Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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874
FXUS63 KOAX 272334
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
634 PM CDT Tue Jun 27 2017

.SHORT TERM...(Tonight through Friday)
Issued at 252 PM CDT Tue Jun 27 2017

Shortwave trough and mid-level speed max currently over the intermountain
West, evident in water vapor loop. Expect these forcing
mechanisms to move eastward the rest of the is afternoon into the
evening, moving into central plains after 00UTC. CAM and Synoptic
models all develop convection along attendant surface front that
will be located western Dakota-Nebraska panhandle into eastern
Colorado. There also is a hint of isolated convection developing
along surface theta-e/thermal ridge axis that currently extends
from west of Wichita to Kearney, but expect any of this activity
to be diurnally driven.

As mid-level forcing/ascent increases this evening, storms that develop
in high plains will trudge eastward, with expected quasi-linear
storm mode. CAM grow system upscale, with sustain instability of
at least 2500 J/Kg, 0-6km bulk 35-50 and 0-3km bulk shear 35-50.
Forecast soundings depicting mostly straight-line hodograph, and
given magnitude of low-level bulk shear values, damaging winds
appear the primary hazard. Weak low level backing per forecast
sounding suggest potential for meso-vortex development along
leading edge. Cannot discount large hail, models advect H7
temperatures 12-14C into area by 06UTC, which could mitigate large
hail growth. Used CAM and NCAR Ensembles as basis for PoP, with
arrival of leading edge of QLCS MCS into western parts of CWA by
02 UTC...LNK-OMA 05-06 UTC and moving into southwest Iowa 07-08
UTC. As storm moves east of primary instability axis, storms
should weaken as they move into Iowa.

Lingering showers and storms will exist across eastern Nebraska
and western Iowa in the morning, though expect coverage to
diminish due to meso-scale subsidence in wake of MCS and large
scale subsidence as main short wave feature moves into Great
Lakes. Reprieve will be short, as expect storms to redevelop
Wednesday afternoon along fused synoptic/meso-scale boundaries
left from overnight storms. Even greater SBCAPE being generated by
synoptic models, with axis of 3-4.5K J/Kg extending from north
central Iowa into northeast Kansas. As such will place highest PoP
and storm threat across southeast Nebraska and western Iowa.

Evap cooling and weak cold pool action may shove boundary a little
south overnight Wednesday, though flat upper level flow will keep
boundary from moving too far south. More vigorous upper level
trough will dig southward from Saskatchewan, thus boundary
forecast to lift back into area.  Moisture pooling and relatively
warm temperatures will set the stage for additional storm
development, with persistent moisture advection sustaining storms
into the evening and overnight. Models are generating healthy QPF,
so may have to consider flood/flash flood headlines; however,
given the nearly constant convective activity over the next couple
days, waiting to see where boundaries and thus heavy precipitation
axis will lay.

Finally on Friday, there looks to be a window when storm threat
will exit, as upper trough over northern plains moves east. Medium
range models are differing on timing of trough movement from
northern plains into Great Lakes, with GFS/GEFS slower than
EC/GEM. Though there are timing differences, any lingering storms
should be east of CWA by mid-day, with Friday night looking dry.

Temperatures during the short term will be driven by convection,
and convective debris. Wednesday looks to be a little warm in wake
of MCS, with thermal ridge setting up across southern NE. Have
bumped up temperatures as such. For the rest of the short term
followed consensus more or less.

.LONG TERM...(Friday night through Tuesday)
Issued at 252 PM CDT Tue Jun 27 2017

Forecast area will remain under influence of large scale trough
located over eastern U.S. through bulk of extended range. Several
weak waves will ripple from northern plains across the area via
northwest flow on the back side of the trough. Medium range
models & ensembles are not being too bullish on precipitation
chances through Sunday, with consensus models generating some
slight to low chance PoPs somewhat randomly. Return flow and a
more substantial short wave moves into area Sunday night into
Monday, with medium range models hinting at MCS development. Exact
timing and progression hard to pin down this far out.

Temperatures appear season through the extended range per
consensus and statistical guidance.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 631 PM CDT Tue Jun 27 2017

Areas of thunderstorms will affect eastern Nebraska TAF sites
between about 05Z through 12Z. Initial surge of storms will
likely produce wind gusts over 40kt with associated MVFR to IFR
cigs/vsbys within storms. Outside of storms, generally VFR
conditions are forecast with gusty south winds ahead of
convection, and southwest to west winds following. More storms are
possible late in the TAF cycle, most likely just south of KOMA
and KLNK sites.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&

$$

SHORT TERM...Fortin
LONG TERM...Fortin
AVIATION...Dergan



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