Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 201134

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
634 AM CDT Wed Oct 20 2021

Issued at 625 AM CDT Wed Oct 20 2021

Updated to include 12z aviation discussion below.


.SHORT TERM...(Today through Thursday)
Issued at 338 AM CDT Wed Oct 20 2021

Surface analysis early this morning shows an elongated surface front
from the Hudson Bay through the northern Great Lakes, across southern
MN and continuing SW through the central Plains and the TX/OK
Panhandle regions. Aloft, a compact upper level low, associated with
surface cyclogenesis over the front, is situated off the lee of the
Rockies near the WY/SD/NE corder triple-point with a flattening ridge
axis to the east of the Mississippi River and another weak ridge
axis on the windward side of the Rockies.

Over the next few hours, the upper level low will rotate nearly due
eastward, dragging Gulf moisture in advance of the low into the Upper
Mississippi River Valley and shift a developing surface low from
northeastern NE into northwestern IA. As the upper low then moves
over the IA/MN border by this evening, the surface low will be nudged
along just in advance of the upper system. Where the warm front in
advance of the low ends up settling will be key to seeing how far
north warmer temperatures move into southern/southeastern MN but the
bulk of the WFO MPX coverage area will have highs in the upper 40s to
upper 50s, with locales south of the aforementioned warm front
reaching the mid 60s. In addition, areas to the east and south of
this surface low will have dewpoints into the low-mid 50s with
dewpoints in the 40s elsewhere. Strong lift with the offset stacked
system plus frontal influences will make for efficient and fairly
widespread rainfall across most of the coverage area from the pre-
dawn hours through the daylight hours into this evening. There may
well be a dry slot over far southern and southeastern MN at points
today, owing to the locations of the fronts and surface low itself.
However, any dry time in southeastern MN, and particularly with and
breaks in the overcast allowing for additional instability to
develop, this may allow for a few robust thunderstorms to develop.
The arrival of the upper low, allowing for colder temperatures aloft,
combined with stronger shear in the low-mid levels (bulk shear in
the 30-40kt range) may well allow for some thunderstorms to produce
marginally-severe hail this afternoon. This portion of the coverage
area is placed within a Marginal Risk of severe weather so this
thinking is reasonable. As for rainfall, a general 0.25"-0.50" can be
realized area-wide throughout the day today while northern and
western portions of the coverage area, away from any dry-slotting and
more underneath good moisture advection, are more susceptible to
higher rainfall amounts, closer to 1".

Rainfall chances will steadily lower overnight through daybreak
Thursday morning as the surface low crosses WI on its way to MI and
the upper low shifts east across the MN/IA border and into southeast
WI. This will allow surface high pressure from central Canada to drop
into the Northern Plains and Upper Midwest, in conjunction with NW
flow aloft. Thus, colder and drier air will be in place for Thursday
on the backside of the aforementioned synoptic low pressure features.
Highs area-wide will only range from the mid 40s to lower 50s, and
this after lows tonight drop to the lower 30s in northwestern
portions of the coverage area to the lower 40s along and east of a
line from Ladysmith to the Twin Cities to Fairmont. As such, frost
formation looks to be a pretty good bet for northwestern portions of
the coverage area pre-dawn Thursday morning.

.LONG TERM...(Thursday night through Tuesday)
Issued at 338 AM CDT Wed Oct 20 2021

The end of this week is forecast to be dry and cool as northwesterly
flow (on the backside of a large, bowling ball upper low south of the
Hudson Bay) continues to funnel cold air into the Upper Midwest.
Highs Friday to Sunday will range from the upper 40s to lower 50s.
Meanwhile, lows are expected in the upper 20s to lower 30s Friday
morning, followed by a couple of degrees cooler Saturday morning. NBM
has all of the MPX CWA below freezing Saturday morning, except for
the Twin Cities. The heat island effect currently has the metro just
above freezing, though this is still not a certainty. Outskirts and
low-lying areas within the metro could very well drop below 32 F.
Thus, most areas will see a freeze at least one of the mornings to
end the week.

Temperatures will begin to moderate by Saturday night as the upper-
level low south of the Hudson Bay moves east. A weak impulse within
the zonal flow may bring rain to the region on Sunday. However,
models are having a rather difficult time handling this feature and
QPF placement. This leads to the NBM just smearing 30-40% PoPs across
the MPX CWA Sunday into Sunday night. Hopefully, more clarity is
given by forecast models as we draw closer to Sunday. The uncertainty
of rain also has an effect on our highs Sunday. Have slightly bumped
up highs to lean towards an ECMWF outcome (less rain).

Forecast models show the upper-level pattern becoming much more wavy
and progressive throughout next week after a strong westerly
jetstreak arrives on the West Coast. With each wave would come a
swing in temperatures and chance for precipitation. However, forecast
uncertainty grows with time as each model suite handles the
developing waves differently. As of now, guidance tends to agree that
we warm up Monday into Tuesday as an amplifying ridge moves east
over the Northern Plains. How warm we get is in question but,
ensemble and deterministic models range from mid 60s to near 70 for
highs at MSP Tuesday. Models hint that a trough (perhaps strong) will
follow the ridge sometime mid-next week. However with the growing
model spread, forecast predictability is much too low at this time to
delve into any finer details.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 625 AM CDT Wed Oct 20 2021

A deep incoming area of low pressure entering SW MN this morning will
continue to slide east over the next 18-24 hours, bringing low clouds
and rain to the entire coverage area, with some thunderstorms for far
southern MN. Ceilings will drop into MVFR level, and likely to IFR
level from time to time, particularly this evening into tonight. As
the rain develops and overspreads the area, some pockets of mod-to-
heavy rain are possible which may well reduce visibility into MVFR
level, although IFR visibility cannot be ruled out. In addition, some
thunderstorms are possible mainly for terminals in southern/eastern
MN into western WI. Rainfall will gradually end overnight into
Thursday morning with conditions likely returning to VFR during the
day tomorrow. Winds will remain generally NE with speeds topping out
around 15G25kt. Could be some higher gusts in thunderstorms.

KMSP...Conditions will likely remain within VFR levels through the
morning push the deteriorate thereafter. Rainfall is likely to start
by late morning then persist through the rest of the day into this
evening. Best timing for CB/TS at this point looks to be early-to-mid
afternoon but could easily vary by a couple hours either way of the
advertised TEMPO group so will monitor radar trends and adjust as
needed. Conditions likely to drop to IFR by the tail end of the
evening push and remain degraded into the overnight hours with some
improvement possible by daybreak Thursday.

Fri...VFR. Wind N 5 kts.
Sat...VFR. Wind E 5-10 kts.
Sun...Chc MVFR/SHRA. Wind E 8-12 kts.




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