Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 211808 AAB

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
108 PM CDT Wed Jun 21 2017

.SHORT TERM...(Today and tonight)
Issued at 355 AM CDT Wed Jun 21 2017

Surface analysis early this morning shows a low pressure center
over southern Saskatchewan province. From this low pressure
center, a leading warm front extends southeast through eastern MT
and the western Dakotas through Nebraska into northern MO while a
trailing cold front meanders through western MT into southern ID
and northern NV. Aloft, a modest upper level ridge extends north
from the central Plains through Manitoba province while a
northern-stream trough moves across Alberta province and the CONUS
Rockies. The upper level trough is expected to continue shifting
southeast and deepen, forming an upper level low over southern
Manitoba by daybreak Thursday morning. The associated shortwave
trough axes rounding this system will help drag the surface low
and its associated fronts eastward. The key will be when and how
much north the warm front of this system moves. The consensus and
current thinking is that the warm front will generally remain to
the south and west of MN through much of the day, possibly nudging
northward into far southwestern MN by late afternoon (which would
help push highs temperatures in that corner of the state to the
mid-upper 80s). By late afternoon into early evening, as the upper
low drops south and helps enhance low-to- mid level jetting, the
warm front will lift north into southern MN while the cold front
enters western MN. The cold front will then continue to sweep
through the state overnight, exiting all but far southeastern MN
by daybreak.

There are several factors at play which may allow for
strong/severe convective development today. First is the arrival
of the warm front which will act as a low level convergence
boundary. Secondly, the stronger low-mid level jetting will
enhance shear. Third, depending on how far north the warm front
pushes, instability will increase over far southern MN. Lastly,
the incoming cold front will again provide increased low level
lift. All things being equal, within a fairly deeply moist
atmosphere, these would contribute to a somewhat robust period of
convective development. However, there are some key mitigating
factors. Primarily, deep moisture will be lacking which, although
that would contribute to an enhanced hail threat, will overall
limit the coverage of any strong/severe storms. Secondly, most
models are indicating fairly good capping over much of the area
which will inhibit convective development. Third, much of the
concern will surround the northward progression of the warm front
which looks to be fairly late today into this evening which would
not be diurnally optimal for capturing available instability and
also which looks to be subject to the zonal flow developing aloft
and not have much punch to lift north. There still will be a large
hail threat due to the strong lapse rates and even isolated
elevated convection with the warm front, while any damaging wind
threat will be confined to southwestern MN due to diurnally driven
convection which could take the form of strong/severe clusters in
advance of the cold front.

Adding all this forecast thinking together leads to scaling back
pops to the low-chance category and only for far southern portions
of MN for much of the day today. As the warm front moves east and
nudges north later this afternoon and evening, chances for precip
increase into the mid-high end chance category for southeastern
portions of the WFO MPX coverage area, but duration will be
limited as the cold front will be in close proximity behind which
will close down precip chances through the overnight hours.

Because of the uncertainty of how far north the warm front will
lift, this will create the potential for a large northeast-
southwest gradient in high temperatures today. There may also be
some variance with there the warmest air may reside. At this
point, highs looks to range from the mid-70s north and east to the
upper 80s far west and southwest. Lows tonight will not be
subject to ideal cold air advection in the wake of the cold front
(better tomorrow), so will look for lows ranging from the mid 50s
to mid 60s north-to-south.

.LONG TERM...(Thursday through Tuesday)
Issued at 355 AM CDT Wed Jun 21 2017

The longer term concerns remain the thunder/rain chances into
Thursday night and overall development of the eastern CONUS trough
through the weekend.

The surface front is progged along the MN/IA border Thursday
morning. There may be at least some left over scattered
convection over the eastern and southern areas Thursday morning.
The western trough is forecast to move east and attendant height
falls/forcing and F-gen along and north of the boundary...any
capping potential will be eroded quickly. We expect a good shot
of rain/thunder especially across the southern third of the area
Thursday afternoon...and then shift east with the upper trough
into Thursday evening. There will be some elevated showers/isolated
thunder into central MN especially during the morning/early
afternoon as the trough and forcing arrive. The severe weather
threat should be limited to the vicinity of the frontal boundary
with mainly elevated hail and wind threat. This matches SPC Day2

Following this front...cooler and drier conditions should envelop
the region as the eastern conus trough digs south and east. THis
draws in very cool canadian air with the GFS bringing in +4 to +8c
850mb temepratures for the weekend...with the ECMWF even a little
cooler. The deterministic models differ somewhat on how they
handle the short wave that moves into the region in the northwest
flow pattern for Saturday/Sunday. We will retain the small chance
PoPs and follow the GFS for now which has been most consistent
with this feature. Still looks cool into early next week as well.
There is some indication of the western ridge moving east into the
northern plains region as the eastern CONUS trough propagates
farther to the east. This should bring in warmer condition along
with another threat of showers/storms to the area by midweek.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 108 PM CDT Wed Jun 21 2017

VFR conditions are expected through this evening, then MVFR cigs
are possible across western Wisconsin overnight. Chances for
thunderstorms are diminishing across much of the area through
tonight, with two regions most favored. The first will be over
western and central Minnesota this evening with the cold front,
but these should weaken or dissipate before reaching I-35 toward
midnight. The other region will be over southeast MN and western
WI with the strengthening LLJ this evening. This may affect Eau
Claire overnight. Took mention of precip out of RWF/MSP/RNH.

KMSP...Best chance of rain will be after 19Z Thursday. Probability
of thunder with that activity is pretty low. A front will be
nearby tomorrow afternoon, so wind direction is highly uncertain
and may be light and variable if the front is overhead.

Fri...VFR. Wind NW 10 kts.
Sat...Mainly VFR. Slight chc -TSRA/MVFR. Wind NW 5-10 kts.
Sun...Mainly VFR. Slight chc -TSRA/MVFR. Wind NW 5-10 kts.




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