Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241026
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
526 AM CDT WED AUG 24 2016

.SHORT TERM...(Today and Tonight)
Issued at 410 AM CDT Wed Aug 24 2016

Quiet weather is expected over the next 24 h. As post-frontal air
continues to move into the eastern Plains, the atmosphere will be
well-mixed today. This will allow dew points to fall into the upper
40s to lower 50s. Despite cirrus moving in during the day, it will
also be warmer than yesterday with highs from the upper 70s to lowr
80s. Winds during the afternoon should mix well above 1 km allowing
for gusts of 20 to 25 mph -especially in more exposed areas north of
I90.

Winds will rapidaly drop off tonight as the boundary layer
stabilizes. A secondary cold front will drop south through the
night. Winds may pick up a bit behind the front but will still
remain at or below 10 mph. There will also be cirrus which will
spread over most of the area this evening. Clearing will move into
the area after midnight. Lows overnight will be in the 50s.

.LONG TERM...(Thursday through Tuesday)
Issued at 410 AM CDT Wed Aug 24 2016

Cirrus shield with secondary wave passing to the south will blanket
around the southeast third of the CWA to start the day, but thin as
it pulls away through the early afternoon. Secondary push of cooler
air as surface ridge settles across South Dakota, with winds likely
to outpace that expected from surface gradient through mixing upward
to around 20 knots - strongest across east central South Dakota and
into southwest Minnesota.  Mixing should be enough to bring a
scattered flat cumulus field by later morning and through the late
afternoon mainly north/east of KFSD area, where forecast soundings
off both GFS/NAM indicate a shallow smidge of capped positive area.
Temps will be cool, ranging from near 70 at elevation in southwest
MN to mid 70s in the Missouri River valley.  Under the sprawling
surface ridge Thursday night, likely to see temps fall back into
the upper 40s for a few spots given the fairly dry air mass.

Friday will be the transition day as trough begins to push across
the Rockies and into the high plains Friday evening. Will begin to
see an increase in the low- to mid-level theta-e advection from west
to east through the day, but a long battle will be waged to spread
into the relatively dry ridge to the east. Therefore, not looking
for much of a chance through the morning hours for precipitation,
but will see increase in mid and high clouds. Spotty showers and
isolated thunder will start to take shape toward central SD where
advections will be maximized during the afternoon. A small threat
for showers and storms may also creep toward KSUX area by very late
afternoon.

Remains a fair difference between pacing and strength of system
between the various models, but a weak shift of faster solutions
toward slower lends a bit more confidence toward the slower set.
Almost looks to be a subtropical wave which shoots up ahead of the
main wave on Friday night, which on the track it shows would enhance
rainfall threat across northwest Iowa and into southwest Minnesota.
Am somewhat concerned about location of the surface-850hPa warm
front between I-80 and I-70 as we head into Friday evening, which
would potentially keep focus of more widespread precipitation to the
south of the CWA. Still, a fairly potent dynamic forcing in mid
levels overnight.  Not overly impressed with degree of instability,
although deep layer effective shear would support organization to
storms. Friday night certainly would look to be a wetter period, and
perhaps one that will need to watch for locally heavy rainfall again
toward northwest IA.

Saturday actually looks to have potential to have a more extended
dry period than might appear in the forecast being between the
exiting leading wave and the approaching main trough to the west.
ECMWF solution represents strongest and slowest to main trough, but
even a somewhat quicker and weaker solution such as NAM would allow
for another decent chance for thunderstorms by later afternoon and
evening around location of low-level convergence as it sits across
eastern SD. Again, not overly impressed with potential instability
and likely to have fairly limited recovery potential with clouds
present, and no more than an isolated severe threat would appear in
order. These storms would progress fairly quickly to the east as the
trough wraps up and pushes some fairly good mid level drying
eastward.

Sunday through Tuesday would generally be characterized by a minimal
threat for any organized precipitation system as zonal pattern
trends toward an early week ridge aloft over the northern plains.
Weak surface ridge Sunday will quickly shift east and put the area
into a persistent southerly flow through midweek, with the synoptic
warm frontal boundary redefining toward the International border
before sagging back southward late in the period. The main low level
moisture will be tied up for a couple days in the southern plains,
but by Tuesday could see a bit better increase in deeper low-level
moisture with the persistent southerly flow. Temperatures near
normal on Sunday should moderate to above normal Monday and
Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 526 AM CDT Wed Aug 24 2016

MVFR and IFR ceilings will persist in parts of northwestern Iowa
through 15Z. There may also be patchy fog with 3-5SM visibilities
through early this morning. Both low clouds and fog will remain
east of KSUX and KFSD. After the low clouds and fog lift and move
east, VFR conditions are expected through 12Z Thursday. Northwest
winds may gust up to 20 kts at KHON and KFSD this afternoon
before rapidly diminishing this evening.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...Chapman
AVIATION...Schumacher


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