Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221746
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1246 PM CDT FRI JUL 22 2016

.SHORT TERM...(Today and Tonight)
Issued at 352 AM CDT Fri Jul 22 2016

Another day of heat and oppressive humidity will be featured across
the area.  Shallow outflow boundary from the MCS which moved across
areas well east in Minnesota/Wisconsin has backed through much of
the CWA, providing a greater easterly component to the near surface
airmass.  Very moist low level airmass remains tonight with
sharpened inversion, and have seen some so far sporadic patches of
lower clouds develop mainly in areas north of I-90 very early this
morning. This area will generally continue to be favored for some
spotty cloud development through the day -- starting this morning as
patchy stratus and even some fog along eastern slopes of elevated
terrain in southwest MN and east central SD. Development of shallow
fog also possible across parts of northwest IA.  Likely that flat
cumulus field will eventually populate skies after a several hours
of diurnal heating after dissipation of earlier day clouds.
Evapotranspiration from maturing corn will contribute strongly to
maintenance of very high dewpoints, aided by off surface inversion.
While temperatures will again keep far from a deeper mixing
potential, the combination of 90s temperatures and dewpoints in the
70s will again produce widespread 100-110 degree heat index values
by this afternoon.  Convective potential through the day given the
continued hot temperatures aloft remains low, but non-zero (+13 to
+15 at 700 hPa but more likely due to 850-800 hPa warm nose).

Confidence in convection does not gain much ground going through
tonight.  While there is evidence that larger scale wave may
approach from the southwest later in night and that low level jet
will increase, convergence from the latter feature does appear to be
somewhat lacking much of the night across the FSD CWA, displaced to
the north.  Models, other than latest look at the extended
experimental HRRR which develops a large complex centered on KFSD by
12z Sat, have been pretty skittish on suggesting either a consistent
preferred location, timing or coverage through tonight for
convection.  Generally, have maintained some modest pops for
thunderstorms starting late evening, spreading from west to east
through the night.  Warm front/outflow boundary works gradually
northward toward I-90 after midnight, meaning again another very
mild night with lows mostly in the lower to mid 70s.

.LONG TERM...(Saturday through Thursday)
Issued at 352 AM CDT Fri Jul 22 2016

Difficulties persist in the forecast for Saturday, with several
unknowns, including the degree of elevated nocturnal convection to
start the day, the degree of warm air aloft through the day, and the
amount of forcing later in the day owing to potential convection
redevelopment and severe weather.   Generally speaking, high
resolution models continue to struggle on Saturday, all suggest the
potential for elevated convection to persist through the morning
hours of Saturday on the nose of an 850 mb jet.  However that jet is
rather weak, and remains somewhat unfocused.  Believe the highest
potential will exist along the Buffalo ridge, with lower showery
chances trailing southwest into the MO river valley through mid-
morning.

We should see a return of hot and potentially very humid afternoon
conditions.  Have trended PoPs downward through the middle of the
day, as LLJ convergence shifts eastward and warm capping layer takes
over.  A frontal boundary will approach and move through the CWA
late in the afternoon and in the evening.  Again, cap remains very
strong especially in areas along and south of I-90. Less inhibition
will occur over NE South Dakota and adjacent areas of MN in the
afternoon.  Additional convection further south on the boundary may
need to wait until temperatures aloft begin to cool after midnight.
Despite rather weak shear profiles, could see a few strong to severe
storm clusters given DCAPE and warm air aloft.

High pressure will slide southward for Sunday, as a shortwave trough
digs through the western Great Lakes. Clouds will be slow to clear
on Sunday, but winds should remain favorable for outdoor activities.

As we move into next week, the mid-lvl ridge will gradually flatten,
allowing numerous wave to pass through the Central US. Rain chances
will increase Tuesday night into Wednesday as an MCS rolls off the
Western Plains. This weakness in the flow will remain near the area
on Thursday, before brief mid-lvl ridging arrive for Friday. Yet
another shortwave will sink into the Northern Plains for next
weekend.  Temperatures next week will settle back towards and
perhaps slightly below seasonal normals, an appreciated change after
this week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1241 PM CDT Fri Jul 22 2016

mainly vfr conditions are expected through the forecast period. A
scattered deck of flat cumulus at around 1500 to 3000 ft agl is
expected to continue burning off through the mid afternoon. Deck
is likely to be broken to overcast across northwest Iowa, but
should also decrease in coverage this afternoon. Scattered
thunderstorms are expected to develop mainly after midnight, with
the best chances across the highway 14 corridor, however a 20 to
30 percent chance exists elsewhere through 15z. Given the spotty
chances of storms, will leave out mention in tafs for now.

&&

.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Warning until 7 PM CDT this evening for
     SDZ038>040-050-052>071.

MN...Excessive Heat Warning until 7 PM CDT this evening for MNZ071-
     072-080-081-089-090-097-098.

IA...Excessive Heat Warning until 7 PM CDT this evening for
     IAZ001>003-012>014-020>022-031-032.

NE...Excessive Heat Warning until 7 PM CDT this evening for NEZ013-
     014.

&&

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...


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