Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 211102

Area Forecast Discussion
National Weather Service Omaha/Valley NE
602 AM CDT Mon Aug 21 2017

.SHORT TERM...(Today through Wednesday)
Issued at 225 AM CDT Mon Aug 21 2017

Rapid MCS formation has occurred early this morning over western
IA near the intersection of a synoptic surface front and residual
outflow boundary, per mesoanalysis. The presence of a 40-45 kt
southwesterly LLJ will aid in back-building storms across our IA
counties through 12z with a net south or southeast displacement of
the MCS through that time frame. Elsewhere, IR satellite data
indicate a plume of mid to high-level moisture (and associated
clouds) advancing northeast through the High Plains in association
with a subtropical-branch shortwave trough over NM.

--Eclipse forecast--

While it appears that the ongoing storms across IA will shift
east of the area by mid morning, model guidance remains consistent
in showing the High Plains cloud shield steadily advancing
northeast into the mid-MO Valley today. Model cross sections of
relative humidity along the path of totality indicate the
potential for considerable cloudiness at 1 PM CDT at or above the
18-20 kft level. At present, we remain uncertain of the opacity of
these clouds, and this is something visible satellite should
clarify in the hours leading up to the event. High temperatures
will be generally in the lower to mid 80s with afternoon heat
index values rising into the lower 90s across southeast NE and
southwest IA. Precipitation chances will remain low through early

By mid afternoon into this evening, height falls associated with
an amplifying polar-branch trough over the ND-MN Red River Valley
coupled with the influence of the subtropical-branch system
tracking into the central Plains will yield an uptick in
convective activity across the forecast area. The most intense
storms will likely form along a cold front moving south into
northeast NE and west-central Iowa where forecast soundings
indicate ample instability and vertical shear for organized severe
weather. Damaging winds, hail and heavy rain will be the primary
hazards with these storms as they advance southeast across the
area this evening and tonight. An isolated tornado is possible,
especially with any storm that can favorably interact with an
outflow boundary remnant from the MCS currently ongoing across IA.

A cooler and drier Canadian air mass will reside across the mid-MO
Valley on Tue and Wed with dry conditions expected.

.LONG TERM...(Wednesday night through Sunday)
Issued at 225 AM CDT Mon Aug 21 2017

A longwave trough will persist across eastern North America from
the middle to latter part of this work week before a more
substantial upstream trough and lower-latitude perturbations move
into the Canadian Prairies and Great Plains this weekend. This
large-scale pattern will yield continued mild and dry weather
through Friday, before precipitation chances increase this


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 602 AM CDT Mon Aug 21 2017

A back-building MCS is in progress as of 1045z across western IA
with more recent storm development along the western edge of the
associated cold pool between KOMA and KLNK. Expect storms to
persist for the next two to three hours before dissipating with
the cessation of the nocturnal LLJ. We will mention prevailing VCTS
at KOMA and KLNK with a TEMPO group for MVFR TSRA at KOMA between
12 and 14z. From mid morning through early afternoon, we expect
prevailing VFR conditions across eastern NE with increasing high-
level cloudiness. Thunderstorm potential will increase by mid to
late afternoon at KOFK and early evening at KOMA and KLNK as a
cold front moves south through the area.




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