Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 192030

Area Forecast Discussion
National Weather Service Omaha/Valley NE
330 PM CDT Mon Jun 19 2017

.SHORT TERM...(Tonight through Thursday)
Issued at 326 PM CDT Mon Jun 19 2017

The return of thunderstorm chances Wednesday and Thursday is the
primary forecast concern.

Surface high pressure had settled over the region this afternoon,
maintaining a mild and relatively dry atmosphere in the Plains.
Upper trough dipping into the Great Lakes was providing northwest
mid level flow across the central CONUS, and cyclonic flow in the
lower atmosphere aided in cumulus development in our area. These
diurnally driven clouds should dissipate this evening, leaving
mainly clear skies, light winds and dry air to allow another cool
morning Tuesday.

Potent mid level wave currently diving southeast through eastern
Saskatchewan is forecast into the Minnesota and Iowa Tuesday.
Associated showers will remain well northeast of our area, but wave
will nudge a backdoor cool front east to west across eastern
Nebraska Tuesday afternoon. A secondary shortwave dropping across
the High Plains may trigger a few showers or thunderstorms in our
far southwest late in the afternoon or evening.

The cold front is forecast to retreat back north as a warm front
on Wednesday as broad southerly flow returns to the Plains in
response to a significant shortwave spreading east through the
southern Canadian Plains. Rapid theta-e influx south of warm front
and just ahead of trailing cold front will lead to increasingly
unstable airmass over much of eastern Nebraska late afternoon and
evening. GFS is the most bullish on moisture return, pulling low
70s dew points into northeast Nebraska. But even the more bearish
ECMWF and Canadian models show upper 60s surface dew points
available for convection, yielding 2500 J/kg of MLCAPE. Sufficient
shear profiles will also exist with 35-40kt of bulk shear vectors
perpendicular to convergence axis. Thus if storms can get going,
organized convection with initial large hail threat and later wind
risk is likely. Tornado risk will likely be confined to
intersection of cold and warm fronts which should be just north of
our area. However convective inhibition may be a bit much to
overcome in a good part of eastern Nebraska as 700 mb
temperatures approach 14C much of the southern half of the area,
with cooler temps and less inhibition making initial development
more likely in our north.

Convection will likely continue and expand through the evening as
cold front progresses southeast and low level jet strengthens,
maintaining a wind and hail threat through midnight. Much of the
activity will be moving off to the east by that time.

On Thursday, models have been offering varied solutions on how far
south the Wednesday cold front will travel. The latest consensus
suggests front will hang up in our southern CWA, and perhaps lift
north a bit during the day. Pooling of moisture near and south of
front will lead to MLCAPEs again topping 2500 J/kg Thursday
afternoon, with associated large hail threat. Also, mid level flow
nearly parallel to surface boundary and only weak low level jet
action suggests training of storms may yield heavy rains across much
of the area near stalled frontal boundary.

.LONG TERM...(Thursday night through Monday)
Issued at 326 PM CDT Mon Jun 19 2017

Convection should be ongoing Thursday evening along stalled cold
front, and will likely continue into Friday as upper trough swings
across the Dakotas and northern Nebraska, inducing continued theta-e
advection over frontal zone. Heavy rain looks likely in this
scenario, but exactly where will depend on surface frontal position.
And timing may be altered as longer term models move upper trough
through the region at different times. GFS says front should be
shoved south Friday night on back side of Dakota trough, effectively
pushing rain chances out of eastern Nebraska and southwest Iowa.
However ECMWF is slower, and keeps rain chances in for much of
Saturday. With either scenario, a chilly forecast will follow for
the weekend when high temperatures linger in the 70s Saturday
through Monday.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1211 PM CDT Mon Jun 19 2017

VFR conditions are forecast through Tuesday morning with scattered
afternoon clouds near FL050 and northwest winds near 15kt both
diminishing this evening.




LONG TERM...Dergan
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