Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 212027
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
327 PM CDT Fri Jul 21 2017

.SHORT TERM...(This evening through Saturday)
Issued at 325 PM CDT Fri Jul 21 2017

Surface analysis shows a discrete boundary located just south of our
forecast area, left over from convection last night and early this
morning. Upper air analysis shows a short wave trough currently
in southern Saskatchewan with a flat zonal flow over the northern
plains.

First of all, will shave off more of the heat advisory for areas
around Storm Lake, and a little west and south of Sioux Falls. Just
too much cloud debris, and in fact low centroid convection has fired
up near the aforementioned boundary in our far southeast zones which
will limit heating today. Will leave it going for Sioux City as they
will at least partially clear out, and with mid 90s and very humid
air just south of KSUX, they still could hit head advisory
conditions late this afternoon and early evening. Otherwise, will
also leave it going for Yankton to Huron and points westward.

Convective forecasting tonight is a real challenge and overall
confidence is not overly high. Currently, +12 to +14C air resides
across the forecast area helping to hinder convective development
where boundaries do not exist, which is basically everywhere except
our far southeast zones. The current high resolution NAM sounding
for Sioux City shows 0-6km shear of only 25 knots which could
explain why updrafts have had trouble maintaining. It is forecast
that this boundary lingers a bit in that area, therefore have
scattered redevelopment across the same area late this afternoon and
early this evening, but it may not become severe this close to the
Missouri River. Will monitor of course. Now to the north is where
the better dynamical regime exists. The aforementioned upper
trough/short wave in south central Canada edges southeastward
through the night. Height falls occur north of I 90 due to this wave
along with cooling air at 700mb down into the +10 to +12C. Therefore
convection which is already beginning in North Dakota could very
well ooze southward as the night progresses and eventually impact
our northern zones overnight tonight. Those cells could be severe as
the pre convective sounding in east central SD shows deep layer
shear of 52 knots, coupled with 0-1km ML cape of over 3000 J/kg and
Dcape values of over 1500. Therefore both wind and hail are a
threat, especially north of I 90. Along and south of I 90, only
skittish 20 percent pops are warranted at this time for rogue
development on the south side of the wave, except for the higher
pops mentioned for the early evening hours for the boundary in our
far southern zones.

Winds are quite challenge, but overall light tonight. Consensus
shows a pre frontal wave moving across our southern zones late this
afternoon and evening. But the dry air remains north of our forecast
area promising another warm and humid night. Finally by late tonight
and Saturday morning, north to northwest winds kick in as the short
wave to the north slides southeastward. The cooler air on Saturday
remains north of our forecast area, however dew point temperatures
will drop substantially during the day. Left convective chances
Saturday afternoon away from our southern zones for now despite a
couple of cams such as the NMM showing some isolated TSRA. It
appears the dew point gradient will be south of our forecast area by
the afternoon hours.


.LONG TERM...(Saturday night through Friday)
Issued at 325 PM CDT Fri Jul 21 2017

This period begins on a dry note, both due to lacking precipitation
and relatively low humidity levels/dew points, especially compared
to the widespread lower 70s of recent days. Shortwave trough which
is responsible for the drying will scoot east into the Great Lakes
Sunday, allowing broad ridging to work a little farther eastward
into the Dakotas Sunday night. Models depict a subtle shortwave
sliding through the area Monday, but confidence in precipitation
development is low, so will leave the period dry for now. GFS/NAM
indicate strong warming in south-southwest flow working into our far
western areas, so have nudged temperatures west of the James River
up a few degrees from blended guidance for highs Monday.

Upper level jet moves into North Dakota/far northern South Dakota by
late Monday night, strengthening as it continues eastward into the
Upper Mississippi Valley later Tuesday/Tuesday night. This could
help generate some spotty showers or storms across the area, though
confidence in timing/location is low, so pops will likewise be on
the low side in the 20-30% range. Humidity levels will once again be
on the rise Tuesday, though at this time, temperatures do not look
to be excessively warm, with highs in the mid 80s to mid 90s.

By midweek, 12Z models are showing surprisingly good agreement in
lifting a warm front into the forecast area late Wednesday/Wednesday
night, ahead of a moderately strong mid-level wave sliding through
South Dakota. Because of this agreement, blended pops were in the
higher end of likely range across the northeast half of the forecast
area. However, model consistency at this range tends to be less than
stellar and confidence in this solution is moderate at best. Thus
coordinated with neighboring offices to cap precip chances at the
higher end of the chance range for now. Plenty of time to trend back
toward the likely range if the models remain consistent with this
scenario in later runs.

Latter part of next week sees a return to somewhat drier pattern
with seasonable, to maybe even slightly below normal temperatures.


&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1236 PM CDT Fri Jul 21 2017

Spotty convection remains possible through the afternoon/evening.
However, even at this short range, confidence is low on timing and
location of development. Thus opted to leave mention of thunder out
of the TAFs at this time, but will monitor closely and amend as
needed. Greatest potential for convection appears to wane after
22/09Z.

In general, VFR conditions are expected through this period.
However, some models continue to develop very low stratus/fog
across eastern portions of the area overnight into early Saturday
morning. Greatest consensus keeps this development east of I-29,
and thus east of KFSD/KSUX, but will be something for later shifts
to examine.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory until 8 PM CDT this evening for SDZ038-050-052-053-
     057>060-063>065-068>071.

MN...None.
IA...Heat Advisory until 8 PM CDT this evening for IAZ020-031.

NE...Heat Advisory until 8 PM CDT this evening for NEZ013-014.

&&

$$

SHORT TERM...MJ
LONG TERM...JH
AVIATION...JH



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