Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 161754
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1254 PM CDT Wed Aug 16 2017

.UPDATE...
Issued at 1138 AM CDT Wed Aug 16 2017

Decaying convection/MCS has started to lift north-northeast
bringing most of the showers and storms across northwest Iowa and
southwest Minnesota this morning. Hi-res models such as HRRR/ESRL-
HRRR/NAMnest models are performing good on the current convection,
suggesting most of the showers and thunderstorms along and east of
the I-29 corridor through early this evening. Given the deep
saturated atmosphere as the system moves through, the main concern
is the potential for locally heavy rain with an additional rainfall
amounts from 2 to 3 inches over southwest Minnesota this
afternoon. Leaned towards these models, and have raised PoPs and
QPF for these areas through OOZ Thursday. At this time, flash
flood risk is low to moderate, but minor flooding cannot be rule
out in urban areas. The severe weather risk remains marginal, but
a few isolated strong storms could develop this afternoon through
early evening. If strong storms do develop, isolated large hail
and damaging winds will be the main threats.

&&

.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 18Z TAFS through 18Z Thursday afternoon)
Issued at 1226 PM CDT Wed Aug 16 2017

Ongoing showers and isolated thunderstorms will continue to move
north-northeast, affecting ceilings and visibilities at all sites
during the TAF period. Occasional MVFR to IFR ceilings will be
prevailing through midnight, then a gradual improvement is
expected as the system slowly moves out of the area. Another
concern is low visibility, dropping conditions to MVFR/IFR
conditions with some moderate to heavy rain. Showers and storms
will gradually taper off from southwest to northeast after 00Z
Thursday.


&&

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

$$

UPDATE...05
SHORT TERM...JH
LONG TERM...BT
AVIATION...05



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