Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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920
FXUS63 KFSD 200850
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
350 AM CDT WED JUL 20 2016

.SHORT TERM...(Today and Tonight)
Issued at 350 AM CDT Wed Jul 20 2016

Nose of subtle isentropic lift axis on the 315k sfc will gradually
begin to produce showers and isolated thunderstorms just northeast
of the CWA along the Minnesota River valley.  Further southeast,
seeing gradual buildup of low level stratus as moisture in the
lowest levels remains elevated. Across the Nebraska Panhandle, a
weak shortwave continues to lift northeast.  Gradual downstream
shower development seems likely through the central part of the
state.

For today:  The big story will be the heat. The lesser story will be
a low end risk for enhanced cloud cover and perhaps a few
thunderstorms through mid-morning near the Missouri River. Depending
on how the Minnesota develop grows, could see a weak backdoor
outflow move into the eastern CWA, potentially tempering the heat
buildup briefly this morning.  The feature with perhaps the most
influence could be the wave in western Nebraska.  Have generally
increased mid-lvl cloud cover in most areas through mid-day.  Again,
this likely won`t prevent the heat to build, but could lower highs a
few degrees and slow the heating to some degree.  At this point, do
not plan on making any changes to current headlines given rather
quick recovery potential today.

Tonight: A disorganized wave of energy will continue to rotate
through central Nebraska.  Again, would expect an increase in both
mid-lvl clouds and even stratus once again.  However, will keep PoPs
out of the forecast for now. Temperatures tonight will struggle to
fall into the mid-70s.


.LONG TERM...(Thursday through Tuesday)
Issued at 350 AM CDT Wed Jul 20 2016

The heat and humidity continue into Thursday with southerly flow
continuing across much of the area.  Models suggest that a front
will approach from the west, but timing and location of the front
are quite uncertain.  With the lack of upper level support this far
south from the main wave, hard to discern how far it will go.  For
now, have sided close to consensus on timing and location.  Ahead of
the front, could see some moisture pooling east of the front, adding
to concerns about the heat index. There could be a veil of mid-high
clouds across the southern portions of the forecast area keeping
temperatures down slightly.  Regardless, have heat index values
forecast well above 100 across much of the area and will continue
the excessive heat warning.

Models continue to produce sporadic precipitation throughout the
Thursday into Friday time frame.  It is hard to discern what is a
real short wave and what is model generated, but the pattern is not
supportive of precip.  As such, have left the forecast dry on
Thursday.  Contemplated adding pops on Thursday night with orphaned
boundary lingering around and low level jet developing across the
plains.  This sounds like a good set up in theory, but when one
starts to look at the amount of convergence it is quite week with
the core of the jet focused well to the south, and turning to the
east as it approaches the boundary across the region.  As such, have
continued dry forecast on Thursday night.

Friday looks to be another warm day across the area. May have some
clouds across the eastern portions of the forecast area. With 925
hpa temps in the mid and upper 20s C, still will see temperatures in
the 90s and lower triple digits. It is possible at this point that
portions of southwest Minnesota do not reach extreme heat warning
criteria, but is close enough at this time that I left the headline
as is.

Friday night, it appears that more robust ridge riders will move
from southwest to northeast across the southeastern half of the
forecast area.  These disturbances knock the upper ridge back enough
to warrant some chance pops at this point.

A cold front associated with a short wave along the US/Canadian
boarder approaches from the west on Saturday.  Ahead of the front,
atmosphere is unstable to warrant chance pops again.

Drier and cooler conditions are expected on Sunday into Monday ahead
of a more unsettled pattern Tuesday into Wednesday as a series of
short waves move through under west-northwest flow.  Have raised
guidance pops slightly as a starting point.


&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1034 PM CDT Tue Jul 19 2016

VFR conditions should be the dominant category through Wednesday
evening, however there are a couple of challenges. Due to the very
humid air mass and small dew point depressions currently in place,
certainly cannot rule out some fog development, especially east of
Interstate 29 in southwest MN and parts of northwest IA late
tonight and very early Wednesday. Due to a south to southeast
breeze, it is anticipated that the fog will not become real
widespread at this time. But another challenge is low stratus
development. Already patchy stratus around 2500 feet has
periodically developed here and there. Model soundings show that
as the inversion lowers for the rest of tonight coupled with tight
dew point depressions, it is plausible that low stratus in the
LIFR category could develop in areas right around sunrise before
mixing out by mid morning. This appears to be a threat especially
at the KFSD and KSUX TAF sites, therefore hedged and included a
scattered deck at 500 feet near sunrise as a heads up that this
could happen. Otherwise from mid morning onward, conditions should
be VFR Wednesday and Wednesday evening. Lastly, some models were
suggesting some sporadic non convective low level wind shear
tonight and also Wednesday evening. For now, left it out of the
TAFs due to the surface winds being somewhat brisk.


&&

.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Warning until 7 PM CDT Friday for SDZ038-050-052-
     053-057>060-063>065-068-069.

     Excessive Heat Warning from 1 PM this afternoon to 7 PM CDT
     Friday for SDZ039-040-054>056-061-062-066-067-070-071.

MN...Excessive Heat Warning from 1 PM this afternoon to 7 PM CDT
     Friday for MNZ071-072-080-081-089-090-097-098.

IA...Excessive Heat Warning from 1 PM this afternoon to 7 PM CDT
     Friday for IAZ001>003-012>014-020>022-031-032.

NE...Excessive Heat Warning from 1 PM this afternoon to 7 PM CDT
     Friday for NEZ013-014.

&&

$$

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



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