Area Forecast Discussion
Issued by NWS Sioux Falls, SD

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KFSD 161137

Area Forecast Discussion
National Weather Service Sioux Falls SD
637 AM CDT Wed Aug 16 2017

.SHORT TERM...(Today and Tonight)
Issued at 423 AM CDT Wed Aug 16 2017

With the exception of isolated storms in the James River valley,
convection to our south appears to be having a significant impact on
northward expansion of rain into the area thus far, with northeast
edge continuing to erode as it tries to cross the Missouri River as
of 08Z. The continued southward push of deep convection through
south central Nebraska/northern Kansas could further impede rainfall
spread into our area. However, as the main mid-upper trough over the
central Rockies begins to lift northeast into Nebraska/South Dakota
through the day, we should be able to see deepening moisture and
lift spread rain over much of the forecast area.

GFS/NAM actually develop a weak TROWAL signature aloft, with a dry
slot pushing into northwest Iowa by this afternoon, and widespread
cloud cover and precipitation wrapping around the northwest side of
low pressure as it tracks northeast through southeast SD/southwest
MN today. This results in much cooler temperatures for our western
forecast area, with a number of models holding temperatures today
near current readings in the upper 60s for areas west and north of
Sioux Falls. With fairly good consistency in this signal, have
lowered temperatures and raised pops across our west and north into
this evening. No longer looking at heavy rain potential in these
areas, and may even see minimal thunder as the cooler temperatures
limit instability. Instead, could see just a steady light-moderate
rain producing up to a half inch of much-needed moisture.

Area to watch will be in northwest Iowa today, as this area ahead of
the surface low could see more active convection prior to the dry
slot pushing into the region by mid-late afternoon. Some question on
timing/location of storms as boundaries are ill-defined and highly
dependent on earlier convection. However, hi-res model consensus
suggests this area remaining fairly dry through late morning, with a
line of convection then developing rather quickly through the early
afternoon along/ahead of a pre-frontal trough pushing east-northeast
through the area. Overall, deep layer shear looks weak, around 30kt
or less, and mid-level lapse rates are likewise pretty anemic.
However, if temperatures are able to sufficiently warm to allow for
greater instability as GFS/NAM suggest, locally enhanced shear near
boundaries could result in an isolated strong-severe hail potential.
Will also have to watch for locally heavy rainfall, as precipitable
water values remain high, and the projected warm cloud depths in
excess of 10kft could again result in very efficient rainfall rates.
Area of greatest concern in this regard would be our far southern
counties, which did receive some heavier rains yesterday.

Tonight should bring a general decrease in precipitation across the
area, although aforementioned wrap-around rains could linger across
our northeast well into the night. However, expect most areas to be
dry by daybreak as the surface and upper low lift north of the area.

.LONG TERM...(Thursday through Tuesday)
Issued at 423 AM CDT Wed Aug 16 2017

Near normal temperatures are expected to prevail throughout the
extended with periodic chances of precipitation.  Could see a
lingering shower or two through the early morning hours on Thursday
as slow moving system that has impacted the region moves northeast
towards the western Great Lakes region. Warm conveyor ahead of the
next system may lead to a few elevated showers and thunderstorms on
Friday morning across Central South Dakota. Potential for additional
storms on Friday as a weak boundary works through the area. Surface
based CAPE looks to be in the 1-2K range, but wind shear looks
relatively weak through the column limiting the severe potential.

Dry, seasonal and pleasant conditions are expected across the region
on Saturday as surface ridge across the central Plains shifts east.
As this system shifts east, southerly winds will increase. Weak low
level jet of 20-25 may create an outside chance for an elevated
storm or two early in the morning hours on Sunday. More widespread
activity is possible Sunday night into Monday as a series of
shortwaves track through the region under zonal flow.  Models have
little agreement on the precise location of the waves at this point,
so left broad brushed pops as is for now. Timing will be critical,
but this may impact viewing round the area of the solar eclipse.

Drier, more tranquil conditions are expected on Tuesday into
Wednesday as surface ridge moves from Saskatchewan towards Kansas
City by Wednesday evening.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 634 AM CDT Wed Aug 16 2017

Variable conditions will be seen across the region today, as a
slow moving storm system lifts northeast through Nebraska and
eastern South Dakota. Periods of MVFR-locally IFR ceilings will
be possible, and due to variability, have remained pessimistic
with the ceiling forecast for KHON/KFSD. The greater threat of
thunderstorms will be focused from northwest Iowa into southwest
Minnesota, and thunderstorms will be accompanied by MVFR-IFR
visibility in moderate to heavy rain. This thunderstorm activity
should exit east of the region after 17/00Z.




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