. Wind SE at 5-10 kts.
Wed...VFR. MVFR/IFR early. Chc TSRA. Wind S at 5-10 kts Bcmg W.




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