Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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922
FXUS63 KFSD 181957
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
257 PM CDT Tue Jul 18 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 257 PM CDT Tue Jul 18 2017

Upper air analysis shows a ridge of high pressure centered over the
central and southern through Wednesday, with a weak surface cold
front extending from east to west just south of Interstate 90. The
forecast area will remain on the southern edge of some pretty strong
westerlies which reside to our north, and the surface front will
make progress only to about Sioux City to Storm Lake before
stalling. Spotty convection has broken out near the surface boundary
across the heart of northwest Iowa. There is not a lot of upper
forcing either right now or tonight to work with. PV along the 1.5
pressure surface is virtually non existent, so current thoughts are
that convection will remain tied to the surface to 850mb wind shift
line this evening. It is unstable, but with weak wind shear in the
lowest couple of kilometers, do not think that severe potential will
become widespread late this afternoon or evening due to a lack of
upper forcing. Still, its certainly worth watching for the next
several hours. Introduced some chances for TSRA late tonight between
06Z and 12Z Wednesday near the I 90 corridor as the low level jet
shifts back to a southeasterly direction. The I 90 corridor is
forecast to be along a wind shift late tonight from a southerly, to
easterly direction, and fairly strong thetae advection maximized
along 925mb is noted as it shifts off to the northwest late tonight.
Did not want to go very high with the rain chances late tonight
however due to rising heights and mid level warm air advection.

The rising heights and mid level warm air advection continue into
Wednesday. In fact by afternoon, both the nam and gfs have a bulk of
of our forecast area at +14 to +15C at 700mb. With our frontal
boundary currently where it is right now shifting back northward
tonight, Wednesday looks to be hot and humid, with a heat advisory
warranted for southeast and south central SD, as well as parts of
northwest IA and extreme northeast Nebraska for heat index values of
100 to 105 degrees. Due to the very warm mid level temperatures,
convective chances during the Wednesday day period are extremely
small, although a few morning thunderstorms could be lingering in
our northwest zones with the aforementioned thetae boundary lifting
northward. Although highs will be in the 90s on Wednesday,
convective temperatures are 100 to 105 degrees, so convective
initiation in the afternoon is pretty remote.


.LONG TERM...(Wednesday night through Tuesday)
Issued at 257 PM CDT Tue Jul 18 2017

Unsettled pattern continues through the end of the week as a series
of disturbances continue to slowly break down the ridge.
Disturbances will kick off a series of chances for showers and
strong to severe thunderstorms while temperatures will slowly become
more seasonable as the ridge breaks down.

Wave swings through Wednesday night bringing chances of rain to the
Highway 14 corridor and southwest Minnesota. Instability will be
plenty sufficient for thunderstorms with CAPE values over 3,000 J/kg
expected. Deep layer shear is strong, with over 50 knots 0-6km
shear, so expect convection to be well organized. However,
convective inhibition remains a concern across our CWA with a fairly
strong cap from 700-800mb seen on model soundings at BKX and MML.
Models have trended accordingly and are now keeping most of the
convection just north and northeast of our CWA. Nevertheless, will
need to keep on eye on this impacting our northern and northeastern
border, especially for severe wind gusts with DCAPE around 1,000
J/kg.

Expected lack of lingering convective debris from Wednesday night
will lead to a hot and muggy day in our southern zones. Heat indices
are likely to exceed 100 around Sioux City. Will go ahead and issue
a heat advisory for Thursday afternoon. As for precipitation, it
will be deja vu all over again Thursday night, this time with only
our edge in southwest Minnesota possibly being affected. Instability
is sufficient but 700-800mb capping may once again keep the
convection out of the CWA save for parts of Lyon and Cottonwood
counties.

Pattern calms considerably by Friday evening. GFS suggests a round
of storms along and south of I-90 Saturday evening but will discount
that for now as it is a significant outlier. But, the upper jet does
linger just to our north through the weekend, so can`t totally rule
out showers and storms. However, QG support for broad lift and
moisture are lacking so will keep chances low for now.


&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1213 PM CDT Tue Jul 18 2017

Main issue today and tonight is a weak cold frontal passage and
associated TSRA chances. Currently at 17Z Tuesday, the frontal
passage is close to Interstate 90, but it is difficult to
ascertain whether this is the primary frontal wind shift, or
residual outflow from thunderstorms in northern SD this morning.
At any rate, the surface winds should generally veer to an
easterly direction as the day progresses into tonight, with Sioux
City`s wind direction possibly the most variable, albeit fairly
light in speed. Convective chances associated with the frontal
boundary are quite a challenge. Right now, timed the TSRA as best
as possible in the KFSD and KSUX TAF sites with not enough
confidence to go a TEMPO group at this time. One final item to
watch for, is if rainfall becomes widespread to the east of the
Sioux Falls forecast area, we will have to watch for MVFR stratus
in rain cooled air backing into the forecast area late tonight and
early Wednesday.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...Ferguson/
AVIATION...MJ



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