Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181720

Area Forecast Discussion
National Weather Service Sioux Falls SD
1220 PM CDT Tue Jul 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 401 AM CDT Tue Jul 18 2017

Potentially active day across the area with several chances of
convection to discuss.  Two ongoing areas of convection across the
area- one across southwest Minnesota which has an MCV associated
with it and the second across south central South Dakota.  These are
expected to continue to decrease in intensity over the next few
hours. Portions of northwest Iowa could get clipped by MCS across
eastern Nebraska, though this activity is expected to remain below
severe limits.

Models suggest that stationary front across central South Dakota
will shift east this morning and become the focusing mechanism for
thunderstorm development this afternoon as a shortwave currently
across eastern Montana moves into the region.  0-6 km bulk shear
along and ahead of the front is 30-45 kts, with the strongest shear
along and north of Interstate 90.  Surface based CAPE values climb
throughout the day and are generally in the 1500-2000 J/kg by early
this afternoon.  Expect the most severe activity to be north of I-90
where the more robust shear is located. CAM solutions have trended
much drier over the last few runs, so have kept pops in the mid
range for now.  As previous forecaster mentioned, cloud debris
should keep highs cooler today, with a few 90s possible in the
Missouri River Valley.

Convection is expected to drive the front through the evening hours.
With a deep warm cloud layer and precipitable water near 2 inches,
certainly have a heavy rain potential through the afternoon and
evening.  Flash flood guidance values are fairly high given the dry
conditions as of late.

.LONG TERM...(Wednesday through Monday)
Issued at 401 AM CDT Tue Jul 18 2017

Boundary projected to lift back to the north on Wednesday, as a
couple of waves slide east across the Dakotas in the afternoon and
overnight. Some question regarding how far north the boundary will
progress, and exact timing/trajectory of the forcing aloft. Due to
these uncertainties, will stick with chance range pops through the
evening hours, but still ramp up to likely in parts of southwest
Minnesota later Wednesday night, where models show somewhat better
agreement in tracking a potential overnight MCS. By this time, 850mb
wind flow becomes more west-southwest, becoming more parallel to
the surface boundary. Meanwhile Corfidi vectors become light, at
times nearly opposing the 850mb flow, suggesting potential for
backbuilding/training storms. All this in a very moist atmosphere,
with projected precipitable water values nearing 1.75 to 2 inches,
or above the 95th percentile for mid July. Air mass is also quite
warm, with a freezing level near 14kft and warm cloud depth over
4km. While forecast QPF within our forecast products will not be
excessive due to uncertainty in location at this time, potential is
certainly there for pockets of heavy rain with this system Wednesday
night, and will continue to highlight that in HWO and social media.
Along with the heavy rain threat, could see isolated strong/severe
storms, with damaging winds the primary threat given the deep warm
layer minimizing the large hail potential.

After any morning convection shifts east of the area, anticipate a
relatively quiet Thursday. However, with model consensus depicting
the low level boundary lingering across the area, and another wave
or two tracking across the Dakotas/Minnesota Thursday night, cannot
rule out a repeat performance in some part of the region. Confidence
in pattern details, specifically in location of lingering boundaries
and potential MCS at that range is quite low. Thus, will stick with
blended consensus which indicates chances ramping up again during
the overnight hours across the northeast half of the forecast area.
Moisture/warmth parameters remain similar to those mentioned above,
and although low level jet influence appears to be less favorable,
still looks like a potential for locally heavy rain wherever MCS
eventually decides to track.

Midweek temperatures again will be strongly dependent on convection
and resultant boundary locations, but generally looks to remain hot
and humid through the Missouri Valley into southern portions of
northwest Iowa, with more seasonable readings across the northeast
portions of the CWA. Heat headlines will need to be considered for
Wednesday/Thursday across our southern areas in later forecasts, but
given potential for convective effects on boundary position and
temperatures, will not be issuing anything at this time.

Lagging jet energy to our northeast, along with indications of a
lingering mid level trough, could result in precipitation chances
lingering into Friday/Friday night, but thereafter, transition to
somewhat calmer pattern still looks on track for the weekend
into early next week, as the strong upper jet shifts eastward and
slight ridging retreats westward into the Rockies. Could still see
spotty storm during this period, but in generally looks drier with
seasonable temperatures.


.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.




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