Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 010348
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1048 PM CDT WED AUG 31 2016

.SHORT TERM...(This evening through Thursday)
Issued at 355 PM CDT Wed Aug 31 2016

Afternoon has quieted down with dissipation of earlier area of
showers and thunderstorms into the mid James valley, and high
pressure axis stretched fairly near the I-29 corridor. Upper-level
ridge will continue to take stronger hold on weather for tonight.
However, a narrow zone of mid-level theta-e advection will drift
into eastern North Dakota tonight, with just enough instability and
convergence to maintain development of a few thunderstorms.
Different for tonight will be our location somewhat closer to center
of mid to upper level ridge, suggesting less potential for any
storms to drift southward into the CWA. Will have increased chance
for additional enhanced mid- to high-level clouds spreading into
areas north of I-90 by later tonight and Thursday morning.
Otherwise, surface ridge axis will settle across Minnesota and Iowa
through tonight.  Has been a day of mixing to drive early afternoon
dewpoint readings into the mid to upper 50s. While this should
preclude any threat for widespread fog, some lower locations across
northwest Iowa mainly along and east the Floyd River at higher risk
to have some spotty fog.

High pressure shifting eastward Thursday will allow increase in
southerly gradient, with winds west of the James River reaching a
bit gusty levels of 15 to 25 mph during the afternoon. While the
flow around exiting ridge could bring another couple degrees cooling
into areas near/east of the James, should be fairly close to today
for south central SD.

.LONG TERM...(Thursday night through Wednesday)
Issued at 355 PM CDT Wed Aug 31 2016

Not a lot going on in the Thursday night through Friday night time
frame. Large upper ridge of high pressure passes slowly eastward
across the plains. Cool surface high pressure will still greatly
impact our eastern zones Thursday night with lower to mid 50s common
east of Interstate 29. Conversely, an increasing southerly flow of
air will occur west of I 29 moderating low temperatures into the
lower 60s in central SD. Friday will follow with mild temperatures,
with mixing from near 850mb still strongly suggesting highs from the
mid 70s east of I 29, to the mid 80s around Chamberlain SD. The main
story on Friday will be the strength of the southerly winds, with
windy conditions of 20 to 35 mph generally prevailing along and west
of the James River valley, then progressively becoming lighter
heading eastward from there. Breezy conditions will continue west of
I 29 Friday night, where trending toward the warmer guidance
readings which are the raw model values is a good idea.

On Saturday, the upper flow begins to radically change as a large
trough of low pressure carves out through the intermountain west.
This will bring an active rainy pattern to our forecast area from
Saturday night, potentially all the way through mid week next week
at the end of the forecast period. There are a couple of things that
the deterministic GFS and ECMWF are in agreement on. First, Saturday
night still looks like there will be broad isentropic ascent along
with thetae advection which will produce a decent chance for
convection. The chances of rain are good through Sunday morning,
which will be followed by a short wave passage Sunday night bringing
a surface boundary through Sunday night into Monday. As expected,
the GFS is about 6 hours quicker with the frontal passage, as it is
on Tuesday also. Therefore active weather is expected Sunday night,
and possibly Monday night also as the aforementioned second boundary
begins to work its way through.

For temperatures, just followed consensus Sunday through Wednesday.
On Saturday, warmed up consensus highs a few degrees given the model
850mb temperatures.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1048 PM CDT Wed Aug 31 2016

Predominately VFR through the forecast period. Patchy fog will be
possible late tonight through shortly after daybreak in portions
of northwest Iowa. Right now it looks like the fog will develop
east of KSUX so will leave out mention in the TAF.


&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Chapman
LONG TERM...MJ
AVIATION...



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