Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201145
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
645 AM CDT Sun Aug 20 2017

.SHORT TERM...(Today and Tonight)
Issued at 402 AM CDT Sun Aug 20 2017

Stronger zonal flow aloft over the northern sections of the region
as a wave zips along the International border early this morning.
Another wave closer to the synoptic warm frontal boundary was
located upstream of a large complex of thunderstorms which continues
to press southeast through southeast Nebraska and southern Iowa.
This has likely disrupted the low-level jet enough to only keep a
minimal chance for continued showers/thunderstorms early morning
mainly along/east of highway 60. Otherwise, the northern wave is
promoting an increase in mid-level frontogenetic forcing across
central/northern South Dakota, which has resulted in a quicker and
more expansive coverage to convection than even high-res model
solutions would suggest. Elevated instability on order of 300-600
J/kg, so would expect at least some spotty thunder in band which
will streak eastward through early afternoon. While the best forcing
looks to remain just north of the area, often times there is a
tendency for these bands to sag southward with continued convective
development, and have carried some lower chance-level PoPs through
the highway 14 corridor.

Through the day, a frontal zone will move gradually southward,
settling toward I-80 by late day. Once the mid-level band across the
north weakens by midday, much of the day will likely remain quiet
from a convective perspective, with a moderate cap becoming
established with the differential thermal advection. Will maintain a
later afternoon low chance for convection near and just north of the
frontal boundary generally south of a KTKC to KYKN line, as heating
produces a greater potential instability, but concerns for capping
remain and boundary likely to sink far enough south.

As we advance toward the late evening and into the early overnight
hours, the low-level jet inducing warm advection into areas north of
the stalled frontal boundary should feed an increasing threat of
thunderstorms from far southeast South Dakota and northeast Nebraska
into northwest Iowa, spreading slowly northward toward southwest MN.
The airmass to the south will feature at least 2000 J/kg elevated
instability, and there is effective shear which increasing
considerably heading northward. Storms could organize within the
elevated regime during the evening and early overnight, with the
primary threat likely large hail, although development of
mesocyclones and more shallow stable layer toward the south could
bring some damaging wind threat into parts of northwest Iowa.
The storms should begin to decrease later in the night as wave pulls
eastward along with veering low level flow, and this could open the
door for some patchy fog development south of a KSPW to KYKN line.

.LONG TERM...(Monday through Saturday)
Issued at 402 AM CDT Sun Aug 20 2017

Monday should be the most active day of the mid-long range,
primarily late afternoon into the evening. Despite what should be a
relatively dry period during the middle of the day for the eclipse,
model trends are looking less favorable for clear viewing, with most
indicating the potential for extensive stratus, at least for areas
near and south of I-90, in the wake of any overnight/early morning
convection. By midday, stratus shifts slightly northward with slow
moving warm front, while high clouds increase from the northwest in
advance of approaching wave.

This wave will bring increasing chances of storm development by late
afternoon, as it swings southeast across the Dakotas/Minnesota and
interacts with frontal boundary located south of I-90. Stability
forecasts vary, depending on whether individual model breaks out of
the stratus and allows sufficient surface warming. Will also have
added effect of eclipse keeping temperatures a few degrees cooler
into the early afternoon than would otherwise typically occur.
However, even in the absence of strong surface warming, should see
abundant elevated instability above any stable/stratus layer,
with steep mid-level lapse rates contributing to elevated CAPE
values in excess of 1000-1500 J/kg north of the boundary, and
mixed layer CAPE in excess of 2000 J/kg near and south of the
boundary. This along with increased lift from mid-upper level
trough swinging through the region should support development of
strong to severe storms during the late afternoon and evening.
Initial threats would more likely focus on large hail due to the
mid-level lapse rates, with isolated strong winds gusts near/south
of the boundary where storms are more likely to be surface-based.
As the evening progresses, could see evolution to a locally heavy
rain threat for portions of northwest Iowa, before storms shift
southeast of the area with advancing surface front by 06Z or
shortly after.

Beyond Monday evening, midweek pattern will feature cool surface
high pressure over the Upper Mississippi Valley, with continued
northwest flow aloft ahead of building ridge over the Rockies and
High Plains. This should allow for below normal temperatures and
generally dry conditions across the region through Wednesday.

Low level ridge and upper trough shift far enough east by Wednesday
night that an increasing southerly flow/associated warm advection
beneath slowly eastward advancing upper ridge will begin to have
greater influence across western portions of the forecast area.
Precipitation chances late in the work week will remain spotty, but
temperatures should gradually warm from mid-week levels. Could see
more unsettled pattern return by next weekend, as the upper ridge
shifts into the Great Lakes and allows another trough to drop into
the Northern Plains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 645 AM CDT Sun Aug 20 2017

Frontal boundary will be pushing slowly across the area today,
reaching KFSD by mid morning, and KSUX midday. Area near the
frontal boundary will have some moisture convergence and should
see some flat cumulus develop, generally at VFR heights.
Convection will become most significant 05z-10z across northwest
Iowa, and will likely contain brief periods of MVFR visibility.
Ceilings likely to lower into MVFR range in proximity to weak
boundary across northeast Nebraska and northwest Iowa. If
convection begins to drop off as expected, areas around KSUX are
likely to see development of stratus/fog reaching likely MVFR
levels, and potentially IFR visibilities.

&&

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

$$

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


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