Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 252331
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
631 PM CDT Mon Sep 25 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 401 PM CDT Mon Sep 25 2017

Deep trough through the Rockies will finally eject the main impulse
northeastward across the area later this evening, leaving behind a
closed portion around western AZ.  For the most part, thermodynamic
profiles are deep moist neutral, so any lift forcing should be more
than sufficient to get a showery response. Even at periods of before
the main synoptic lift toward early evening, persistent
frontogenetic forcing/850-700hPa theta-e advection will maintain at
least some shallow precipitation/drizzle. About the only chance to
perhaps get a rumble of thunder will be in parts of northwest Iowa,
where a slight bit of elevated instability will linger into the
early morning hours. Carrying likely to categorical PoPs seems in
order, and also have increased PoPs during the evening across the
James River valley with most solutions indicating development of
additional banding along a higher surface as the main jet entrance
lifts past the area. Persistence of the modest northerly winds will
likely keep fog at bay overnight, with temperatures sluggishly
falling toward the mid 40s to lower 50s.

While clouds will likely hang around on Tuesday, decreasing
gradually from the west/southwest through the afternoon, it does
appear the precipitation chances should wane quickly during the
early morning hours. Northwest winds should freshen a bit more, and
eventually the subsidence with cold advection and passage of the
upper trough should work on breaking clouds, but still a seasonably
cool day in the upper 50s to mid 60s.

.LONG TERM...(Tuesday night through Monday)
Issued at 401 PM CDT Mon Sep 25 2017

Weak surface ridge will settle across the region Tuesday night,
sinking into Nebraska by early Wednesday. With likelihood of
getting at least a bit of light west/southwest wind north of the
ridge axis, this will likely limit the threat of temperatures
dropping too far, with perhaps a few lower spots eking into the
upper 30s. Wednesday will be a dry, pleasant and seasonable day in
dominant northern branch of broad confluence of northern stream
and flow around the closed low in the southwestern U.S. With
surface ridge settling south of the area, a westerly component
should help drive highs upper 60s to lower 70s, and again allow
for a seasonably cool Wed. night in the mid to upper 40s.

A stronger wave in the northern branch will push toward the Great
Lakes on Thursday, driving a back door front southward into the area
late day. Temps probably warmest in the short term on Thursday right
ahead of boundary, a bit above normal in the lower to mid 70s.
Should get a few more clouds along this boundary, but deeper forcing
and any marginal hints of instability look to remain east of the
area along the front.  Reinforcing dry and cool low level ridge will
settle through Minnesota through Friday night, but a quick return to
stronger southerly flow by Saturday which will persist through the
weekend.  The reason for the increase in winds is the strong
troughing developing across the northern Rockies as the southwest
closed low is picked up and absorbed by deepening Pacific NW trough.
The impacts of flow out of the strong ridge to the east will likely
hold back precipitation chances until at least Sunday night.

Not a great deal of agreement run-to-run or model-to-model in timing
stronger impulses rotating through the western trough, so have not
deviated from initial blended PoPs at this time, which suggest
greatest chances developing Sunday afternoon and night as deeper
moisture becomes involved and low-level boundary may push out far
enough east across the plains to start to enhance convergence. Some
threat of thunder with this feature, with EML starting to push out
across the area on Sunday.  Some lower PoPs Monday more a signal of
the uncertainty of how progressive the system may be than a solid
precipitation threat.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 629 PM CDT Mon Sep 25 2017

Persistent light rains or drizzle will move across the forecast
area overnight. Ceilings will remain MVFR to IFR and may even
flirt with LIFR in a few locations. Visibility may fall to a mile
or less in a few areas. Generally though we should see
improvement as far as rain is considered as daybreak approaches.

A gradual and slow improvement in ceilings is expected during the
day on Tuesday, with VFR conditions returning Tuesday afternoon.

&&

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

$$

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



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