Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 171753
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1253 PM CDT Sun Sep 17 2017

.SHORT TERM...(Today and Tonight)
Issued at 359 AM CDT Sun Sep 17 2017

A crisp mid-September early morning underway with most readings in
the 40s along with a light west to north breeze. High pressure will
become centered over the area by midday, then shift slowly toward
the upper Mississippi valley early this evening, allowing return
flow to develop.  While sunshine will greet what will be a
pleasantly cool day with highs in the mid 60s to around 70, there
will be an increase in clouds heading into the afternoon as a quick
moving wave from Pacific NW moves into western SD by 00z. Confluence
of flow will stream mid and high level moisture eastward across the
area, while low levels remain much drier under the influence of the
retreating ridge.  Mid clouds could become fairly thick toward the I-
29 corridor by evening, but for now, have kept any precipitation out
of the forecast with a fairly stable profile in the cloud bearing
levels and the presence of the dry air below. If it were to be a bit
more unstable, could perhaps see a few sprinkles KFSD-KSUX for the
last couple of daylight hours which scoot across mainly NW IA.

As we progress through the night, a well-defined jet entrance will
drag across the area, increasing the mid-level warm advection.
Fairly confident of keeping dry through 03-04z, but then rapid
increase in warm advection should yield a quick development of
precip from south central SD southward into NE, which will stream
along with broad jet entrance east and northeast toward southeast
SD/northwest IA/southwest MN.  Some of the higher resolution
solutions are hitting rainfall pretty hard across northwest IA, and
areas from KYKN-KFSD-KMWM and south are a fairly good signal for
lift during the last third to half of the night. Elevated
instability remains fairly minor, so looking more at embedded
thunder in general showers, mainly south of I-90.  Clouds and the
increased moisture trends with southerly wind are likely to keep
lows around the lower 50s tonight.

.LONG TERM...(Monday through Saturday)
Issued at 359 AM CDT Sun Sep 17 2017

Three main areas of focus with the medium and extended portions of
the forecast; rain chances on Monday, severe weather potential
Tuesday, and a still uncertain end of the week.

Shortwave energy moves through quickly Monday morning, enough
instability to warrant continuing thunder chances, but in general
this wave and associated rain should move east by mid-late morning.

Cooler easterly flow should allow stratus to develop Monday night
into Tuesday morning, especially east of I-29. Then attention turns
to a strong frontal boundary moving eastward across the Dakotas
Tuesday afternoon and evening as an impressive upper trough and
strong jet nose into the Northern Plains.

Model guidance has varied by 6 hours with the timing of the front by
18-21Z, but have somewhat settled on a location near or west of
Pierre by mid-afternoon.  Soundings ahead of the boundary in the MO
River valley show modest MLCAPE of 1500-2000 J/KG should sfc dew
points reach the lower 60s.  There is a bit of a capping inversion
present through the afternoon, which could pose an issue with
convective initiation. That said, as deep forcing ejects eastwards
and 500 mb speed max moves over the front, model consensus points to
rapid convection potential along the boundary in central SD.   Point
soundings are certainly impressive from the initiation point into
areas west of the James river into the evening. Deep layer shear
approaching 50+ knots, "inverted V", mid-level dry layer, and sharp
backing of the sfc winds, are all present with the sounding.
Scattered development on the front should quickly turn into a
progressive linear line as the front and very strong dynamic forcing
moves eastward. Given shear vector orientation, would expect bowing
segments as any formed convection moves east. Hodographs would
certainly suggest a spin-up potential. Instability further east
towards I-29 remains a bit question and convection should trend
elevated as it moves east in the evening. All that said, we still
have 2-3 days to go, but it`s a time period that bears monitoring
especially west of the James River.

Sweeping high pressure scours boundary layer moisture away from the
area for Wednesday and Thursday.  Model differences continue towards
the end of the week, with GEM/ECMWF trending slower and GFS
maintaining a late Friday front arrival.  Would prefer a slower
solution given the synoptic pattern.  Still looks like a reasonable
severe weather risk late Friday and Saturday, but so many questions
about timing to be any more specific.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1240 PM CDT Sun Sep 17 2017

Mid-level cloud cover will increase through the afternoon hours,
with light winds at the surface. Later tonight, roughly after
18/0600z, precipitation will begin spreading into the area ahead
of an upper level shortwave trough. Cloud ceilings will lower
during this timeframe, with some visibility restrictions possible
in NW Iowa with the heavier rain. Widespread lightning is not
expected with showers, but the strongest showers could produce a
few strikes. Rain chances quickly decrease by late morning
Monday.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...VandenBoogart



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