Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181731

Area Forecast Discussion
National Weather Service Sioux Falls SD
1231 PM CDT Sat Mar 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 320 AM CDT Sat Mar 18 2017

Pleasant and mild day expected throughout the region with upper
level ridging building over the high Plains. Surface high pressure
slowly slides eastward today, with return flow tapping into to
southerly winds by the afternoon. Gradient tightens some midday in
south central SD where southeast winds will be breezy in the
afternoon, otherwise, winds will generally be less than 15 mph.
Cirrus streaming across the area this morning will thin, leading to
abundant sunshine and high temperatures in the mid 40s to lower 60s.
Again, the coolest readings will be across portions of northwest
Iowa and southwest Minnesota where deeper snowpack and the later
arrival of return flow will hold back readings. The mild readings
however should continue to eat away at the remain snow cover.

Breezy south southeast winds will expand across the rest of the
region this evening and tonight and the pressure gradient tightens
over the northern Plains. With warmer air building aloft, the breezy
winds will create mixy conditions. This will keep the overnight lows
quite mild, especially across south central SD where lows could hold
in the upper 40s. Elsewhere, low will generally be in the mid 30s to
lower 40s. Nudged lows up slightly using the BCCONSMOS guidance.

.LONG TERM...(Sunday through Friday)
Issued at 320 AM CDT Sat Mar 18 2017

Strong warming still expected on Sunday, ahead of and immediately
preceding cold front pushing into the Lower James River Valley by
late afternoon. Some models continue to hint at potential for some
stratus streaming northward into parts of northwest Iowa/southwest
Minnesota in the morning, which could impact initial temperature
rise. However, good mixing ahead of the front and partial sunshine
through the afternoon should be able to yield highs in the 60s.
Southwest portion of the forecast area, along/south of I-90 and west
of the James Valley, have potential for warmest temperatures, with
highs near 80 not out of the question in some areas. Begin to see
cooling aloft behind the cold front by 21z, but with 850mb readings
still in the teens, mid to upper depending on model preference.
While areas ahead of the front may be hampered by strong inversion,
the post-frontal cooling should allow for some deeper mixing and
burst of warming before readings cool more rapidly toward evening.

Temperatures settle closer to normal by Monday with highs in the
upper 40s to lower 50s most areas, followed by even cooler days for
Tuesday/Wednesday as increased cloud cover and cooler temperatures
aloft hold daytime highs in the 40s. Precipitation chances through
Tuesday evening look minimal for most, with a chance spreading into
the Missouri River Valley late Monday night/Tuesday morning as a
weak wave slides into the central Plains.

Still some questions regarding eastward extent of precipitation
chances as a stronger wave moves into the Dakotas later Tuesday
night/Wednesday. GFS remains drier with this event, likely due to
weaker wave than seen in the Canadian/ECMWF, as low level moisture
looks fairly similar among the solutions, with deepest moisture
remaining largely west of I-29 late Wednesday. Given disparity in
wave strength, will keep pops in high chance range, again focused
west of I-29 through 18Z Wednesday, then weakening as it lifts
northeast through the afternoon. Temperatures aloft cold enough to
support snow initially, possible transitioning to rain through the
day before ending, though amounts in either case appear light.

Stronger system slated for the end of the week, and still seeing
disparities among the models in how they develop and track this
system east across the Plains. GFS has weakened a bit from its run
24 hours ago, now showing an open wave like the Canadian/ECMWF.
However, it is now much farther north than previously, allowing for
even warmer air and a wedge of instability building northward into
the area by late Thursday/Thursday night. Canadian seems to have
maintained some consistency with previous solutions, while latest
ECMWF has flipped to the solution with the stronger/more southern
wave. All solutions continue to point to a good chance at a wet
Thursday/Thursday evening, and will continue with our likely pops in
these periods. Thereafter, GFS and Canadian point to potential for
dry slot impacting this area later Thursday night/Friday, and will
trend pops downward accordingly. As for temperatures, even the
cooler Canadian/ECMWF look to support mainly rain for this forecast
area, though those models do indicate potential for some light snow
at the onset early Thursday before the warmer air moves in from the
south. Have also added a mention of isolated thunder to Thursday
night, as both GFS/Canadian have trended toward instability farther
north than previously seen.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1224 PM CDT Sat Mar 18 2017

VFR conditions through the period. Surface ridge will scoot east
this afternoon, with southeasterly winds increasing. Flow
increases aloft overnight tonight, leading to low level wind
shear. The threat will abate on Sunday morning as breezy
conditions develop at the surface.




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