Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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022
FXUS63 KFSD 200242
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
942 PM CDT TUE JUL 19 2016

.UPDATE...
Issued at 940 PM CDT Tue Jul 19 2016

Due to small dew point depressions, will be adding patchy fog to
much of southwest MN and northwest IA to the forecast shortly.
Although will likely keep the patchy fog east of Sioux City, more
in the Iowa Great Lakes and Storm Lake area. Hopefully any fog
development will not get too out of control with the southerly
flow we have on going, but one wonders about stratus development
around sunrise also.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 311 PM CDT Tue Jul 19 2016

Gradually warming and becoming more humid today as MCS to the south
slow to exit. Low level flow east of the James River Valley coming
out of this cooler air left behind which has slowed warming. The
main concerns overnight will be the low to mid level boundary left
behind by this MCS and whether or not any convection can initiate.
At this time will not include any thunderstorms in the forecast but
will need to keep an eye on locations to the east of Interstate 29
as southwest winds across the mid level thermal boundary could spark
an isolated storm or two. Not much in the way of instability or
shear so not expecting any severe weather. Southerly flow will
remain about 10 to 15 mph through the night so will keep lows on the
warm side. Did lower a degree here and there since the southerly
flow is not exceptionally strong but still very warm in the mid 70s.

Will keep the extreme heat warning in place starting at 18z
tomorrow. Still a small question on the potential for some mid level
clouds to keep highs a bit lower than expected in places, but
confidence is not high enough to make any changes. And in some
locations where there could be some mid level cloud cover we are
well into the heat that the temperatures would need to be 10 degrees
cooler. So, shooting for highs from the lower 90s in northwest Iowa,
with dew points of 75 to 77, to highs close to 105 near Chamberlain
with dew points close to 60. This should give us heat indices of
about 100 to 110 across the area with the highest potential readings
in the James River Valley where the extreme heat should meet with
the higher dew points.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 311 PM CDT Tue Jul 19 2016

For starters on the Wednesday night through Tuesday period, both
Wednesday and Thursday nights will have the very warm and moist air
still over our area, but we also still have the very warm
temperatures across the mid levels and capping even for elevated
convection. There is a decent low level jet indicated by both the
NAM and GFS that develops over the southeastern two thirds of the
area Wednesday night. A less decent LLJ is pictured by the models
for Thursday night. Elevated CAPE of 1000-2000 J/kg appears to
be over the eastern half of the area Wednesday night, while
somewhat stronger CAPE/instability is progged in a narrow band
Thursday night south of Interstate 90; a band that seems
coincident with a weak surface boundary shown by the models. the
presence of this weak boundary which settles over the Missouri
River area Thursday night, then pushes back north Friday, appears to
tip the hand in favor of Thursday night for any isolated nocturnal
storms. At this point, with wind fields above H8 weak, and moisture
fields aloft not too favorable, have given the capping the edge and
kept POPs below mention threshold.

Thursday and Friday daytime should be mostly sunny hot and humid, and
the heat warning appears to remain appropriate through Friday
daytime. Models especially the NAM hold a somewhat cooler, or less
hot, bubble northeast of the Thursday night boundary, eroding it
Friday, but leaving enough effect to keep the heat index a little
lower. Current projections do have it down a tad, but still close to
100 thursday afternoon, even in southwest Minnesota. In short,
though it looks like Thursday will be the hottest day overall in
terms of both temperature and heat index, and this goes for much of
the area, Friday still looks very well in the warning category,
especially since we will be on the third to fourth day of hot and
humid weather. The lower Missouri River area, Yankton to Sioux City,
could threaten to touch 115 on the index Thursday afternoon. The
hottest actual temperatures will be west of the James River, but
dew points will be higher in the east.

Arrival of the real upper wave and cold front Saturday, besides
bring a threat of storms, will be accompanied by a breaking down
of the upper ridge at our latitude, and should keep the heat index a
little lower even with any slowdown in cold frontal timing.
However,it is plausible that we may need to go back to a heat
advisory in the southeast one day. In any event, the heat and
humidity will be less excessive.

More noticeable relief looks guaranteed for the Sunday through
Tuesday period, with cooler and drier air taking temperatures down
close to or even a tad below normal. The extended guidance reflects
model depiction of following weak waves in the flattened
westerly flow, and shows a small chance of storms for Monday and
Tuesday, and will go with that for now.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1221 PM CDT Tue Jul 19 2016

Scattered to occasional MVFR ceilings will be possible along and
east of Interstate 29 into the early afternoon. Scattered showers
and isolated thunderstorms will be possible into the afternoon
across northwest Iowa and southwest Minnesota as well. After this
VFR conditions are expected. May be some LLWS late tonight.


&&

.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 Wednesday to 7 PM CDT Friday
     for SDZ039-040-054>056-061-062-066-067-070-071.

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

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

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

&&

$$

UPDATE...MJ
SHORT TERM...08
LONG TERM...
AVIATION...08



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