Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 240349
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1049 PM CDT FRI SEP 23 2016

.SHORT TERM...(This evening through Saturday)
Issued at 310 PM CDT Fri Sep 23 2016

Not a lot going on in the near term tonight, other than low clouds
and possible patchy fog hanging on north of the warm front in the
northern sections of southwest MN. But otherwise the weather looks
quiet with much above normal temperatures for lows. Bias corrected
values are too chilly for tonight due to the unusual pattern going
on. Therefore raw model and conventional MOS values were much better.

Still looking at the well advertised and very large upper low
beginning to greatly influence our weather for Saturday. The center
of the upper low should move slowly from central WY to extreme
northwest SD. Dealing with basically mass moisture transport in a
deep layer. Rainfall will first begin late tonight and early
Saturday in our western zones from the James Valley westward, then
slowly migrate eastward through the day. There is a lack of
discernible short wave moving northward with the upper flow, just a
broad area of mid and upper QG forcing moving eastward across the
forecast area with the upper low. Due to the slow progression of the
accompanying surface cold front and thus drier air behind it,
worried about areas of heavy rainfall, especially east of the James
River valley late morning and afternoon Saturday. The 0-6km mean
flow is very southerly to south southwest, therefore am anticipating
bands of showers and embedded TSRA forming in north to south
orientated lines, and then moving nearly northward. Seeing that the
FSD airport recorded 0.70 tenths of an inch of rain in 15 minutes
early this morning, one can see that it would not be a stretch of
the imagination to see some areas get 2 to 4 inches of rainfall if
you get under a heavy band of rain which lingers over your area for
an hour or two. But due to the showery nature of the precip with the
banded rainfall, not everyone will see these kinds of amounts of
course. Did not put out a flood watch at this time because the
forecast area`s rivers are well below flood. So this could be more
of a periodic urban flood event in poor drainage areas, again
getting under a heavy band of rainfall. PWATS are of course high,
but they are contaminated by the convection. More interesting
are the soundings are conditionally unstable with moderately thin
cape. The thetae advection and warm air advection is disjointed and
hard to follow. But it was last Thursday also when parts of the
forecast area got 3 to 6 inches of rainfall. So not letting that
alter the rain potential for Saturday. Finally, due to the rainfall
and abundant cloud cover, not too concerned about severe weather on
Saturday, just the aforementioned areas of heavy rain threat. The
mid level winds are quite stout. But the unidirectional wind flow
suggests a lot of multicellular clustering, not conducive for strong
rotational characteristics.

.LONG TERM...(Saturday night through Friday)
Issued at 310 PM CDT Fri Sep 23 2016

The cold front as well as the associated band of showers and storms
will move steadily east out of the area Saturday night, although
there may be decent early evening storm coverage in the far eastern
parts of our northwest IA/southwest MN areas. Everything should be
gone before daybreak Sunday and clouds will temporarily decrease
behind the front. temperatures will cool into the 50s.

The trailing short wave and surge of cooler air should combine to
bring an area of cloudy to mostly cloudy skies across the northern
and eastern parts of the area Sunday. This combination with the
breezy to windy conditions will also make it feel chilly, with highs
in the 60s. A few sprinkles or light showers seem at least a little
possibility, but after coordination will leave out for now with the
limited instability from mid levels on up preventing anything from
going very high up above bases. High temperatures will be in the
60s, but after the current warm and humid weather, people outdoors
will probably think afternoon temps feel like 50s at best.

Sunday night through midweek will see a warming trend and
developing strong ridging. Clouds and cool temperatures may hang
close to the east side of the area for a day or so, otherwise the
coming week should be mostly mild and dry. Feel model high
temperatures are too low for Tuesday on, but we are past the
equinox in terms of the sun heating potential, so after coordination
am leaving guidance high temperatures alone for now.

There appears to be a small chance of showers or storms developing
near the end of the week, as moisture begins to increase, but do not
see enough upper support and moisture combination to go any higher
than that very low chance mention.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1038 PM CDT Fri Sep 23 2016

Stratus hanging very close to FSD taf late this evening, but
should remain just northeast. Weak isentropic lift will continue
to develop ACCAS across Kansas and Nebraska overnight, with
potential that we could have very light showers move into the area
after 11z and continue into mid-morning. Coverage may be on the
lower end, so will only mention a VCSH ATTM.

Some of the higher resolution CAMs indicate a break in the late
morning hours, before an increase in convection after 17-18Z. Will
include a window in the afternoon with prevailing thunder.

The initial activity should slide late in the afternoon on the
prefrontal trough, but the primary cold front won`t arrive until
late in the evening. This front may also provide isolated to
scattered storms along with it.

&&

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

$$

SHORT TERM...MJ
LONG TERM...
AVIATION...Dux


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