Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 212040

Area Forecast Discussion
National Weather Service Sioux Falls SD
340 PM CDT Fri Apr 21 2017

.SHORT TERM...(This evening through Saturday)
Issued at 339 PM CDT Fri Apr 21 2017

Deep-layer ridge axis will gradually settle southward over the next
24 hours, setting the area up for a quiet and pleasant period, with
the greatest concern in the dealing with frost potential tonight.

Heading into the evening, diurnal cumulus field will quickly fade,
and thicker cirrus field will continue to pull away to the east and
southeast.  Upper ridge will continue to fold over and elongate more
west to east across the northern plains, settling southward to
assist in clearing overnight. With clearing skies and winds easing,
will be watching how low temps dip tonight.  Track record for
guidance was not strong last night, as temps were well to the cool
side of guidance values. Expect more of the same tonight as clouds
vacate and the surface winds drop to light, especially north of I-
90. May maintain just a bit more northeast low level winds across
northwest Iowa, which is probably the area of greatest uncertainty
tonight. Overall, there is probably an equal chance that conditions
will turn out as a more heavy dew at 34-35 degrees rather than any
modest frost threat. In this radiative cooling environment, and
frost development would certainly favor more protected/lower areas,
and thus should remain fairly patchy. For now, have limited any
patchy frost mention to time/area of temps cooler than 35, generally
spots north of I-90, around the Iowa Great Lakes, and perhaps the
lower Missouri valley outside the Sioux City metro. Still, it is a
wise move to protect any early vegetation as temps fall into the mid
30s overnight.

Surface ridge axis will start the day near a Marshall to Pickstown
line, with a gradual southward sink through the day. Winds north of
the axis will eventually favor a more west to southwest direction,
while ahead of the ridge axis, north to northeast flow aloft will
mix down during the later morning and afternoon. All in all, plenty
of sun should drive temperatures into the mid 60s for most, a bit
warmer west of the James, and a shade toward the cooler side of
the range in parts of northwest Iowa.

.LONG TERM...(Saturday night through Friday)
Issued at 339 PM CDT Fri Apr 21 2017

On Saturday night and Sunday, a positively tilted upper ridge of
high pressure nudges off slowly to the southeast. In response,
surface high pressure will move off to the south, allowing our winds
to shift to the south and southwest and will begin to increase in
speed Saturday night, becoming quite breezy on Sunday. To the north,
a strong cold front makes it southward close to the ND/SD border
where it stalls. Due to the southwesterly surface flow, lows will
not be as chilly Saturday night, with widespread temperatures in the
upper 30s and lower 40s. Sunday will follow with very warm
temperatures. Given the 850mb temperature profile, boosted highs
over the superblend readings a little bit, giving upper 60s in
southwest MN, and lower to mid 70s elsewhere. With southwest winds
of 20 to 30 mph, there is a chance that we could get warmer. However
am a little concerned about thicker high clouds moving in through
the afternoon hours which may slow down the rise in temperatures.
Something to keep an eye on at any rate.

On Sunday night, the next short wave moves into the western high
plains by late at night, advancing quickly eastward to the I 29
corridor by early Monday evening. Kept a low risk for skittish
thunder possibilities Monday afternoon along the accompanying low
level boundary, give the amount of elevated MU CAPE. There is a
pretty stout inversion on both the NAM and GFS soundings in the 1.5-
2.5km layer. Therefore elevated CAPES are rather thin, and projected
0-1km ML CAPE is slim to none. In addition, this time around
moisture is limited, and 0-6km shear is weak, so not too concerned
about severe weather prospects at this time. In fact the boundary
could go through rather dry. Monday will be one more above normal
day temperature wise, with a lot of mid 60s to around 70 in store.
And the surface winds will continue to be pretty brisk ahead of the
cold front along and east of I 29 at 20 to 30 mph.

This short wave moving through on Monday will allow the
aforementioned stalled frontal boundary to our north to move through
Monday night, giving cooler temperatures for the week. Rain chances
are now lower on Tuesday as dry low level air continues to filter
down from the north, so now the rain looks really light and a bit
spotty. One exception may be in our central SD zones where rain
chances are a bit better where a short wave has more impact when
compared to areas east of there. When this next short wave moves
east Tuesday afternoon, it encounters increasingly dry low level air
coming down from Canada.

There continues to be a stronger system lurking out there late next
week. Have the usual major model discrepancy in how the upper low
evolves. But looking at the large scale, it does look chilly and
rainy Thursday through Friday morning. No doubt the GFS is an
outlier with the surface and upper lows when compared to the GEM and
ECMWF. But that far out its most wise to use superblend because
there is no guarantee that the foreign solutions will be right
either. So blended the solutions. Temperatures look below normal
with ample rainfall possible.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1243 PM CDT Fri Apr 21 2017

Model underestimation of the moisture in the boundary layer today
and have seen a much greater expanse of diurnally driven cumulus
at VFR levels as mixed layer deepened late morning. Expect
coverage should peak in the next 1-2 hours, and dissipate fully
by around 00-01z.




SHORT TERM...Chapman
AVIATION...Chapman is the U.S. government's official web portal to all federal, state and local government web resources and services.