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Area forecast discussion
National Weather Service Sioux Falls South Dakota
1244 PM CDT Sat Jul 20 2019

Short term...(today and tonight)
issued at 358 am CDT Sat Jul 20 2019

The humidity is still here for now, with 08z dew points still in the
70s to around 80 across much of the forecast area. Relief is on the
way, though as lower dew points gradually filter southward into the
area behind a cool front today. By late afternoon, expect dew points
around 8-10 degrees lower than current readings. With decrease in
humidity and cooler temperatures, attention for today shifts from
oppressive heat to convective chances and associated risk of severe
weather and heavy rain.

Ongoing convection across northwest quarter of South Dakota will be
immediate concern. This driven largely by a mid level wave across
northwest South Dakota which will continue moving across South Dakota through
the morning. Storms should remain largely elevated as they move into
and across our northern County Warning Area this morning, with surface boundary over
our southern counties from northeast Nebraska into northwest Iowa
between Spencer and Storm Lake as of 08z. This activity will work
into area with 1-2kj/kg elevated instability and 40+ kt deep shear,
which should support at least isolated severe threat near/north of
I-90 as the storms advance east across our forecast area early to
mid morning. Hail still seems to be primary threat given elevated
nature of storms, but have already seen isolated severe wind
gusts with the storms in central South Dakota, so this certainly
cannot be ruled out with west-southwest oriented 0-3km shear vectors of
30-45kt and dcape values in excess of 1000j/kg indicated through
mid-morning by the latest rap.

Approaching wave will provide little additional southward push to
the front through the morning, with the boundary becoming reinforced
by cooling from the rain/storms over our northern counties. This
could result in a fairly strong thermal gradient by mid afternoon
with readings in the upper 60s-lower 70s in southwest Minnesota, to lower
80s in our far southern counties toward Sioux City-Storm Lake, with
surface front likely pushed to just south of the forecast area by
20z. Despite slight surface cooling through the late afternoon,
enough lift from elevated front to keep a threat of scattered storms
across our southeast forecast area through the late afternoon and
evening. Still some elevated instability to work with, but deep
shear weaker in this area with strongest winds aloft remaining
farther north, so would expect a more isolated severe threat due to
less organized storm potential.

In addition to any severe threat, deep warm layer and precipitable
water values near 2 inches still at or above the 90th percentile in
climatology, heavy rainfall rates and potential flash flooding
remain a large concern with any storms through this evening. This
highlighted well by wpc excessive rainfall outlook, as well as
recently issued mesoscale precip discussion.

Storm threat should gradually wane from north to south through the
late evening and overnight hours today as front finally makes a
stronger southward push into the Central Plains.


Long term...(sunday through friday)
issued at 358 am CDT Sat Jul 20 2019

Much cooler and less humid air mass settles into the region for the
latter half of the weekend. Though overall air mass will be drier
than recent days, a couple of waves moving across the region could
produce isolated to scattered showers or non-severe storms, mainly
in the afternoon.

Cooler than normal temperatures remain in place through early next
week as an upper ridge builds over the intermountain west, placing
US in northwest flow pattern. Ridging begins to work east during the
latter half of the week, which should spread more seasonable
temperatures back into the region from Wednesday Onward. Despite
weak disturbances dropping through the region, a lack of deep
moisture in this pattern will keep precipitation chances on the low
side.

&&

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1233 PM CDT Sat Jul 20 2019

Scattered showers and thunderstorms will remain possible this
afternoon into early evening mainly east of I-29 and into the
Sioux City Metro area. A lingering shower or thunderstorm may
persist in the Highway 20 corridor into the early morning hours.
Reduced visibility and ceilings will be possible with any
thunderstorms, with gusts to 35 kt possible in the stronger storms
this afternoon. Otherwise, breezy winds decrease late afternoon
and this evening.

&&

Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&

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