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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KOAX 280544
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1244 AM CDT Wed Jun 28 2017

.UPDATE...
-- Changed Discussion --
Issued at 1241 AM CDT Wed Jun 28 2017

Have decided rather abruptly to include counties along the
Missouri River in eastern Nebraska for Severe Thunderstorm Watch
374. Apex of bow structure appears to be point toward or just
south of the Omaha Metro. Thus included counties from Washington
to Otoe, basically Blair to Nebraska City, along the Missouri
River.

UPDATE Issued at 1229 AM CDT Wed Jun 28 2017

Have added a couple of counties, namely Saunders and Lancaster, to
Severe Thunderstorm Watch 374 through 2 AM. These counties appear
to be in line with the strongest reflectivities and velocity
signatures moving in from the west. Wind reports have been hard to
come by within this line as it moved out of central Nebraska,
however radar velocities are painting greater than 70kt about
3000ft off the deck. Will be monitoring evolution of the line, and
may have to add counties farther east, including the Omaha Metro
area.
-- End Changed Discussion --

&&

.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 06Z TAFS through 06Z Wednesday night)
Issued at 1229 AM CDT Wed Jun 28 2017

Line of thunderstorms will move through the KLNK and KOMA TAF
sites early in the TAF cycle, generally between 06Z and 10Z.
Strong winds gusting over 40kt are possible, with some potential
for greater than 50kt with initial surge of convection. Lingering
rain and thunder is likely to hold through 12Z or so. Farther to
the north, KOFK already saw the strongest storms move through that
area, but several hours of rain and isolated thunder is likely
there. After 12Z to 15Z time frame, most of the activity will have
moved out of the area. However redevelopment is likely along an
outflow boundary expected to set up near or southeast of
Interstate 80. Thus KLNK and KOMA could see another round of
thunderstorms between 20Z and 00Z.


&&

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

$$

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