Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 171725
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1225 PM CDT Mon Apr 17 2017

.SHORT TERM...(Today through Wednesday)
Issued at 257 AM CDT Mon Apr 17 2017

One more relatively tranquil weather day today will be followed by
more unsettled weather as the chance for thunderstorms enters the
forecast tonight through Wednesday. Severe storms are possible in
the southern half of our area Wednesday.

Surface high pressure that brought mild conditions to the area on
Sunday was moving off to the east across the Mississippi River
Valley region this morning. The return of southerly low level flow
was beginning in western Nebraska, and will overspread all of the
Plains later today as mid level shortwave trough approaches the
Dakotas. Ribbon of low and mid level moisture will ride the High
Plains north today and tonight as surface low deepens then ejects
east across the North and South Dakota border overnight. Trailing
cold front should be approaching northeast Nebraska after midnight
tonight, triggering convection from central Nebraska into the
Northern Plains where elevated instability is characterized by over
1000 J/kg of MUCAPE. Shear and forcing will be maximized well north
of our area, however nearly 40kt of bulk shear is forecast by GFS to
coincide with axis of instability. A few strong storms are possible
in our northern CWA before sunrise until shortwave and attendant
forcing shifts east.

Frontal boundary will continue to slide southeast during the day
Tuesday, and settle over southeast Nebraska and southwest Iowa in
the afternoon. GFS and Canadian models show low to mid 60s surface
dew points pooling ahead of front, yielding SBCAPE values over 1500
J/kg by late afternoon. Shear profiles are not impressive, 25-30kt,
so am not expecting widespread severe at this time.

However better severe chances are likely overnight Tuesday night and
then again Wednesday afternoon. Another mid level shortwave is
forecast to move into the Central Rockies overnight, inducing low
level cyclogenesis in the High Plains, leading to strong theta-e
advection over stalled frontal boundary. Impressive elevated
instability and increasing shear is forecast to spread through
eastern Nebraska and western Iowa overnight as southwest mid level
flow develops ahead of approaching trough. Severe hail will be
the main threat given low level stable layer north of front.

Storms should be progressive during the day Wednesday, lifting
northeast of our CWA by early afternoon as shortwave enters our
region and pushes frontal zone north. This puts much of the southern
half of our area in warm/moist sector for the afternoon when GFS
shows SBCAPE over 2500 J/kg and 50kt of bulk shear, generally south
of Interstate 80. Expect storms to fire by late afternoon as surface
convergence increases when frontal boundary sweeps southeast on back
side of exiting shortwave. All modes of severe are possible in this
environment as supercells are likely given large angle between
shear vectors and convective line orientation.

.LONG TERM...(Wednesday night through Sunday)
Issued at 257 AM CDT Mon Apr 17 2017

The potential for strong to severe thunderstorms in southeast
Nebraska and southwest Iowa will likely end Wednesday evening as
front slides southeast. Then attention turns to a more pronounced
trough moving onto the West Coast Thursday which deepens into a
mid level low over the Rockies by Friday morning. After a mainly
dry day Thursday, southwest flow aloft will promote northward
transport of moisture again, sufficient for re-introduction of
precip chances in our south and west on Friday. Chances increase
Friday night and Saturday with low tracking into the Plains, with
decreasing chances Saturday night as system moves east. There are
wide differences between the ECMWF and Canadian/GFS solutions with
former keeping most precipitation to our south with southerly
surface low position, with the latter painting widespread
precipitation over Nebraska and Iowa. With either solution,
highest thunderstorm potential remains well south as we remain on
cooler side of system with broad east or northeast low level flow
keeping moisture limited and temperatures in the 50s or 60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1223 PM CDT Mon Apr 17 2017

VFR conditions initially, but all TAF sites drop to MVFR by the
end of the period, beginning by 11z at KOFK, and 14-15z at
KLNK/KOMA. Thunderstorms could also develop vicinity KOFK 08-11z,
with showers 11-16z. Storms could affect KLNK 10-12z, and unsure
if storms would reach KOMA, thus just mentioned showers at KOMA
11-15z. Some LLWS is also forecast, 07-13z at KOFK and KOMA.

&&

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

$$

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



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