Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 211746

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1246 PM CDT Thu Sep 21 2017

.UPDATE...For 18z Aviation discussion below


.SHORT TERM...(Today and tonight)
Issued at 418 AM CDT Thu Sep 21 2017

Surface ridging overhead has provided clear skies and light winds
across the region this morning. Some patchy fog has developed, but
it should burn off quickly after sunrise. It`s a bit chilly with
temperatures in the low 40s to low 50s. This is actually where we
should be this time of year. However, the next 24 hours will change
dramatically with dew points and low temperatures in the lower 70s
tonight - 20 to 30 degrees warmer.

The main feature to watch today will be the surface warm front
lifting north from southern Iowa and southern Wisconsin. Strong
moisture advection behind this boundary will send dew points into
the lower 70s by early evening across southern Minnesota and
elsewhere by mid evening. This moisture advection will erode the cap
and could allow isolated storms to develop near the front mid to
late afternoon. They could dissipate this evening with the loss of
daytime heating before the next round of elevated convection
develops overnight.

The low level jet is expected to strengthen this evening. Most
recent model runs show a little better defined nose extending up
into southwest Minnesota by 06z. This should focus the best risk of
convection along and north of I-94 over central MN after 07-08Z,
propagating toward northwest WI during the predawn hours. Steep
lapse rates atop a very warm/moist boundary layer could lead to
MUCAPE greater than 3000 J/kg overnight. Effective shear is rather
meager, however, which will limit the extent of severe potential. It
will also lead to loosely organized storm clusters and motions will
be mostly dictated by outflow boundaries. Some large hail is
possible with the stronger cells/clusters, otherwise locally heavy
rainfall is likely with any cell training.

.LONG TERM...(Friday through Wednesday)
Issued at 418 AM CDT Thu Sep 21 2017

There are four main concerns during the long term period. Potential
for excessive heat and the need for heat related headlines Friday,
potential for severe storms Friday evening in western MN, placing
the baroclinic zone and the eastward extent of PoPs through the
weekend, and the potential for excessive rainfall this weekend for
western and central MN.

Models remain in good agreement on the large scale pattern evolution
through the weekend, with a blocking high building into the Great
Lakes, with a positively tilted long wave trough extending from the
Great Basin to Hudson Bay. This upper air pattern will create a
strong surface baroclinic zone from far western into northern MN
that will move very little from Friday night through Sunday night.
Next week, the ridge finally starts to weaken, allowing for the
trough to our west to finally kick east, the end result of which
will be northwest flow by the end of next week. So over the course
of the next week, we`ll get to enjoy summer and fall, with a little
bit of rain in between.

For Friday, the only thing that will slow temperatures down is
potential cloud cover from what, if anything we see tonight. Deep
southerly flow will bring very warm air into the area. Based on 925-
850 temps, we went several degrees above the blended guidance for
temperatures for areas east of the baroclinic zone through the
weekend, though Friday will be the warmest of the days. The NAM/GFS
show a tongue of 925-850mb temps of nearly 28c coming up into
western MN ahead of the front Friday afternoon, and this area could
see warmer temps than we have now. In fact GEFS temperature plumes
for RWF top out at 98 on Friday, so there is potential this forecast
is conservative out in southwest and west central MN. Beside the
warm temperatures, these south winds will bring dewpoints in the
low/mid 70s that are currently in the mid-Mississippi Valley this
direction Friday. With highs in the low/mid 90s with dewpoints in
the low 70s, we now have apparent temperatures up around 100 for
Friday afternoon. Getting a full picture of the heat health with the
WBGT, you see we have one saving grace in the excessive heat
department and that`s the breezy south winds expected. These strong
winds combined with a lower sun angle are helping keep WBGT values a
bit lower and than what you expect with similar temp/dewp
values in say July.

For the severe threat Friday, though very unstable, the warm sector
will also be securely capped and any thunderstorm development will
occur along the cold front. Eventually, convergence along the front
will overcome the cap, but likely not until after 5pm Friday. Current
SPC day 2 outlook matches up pretty good with where the SREF has
it`s greatest corridor of severe potential and see no reason to
argue against the slight risk placement. Shear vectors are pretty
much lined up with the front, so linear looks to be the mode of
choice. Shear/instability parameters certainly support all modes of
severe weather being possible, but the expected storm mode likely
means wind and hail will be the main treats.

For the weekend, the biggest change in this forecast was to trim the
eastern extent of PoPs as the baroclinic zone will move very little,
with repeated rounds of thunderstorms expected across western into
parts of central MN. We now have a dry forecast through Sunday night
from south central MN to the southeast Twin Cities metro and over to
Eau Claire. The heaviest rain still looks to fall along and west of
a Redwood Falls, to St. Cloud, to Mille Lacs line, where 3-6" of
rain still looks likely through the weekend. When looking at GEFS M-
Climate QPF percentiles, the two periods that look to have the
greatest risk of excessive rainfall are Saturday night and Sunday
night when QPF values are forecast to be above the 99th percentile
in west central MN. Fortunately, our severe weather potential over
the weekend looks considerably diminished as the steep mid-level
lapse rates of Friday will not stick around for the weekend, which
will greatly reduce instability and in turn the severe threat.

Monday, as heights in the ridge start to fall, the trough to our
west finally comes east. This will drag the moisture plume slowly
east across the area Monday and Tuesday, which is when eastern areas
finally look to get some rain, though by then this system will begin
to lose some of it`s moisture and the forcing mechanisms will be on
the move, so rainfall amounts to the east will be much less than
what we see west.

Eventually, this system moves off to our east on Wednesday and we
get a dry second half of next week with temperatures back down near


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1247 PM CDT Thu Sep 21 2017

A narrow band of low clouds will continue to lift across central
MN/WI during the early afternoon hours. While this band is
scattering out to some degree, expect KMSP and KEAU to have the
potential for brief broken ceiling (circa 2KFT at KMSP and 1KFT at
KEAU). Should only last for 1-2 hours, and then VFR conditions are
expected through this eve. Shower and thunderstorm chances
increase after midnight along a warm frontal boundary over central
MN/WI, but the main area of potential looks to be north of most
sites except possibly KAXN/KSTC/KRNH, where PROB30 mentions have
been maintained. The other concern overnight is the potential for
low level wind shear as surface gusts subside and winds just above
the surface increase. Have included a mention at KMSP/KRWF/KMKT.

Ceilings may briefly go BKN020 between 18z and 20z, but after that
VFR conditions are expected through the period. Thunder threat
overnight still looks to be to the north.

Sat...Mainly VFR. Slight chance MVFR/TSRA. Winds SSW 5-10 kts.
Sun...Mainly VFR. Slight chance MVFR/TSRA. Winds SW 5-10 kts.
Mon...MVFR/TSRA. Winds S 5-10kts.




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