Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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965
FXUS63 KFSD 181732
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1232 PM CDT Fri May 18 2018

.SHORT TERM...(Today and Tonight)
Issued at 314 AM CDT Fri May 18 2018

Early this morning, we`re continuing to see convection over Nebraska
intercept the low level jet. This is resulting in ongoing convection
focusing and feeding further south of the area, slowing to even
stopping the eastward trajectory of rain, and allowing the
northern stratiform region to diminish this morning. A separate
shortwave across northern South Dakota moving northward is also
helping to split precipitation across the state.

Today: Not an easy forecast when it comes to rainfall projections
through the day and night and high resolution guidance is of minimal
help. We will need to allow convective debris to filter out this
morning to get a better picture of the synoptic features. That said,
envisioning a very slow decay to rain this morning, with the main
focus of very light rain staying in the James River valley this
morning along with continued elevated convection over central
Nebraska.

Eventually we`ll get a better picture of a frontal boundary dropping
southward this morning, which will serve as a focusing mechanism for
convection later today and especially tonight. Atmospheric recovery
may be fairly slow today thanks to cloud debris, which will limit
the overall severe weather potential this afternoon. That said,
soundings show a weakly capped environment across south central
and central South Dakota this afternoon ahead of this boundary,
but also very few other focusing mechanisms at the surface.
Several model simulations pull a weak shortwave northward which
could be enough to spark scattered convection later this afternoon
both along the approaching boundary, but also in the weakly
capped environment from ONeil to Valentine northward through
Mitchell, Chamberlain, and Huron. Again, only looking at upwards
of 1000 J/KG MLCAPE and meager effective shear. The main risks
would be marginal hail, but perhaps the greatest risks would be
heavy rainfall. Very slow storm movement could give the potential
for localized rain amounts greater than 2" in the southwest CWA.

By tonight the front will sag southward across the CWA allowing
cooler air to filter south, but also pushing the greatest risks of
rain southeast along with it. There will be a continued thunder
risk, but instability wanes quickly which should serve to lessen
rainfall amounts.

.LONG TERM...(Saturday through Thursday)
Issued at 314 AM CDT Fri May 18 2018

This weekend:  There remains considerable uncertainty in the track
of the various pieces of energy moving through the Plains.  The
trend over that past three days however has been an increasing risk
for a more prolonged rain chance now pulled through the entire
weekend and into Monday as the parent upper only just moves through
by Monday.  Now that`s not saying that the entire weekend will be a
rain-out, there may be localized breaks each day, but there will be
chance for rain nearly continuously based on each little wave that
moves through. Growing confidence in cooler temperatures this
weekend, especially Sunday, where highs likely won`t see the 60s.

Monday:  There may be a short lived dry period on Monday and Monday
night as we`re in a short period of ridging between upper waves.
Temperatures may rebound back to normal levels.

Tuesday-Thursday: The upper pattern evolves into one that will
provide a daily risk for rainfall through much of next week. As long
as there is minimal troughing over the Eastern Great Lakes, and a
large disorganized trough over the Rockies, we`ll keep a confluence
zone in the Plains. Not everyone may see rain, with the lowest risks
in our Minnesota and northern Iowa zones.  Temperatures will average
near the seasonal values.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1215 PM CDT Fri May 18 2018

VFR conditions will prevail this evening into the overnight hours
until scattered showers and isolated thunderstorms move northward
into the area early Saturday morning. Low MVFR to IFR conditions
are expected to develop at all sites, mainly after 12Z Saturday.
At this point, better chances for scattered showers and
thunderstorms will be possible across KSUX late tonight into
Saturday afternoon. Did not mention showers or thunder across
KHON and KFSD due to low confidence in timing and coverage of
thunderstorms.

&&

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

$$

SHORT TERM...Dux
LONG TERM...Dux
AVIATION...05



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