Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172004
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
304 PM CDT Wed May 17 2017

.SHORT TERM...(This evening through Thursday)
Issued at 304 PM CDT Wed May 17 2017

Active afternoon today as scattered convection has quickly developed
ahead of the upper low rotating northeast out of eastern Nebraska.
Localized 0-1 km helicity maximum approaching 200 along northward
rotating warm front has lead to an increased localized risk of
tornadoes across portions of northwest Iowa and adjacent areas of
South Dakota this afternoon.  Mesoscale environment has turned more
favorable early this afternoon as clearing behind the dry slot has
pushed 0-1 KM CAPE has also spike over 200 J/KG. The result has been
one or two storms anchoring to the warm front with one reported
touchdown (as of this discussion) near Hospers.

Through the afternoon, not anticipating a huge jump to this front
north of the Iowa border, but it could reach up towards Windom.
Generally, feel the tornadic risk may stay rather confined to
extreme NW Iowa and the very adjacent areas of South Dakota through
5pm.

Further south watching more organized clusters beginning to
form over NW Missouri and SW Iowa, which are expected to pivot north
and northeast through the late afternoon and early evening hours.
Guidance is showing an enhancement in the low-level winds this
afternoon, with an increased drying a the 800:700 mb layer which
could suggest a strong wind risk could form.  The greatest risk from
this complex will fall from the Sioux City to Spencer line and areas
southeast into central Iowa where there has been less overturning of
the atmosphere early this afternoon. Still a favorable hail profile
in the southeastern portions of the CWA.

Rainfall has been very efficient this afternoon, thankfully the fast
moving nature of the activity has limited the flash flood risk. Any
very localized flooding should subside rapidly so am not
anticipating any flash flood watch.

The upper low will quickly pivot through central Iowa this evening,
leaving much in the CWA in the westward quarter of the system, with
lingering light rains through daybreak Thursday.

A very brisk and chilly Thursday is expected. High temperatures may
struggle to climb through the 50s.


.LONG TERM...(Thursday night through Wednesday)
Issued at 304 PM CDT Wed May 17 2017

Mid-range portion of the forecast focuses on next upper low, which
will swing northeast out of the Four Corners region across the High
Plains late this week. Models show decent agreement timing a lead
wave rotating northward through the Plains late Thursday night and
Friday. Strong isentropic lift and increasing saturation associated
with this wave will begin to spread rain into areas south of I-90
late Thursday night, continuing northward across much of the region
on Friday. This band of rain will be accompanied by some very chilly
air. Most models are showing 925mb temperatures of 0C to 4C within
the rain band, with spotty locations dipping to sub-zero readings in
some models. Warm enough air above/below this coolest level that
mixed precip is not a concern at this point. However, the cold air
and precipitation will lead to a cold day on Friday, with many
locales likely not reaching 50 degrees! Current record low maximum
temperature values are in the 40s to lower 50s for much of the area
(49 at KFSD/48 at KMHE/47 at KHON/44 at KSUX), and some of these
records could be in jeopardy if some of the raw model output
verifies.

Models have trended toward the slightly faster solution for timing
as the system lifts northeast toward the Northern Plains late this
week. This will keep a good shot at rain across the area into at
least Saturday morning, but the latter half of the weekend continues
to look dry and slightly warmer.

Temperatures remain a shade cooler than normal into early next week,
with low precipitation chances later Monday into Tuesday, as a
northern stream wave drops into the region on the back side of
stagnating upper low over Ontario.


&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1237 PM CDT Wed May 17 2017

Widespread MVFR-locally IFR conditions will be prevalent through
the period, largely associated with bands of rain and scattered
thunderstorms rotating around a storm system moving from eastern
Nebraska into the Upper Mississippi Valley during this period.
Thunder threat is greatest along and east of KYKN-KFSD-KMML line,
with isolated strong to severe storms bringing a potential for
hail to parts of northwest Iowa through this afternoon.

In addition to the low ceilings and scattered thunder threat,
strong northerly winds will gust in excess of 25kt at times,
mainly across southeast/east central South Dakota and southwest
Minnesota.


&&

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

$$

SHORT TERM...Dux
LONG TERM...JH
AVIATION...JH



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