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Area forecast discussion
National Weather Service Sioux Falls South Dakota
626 am CDT Wed Sep 18 2019

Short term...(today and tonight)
issued at 232 am CDT Wed Sep 18 2019

Widespread elevated convection breaking out this morning along the
axis of a 50-60 knot low level jet. Generally this activity has remained sub-
severe. Despite MUCAPE values around 1000 j/kg, effective shear
remains a rather weak 15 to 20 knots. That said, could still see a
lingering marginal hail risk continue for a few hours. The upper
wave responsible for the increased low-lvl convergence continues to
pivot northeast into eastern North Dakota this morning. This motion
will allow convection to ease eastward through daybreak, which
should keep the potential for training slightly lower. Areas from
Spencer to Storm Lake could see the longest rainfall potential after
daybreak.

Today: the upper wave will push a weak boundary through the County Warning Area
this morning. This front will likely either flatten or completely
washout by this afternoon. For most of US, the day will be dry,
with highs remaining above normal, but cooler than Tuesday.

Tonight: guidance shows another minor shortwave moving through the
Central Plains later this evening. Minor convergence along the
aforementioned surface boundary could trigger a few sfc based
storms later this afternoon, but generally feel the greatest risk
for convection will develop through the evening and into the
overnight hours as 850 mb flow backs and begins to again converge
over portions of eastern Nebraska and northwest Iowa. Latest
guidance indicates that the greatest threat from heavy rainfall
has shifted slightly further south, but could still see an area of
increased rainfall potential along a line from Sioux City to
Spencer overnight with amounts of 0.5-1.5" possible. Elevated
instability does increase through the night, but shear again
remains marginal. Could see a stronger storm or two, and it's
possible to see convective outlook potential increase slightly
today along our southern zones.

Long term...(thursday through tuesday)
issued at 232 am CDT Wed Sep 18 2019

Thursday: shortwave ridging arrives for Thursday, which generally
should mean a dry and warmer day. As mentioned previously, will
watch for stratus development behind convection. Temperatures again
rise some 5 to 10 degrees above normal.

Thursday night: mid-lvl flow turns more meridional Thursday
afternoon and night as a trough moves into The Rockies. It's
possible to see elevated convection develop Thursday evening and
overnight as the eml begins to advect northward. This lingering risk
of convection continues into Friday.

Friday into saturday: deep southerly flow brings plenty of humidity
northward on Friday. As low level temperatures rise, convection
should become capped off through the day on Friday. Strong gradient
flow should result in very gusty winds through the day. Have bumped
up with speeds and gusts from Friday afternoon through Friday
evening. We'll turn our eyes to two potential areas for convection.
One area along an advancing frontal boundary in the western Dakotas,
and a second area further east along the I-29 corridor Friday
evening tied to a secondary vort lobe. While still some
uncertainty, there will be an inherited severe weather risk (mostly
focused west of the James River into Friday night. There are
questions remaining as to how organized convection can remain as
it tracks east into a more marginal environment. There will also
be a risk of excessive rainfall given high precipitable water values and
expectation that convection may attempt to train along the
advancing frontal boundary. Given the meridional movement of the
shortwave, have some questions above how far east the heavy rain
risk can extend. Felt the nbm quantitative precipitation forecast was too high and to far to the
east this evening, and trended slightly closer to the wpc
guidance. This would place the greatest excessive rain risks
focused in the western third of the County Warning Area.

Saturday-tuesday: slightly cooler temperatures arrives on
Saturday behind a frontal boundary, but given residual warm air
aloft, highs may still reach near to above normal. A pleasant day
is anticipated for Sunday with dry conditions continuing into the
new week. Temperatures won't stray too far from normal throughout
the extended portions of the forecast.

&&

Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 625 am CDT Wed Sep 18 2019

Scattered convection is moving away from Sioux City this morning,
which will minimize our concerns for the daytime hours of
Wednesday. By tonight, some increasing mid-lvl clouds may develop
with a risk for a thunderstorm or two near Sioux City again.

&&

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

$$

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