Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241135
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
635 AM CDT FRI JUN 24 2016

.SHORT TERM...(Today and Tonight)
Issued at 406 AM CDT Fri Jun 24 2016

Brief return of heat/humidity today, especially in areas along and
west of the James River, although main question will be how quickly
the moisture will move into this area. Early morning dew points in
the mid-upper 50s throughout our forecast area, and upper 50s-lower
60s are prevalent as far south as far northern Kansas as of 08z. as
surface ridge axis currently extending from the mid-Missouri River
Valley into eastern Minnesota continues to shift east, expect that
surface flow will become southerly and increase by midday, which
should start to draw some of the higher moisture from the central
Plains northward. That said, have hedged away from the highest dew
point guidance which brings abundant 70+ readings into southeast
South Dakota by mid afternoon. On the other end of the spectrum,
RAP/HRRR appears too strongly mixed, pushing temperatures well into
the upper 90s-lower 100s while holding dew points in the lower 60s.
While the RAP/HRRR solution not out of the question, confidence is
low and opted toward a middle ground bringing dew points up into the
mid-upper 60s by late afternoon. This will combine with temperatures
ranging from lower 80s in southwest Minnesota to lower-mid 90s near
and west of the James River, with heat index values topping out in
the mid-upper 90s in the latter area. Thus will forgo any headlines,
but next shift will need to monitor closely, as trend toward either
of the aforementioned extremes would likely push heat indices above
100 in some areas west of the James this afternoon.

Tonight will feature increasing low level jet, which should aid in
pooling some higher dew points in southeast South Dakota ahead of an
approaching cold front. While main focus for overnight convection
should be north of our forecast area where low-level convergence is
more favorable, elevated instability and approach of mid-level wave
into south central South Dakota later tonight should be enough to
spark isolated-scattered elevated storms. Overall severe threat
appears low, though, as weak mid-upper level flow results in low
effective shear, and soundings indicate a deep warm layer which
should lessen large hail threat. Continued breezy southerly flow
and increasing low level moisture should hold overnight lows in
the lower-mid 70s, though extreme western areas could drop back
into the 60s if cold front pushes through the Chamberlain-Gregory
areas prior to 12Z as expected.

.LONG TERM...(Saturday through Thursday)
Issued at 406 AM CDT Fri Jun 24 2016

Convective coverage and severe weather potential are the greatest
concerns for Saturday.  The early morning should find an increase in
larger scale forcing across the southern and eastern CWA with
orientation of low level jet and approach of subtle upper impulse
out ahead of main trough pushing into the western Dakotas. Areas
across northwest Iowa and into southwest Minnesota remain slightly
cooler aloft, and result of the increased moisture on veering low
level jet is to produce a modest potential for 1000 J/kg elevated
CAPE to start to day. Could even have some residual convection from
overnight activity to the west wandering through southeast South
Dakota.  As a result, have maintained generally lower scattered pops
to cover at this time as uncertain on how widespread this early
activity may be as most of the greater moisture remains trapped
below strengthening inversion. Early day activity would at best
result in some spotty severe hail threat on instability alone, with
effective shear lagging at 20-30 knots.

During the day, will have a sharp cold front pushing across the CWA
associated with deep upper trough, reaching near the I-29 corridor
around 18z and clearing the CWA by 00z. Pooling of sub inversion
moisture should allow boundary layer to hold upper 60s to lower 70s
dew points, but slightly warmer temps aloft will also be slipping
eastward just ahead of upper trough. Question also remains on how
much morning activity could blossom ahead of the front to interfere
with destabilization process. For the most part, shear parameters
are far from ideal for a larger severe threat, with greater deep
layer values 35-50 knots lagging behind the frontal boundary.
However, suggestion is that could be a small corridor near the front
across northwest Iowa where 0-3km shear reaches 20-30 knots and deep
layer values approach 45 knots. Convective mode would appear to be
fairly mixed for our area, perhaps a few supercell structures along
the front, but likely quite linear before long. Severe threat would
be focused on isolated large hail and damaging wind for parts of
southwest Minnesota through northwest Iowa. Timing will be limited
on the back side by frontal passage, which should provide a clean
ending to the threat.

Precipitation threat should have dwindled across even the eastern
CWA by 00z, as frontal boundary is driven eastward very quickly.
Winds could remain a bit brisk overnight, especially north of I-90
as wave keeps things stirred up a bit. Sunday a classic mixing day
to the southwest of upper wave, with 25 to 35 knots of wind in upper
mixed layer, especially near/north of I-90. Mixing may be a bit more
challenged toward northeastern Nebraska as flow weakens southward,
but temps off surface would still suggest a vast coverage of
temperatures in the upper 80s to lower 90s.  Do get some afternoon
relative humidity to fall to 20 to 30 percent in areas west of the
James River to combine with wind potential, but green enough to keep
fire danger concerns fairly low for established vegetation.

Winds will decouple to lighter southwest fairly quickly Sunday
evening, but will watch progression of secondary backdoor-type
frontal boundary overnight, trending winds toward northerly.  While
the airmass is very dry, dynamics in jet entrance region suggest
keeping an eye on perhaps a rogue storm threat during later Sunday
night toward the Missouri valley in frontal gradient.  Cooler air
filters in on Monday with up to around 10 degrees cooling from a
toasty Sunday.

Working beyond Monday, gradually amplifying upper pattern with
western ridge and eastern trough through midweek. Several
disturbances seen dropping through the pattern across the northern
plains.  While there are subtle differences in timing and track,
there are enough similarities in the pattern to suggest a better
chance for thunderstorms within the Tuesday night through Wednesday
period as induce a decent lower-level southerly inflow on back side
of cool surface ridge, with some lesser chances buffering around in
space/time.  For the time, better chances will cover the Missouri
valley, as areas further north/east toward southwest Minnesota will
need to overcome a drier trajectory.

For the most part, high temps will reside on the cooler side of
normal this week in the 70s to around 80, although some moderation
suggested by Friday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 635 AM CDT Fri Jun 24 2016

VFR conditions will prevail through the TAF period, although MVFR
ceilings may briefly nudge into areas near Highway 71 between KMWM
and KSPW very early in the period.

Isolated elevated thunderstorms will be possible near and south of
I-90 corridor late tonight after 15/06z, though coverage/location
far too uncertain to include in KFSD/KSUX TAFs at this time. These
sites will see low level wind shear develop overnight as well, in
response to increasing 40-45kt low level jet after 04z.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...JH
LONG TERM...Chapman
AVIATION...JH



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