Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 190930
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
430 AM CDT Tue Sep 19 2017

.SHORT TERM...(Today and tonight)
Issued at 430 AM CDT Tue Sep 19 2017

Big concern today is the potential severe risk in the west this
evening. Strong wave that will help drive today`s severe weather
risk is moving across southern Idaho around the south side of a
closed h5 low centered over BC. This wave will move out to the
central Dakotas by late this afternoon and eventually the northwest
corner of MN tonight. A strong low (990mb) will form at the nose of
the jet streak over eastern Manitoba, which will drive a cold front
into western MN this evening. Ahead of the cold front, a warm front,
which was along the MN River Valley at 330am, will not move much
today, but begin to lift northeast this evening as the cold front
moves into western MN.

The problem with the warm front not moving much is that the
widespread stratus that envelopes much of the upper Midwest this
morning will be slow to erode today. However, the stratus won`t
preclude dewpoints from climbing into the upper 60s today, with an
EML and associated steep mid level lapse rates moving in as well.
This combination still looks to result in a very unstable atmosphere
for mid-September standards up here. Model guidance is actually
pretty strong, with not a whole lot of spread in the CAMs with
convective evolution today, with storms firing along the cold front
over the central Dakotas around 6pm. Severe threat certainly looks
high where these storms initiate with a moderate instability/high
shear environment expected. Everything still points to the Dakotas
storms lining up and moving into our west around 9/10pm. Greatest
forcing will be shooting off to our northwest tonight, but a plume
of modest instability (2k-3k j/kg MUCAPE) will reside across
southern MN, which will help keep the line of storms going into the
Twin Cities.

For the severe weather risk, our biggest inhibitor is timing. By the
time storms get here, a nocturnal stable boundary layer will be in
place, with capping issues existing as well with the EML moving in
overhead. Though hodographs display large loops with the southeast
surface winds, there are some questions whether storms will actually
be able to tap into this high helicity air. However, southeast winds
will likely still be gusting upwards of 30 mph out in western MN
this evening. These winds will really limit the strength/depth of
the stable nocturnal boundary layer, leaving this strong low level
helicity available for storms. Based on CAM updraft helicity tracks
and neighborhood probabilities, the Enhanced Risk area from the SPC
looks good for where our highest threat will be. Given expected
linear nature of convection, along with the steep mid level lapse
rates, wind and hail look like our main threats. However, given
extreme curving of low level hodographs, a tornado threat will exist
on the south end of any line breaks we happen to see move into the
area.

As we get out past 6z, instability looks to start diminishing, which
will result in the line weakening as it moves toward western WI
through the night. In fact it may never make it to western WI,
instead, best threat for storms in our east may actually come as the
main line is in western MN. The reason is that as the surface warm
front lifts north this evening, it will have a nearly 60 kt llj
crossing over the top of it, so we will likely see a NW-SE
oriented band of elevated WAA thunderstorms develop ahead of the
main squall line from near the warm front/cold front triple point
over north central MN, southeast toward western WI to the north
of the warm front.  The 6z NAM nest highlights this potential well.

To sum this up, severe weather setup looks best over the eastern
Dakotas, but western MN will certainly have a risk of severe storms
as well. Convective mode looks to be linear, so strong winds will be
our main threat, followed by large hail, with tornadoes a not to
distant third wherever we see breaks in the line develop.

.LONG TERM...(Wednesday through Monday)
Issued at 430 AM CDT Tue Sep 19 2017

...INCREASING RISK OF EXCESSIVE RAINFALL THIS WEEKEND ACROSS
WESTERN AND PORTIONS OF CENTRAL MINNESOTA...

A few thunderstorms may be ongoing early Wednesday morning east of
I-35, but coverage should be waning quickly overnight tonight as
the mid level wave quickly lifts to the northeast away from the
area. A surface ridge axis will pass overhead Wednesday afternoon
with winds expecting to already back southeast Wednesday night.
The front from tonight will wash out across the Mid Mississippi
Valley and southern Great Lakes with broad southerly flow
developing across the entire Mississippi Valley by Thursday. The
Gulf of Mexico will be wide open for several days in advance of
the very slow moving deep western trough. Meridional flow will be
efficient in bringing deep moisture northward through the weekend
with pwats increasing to at least 1.75 inches by Friday, continuing
into early Monday.

With the low level moisture advection increasing and a strengthening
LLJ Thursday night, we cannot rule out storms developing
overnight. However, strong capping associated with the arrival of
thermal ridging aloft will likely keep the lid on much development
across southern Minnesota, with a far better chance occurring
across northern Minnesota toward Lake Superior and the Upper
Peninsula of Michigan. The GFS is the only model to show much
development in our CWA, which does appear much overdone.

Friday looks quite hot and humid. Thicknesses reaching 580 DM,
and thermal ridging characterized by 850 mb temps between 21-24C
and 925 mb temps of 25-28C with deep mixing should send temperatures
into the lower 90s over much of the area. Given unusually high
pwats, this deep mixing will likely not result in a decline of
afternoon dew points and if anything they may continue to rise
with continued moisture advection. Some dew points may rise into
the lower 70s by peak heating with heat indices peaking in the mid
to upper 90s. Unless we really overachieve on temperatures, we
will likely remain at least 2-3 degrees below record highs.

Capping will hang on until the thermal ridge weakens and slides to
the east Friday night. Convection will then begin to develop,
marking the beginning of a very wet period which could lead to
impressive rainfall totals and possible flooding across the
northwestern half of the CWA. Models are in fairly good agreement
through the rest of the period depicting a stationary front due to
the highly amplified pattern. Multiple waves riding along this
boundary will sustain widespread showers and thunderstorms along
and just behind the front. These will be very efficient rain
producers given the anomalously high moisture in place. With pwats
peaking around 1.9 inches in low 570s DM thicknesses, this
scenario falls well above the critical 70% saturated thickness
regression line important for heavy rainfall events. This pattern
is expected to persist for 2 to 3 days. Widespread rainfall
amounts of 3 to 6 inches are expected across western and portions
of central MN, but totals could exceed 10 inches potentially
depending on daily convective evolution and strength. The
temperature gradient across the CWA could be impressive as well,
with the warm sector expected to be largely dry, and areas behind
the front socked in clouds, with heavy rain and northerly surface
flow.

The front will begin making progress eastward Sunday night and
Monday as the ridge to the east breaks down. Cooler and drier air
will follow the frontal passage Monday and Tuesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1100 PM CDT Mon Sep 18 2017

IFR cigs will prevail through Tuesday morning. The exception will
be the eastern third of the area (KMSP-KRNH-KEAU), where cigs will
likely wobble between MVFR and IFR overnight. Stratus deck will
prevent dense fog from forming overnight, but will still have some
1-3SM BR-especially at western sites. Models still indicate
gradual improvement to VFR between 15z and 18z. Southeast winds
prevail through the period, and become gusty during the late
morning and afternoon hours on Tuesday. The next batch of
precipitation works in from the west tomorrow evening. This round
will have the potential for thunder.

KMSP...
Ceilings will be a bit frustrating overnight, likely wobbling
between IFR and MVFR. Expect to stay at least MVFR through most
of Tuesday morning, however, with improvement to VFR Tuesday
afternoon.

/OUTLOOK FOR KMSP/
Wed...Mainly VFR. Chance TSRA/MVFR early. Winds W 5-10 kts.
Thu...VFR. Winds SE 10 kts.
Fri...VFR. Chc MVFR/TSRA late. Winds S 10-15 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

$$

SHORT TERM...MPG
LONG TERM...BORGHOFF
AVIATION...LS


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