Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191128

Area Forecast Discussion
National Weather Service Sioux Falls SD
628 AM CDT TUE JUL 19 2016

.SHORT TERM...(Today and Tonight)
Issued at 401 AM CDT Tue Jul 19 2016

Showers and thuderstorms are just starting to develop within a zone
of elevated warm air/theta-e advection ahead of ashort wave moving
through the area. Given developing convection, will keep discussion
brief.  This wave will be a relative slow mover, leading to the
potential for showers and thunderstorms east of I-29 throughout the
day.  As such, have kept temperatures down in the east.  Further to
the west, hieghts rise behind the wave and with clearing skies
expect temperatures to warm into the 90s. Forecasted dew points in
the mid and upper 70s seem a smidge high, and lowered things
slightly. Regardless, with highs in the 90s and 70+ dew points, have
heat indicies approaching 105 in central South Dakota today.  With a
prolonged heat wave expected across the area over thenext few days,
went ahead and issued an extreme heat warning for today and tonight.

With the low level jet developing across the plains tonight, should
see some very warm lows tonight.  Have heat indicies only falling
into the upper 70s in most locations.  Both the NAM and GFS go
convective tonight creating thunderstorms.  With stout cap in
place,have kept the forecast dry for the time being.

.LONG TERM...(Wednesday through Monday)
Issued at 401 AM CDT Tue Jul 19 2016

The heat will continue to build across the central states on
Wednesday and Thursday with the strong upper level ridge peaking
over the eastern Plains. With strong mid level heating and capping
inversion in place, dry conditions are expected. Humidity will
continue to rise however, further exasperated by evapotranspiration
from maturing crops. Dewpoints surge into the mid and upper 70s east
of the James River with highs in the lower 90s to near 100. While
temperatures will warm to around 100 to 107 to the west of the
James, dewpoints will mainly hold in the 60s. The result however is
area-wide afternoon heat index values of 100 to 110.  In addition,
temperatures will struggle to cool as low as 75 degrees overnight,
bringing little relief to the uncomfortable conditions.

The ridge begins to flatten on Friday with a trough digging into the
northern Rockies and high plains, however the degree of this impact
differs a bit between models. The ECMWF is more aggressive with a
leading southern stream shortwave arriving in the central Plains on
Friday afternoon. The GFS and GEM are slower and further north.
Think that there will be some slight cooling of highs on Friday, but
with continued high humidity and heat index values remaining very
close to 100 to 105, think that keeping the heat headline into
Friday afternoon makes sense.  Considering that this is an extended
period of excessive heat and without much relief at night, think it
is a slam dunk for reaching the excessive heat warning criteria in
our forecast area Wednesday through Friday.  Given the high
confidence, will go ahead and upgrade the watch to a warning.

The aforementioned model divergence continues into the weekend, with
the GFS being far more progressive with the trough than the ECMWF.
Think that there will be a frontal boundary moving through the area
around the Saturday time frame, with the next best shot for showers
and storms late Friday night through Saturday night. The scattered
pops seem reasonable right now.  The good news is that temperatures
also cool into the 80s to near 90 this weekend into early next week.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 625 AM CDT Tue Jul 19 2016

Band of thunderstorms will shift east through the area through
morning hours. Could see some additional redevelopment over
southwest Minnesota into northwest Iowa today. Expect scattered-
broken cumulus to develop along and east of I-29 today. Low level
jet will develop tonight, creating low level wind shear.


.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Warning from 1 PM this afternoon to 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.



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