Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 241956

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
256 PM CDT Mon Jul 24 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 254 PM CDT Mon Jul 24 2017

Afternoon MRMS imagery showed an area of showers and thunderstorms
progressing eastward through northern South Dakota near Aberdeen.
This precipitation may hold together and reach western MN around 23Z
which is in response to a shortwave trough evident at 500H in most
of the available guidance.  For tonight, the low level jet will
increase this evening, so while this area of precip may not have all
that much instability to work with, there should be a decent
increase in theta-e advection out ahead of the shortwave as winds
pick up, enhancing low level convergence and lift to generate
showers with some elevated thunderstorms.  Severe parameters are
quite weak with this acitivity, so much so that the SPC has pulled
the marginal risk from all of Minnesota for tonight.  The current
expectation is for showers and garden variety storms to gradually
work eastward overnight and reach the Twin Cities early tomorrow
morning, before dissipating as they progress eastward from there
thanks to the low level jet weakening during the daylight hours.

As for the remainder of tomorrow, there will be plenty of cloud
cover especially east tomorrow morning but generally expect the
morning activity to move out and for at least a mix of clouds and
sun by mid day.  An elevated mixed layer will nose into western
Minnesota during the day, so steep lapse rates will be available for
any convective initiation to take advantage of.  But, do not expect
to see much during most of the day given the +14C 700H temperatures
overhead placing a solid cap on the atmosphere.  However, the
thermal ridge will lean over to the east as height falls approach
along with the front which will start eroding the cap on the cool
side towards evening. For details on the outlook for tomorrow
evening, see the long term discussion below.

.LONG TERM...(Tuesday night through Monday)
Issued at 254 PM CDT Mon Jul 24 2017

The main issue in the longer term will be convection and potential
for some severe weather and heavy rainfall Tuesday and Wednesday. In
the big picture, the fairly low amplitude flow over the country at
the start of the long term looks to eventually give way to a more
amplified pattern by the end of the period with western ridging and
eastern troughing. But, before things transition to what will be a
drier large scale pattern for us, we will need to navigate our way
through 12-24 hours of potentially very wet weather for some.

Warm frontal boundary will be laced across the area from west to
east Tuesday evening, with its exact position still being somewhat
uncertain and guidance having it anywhere from south of the
Minnesota River Valley to near the I-94 corridor. It`s position will
be somewhat depending on any convection earlier on Tuesday, but also
on the large scale forcing, where some uncertainty remains in the
guidance. The NAM is perhaps most aggressive in pushing the boundary
north, while the GFS is least aggressive, and the ECMWF is somewhat
in-between as is the Canadian. There isn`t much in the way of CAM
guidance yet available for that period, and what is available (HiRes
Window runs) is somewhat similar to the NAM. However, even with the
uncertainty it appears somewhere in the forecast area is under the
gun for convection to become widespread during the evening hours,
with initially discrete cells likely congealing into an MCS with
some potential for training storms along and north of the warm
frontal boundary. There is some potential for severe weather, most
likely early on before upscale growth occurs and heavy rain becomes
more of a concern. The main upper jet is pretty far to the north of
the area, but deep layer shear is still progged around 35 kt, so
enough for some organization, particularly with MLCAPE AOA 2500
J/Kg. The 0-3km shear looks to be around 30-35 kt, so some bowing
segments and wind certainly can`t be ruled out in addition to the
hail potential. Tornado potential looks relatively low, especially
once things grow upscale, but would at least be of some concern
initially with the warm frontal boundary in the area and 0-1 km bulk
shear of around 20 kt forecast. In terms of heavy rain potential, it
certainly seems a possibility given the potential for training
storms, and precipitable water values AOA 2" over much of the area,
along with good 850 mb moisture transport and frontogenesis
north/northeast of the theta-e ridge. Most of the guidance hints at
two rounds of convection, once during the evening into the overnight
hours, and a second on Wednesday as the shortwave pushes through to
our north and the boundary moves southeast through the area. That
will hopefully keep the same area(s) from getting hit with the
heaviest precipitation twice. At this point, given things are still
24+ hours away, along with the convective nature of things and
uncertainty on where the greatest precipitation will occur, will hold
off on the issuance of a flash flood watch. However, it will
certainly be something for subsequent shifts to consider,
particularly once they have additional CAM guidance to review and a
better sense of convective trends heading into Tuesday.

It looks like any lingering convection will push southeast of the
area Wednesday evening, then we will get into a prolonged period of
slightly cooler and dry weather from Thursday through Saturday as
high pressure slowly drifts across the region. Return flow
eventually returns Saturday night or Sunday, and along with that
will be the chance for some showers and thunderstorms. Although the
large scale guidance is in reasonable agreement on the evolution of
the large scale pattern at that time, there are definite differences
in timing and position of features, resulting in some low chance
PoPs from Saturday night into Monday.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 110 PM CDT Mon Jul 24 2017

A rather widespread MVFR level stratocu field formed this morning
beneath the inversion across the area and continues to linger
primarily across southern and eastern Minnesota. The dense cloud
cover is located from Mille Lacs through the Twin Cities and south
through southeastern MN, and is in the process of eroding from
both the west and the east. As the day continues, expect a
combination of mixing and subsidence to erode this cloud layer

Expect a decent shot at showers and a few thunderstorms tonight
primarily after 03Z in far western MN, and progressing eastward
overnight. There is some doubt pertaining to the cap in the
Dakotas but as the low level jet increases tonight, so does
theta-e advection which could be enough of a push to overcome the
cap and at least initiate some elevated convection.

KMSP...confidence on placement of precip overnight is relatively
low, hence went the PROB30 route for now until the picture becomes
more clear.

Wed...VFR with TSRA possible early. Wind NW at 05kt
Thu...VFR. Wind NE at 10kt.
Fri...VFR. Wind E at 5kt.




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