Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241719

Area Forecast Discussion
National Weather Service Sioux Falls SD
1219 PM CDT Tue Apr 24 2018

.SHORT TERM...(Today and Tonight)
Issued at 402 AM CDT Tue Apr 24 2018

Compact wave dropping south through western South Dakota will bring
showers to mainly western parts of the forecast area into tonight.
Leading line of showers appears to be fighting dry air intrusion
thanks to the southeasterly low level flow, but expect this will
gradually be overcome by the increasing lift through the morning, so
likely-categorical pops still look reasonable for our far western
areas today.

HRRR is trending farther east with development across our southern
areas as the wave drops into Nebraska this afternoon, but this not
presently supported by other hi-res models, so will stick with just
chance or lower pops for all areas along/east of I-29 for now.

Rain will end north to south tonight as the wave continues to drift
slowly southward, followed by clearing skies. Temperatures today
will be somewhat non-diurnal, especially in southern areas which
should see midday highs followed by cooling as rain expands and cold
advection takes over. Northern areas more likely to see typical late
day highs as rain begins to diminish through the afternoon. Tonight
temperatures will fall into the 30s most areas, with sub-freezing
lows for some areas near/north of I-90.

.LONG TERM...(Wednesday through Monday)
Issued at 402 AM CDT Tue Apr 24 2018

Temperatures will be the focus for the mid-long range portion of the
forecast, with trends continuing to point toward a stretch of warmer
weather as we close out this month. Given proven ability to warm
nicely the past couple of days, and continued cooling influence of
bias-corrected grids, have nudged highs up a bit most days, but the
most dramatic shift from our consensus blend comes Sunday/Monday.

Surface ridge drops south across the area Wednesday, which will
bring light winds and plenty of sunshine. Expect temperatures to be
close to normal, with highs in the upper 50s-mid 60s. A weaker and
faster wave slides across the region Thursday, accompanied by a weak
cool front. Current timing of the front places it across northwest
Iowa during peak heating, and could see some spotty showers develop
there in the afternoon. NAM and GFS hint at some weak instability
ahead of the 850mb boundary late Thursday afternoon, so cannot rule
out an isolated clap of thunder, but not confident enough in timing
of the boundary to introduce mention at this time.

Friday/Saturday should be quiet with the start of the warming trend
as the upper ridge builds into the Plains.

Early next week, upper ridge begins to shift east, as a trough
develops across the Rockies. Increasing southerly flow ahead of the
trough will support good warming Sunday/Monday, as thermal ridge is
pumped northward into the Upper Midwest. Some difference in strength
of the Rockies trough, and thus with timing of when it swings out
into the Plains, but all models indicate the associated cool front
will remain west of our forecast area through at least Monday. Given
low level temperatures, highs in the 70s seem a good bet, and may
even be on the cool side in some areas, especially on Monday.

With the stronger south winds and warm temperatures, will have to
watch humidity levels Sunday/Monday. Models currently pointing to
afternoon dew points in the 40s-lower 50s spreading northward into
the area, but with the late start to the growing season and good
mixing, may see these values mix lower in the afternoon. Already
seeing the fire danger hit Very High in many areas based on the
winds and MinRH of 35-45 percent, so warmer temperatures and/or
lower dew points could push the fire danger higher.

Aforementioned differences in the handling of western trough lead to
significant differences in the models by next Tuesday, so no changes
to the forecast there at this point. If faster EC verifies, highs in
the upper 50s/60s are reasonable, but if the slower GFS pans out,
Tuesday could be another warm day in the 70s/lower 80s. Too much
uncertainty to select one over the other just yet.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1215 PM CDT Tue Apr 24 2018

Although light rain will be in the area this afternoon and
evening, the rain is light enough that we are not overly confident
that it will cause IFR or MVFR visibility. If it does, it likely
will not last very long. That said, MVFR ceilings are definitely a
threat at KHON this afternoon. There is a chance that some MVFR
ceilings could develop also this afternoon and evening at KSUX so
will monitor.




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