Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 282315
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
615 PM CDT Sun May 28 2017

.SHORT TERM...(This evening through Monday)
Issued at 415 PM CDT Sun May 28 2017

Very breezy afternoon across the region, as deep mixing is allowing
20 to 30 knot gusts to mix downward. Relatively quiet short term
continues, but we are watching convection across North Dakota
quickly race southeast. This convection is tied to a PV anomaly
pivoting across eastern North Dakota this afternoon, and focused on
the nose of an upper jet streak plunging down the western side of
the broad Canadian upper trough.

High resolution models are doing a good job handling this
convection, with expectations that the leading edge of this activity
may push into the northwestern portions of the CWA along the Highway
14 corridor by 6pm. With this activity no solely diurnally based,
expecting this scattered activity to persist southeast towards NW
Iowa by the late evening hours.  While instability really isn`t that
high, will maintain an isolated thunder wording in the grids.
Perhaps the biggest concern would be the continuing steep low level
lapse rates and high storm bases, which could support brief wind
gusts up to or higher than 40 knots this evening. Simulated HRRR
gusts speeds support this idea as well.

Memorial Day could turn into a somewhat cloudy and cool day, as
strong cold air advection pivots southwest in the morning.  With the
large upper trough dropping southward into the western Great Lakes,
any breaks in cloud cover should result in scattered showers or even
a few thunderstorms over western Minnesota and far eastern South
dakota by mid-day. Increased cloud cover, combined with cold
advection, will force temperatures slightly downward into the lower
and middle 60s. Afternoon winds will again be rather gusty, have
have increased speeds a few knots over populated guidance.

.LONG TERM...(Monday night through Sunday)
Issued at 415 PM CDT Sun May 28 2017

The showers Monday will rapidly wane as the daytime heating subsides
in the evening, enough to not warrant a mention of pops. As the
winds subside, cool high pressure will produce below normal lows in
the 40s. On Tuesday, the very large upper cyclone is still northeast
of our CWA greatly influencing our weather with an upper cyclonic
flow. Despite a high degree of mixing, cumulus and cool temperatures
above the surface will help to keep highs below normal with readings
in the 60s. The one exception is in our MO River valley zones where
highs should reach the lower 70s. Despite the upper cyclonic flow,
at this time decided to leave the northeast half of our forecast
area with no mention of pops in the afternoon as the cumulus field
builds. There is some subtle warm air advection in the 700-500mb
layer of about +2 to +4C which could keep light showers at bay, and
a lack of discernible short wave. It will be breezy again however,
with mixed layer winds of 20 to 30 knots suggesting a lot of 15 to
30 mph winds at the surface from a northwest direction. As surface
high pressure moves close to the forecast area Tuesday night, lows
will be very chilly for this time of year. Hedged toward cool
guidance readings such as MOS and the ECMWF to give a lot of 40 to
45 degrees under light winds.

The pattern then begins to warm on Wednesday with less wind. The
large upper cyclone is still situated over southeast Canada and the
northeast United States Wednesday through Friday. To the west, a
weak upper ridge of high pressure moves into the western plains. As
this happens, the low level jet increases Wednesday night with warm
air advection. This CWA is placed in speed convergence with 40 knots
in south central Nebraska feeding into only 20 knots in our southern
zones which gives some broad ascent. Furthermore, there is some
model consensus in showing MCS development to our south toward I 80.
The question is, will any rain fall on the northern side of it this
far north. This is similar to what happened last late Thursday night
and Friday morning. The GFS and GEM Global are similar in producing
some light rain and general TSRA in our forecast area Wednesday
night and Thursday morning, with the ECMWF less bullish. Solid
chance pops are certainly warranted with this disturbance, and if
the ECMWF comes more into line with the GEM and GFS in future model
runs, likely pops may be warranted at least from I 90 southward.

From Friday through Sunday, an upper trough begins to form behind
the upper ridge over MT, then moves southeastward into the plains on
Saturday. Friday is likely the most problematic in terms of highs as
to where the warm front sets up. The GEM Global and GFS has the warm
front well south of our area, whereas the ECMWF brings up warm air.
For now, used superblend with this much discrepancy which produces
highs in the upper 70s to lower 80s. If the ECMWF pans out as is,
then readings will be 5 to 10 degrees warmer. But as the upper
trough moves through on Saturday, with the ECMWF the quickest,
temperatures will cool some with a weak cold front following
Saturday night. At any rate, chance pops are certainly warranted for
rain and some TSRA late Friday night and Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 611 PM CDT Sun May 28 2017

Scattered showers and isolated thunderstorms will linger through
the evening, then taper off after 02Z. Gusty northwest winds will
also quickly decrease in the evening with loss of daytime
heating. Northwesterly winds will again pick up by mid morning on
Monday, gusting around 20 to 25 kts into the afternoon.

&&

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

$$

SHORT TERM...Dux
LONG TERM...MJ
AVIATION...JM



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