Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 142338
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
638 PM CDT Thu Sep 14 2017

.SHORT TERM...(Tonight through Sunday)
Issued at 336 PM CDT Thu Sep 14 2017

Cold front sweeping through the area Saturday, with warm temps ahead
and cooler temps behind the front one highlight of the forecast.
Several sporadic thunderstorms chances through Monday are also a
forecast concern.

Broad upper trough axis extended from Hudson Bay into the Pacific
Northwest this afternoon, with weak ridging noted from eastern
Nebraska into Mexico. Significant shortwave was noted near Las
Vegas, which is forecast to eject northeast into the Northern Plains
Friday afternoon. In its wake, trough will build south across much
of the Western U.S. placing our area under southwest flow aloft
regime for the weekend. During that time, a large chunk of energy
will rotate east and northeast from core of western trough, forcing
a cold front through the Central Plains on Saturday. Then persistent
west-southwest flow will emerge for early next week.

Currently at the surface, a weak frontal boundary was noted from
eastern South Dakota into southwest Nebraska. That boundary is not
expected to move much overnight as upper flow nearly parallels it.
Modest theta-e advection on increasing low level jet ahead front
will likely spark scattered elevated convection tonight, favoring
areas just to the west of our CWA, and may affect northeast Nebraska
after midnight. Then much of Friday should be dry and warm as
southerly flow and warm mid level temps suggest highs will top 90 in
much of the area.

Effects of shortwave moving into the Northern Plains on Saturday
will begin Friday night as even stronger low level jet is induced
and cold front begins moving into northeast Nebraska. Most
convection should be confined to near the front, but broad theta-e
advection zone could trigger an isolated thunderstorm just about
anywhere in eastern Nebraska and southwest Iowa late Friday night.
In fact most synoptic-scale model output shows QPF near and east of
the Lincoln and Omaha areas a few hours either side of 12Z Saturday.
Any morning convection will likely be short-lived, pushing to the
northeast rather quickly, with another potential round setting up in
the afternoon as front advances southeast. Timing of that convection
appears to favor after 3 pm when convective temperatures are
reached, generally along and south of Interstate 80. GFS shows over
2000 J/kg of MLCAPE and 30kt of bulk shear during this period,
suggesting some risk for storm organization leading to potential
severe weather.

That risk should wane overnight as frontal boundary slides southeast
of our area, then leaving much of Sunday relatively dry. However
frontal boundary is expected to lift back north rather quickly
Sunday afternoon as persistent southwest flow aloft lowers surface
pressures in the northern High Plains. Thus there is a chance for
more thunderstorms later Sunday afternoon in our south, but more
likely Sunday night.

After warm temperatures near or above 90 on Friday, cooling behind
Saturday front will be realized Saturday afternoon in our north
where temps will struggle to reach 70 in far northeast Nebraska.
Later afternoon arrival of front should allow temps to rebound back
into the mid and upper 80s in our south. All of eastern Nebraska and
southwest Iowa will see highs in the 70s for Sunday.

.LONG TERM...(Sunday night through Thursday)
Issued at 336 PM CDT Thu Sep 14 2017

As cold front lifts back north as a warm front Sunday night and
Monday, widespread showers and thunderstorms appear likely between
midnight and noon Monday. Fairly high consistency between all models
suggests more than an inch of rain is possible, with some chance for
elevated severe hail storms given 1500 J/kg of MLCAPE.

Southwest flow aloft is forecast to continue in our region much of
next week, however models begin to diverge by Tuesday night and
Wednesday on amount of energy ejecting through the Plains, and thus
many aspects of sensible weather for eastern Nebraska and southwest
Iowa. ECMWF and Canadian are similar in reforming western U.S.
trough and maintaining its general position well to our west through
Thursday. GFS has the idea to keep more energy in the northern
Rockies and Alberta, then ejecting a strong shortwave into the
Northern Plains which slams a cold front across our area on
Wednesday. The EC/Canadian solutions keep frontal system to our west
through Thursday, thus warmer with much reduced precipitation
potential.

Our going forecast, and what is going out today, favors the
EC/Canadian solutions, keeping our are in warm sector of frontal
system through Thursday, and thus lower precip potential and warmer
temperatures.

However we will still have chances for thunderstorms through the
period as theta-e advection persists on lower atmospheric southerly
flow regime. Periodic impulses ejecting into the region will enhance
chances, most notably Monday night and Tuesday night when low level
jet increases help drive convection.

Otherwise will keep temperatures generally in the low to mid 80s
Tuesday through Thursday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 615 PM CDT Thu Sep 14 2017

VFR conditions through the period. LLWS could develop at all TAF
sites 04-14z, with winds up around 2000 35-40 knots. Could also
see a few showers in the vicinity of KOFK 07-14z as well. Otherwise,
gusty south winds redevelop at all sites 14z and beyond.

&&

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

$$

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



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