Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180532

Area Forecast Discussion
National Weather Service Sioux Falls SD
1232 AM CDT Tue Jul 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 324 PM CDT Mon Jul 17 2017

Unsettled weather returns tonight and continues tomorrow as several
disturbances move across the region. A weak shortwave over central
Nebraska will move northeast this evening, breaking down the ridge
over the area. At the same time, a cold front extending along the
Northern Plains/Central Dakotas will approach the area early this
evening; bringing isolated/spotty thunderstorms along and ahead of
the front. Strong deep layer shear is lacking, but with strong
heating, and afternoon dewpoints in the upper 60s to lower 70s, the
environment could support some marginally strong to severe storms
this evening. The greatest threat will be west of the James Valley
and along/north of Highway 14. If storms do develop, strong gusty
winds will be the main threat given the inverted V at KHON model
soundings, and DCAPE values > 1,200 J/kg. The risk for strong/severe
storms gradually diminishes through midnight.

Attention then turns to the increasing chances for scattered
thunderstorms and potential for locally heavy rain early Tuesday
morning through the rest of the day. Model soundings suggest an
increase in low-level moisture with the GFS/NAM showing increasing
southerly inflow of PW to over 1.5 inches. There are still
uncertainties in terms of timing/placement of this event. The
GFS/Canadian/ECMWF prog most of the precipitation over the CWA, with
highest QPF residing east of I-29 in the afternoon hours. The NAM is
slightly faster, and places most of the PoPs north of I-90. With
the passage of the frontal boundary along with the vort max at
mid-upper levels, and MUCAPE values of 3,000+ J/kg, the dynamics
will be conducive to strong/severe storms in the afternoon and
evening. Abundant cloud cover will hold back high temperatures to
the 80s. The exception will be along the Missouri Valley, where
highs could reach the lower 90s. Overnight lows will remain in the
upper 60s to lower 70s.

.LONG TERM...(Tuesday night through Monday)
Issued at 324 PM CDT Mon Jul 17 2017

Main concern with the long term will be the heat and humidity along
with chances for periodic thunderstorms through the end of the week.
General pattern puts our CWA on the north side of a flat ridge with
several waves rounding the ridge. Timing of waves and position of
thermal boundaries will be paramount toward determining the exact
placement and timing of convection. All told, could get up to 3 MCS
events affecting our CWA through the end of the week.

In the wake of Tuesday night`s convection, thermal boundary will be
pushed south into Nebraska. Boundary builds back northward on
Wednesday but wave aloft will likely hold off until at least
Wednesday evening. Resultant lack of afternoon convection will allow
heat and humidity to build, especially in the southern half of the
CWA. Will likely need to eventually issue a heat advisory around
Sioux City and the Missouri River valley on Wednesday.

Model timing and placement differences begin to rear
their head starting with convection Wednesday night. General
agreement that approaching wave will kick off an MCS along SD/ND
border which will then dive southeast and could impact southwest
Minnesota. Given the large amount of instability and deep layer
shear this is a reasonable solution. Main differences are in the
timing of convection. Regardless, progged DCAPE values over 1000
J/kg would support a threat for damaging winds. Overall warm
atmosphere will keep hail threat to a marginally severe level.

A repeat event is possible Thursday night with general agreement
between the GFS and EC for convection across southwest Minnesota.
Fly in the ointment will be the placement of the thermal boundary in
the wake of the aforementioned rounds of convection. However,
dynamic support for this event is greater than Tuesday and Wednesday
nights with a more vigorous wave swinging through. Once again, would
be looking at a severe wind threat along with marginally severe hail.

After Thursday night, model differences and their general poor
handling of boundaries resulting from convection preclude dialing in
on any specific times or areas for convection. General consensus is
that additional precipitation may occur east of the James River
sometime Friday into Saturday, unfortunately of little use to
drought stricken central SD. Aside from precipitation, the ridge to
the south slowly translates eastward by the weekend, resulting in
more seasonable temperatures across the CWA this weekend.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1230 AM CDT Tue Jul 18 2017
Compex of storms working east across northwest Iowa into southwest
MN. Could see some gusty winds with gusts up to 40 knots or so
with convective activity. Thereafter, additional rounds of storms
possible during daylight hours on Tuesday. Timing is ill defined
at this point, so have not included mention in TAFs.




LONG TERM...Chapman/Ferguson
AVIATION...BT is the U.S. government's official web portal to all federal, state and local government web resources and services.