Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201702
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1202 PM CDT Sat May 20 2017

.SHORT TERM...(Today and Tonight)
Issued at 410 AM CDT Sat May 20 2017

Strong upper low will continue to lift northeastward today and
tonight right across our eastern zones as evidenced by upper QG
forcing. Surface low pressure, currently centered in northeast KS,
will move into central MN by tonight and occlude while doing so. A
trowal based in the 300-315K layer lifts northward this morning
along a short wave, then wraps around into our northern and western
zones this afternoon and tonight. Typical for a mature cyclone, the
shower activity will become a bit more scattered on the southwest
flank of the occlusion as the day progresses, especially from Sioux
City to Storm Lake, and northward to Sioux Falls and Yankton.
Elsewhere near the trowal, pops will remain either in the likely or
categorical range. With elevated CAPE centered in northwest IA,
mentioned a chance for TSRA in that location and immediate
adjacent locations, aided by a 100-110 knot jet streak moving
eastward across our southeast zones today. Despite the high
pops in the trowal locations, opted for an occasional wording to
the showers where categorical pops are present today, as the radar
is showing a rather intermittent rain regime. High pops will linger
tonight in the trowal area, with likely to categorical pops
remaining except in northwest IA where chance pops are warranted.

Highs today are a bit of a challenge. Warmed up the lower pop areas
forecast for this afternoon, basically in northwest IA and extreme
southeast SD over guidance values. Once again, the raw model and
bias corrected values appear too cold in these locations as the
rainfall could be quite scattered later today. And given current
temperatures, just cannot see going lower 40s for highs. Indeed, the
guidance temperatures were too chilly yesterday for many locations
minus southwest MN. Still, it will be very cold for this time of
year with a lot of mid to upper 40s present, perhaps lower 50s in
the Storm Lake area. This will once again put many highs in the top
5 coldest maximum readings for May 20. Tonight, introduced just a
bit of rain/snow mix for east central SD, generally in Brookings and
Kingsbury counties. But with lows in the upper 30s in that
location, not looking at any accumulations nor a freeze.

.LONG TERM...(Sunday through Friday)
Issued at 410 AM CDT Sat May 20 2017

Sunday is a little tricky across the east as it looks like the
potential for self destructive sunshine because of steep lapse rates
aloft. With the cloud cover hanging tough across the area, have
lowered forecast highs across the east. There is also the possibility
of an isolated shower or two developing across southwest Minnesota
due to diurnal heating. Further to the west, should see some nice
warming leading to highs in the 60s.

On Monday, models suggest shortwave energy wrapping cyclonically
around an upper low across the Great Lakes Region.  As this
disturbance moves in, expect showers and thunderstorms to develop
across the region.  With tall thin CAPE profiles and weak wind
shear, am not expecting severe weather but could be a fairly
efficient for rain production given the deep saturation.

On Tuesday, much cooler and breezier conditions are expected as cold
air advection couple with 20-30 knots of momentum at 925 hpa. Again,
fairly steep lapse rates in the 800-700 mb layer could result in
instability showers/thunderstorms.

Warm air advection develops across the area on Thursday bringing
widespread 70s back to the region.  A series of leading shortwaves
ahead of a slow moving trough across the Pacific NW will create the
chance for thunderstorms on Friday.  As low pressure intensifies
across the Plains Memorial Day weekend, unsettled conditions are
expected before cooler conditions develop in the latter portion of
the weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1157 AM CDT Sat May 20 2017

IFR/MVFR ceilings will dominate through the TAF period. Scattered
showers, with some MVFR visibilities, will continue across the
area into this evening, then taper off overnight. Northwesterly
winds will pick up on Sunday morning, gusting 20 to 25 kt at
times.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...BT
AVIATION...JM



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