Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181741

Area Forecast Discussion
National Weather Service Sioux Falls SD
1241 PM CDT Mon Sep 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 356 AM CDT Mon Sep 18 2017

Convection on the uptake across northeast NE, far southeast SD and
western portions of northwest IA early this morning, as combination
of larger scale lift in jet entrance region and increase in the
theta-e advection ahead of sheary trough. Next 6-9 hours is likely
to focus the best precipitation coverage across the southeast half
of the area (approximately south of a KYKN-KFSD-KMWM line) as the
advective parameters shift gradually east and northeastward across
the area. Thunder threat would look to mainly be contained south of
I-90 and perhaps a bit more north across southwest MN, but the thin
and meager profile should keep coverage much less than the showers.
Sharper back edge to lift forcing should bring precipitation to an
end around KFSD toward midday, shifting quickly eastward through the
afternoon.  Clearing should build east and start to displace the
lower clouds west of I-29 during the afternoon as flow off surface
turns more westerly and shallows out residual moist layer. Clouds
will drive a large thermal gradient over the area today, with lower
20s at 925 hPa west of the James River likely bringing near 80 into
the mix, while clouds and later exit of precipitation keep readings
in the lower to mid 60s through parts of southwest MN and northwest
IA. Between areas near/west of I-29 are the most challenging with
clearing in question, and could see large gradient in temps from
upper 60s into the 70s.

Since areas to the east are not likely to mix to a great depth,
pattern is very favorable for the rapid northwestward expansion of
the stratus field overnight with near-surface winds retaining an
east of south component. Would not be surprised to have stratus
build all the way west through the CWA.  In addition, potential for
fog will be the fly-in-the-ointment tonight, as either the lowering
of stratus on higher elevations of eastern SD/southwest MN/northwest
IA, or some potentially lower visibilities with dense fog on the
edge of the expanding stratus field.  Winds not terribly strong, but
clouds/persistence of southeast winds suggested blending in some
CONSRAW data to min temps overnight.

.LONG TERM...(Tuesday through Sunday)
Issued at 356 AM CDT Mon Sep 18 2017

The three main storyline in the medium and extended portions of the
forecast remain focused on severe weather chances Tuesday, a strong
warmup by the end of the week, and potential for widespread rainfall
as next weekend approaches.

While certainly a very potent risk, a few flies in the ointment
continue for Tuesday`s severe weather risk.  First begin extensive
stratus that may build overnight and persist along and east of the
James River into Tuesday morning.  Some models are even more
aggressive pulling this stratus west of the Missouri River.
Secondly, model guidance continues to hint at a very stout capping
inversion, especially west of I-29 which will need to be overcome
before convection can initiate further west. Lastly, we`ll have some
concerns about frontal timing and when deep layer forcing arrives in
the evening.

Model guidances remains fairly settled on a frontal location near or
west of Pierre by mid-afternoon. Soundings ahead of the boundary in
the MO River valley show modest MLCAPE approaching or locally
exceeding 2000 J/KG.  This instability is of course built on the
idea that sfc dew points reach the lower to middle 60s.  The mixture
of high-resolution guidance and synoptic models continues to point
at late afternoon or early evening initiation along and ahead of the
boundary as deep forcing moves overhead. Scattered cells should
quickly transition into a a progressive linear squall line with LEWP
structures at times. The biggest question is just how far south
along the front storms can develop.  Currently feel the greatest
potential for development would be along and north of I-90, where
the CAP may become the weakest and forcing strongest.

Greatest risks with storms will be damaging winds, as DCAPE values
approach 2000 J/KG in the evening and deep layer shear intensifies
and 850:700 mb winds greatly increase after 00Z. Also concerning is
the very high 0-1 KM helicity and shear values progged to climb
between 200-3000 M2/S2 and the latter up to 20 knots respectively. 0-
2 KM shear vectors have shown a tendency to turn perpendicular to
the line, so I would anticipate an elevated tornado/mesovortex risk,
especially should the line become more developed into LEWPs.  By
late evening, the eastward progression of the severe risk remains
questionable. Most models continue to show lingering stability from
longer lasting stratus across Minnesota and Iowa, meaning any
organized system may trend more elevated in nature, decreasing the
wind threat, but leaving an elevated hail threat.  All in all
though, while the severe weather risk looks potent, we`ll have to
overcome the aforementioned issues first. Have added in severe
wording to the gridded forecast.

High pressure sweeps into the area for Wednesday, significantly
lowering dewpoint in the region. However, the synoptic pattern
evolution would suggest a rapid return of warmer and more humid
conditions beginning late Thursday but amplifying Friday.
Similarities in all extended guidance indicating a frontal boundary
approaching the area on Friday with modest instability and another
sharp increase of shear potentially leading to another severe
weather risk.

The eventual evolution of the high amplitude longwave pattern
remains a bit in question, but an intensifying eastern ridge
combined with two Atlantic hurricanes could mean that this frontal
boundary remains stalled near or over the CWA into the weekend.
Similarities in guidance have increased confidence to jump
temperatures significantly upwards over blends for both the
overnight and day periods of Friday and Saturday.   Have also
continued likely PoPs into the weekend, with a potential of locally
heavy rainfall Friday night through Saturday night.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1237 PM CDT Mon Sep 18 2017

Low lying stratus east of I-29 will be quite stubborn but will
attempt to shift the focus east through the afternoon. With
sunset, expect fog and stratus to expand back westward, and have
fog/stratus in both KFSD and KSUX terminals. Fog could become
dense overnight, but TAFs do not reflect full visibility

Low level moisture is somewhat deep on Tuesday morning and have
kept stratus and fog east of the James River through better part
of the morning. Strong southeast winds will develop during the




SHORT TERM...Chapman
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