Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 161139

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
639 AM CDT Sat Sep 16 2017

.SHORT TERM...(Today and tonight)
Issued at 345 AM CDT Sat Sep 16 2017

Little has changed overnight with our upper air pattern, with the
upper MS Valley remaining in southwest flow to the east of a broad
trough centered near Yellowstone. The main precip generating waves
on water vapor show up over western Neb and northwest IA. At the
surface, there`s a double barreled low, with one surface low in the
Park Rapids area and the other over northwest KS, with a cold front
that is slowly moving east across western MN that arcs from NW IA
across Neb to the KS low. East of the cold front, a strong LLJ has
been in place all night, with plenty of accas developing in response
to the persistent isentropic lift. We`re seeing quite the increase
in convection ahead of the Nebraska wave and like the idea of the
HRRR that we see fairly widespread thunderstorm activity come up
across south central into east central MN and eventually western WI
through the morning.  We`ll likely see a lull in precip generation
for several hours behind this first wave, before another round of
storms rides up along the front. The afternoon storms will develop
in response to height falls overspreading the area as the main upper
trough over the Rockies this morning slides into northwest MN by
tonight. Looking at soundings though, it looks like the warm sector
will remain capped, so these actually look to be post frontal. Even
though storms look to be post frontal, we`ll still have steep lapse
rates and ample elevated instability (00z NAM at 3z this evening has
about 2500 j/kg of CAPE for a parcel lifted from h8 at MSP) to have
at least a marginal threat for severe hail. If some storms were able
to be rooted near the surface along the front, there will be ample
shear in place to warrant a wind and tornado threat as well, which
SPC does show in the initial rendition of their Day1 convective
outlook. Current forecast spikes likelies along the front by 00z and
carries the likelies east with the front through 6z, which is
similar timing to what we are seeing with the HRRR and NSSLwrf.
Highs will be tricky today with this boundary across the area as
well, with conditions expected to range from cool fall to middle of
summer from west to east as west central MN will likely stay in the
50s, with western WI again warming into the low 80s.

Tonight, the cold front will finally start pushing east of the MPX
area, ushering in much cooler and drier air for Sunday. Although
this morning we see stratus extend from the cold front in western MN
all the way back to central Idaho, it will not be as expansive
tonight. In fact, we should see clearing skies from west to east
tonight as we get into the dry air feed to the south of the upper
low that will be going from the northern Rockies this morning into
central Canada tonight.

.LONG TERM...(Sunday through Friday)
Issued at 315 AM CDT Sat Sep 16 2017

The large scale flow pattern changes and becomes more amplified
with time in the longer term. Timing of main upper troughs/surface
fronts will be the main concern overall.

Initially we see a much cooler period following the Saturday
trough/cold front. Temperatures will struggle to warm through the
60s both Sunday and Monday. Monday could see an increase in
clouds during the day as return flow brings at least some mid
clouds and a chance of showers.

Tuesday still looks to be a dry day with the main warm front south
of the area. This does eventually lift north as the next more
amplified trough moves east across the Dakotas Tuesday night into
Wednesday night. The deterministic models continue to diverge on
timing of the is feature with the 00Z ECMWF a bit lower than the
GFS. There could be some interaction with the tropical system Jose
over the eastern seaboard at this time, slowing the troughs
eastward progression across the upper midwest.

Following Jose we see strong ridging continuing over the eastern
conus with a deep trough developing to the west. This will
eventually make slow eastward progress over the upper midwest and
we should see an increasing threat of showers and storms moving
back into the area especially Friday into next weekend. Until that
time, look for temperatures warming above the seasonal normals
by Tuesday and this should last through the week.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 639 AM CDT Sat Sep 16 2017

Front right now stretches from about MKT to MSP. Showers and
thunderstorms will continue through mid morning to the east of
the front and will primarily impact WI, though MSP will may see a
passing shower before this plume of moisture finally pushes off to
the east. Will see a lull in precip, but another round of
shra/tsra is expected to develop during the early afternoon over
central/western MN and work east through the evening. Only change
to TAFs for this second round was to speed up onset an hour or two
based on timing from the HRRR. Cloud forecast is tricky from the
MKT to MSP corridor as they look to be on the edge of the stratus
shield today and may bounce between MVFR and VFR cigs quite a bit.
Question for tonight is how quickly do we see VFR conditions move
in from the west behind the front. Have timing similar to the
LAMP right now, but would not be surprised if low clouds tonight
hang on a bit longer than currently forecast.

KMSP...MSP looks to stay west of the main thunderstorm plume
coming out of IA into southeast MN. However, we are still in a
plume of a moisture rich LLJ and until that moves through, MSP
will have the threat of getting hit by a stray shower. Low
confidence on CIG forecast, though with MVFR cigs now in the west
metro, MSP could easily spend most of the day with MVFR cigs.
Confidence on second round of storms is moderate, as current cloud
situation may result in this round developing a bit east of where
the HRRR has it.

Sun...MVFR cigs possible early. Winds WNW 5-10kt
Mon...VFR with TSRA/MVFR possible. Winds SE 10-15kt
Tue...VFR with TSRA/MVFR possible. Wind SE at 10-15kt




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