Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202339

Area Forecast Discussion
National Weather Service Sioux Falls SD
639 PM CDT Sun May 20 2018

.SHORT TERM...(This evening through Monday)
Issued at 311 PM CDT Sun May 20 2018

Surface high centered over northern Minnesota will continue to shift
eastward into the Great Lakes, while a surface trough and vort max
continue to slide east-northeast through the forecast area this
evening. Soundings indicate decent forcing and deep saturated layer
along and east of I-29 tonight into Monday; therefore have kept
spotty/isolated showers across this area. With easterly surface flow
prevailing tonight, expect mostly cloudy skies tonight and for most
of the day on Monday. Overnight lows will generally be in the upper
40s and lower 50s.

Upper-level ridge will dominate the area by Monday, bringing
mostly dry conditions after midday. Return flow of the departing
surface high over the Great Lakes area will bring warm southerly
winds and mid-level moisture back into the region. Highs will
range from the upper 60s to upper 70s.

.LONG TERM...(Monday night through Sunday)
Issued at 311 PM CDT Sun May 20 2018

A more active pattern will take shape this week with multiple
opportunities for showers and thunderstorms to the region, along
with warmer temperatures.

Upper-level flow remains mostly zonal with fast-moving impulses of
energy, at least through mid-week. By Monday night, a surface low
pressure over the central Plains will be lifting a warm front
northward into the area. Guidance continues to suggest late evening
convection along the lower Missouri river valley on the nose of the
LLJ coming from central Nebraska and increasing low-level warm
advection. The shear for this time-frame looks minimal, but decent
dynamic will favor spotty thunderstorms, some of them could become
strong.  Actually, showers and thunderstorms look more favorable
Tuesday evening. As the front lift farther north on Tuesday,
increasing theta-e advection, moderate deep layer shear, MUCAPE
(2500-3000 J/kg) and a reinforced LLJ may induce deep convection
Tuesday night into Wednesday. Again, will need to monitor for severe
weather potential.

There are some differences Wednesday and beyond with timing of
embedded shortwaves and intensity of any convection. Better chances
for showers and thunderstorms look to be Thursday night into Friday
night. Amplified upper trough over the Northern Plains will approach
the area, bringing chances for thunderstorms along and ahead of the
frontal boundary. Still quite of uncertainty regarding the intensity
and extent of precipitation by the end of the week.

With the upper ridge in place for majority of the week, temperatures
will be near to above normal through the period. Daytime highs will
generally be in the 80s, and overnight lows in the 50s and 60s.
With H850 temperatures from 15-20 C, Wednesday and Thursday will
be the warmest days of the period.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 635 PM CDT Sun May 20 2018

MVFR conditions will continue to be widespread this evening before
ceilings reduce to IFR across far southeastern SD and northwestern
IA, including FSD and SUX terminals. Guidance is suggesting LIFR
ceilings will advect northwestward late tonight but, without any
observations to our southeast actually being LIFR at issuance time,
have kept ceilings above 500 feet. Expect west to east clearing
to eventually occur by the end of the TAF period.




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