Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251214
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
714 AM CDT Tue Jul 25 2017

.SHORT TERM...(Today and Tonight)
Issued at 439 AM CDT Tue Jul 25 2017

Strong ridge across the Northern Plains now being flattened by very
strong wave moving through central Saskatchewan into Manitoba, with
strong belt of westerlies across North Dakota, with nice transverse
banding noted on GOES-16 imagery. Strong low level jet punching into
the area with winds continuing to pick up at elevation, and a good
return of surface dewpoints into the mid 60s to around 70. Focus on
this forecast is on the severe convective threat for later today and
tonight.

Much of the day today should remain quiet, as overnight convection
in warm advection brushes through southwest Minnesota in the pre-
dawn hours, and the southerly winds continue to increase eastward
and expand through lower elevations. Should have a fairly clear
window for surface heating through the early afternoon. Wave passing
to the north will push a frontal boundary into the northern CWA by
mid to late afternoon, and this boundary looks to sharpen and act to
pool low-level moisture, driving some surface dewpoint values into
the lower to mid 70s. MLCAPE values will attain healthy 2500-3500
J/kg over much of the area near the boundary. One thing to watch is
a curious pocket of drying from northeast NE into far southeast SD
which does show up on most model solutions, especially the NAM/ARW
and HRRR, which knocks back dewpoints (as low as the 50s in the
NAM/HRRR) and results in a local minimum in potential MLCAPE (sub
500 J/kg) by mid to late afternoon. This area is suggestive of
perhaps a little more mixing, as temps climb into the mid to upper
90s, while some readings around 100 degrees in south central SD
could weaken cap enough to initiate convection around peak heating
time mid to late afternoon.

Dynamic forcing along the frontal boundary along with pooling of
moisture will weaken cap with very rapid convective development
anticipated to commence 21-22z along a K9V9-KHON-KMML line.  Deep-
layer shear parameters are fairly weak with diminished speed to mid-
level winds as one progresses south across the region, with mostly
20-30 knots.  However, CAPE-shear space would not eliminate
potential for HP-type supercells/multicells with initial development
on shear enhanced near boundary. Some large hail will be possible
into early evening, but the main threat would likely be damaging
wind with decent DCAPE in the pre-storm environment.  Will also have
to keep tornadic threat in mind, but temporal window is probably
fairly small and limited to areas near Highway 14, as more
widespread convection develops through the evening across the
western through far northern CWA as low-level jet increases.   Some
evening Corfidi vectors are not very aggressive with progression of
storms, and suggestive of training of storms, and presence of the
drier/more stable airmass could keep things nearly locked in place
for a few hours through at least 03z. Eventually, there should be an
expansion of cold pool to degree where the focus for development
will sink southward through the night.  There is some hint that 0-
1km shear vectors could work toward producing bowing segments along
the line of convection, which could in turn act to enhance wind
threat or potentially a brief QLCS-type tornado through the early
overnight north of I-90.

As another wave slides through southern SD and northern NE later in
the night, could see northern area of precipitation gradually lose
favor to an expanding area of convection through the Missouri River
corridor, or worst case a continued backbuilding on the edge of the
cold pool toward south central SD. HPC idea of a slight risk to
excessive rainfall north of a Tyndall to Sioux Falls to Windom line
seems appropriate given the training potential. Most higher-
resolution model solutions indicate potential for some areas to
see 2-4 inches of rainfall, with obvious discrepancies on where
that might be.

Finally, there will be a combination of heat and dewpoint that
results in an area of 100-105 heat index this afternoon generally
through the James and Missouri River corridors.  With some
uncertainty on how low dewpoint values might mix this afternoon,
have passed on issuance of any heat advisory for the time.

.LONG TERM...(Wednesday through Monday)
Issued at 439 AM CDT Tue Jul 25 2017

Wednesday morning will likely see an MCS/cluster of showers and
thunderstorms along and about south of Interstate 90 as the trough
of low pressure settle into east central SD. This suppress the
surface boundary south as well as the warm and moist advection in
the low to mid levels. As for details not entirely confident on what
will occur Wednesday as the late night MCS will play a role in
location of activity. For now expecting precipitation chances to
diminish north of Interstate 90 through the morning with some mostly
non severe showers and thunderstorms ongoing south of Interstate 90.
Still a concern that heavy rain will be possible with the freezing
level around 14000 feet AGL. Lowered highs just a touch with a
little higher confidence in clouds and precipitation.

After this Thursday through Monday looks like very nice late July
weather as northwest flow aloft keeps the temperatures and humidity
on the lower side. Suspect that temperatures will be a bit below
normal during this time. While there will likely be a couple of
shots of showers and storms during this time as jet maxes wrap
southeast around the ridge, timing them is very difficult so any
chances in this period will be low.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 714 AM CDT Tue Jul 25 2017

Low-level jet early this morning will keep a couple hours of low-
level wind shear for TAF sites, then perhaps a few wind gusts up
toward 25 kts for KFSD/KSUX during the mixing. Main aviation
concern is with severe storm potential from late afternoon into
early evening. Thunderstorms likely to develop rapidly very late
afternoon around KHON, with the focus gradually shifting southeast
toward KFSD around midnight, and KSUX late in the TAF period.
Severe storms could produce large hail and damaging winds around
KHON through mid to late evening, and KFSD around midnight. Storms
will likely contain brief periods of MVFR-IFR visibility and
ceilings.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...08
AVIATION...Chapman



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