Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 111756
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1156 AM CST Sat Nov 11 2017

.SHORT TERM...(Today and Tonight)
Issued at 328 AM CST Sat Nov 11 2017

Focus of the short term forecast will be the good chance for
precipitation in our southern zones Saturday evening. Open wave
currently over the Pacific Northwest will translate eastward
overnight. This will bring a decent amount of PV advection to our
southern zones where a diffuse but frontogenetic 700mb boundary will
be present around 00z. Initial precipitation generation will focus
on this area of 700mb frontogenesis. Models are in decent agreement
for precipitation tomorrow evening from 00z - 12z. Main limiting
factor will be the amount of available moisture, particularly in the
low levels. Southerly winds today, aided by our stratus Friday night
which has kept some of our temperatures from falling too far below
freezing, will allow most of the precipitation to fall as rain, save
for some snow mixing in on the back side late. An outlier of note,
the NAM develops spotty precipitation across NW IA by 18z, causing
modeled surface temperatures, already a cold outlier, to wet bulb
down to below zero, culminating in several inches of raw snow output
once the precipitation cranks up after 00z. Will go with the
consensus and stick with mostly rain.

.LONG TERM...(Sunday through Friday)
Issued at 328 AM CST Sat Nov 11 2017

Sunday marks the beginning of a quasi-zonal flow across United
States lasting through the upcoming week until about Friday. High
pressure at the surface will nudge slowly eastward on Sunday giving
a day with light winds. That said, did boost up winds a little bit
in northwest IA early Sunday morning due to 20 to 25 knots at 925mb
and a moderately tight pressure gradient. But those winds will relax
by late morning. 925mb temperatures suggest highs on Sunday ranging
from the mid to upper 30s in southwest MN, to the upper 40s in our
zones approaching south central SD.

With the high moving off to the east, winds will become southerly
Sunday night and will increase after midnight, giving a quite breezy
day on Monday east of the James River valley. Bumped up wind speeds
late Sunday night along the gradient and strongest low level winds,
and continued that trend into Monday as the gradient and stout mixed
layer winds move eastward across the forecast area. The southerly
flow will help to usher in a more mild day, with highs in the mid
40s to mid 50s. Interestingly, the various models including the GFS,
ECMWF and NAM are all pointing at stratus moving northward toward
areas along and east of I 29 by midday Monday and Monday afternoon,
maximized in the 900-850mb layer. The stratus will likely be
somewhat broken, but could hinder highs throughout the eastern half
of the forecast area, therefore went barely more than isothermal for
highs along and east of I 29. All models then show a lower deck of
stratus developing Monday night and developing further westward, all
the way back to the James River valley. A very high degree of
moisture then exists near 925mb all the way through Tuesday east of
the James until it is ushered out eastward along a wind shift.
Therefore between clouds and a southerly flow, Monday night will be
seasonably mild with lows in the 30s to around 40. Tuesdays highs
still look mild despite again going barely more than isothermal from
925mb because the air mass is so warm. Therefore expecting a lot of
50s for highs, even lower 60s in south central SD where they could
be stratus free. Raised sky cover some both Monday and Tuesday east
of the James to account for the stratus potential.

But as a short wave moves through this area Tuesday and Tuesday
night, the aforementioned wind shift will push the stratus eastward
late Tuesday afternoon and Tuesday night. Precipitation chances are
very limited with this wave passage, with perhaps a very small
chance of light rain Tuesday evening along the wind shift from
Windom MN to Storm Lake IA.

The air mass still will be pretty mild Wednesday through Friday with
an absence of cold air advecting southward from Canada. At this
time, next Thursday looks windy with a return southerly flow of air.
Then after that, we have the usual discrepancies in short wave
timing between the ECMWF and GFS late in the week, with the ECMWF
about 12 hours slower than the GFS. But overall the deterministic
models really are not all that far apart heading into Saturday.
Friday will likely be another windy day, this time with directions
from the northwest. Organized precip chances are not looking overly
strong in the Thursday and Friday time frame right now as the strong
short wave moving through late in the week strengthens east of our
forecast area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1143 AM CST Sat Nov 11 2017

Main aviation concerns this period will be at KSUX and surrounding
areas where MVFR stratus is well entrenched with this expected to
continue into the overnight hours. By this evening, light rain
could impact KSUX at times with perhaps a brief change over to a
mix or snow toward the very end of the precipitation overnight.
Otherwise, improving conditions expected toward daybreak Sunday
morning. KFSD/KHON should remain VFR through the period with some
additional mid or high level clouds returning toward this evening.

&&

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

$$

SHORT TERM...Ferguson
LONG TERM...MJ
AVIATION...Kalin



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