Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202315

Area Forecast Discussion
National Weather Service Sioux Falls SD
615 PM CDT Thu Jul 20 2017

.SHORT TERM...(This evening through Friday)
Issued at 329 PM CDT Thu Jul 20 2017

A relatively quiet and warm day has prevailed today thanks to
surface high pressure centered over central Minnesota. Very light
rain/spotty showers have developed along the Missouri River in
response to a weak surface boundary extending from central NE to
central IA. Thunderstorm chances will increase late tonight, as
an 850-700mb theta-e boundary surges north-eastward into
Minnesota advecting warmer air and more low-level moisture. Not
quite confident in terms of showers/storm coverage, but leaned
toward Hi-Res CAMS tonight. Have up to a 35 percent chance of
precipitation in the nighttime, mainly over northwest Iowa, where
more unstable air prevails.

Very warm and more humid conditions are expected on Friday. GFS/NAM
suggest 850 mb temperatures of 25+ C across the area. Therefore high
temperatures will generally be in the 90s to near 100 along the
Missouri River Valley. Given the warm temperatures, and afternoon
dewpoints in the upper 60s and lower 70s; heat index values could
reach the triple digits. In collaboration, will issue a heat
advisory for southeast SD and northwest Iowa from 12pm to 8pm

Another concern is the increasing chances for thunderstorms in
the afternoon. A shortwave will be moving eastward bringing the
potential for scattered thunderstorms. This is another period to
keep an eye on in terms of severe weather. Steep mid-level lapse
rates in the afternoon, along with strong deep layer shear of 35
to 45 knots, suggest organized supercells over the area by late
afternoon and early evening.

.LONG TERM...(Friday night through Thursday)
Issued at 329 PM CDT Thu Jul 20 2017

Friday night is marked by a cold frontal passage in this forecast
area. The aforementioned short wave energy is maximized just north
of this area per mid and upper QG forcing, with PV forcing very
weak. But the frontal boundary will provide a focus for lift, with
0-6km bulk shear vectors averaging about 25 knots in our southern
zones Friday evening, increasing to 35 to 40 knots in our far
north. This will be enough to sustain some updrafts. Chance pops
are certainly warranted along the frontal wind shift, and some
severe weather is possible due to a high degree of instability, as
0-1km ML cape values are over 3000 J/kg. At this point, cannot
rule out quarter sized hail in the evening, but with +14 to +15C
air at 700mb, the bigger threat would probably be wind if we can
sustain some rotating updrafts.

After Friday night, organized rainfall chances look pretty meager
for the rest of the forecast. Temperature wise, Saturday will still
be quite warm despite being behind the front. With a northerly
wind, dew point values will be a lot lower however. Then high
temperatures will cool off more markedly Sunday and Monday as
another surge of cool air moves down from the north Saturday
night. Highs on Sunday and Monday will be much closer to normal
under the influence of Canadian high pressure, with some locations
actually below normal highs for a change. The warmest locations
will be in our far western zones in south central SD where highs
will still be near 90.

Beginning next Tuesday on day 5, the usual deterministic model
discrepancies begin to exist. The ECMWF brings another cold front
through next Wednesday, with the GFS slower and bringing it
through on Thursday. The Canadian hangs the frontal boundary up
near Sioux City next Thursday. Therefore blended temperatures
between the various models, with highs generally looking at to a
little above normal Tuesday through Thursday.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 613 PM CDT Thu Jul 20 2017

Lone thunderstorm near Sioux City will stay southeast of the
terminals tonight, leaving VFR conditions across the remainder of
the forecast area.

Warm advection commences after 4am and should allow scattered
showers and thunderstorms to develop. Still uncertain on the
coverage of this activity to include prevailing conditions in the
taf. After this activity moves east, quick recovery should take
place in the afternoon.


SD...Heat Advisory from noon to 8 PM CDT Friday for SDZ038-039-050-

MN...Heat Advisory from noon to 8 PM CDT Friday for MNZ098.

IA...Heat Advisory from noon to 8 PM CDT Friday for IAZ001-002-012-

     Heat Advisory until 8 PM CDT this evening for IAZ031-032.

NE...Heat Advisory from noon to 8 PM CDT Friday for NEZ013-014.

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



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