Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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780
FXUS63 KFSD 140359
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1059 PM CDT Fri Oct 13 2017

.SHORT TERM...(This evening through Saturday night)
Issued at 328 PM CDT Fri Oct 13 2017

Mid and upper QG forcing rapidly departs late this afternoon,
only to be replaced by strong isentropic lift in the 295-310k
layer ahead of an approaching upper trough on Saturday and Saturday
evening. Overall in between the QG forcing and the isentropic ascent.
the weather looks pretty quiet tonight. Stratus will likely return
to our southern zones after midnight as a low level southeast flow
of air ushers in low clouds which are currently residing to the south
and southeast. Elsewhere, thicker mid and upper level clouds will
move into our western zones. Therefore for lows, preferred a
blend involving a strong weighting toward the GEM global which
had our far southeast zones and far western zones mild, with
chilly air in between. Other guidance values did not delineate
between these two areas and more or less had everyone in the lower
40s. With the cloud cover, light showers are possible across our
south and southwest zones late tonight, but with little in the way
of accumulations.

The aforementioned upper trough will move into the northern and
central plains on Saturday, with this CWA dominated more by the
southern stream which gives a robust moisture profile from 850-
500mb. As stated above, isentropic lift is a main driver of ascent
on Saturday, with a surface cold front moving through the western
half of our area by late afternoon. This is in response to surface
low pressure organizing and strengthening in Iowa as it moves
northeast. Saturdays highs will be very chilly, mainly 50 to 55,
although it is possible that locations from Sioux City to Storm Lake
may reach near 60 as organized rain chances decrease in the
afternoon and they are still ahead of the cold front. In general,
the likely and categorical pops move from the southwest to northeast
throughout the day, which coincides the deepest moisture profile.
Behind the high pops, it will remain cloudy due to abundant
stratus. But with the mid layers drying out, there could be some
lingering drizzle behind the bona fide rainfall. Overall, rainfall
amounts will average about a tenth to quarter of an inch.

Saturday afternoon and evening, mid and upper QG forcing moves
squarely across our forecast area, with quite a strong PV surge on
its heels. Behind the cold front, winds will be strong for a while
along the PV surge, and raised them above superblend values quite a
bit. Late Saturday afternoon and evening, winds behind the cold
front will average about 20 to 30 mph for several hours at least.
The precipitation will decrease Saturday evening, with the overnight
and late night hours dry except perhaps across our eastern edge of
zones.

.LONG TERM...(Saturday night through Friday)
Issued at 328 PM CDT Fri Oct 13 2017

Sunday will begin the new week of dry and mild weather for this
time of year. Quite a change from early October. Sunday will begin
seasonably cool with highs in the 50s to near 60 degrees. After
that, the southwest United States will have upper ridging early in
the week, replaced by a strong upper trough by very late in the
week developing over the Pacific Coast. 925-850mb temperatures are
very mild, therefore highs Monday through Friday will be in the
mid 60s to mid 70s. If there is an error in temperatures, they
could very well be even warmer. For particulars, increased winds
along and east of a Yankton SD to Brookings SD line on Monday
coinciding with the pressure gradient. Superblend winds were too
light. There is a lot of model discrepancy with the winds on
Wednesday. Short wave energy moves across south central Canada,
with the ECMWF and GEM Global very aggressive in bringing a wind
shift through the entire forecast area to a west and northwest
direction. The GFS is much slower and weaker with the wind shift,
and basically washes it out in northwest IA. In the long run, it
may make little difference as a southerly flow returns on all the
models by early Thursday. Furthermore, there is no chilly air
behind the wind shift on the ECMWF and GEM Global. Overall,
Thursday and Friday look to be the windiest days next week but
there is a big question on where the tightest pressure gradient
will reside those two days.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1055 PM CDT Fri Oct 13 2017

MVFR to IFR stratus will build northward overnight, eventually
impacting all terminals by mid morning Saturday. Scattered light
showers will overspread the area Saturday as well. Stratus will
begin to clear west to east late in the TAF period.

&&

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

$$

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



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