Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 281948

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
248 PM CDT Fri Apr 28 2017

.SHORT TERM...(This evening through Saturday)
Issued at 131 PM CDT Fri Apr 28 2017

Surface analysis this afternoon shows a pair of quasi-stationary
fronts both south of the Upper Mississippi River valley, one from
northern IL westward to northern CO and another from TN westward
into NM/AZ. Each front has a few low pressure centers along it
west of the Mississippi River. In addition, surface high pressure
is straddling the Canadian border with ND. Aloft, a deep upper
level low is sitting atop the 4-Corners region with a deep Bermuda
high pressure feature sitting off the Atlantic seaboard.

Over the next 24 hours, the Bermuda high will remain nearly
stationary while the upper low meanders slowly east into the Deep
South, with the trough surrounding it becoming more pronounced.
Closer to home, the upper level flow will shift from zonal to
meridional, allowing much more moisture into the region ahead of
the deepening system. The two fronts will also slowly merge
through tomorrow, becoming a more solid focusing mechanism for
precipitation over the central CONUS. However, what will delay the
approach of this front and a developing southwestern low into this
region is the international border high pressure. The high will
slide to the east tonight through tomorrow, shifting from northern
Alberta province into Ontario province, but maintaining enough of
a southern influence to effectively keep precipitation from not
pushing north from Iowa into MN/WI. Though a few showers are
possible along the state line this afternoon and evening, the bulk
of the precipitation will hold off until late tomorrow afternoon.
High clouds will push into this region through tomorrow from the
south thus making for more cloudy periods than clear periods.
However, the chances for rain will steadily increase starting
tomorrow evening.

With the Canadian high pressure airmass being the prevailing
feature over this region through tomorrow, temperatures will
continue to run rather cool for the end of April. Highs tomorrow
will be similar to (or possibly slightly warmer than) today, with
highs ranging from the upper 40s to lower 50s. Lows tonight will
be similar to what was reached early this morning, with lows from
the upper 20s to mid 30s.

.LONG TERM...(Saturday night through Friday)
Issued at 131 PM CDT Fri Apr 28 2017

Although it probably doesn`t need saying, the main story in the
later periods remains the significant storm system that will
impact the region during the Sunday-Monday time period. To some
extent there is less to discuss than in previous forecasts since
things haven`t changed too much. Overall, the forecast remains
similar to the previous forecast. Models have come into fairly
good agreement now, with the main differences being in exact
position and amount of heaviest precipitation, and forecast
thermal profiles. However, these differences are not as great as
they were 24-36 hours ago. In comparison to yesterday, the ECMWF
has come a little bit east and the GFS has shifted a bit west. We
should see precipitation begin to lift up into the area from the
south Saturday night, and overspread most of the area by early
Sunday afternoon. This looks to be mainly rain, except perhaps at
the onset in a few locations, where some snowflakes could occur.
The surface low should work into Iowa by Sunday evening, with
deformation zone precipitation (where we remain concerned about
accumulating snow potential) setting up in an area of good coupled
low-mid level frontogenesis and upper level divergence from
southwest Minnesota into central Minnesota. This will shift north
and a bit west overnight Sunday night into Monday, with the
heavier precipitation looking to settle down by Monday afternoon.
As has been previously discussed, where precipitation is heaviest
in the deformation zone, we should see the column cool below
freezing with only the near surface temperatures remaining near or
above freezing. Continue to think this should allow for a
changeover to snow. The big questions remain what we`ll see for
snow ratios, and exactly where that heaviest axis will setup. With
these lingering uncertainties, have highest snow amounts
generally less than four inches forecast from southwest into
central Minnesota. This corresponds well with the 50th percentile
from the WPC probabilistic snowfall guidance, and for now is the
most prudent course of action. However, still feel that amounts
will very likely be higher where things come together, so expect
to see an increase in the highest forecast amounts as things get
closer in time.

Regardless of precipitation type, there will be widespread liquid
equivalent amounts from 1-2", and even where it is snow it will
melt rather quickly, so we will be seeing a response in area
rivers as we work through the upcoming week. Some localized
flooding problems could certainly occur during the precipitating
period, with some river concerns coming thereafter. Some lingering
rain/snow showers can be expected in the wake of the system
through Tuesday. Otherwise, we`ll see a weak ridge of high
pressure build into the region and bring mainly dry conditions
through Wednesday. Some rain at the leading edge of warm
advection/return flow looks possible Wednesday night into
Thursday, but nothing too significant is expected at this time.
Things should continue to moderate, while remaining dry later
Thursday through Friday. There are disagreements in the guidance
by Friday in terms of the amplitude of the upstream ridge, and at
this point will stick close to a consensus forecast which will
likely not be very close to reality depending upon which solution
works out. But, there is certainly insufficient confidence to go
with one extreme (cold frontal passage by late Friday in the GFS)
or the other (surface ridge across the area with highly amplified
upper ridge to our west), particularly given significant spread in
the ensembles.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1207 PM CDT Fri Apr 28 2017

High pressure dropping in from the northwest will keep the frontal
system and associated precipitation to the south of all TAF sites
through tomorrow. High clouds from the system will be the main
impact, keeping ceilings in place. Though there are some isolated
pockets of MVFR-level clouds (such as KRWF at initialization
time), those instances are expected to be few and far between to
impact any other sites. Thus, have maintained VFR conditions at
all sites through tomorrow. N winds around 10 knots with
occasional higher gusts will persist through this evening then
diminish closer to 5 knots while veering to NNE then to NE by
daybreak tomorrow.

KMSP...VFR conditions expected throughout this set with high
ceilings later this afternoon into the early morning hours. There
may be a few occasional periods of FEW-SCT clouds at 2-3 kft
through this evening but no lower ceilings are expected.

Sun...MVFR/IFR with RA. Winds NE 15-20G30kts.
Mon...IFR/RA mixing with SN AFTN/EVE. Winds N 15-20G30kts.
Tue...MVFR ceilings possible. NW 10-15 kts.




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