Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 271722

Area Forecast Discussion
National Weather Service Sioux Falls SD
1222 PM CDT WED JUL 27 2016

.SHORT TERM...(Today and Tonight)
Issued at 345 AM CDT Wed Jul 27 2016

Coverage/timing of convective chances are primary challenge today,
as weak cold front moves southeast through the region. As of 08z,
precipitation still split into two camps. The first is the slowly
waning remnants of MCS which dropped south through south central
South Dakota and central Nebraska last evening into the overnight
hours. Northern fringe of largely stratiform precip shield slipping
east through the Missouri River Valley west of YKN. Has been slowly
diminishing in coverage north of the river as subsidence behind MCV
spins eastward through northeast Nebraska, however could continue to
see this precip scrape our far southern areas through the early
morning hours.

The second is to our north as mid-upper level wave interacts with
currently stationary front from northeast South Dakota into northern
Minnesota. Expect this to be the primary player through the next 24
hours as the wave slides through Minnesota and drags the boundary
southeast across our area. Some of the CAMs have been having quite a
bit of difficulty in the early portions of their respective runs, in
that they continually try to fill the gap between these two areas of
convection. Given the aforementioned subsidence in the wake of the
Nebraska MCV, have leaned toward drier models through the early-mid
morning hours, gradually tapering chances across the south as the
MCV continues east/weakens, while focusing low-end chances across
northeast portions of the forecast area as the upper wave begins to
drop southeast. For now, hanging on to slight chance between the
two areas to account for some lingering uncertainty in evolution.

By midday/early afternoon, most models trend toward modest agreement
in placing the cold front east of KMML-KFSD-KYKN line. Likely to see
fair amount of cloud cover lingering from the morning convection,
which should hold highs down a bit across our east. This along with
primary forcing passing to the northeast of the forecast area should
limit convective coverage through the afternoon/evening. However,
would expect to see isolated-scattered convection slowly developing
southward along/ahead of the front into somewhat more unstable air
mass across northwest Iowa by late afternoon. Forecast soundings
indicate 1500-2000 J/kg MUCAPE with localized 30-40kt of effective
shear along the front in parts of northwest Iowa as it moves through.
This could be enough for an isolated stronger storm, though overall
the threat of severe weather appears low. Front should be east of the
forecast area by 03Z, leaving largely dry conditions for the region
through the overnight hours.

Highs today will generally be in the upper 70s in southwest Minnesota
to mid 80s through the James/Missouri River Valleys. Drier air and
light winds behind the front tonight should allow for lows in the
upper 50s to lower 60s for most by early Thursday.

.LONG TERM...(Thursday through Tuesday)
Issued at 345 AM CDT Wed Jul 27 2016

By 12z Thursday, not much remains for forcing for precipitation to
start the day. By afternoon, remains just a bit of cyclonic
curvature lingering toward the far eastern CWA that just a bit of
heating could be enough to generate a couple of stray showers with a
couple rumbles of thunder with some minimal CAPE potential. Larger
scale is not particularly conducive to lift support, so may be a
tough road.  Regardless, northeasterly low-level flow likely to keep
plenty of clouds in play east of I-29 likely to hold temps back into
the mid to upper 70s, while a bit more sunshine potential should
see a return of lower 80s to the west.

While timing of small features can be characterized as chaotic at
best in the various guidance, once such feature looks to be in play
on Thursday night. However, will be pushing southeast and
encountering a less and less favorable airmass in terms of
stability, so if any precipitation would begin to encroach on the
western CWA, it would likely dwindle fairly quickly, and certainly
remains focused more toward the stronger baroclinicity across
Nebraska.  Cooler surface ridge building toward Minnesota and
Wisconsin will keep a light easterly component to winds, and if not
for the threat of clouds, would probably hedge even a bit cooler
than upper 50s to lower 60s.

Another wave should be around from Friday into Friday night, with a
little stronger eastward progression of the mid-level thermal
gradient.  Should start to see an increase in low-level jet by
Friday night, and with baroclinicity lined up from western South
Dakota southeastward toward southern Nebraska/eastern Kansas, will
continue to find best threat of storms outside the area. However,
enough lift with elevated boundary to get a few more storms
especially near/west of I-29 through 12z Sat.  The overall
instability profile is fairly meager, so should keep severe weather
threat at a minimum through Saturday morning.

Timing of precipitation does not get any easier starting the
extended range.  Wave will be in the process shearing/exiting the
area weakening mid level advections, but also will be encountering a
drier and more stable airmass toward Minnesota under influence of
the cooler ridge to the east. Scattered precipitation mainly earlier
on Saturday, with a tendency to decrease some through the day with
start of ridging impact. Northeastward progress of mid-level
instability gradient and increase in low level jet Saturday night
could see a couple of storms initiate, which would then track
east/southeast toward southwest Minnesota.

One thing for certain will be the increasing humidity by the end of
the weekend, and heat by early next week.  Rising heights through
Sunday as 700 hPa temps warm to +10 to +14C, with moisture return
below inversion as more of a southeast trajectory from the mid
Mississippi valley sets up.  Lower 90s should return to areas west
of the James river on Sunday, build toward I-29 on Monday, and be
most areas on Tuesday. GFS appears too aggressive building ridge and
shifting axis east of the area Tuesday, and prefer slightly more
zonal ECMWF and ensemble mean solution.  With buildup of dewpoints,
heat index readings across southeast SD into northwest IA on Sunday
could be well into the 90s, with some areas Missouri and James
valley approaching 100 on Monday. Much of the area south of I-90
could see a return of 100-105 heat indices on Tuesday, barring any
unforeseen convective outflow issues.

Warming temps aloft should minimize precipitation threat from later
Sunday through Monday night. By Tuesday, with broad zonal ridge over
the northern Plains, could see convection begin to return,
especially near boundary which suggest residence somewhere near the
area.  Severe threat will have to be monitored due to greater
potential instability, proximity to greater shear on the southern
edge of mid-level westerlies.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1220 PM CDT Wed Jul 27 2016

For the most parts, VFR conditions through the period. Parts of
southwest Minnesota and northwest Iowa could see ceilings drop to
low end LVFR on Thursday morning, but at this time not expecting
widespread lower ceilings along and west of Interstate 29. Hit and
miss showers and thunderstorms possible this afternoon and early
evening mainly in northwest Iowa.


.FSD Watches/Warnings/Advisories...


LONG TERM...Chapman
AVIATION...08 is the U.S. government's official web portal to all federal, state and local government web resources and services.