Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 121723
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1123 AM CST Sun Nov 12 2017

.SHORT TERM...(Today and Tonight)
Issued at 340 AM CST Sun Nov 12 2017

Not much to write about the short term forecast. High pressure
builds eastward through the period, being overhead for much of the
day Sunday, leading to pleasant conditions. The light breeze and
mostly sunny conditions will be a repeat of Veterans Day for our
northern zones, where highs were several degrees above guidance.
Have trended the same direction for the entire forecast area Sunday
and increased highs a couple degrees above guidance. Sunday night,
while we do have marginally enough surface moisture for some fog
problems, the high pressure will build eastward enough for gradient
winds to return to the region after midnight, negating much of the
risk.


.LONG TERM...(Monday through Saturday)
Issued at 340 AM CST Sun Nov 12 2017

On Monday, a brisk southerly flow will be found in this area with a
moderately strong pressure gradient coupled with mixed layer winds
of 25 to 30 knots. Therefore continued the trend of boosting
superblend wind speeds along the zone of strongest gradient. The
various models including the ECMWF, NAM and GFS are still all
pointing at stratus advecting northward early Monday morning in
south central SD, then sliding slightly eastward into the James
River valley through the morning hours. The lowest layer of stratus
near 925mb mixes out, to be replaced by a higher layer in the 900-
850mb layer further eastward in the afternoon, first close to I 29
at 18Z, then ushering eastward Monday afternoon. Therefore in the
afternoon, the clearest skies will be found in our western zones
where high temperatures were raised by following MOS guidance. East
of the James River valley, did not alter superblend highs due to the
potential for clouds, with high temperatures slightly more than
isothermal from 925mb. Monday night, the stratus deck in the 900-
850mb layer continues to move slowly eastward along the axis of the
low level jet, and it basically exits our southeast zones from 06z
to 09z Tuesday. However another round of very low stratus then
ushers northward once again, this time taking aim toward the I 29
corridor, with a mid deck of clouds west of I 29. Therefore lows
will be very seasonably mild between the lingering southerly flow of
air and cloud cover. Some of the models such as the NAM were hinting
at drizzle in our eastern zones, at least in the Monday evening
hours. But the other models were pretty dry so did not want to
include that yet at this time. If it occurs, it will be of the
liquid variety anyway.

Highs on Tuesday are quite a challenge. This is because the low
stratus deck engulfs are forecast area ahead of a wind shift which
is tied to a short wave moving into the central and western Dakotas
by afternoon. As the wind shifts to the west and northwest along the
short wave, the stratus ushers eastward but it remains quite cloudy
in our eastern areas for much of the day, only to be replaced by a
lot of mid and upper level cloud cover in the afternoon throughout
the region. Therefore despite the warm air mass, kept highs in the
50s, perhaps lower 60s in south central SD. In addition, the GFS
looks like a warm outlier with its 925mb temperatures compared to
other models, probably due to over mixing.

It still appears that this short wave picks up deep layer moisture
too far east to warrant a precipitation chance in our forecast area
late Tuesday and Tuesday night. Behind this wave on Wednesday, will
have to monitor highs in that they could be cooler than forecast.
The latest round of 925mb temperatures are suggesting highs 5 to 7
degrees cooler then superblend and the weighted model and bias
corrected ECMWF picked up on this. It all depends on the degree of
mixing, so at this time hedged highs very close to normal. The main
feature for the late week is still another short wave passage in the
Friday time frame. The GFS is about 6 hours faster than the ECMWF,
so for that far out, really not that far off. The precip type looks
to be primarily rainfall given the low level thermal field, with any
snowfall likely not to accumulate at this point due to relatively
mild temperatures. The highest pops are warranted late Thursday
night and Friday morning.


&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1122 AM CST Sun Nov 12 2017

Surface high pressure with light and variable winds will be the
main story weather-wise today. High impact weather is not
forecast through the next 24 hours. Overnight, there is a slight
chance of fog formation, with airports sites near rivers such as
KSUX seeing a slightly higher chance for a few hours of patchy fog
Monday morning. Light south winds are expected to be a mitigating
factor though, in addition to high clouds coming in from the
west. By Monday afternoon, low stratus decks could bring MVFR to
IFR conditions as low-level moisture advection from the south
ramps up.


&&

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

$$

SHORT TERM...Ferguson
LONG TERM...MJ
AVIATION...VandenBoogart



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