Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 161712
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1212 PM CDT Sat Sep 16 2017

.SHORT TERM...(Today through Monday)
Issued at 337 AM CDT Sat Sep 16 2017

Early Saturday morning, an area of widespread thunderstorms has
steadily progressed SW to NE across much of the forecast area, but
as of 4 AM will be almost entirely east of the area. Meanwhile, a
strong surface cold front appeared to extend from near Columbus to
West Point to just south of Sioux City. Additional scattered
activity is expected to continue developing through sunrise
though, mainly focused over southeast NE and southwest IA, driven
by LLJ-based ascent into a modestly unstable elevated airmass. May
also see an uptick in elevated shower and thunderstorm activity
closer to sunrise in northwest parts of the forecast area driven
by larger- scale ascent associated with a short wave trough moving
into north central NE. Any convection in this region will be well
behind the surface cold front and probably not particularly
strong as the steepest lapse rates associated with the center of
the wave will remain west of the forecast area.

Through early afternoon, the forecast is focused on the
aforementioned north central NE energy as well as as train of weak
vorticity maxima lined up from north TX to southwest IA at 4 AM.
While the cold front will continue to progress southeast, it will
do so in bursts with the passage of troughs and related influxes
of cold advection. Current indications are that these weak
disturbances will drive relatively small pockets of convective
development amidst the elevated pre-frontal warm advection
regime focused near and south of I-80, which will quickly move to
the northeast. Instability is limited and storms in this region
should remain elevated, rooted around 12000 feet, with the
primary hazard being cloud to ground lightning. North of the
front, there are indications of somewhat steeper lapse rates aloft
but forecast soundings also indicate that elevated parcels become
capped due to an incoming warm EML.

By late afternoon, pre-frontal airmass will become rather unstable
with surface based thunderstorms likely to develop immediately
along the cold front. The combination of moderate instability and
around 30 kts of deep layer shear should allow for some storm
organization with an attendant hail/wind threat, but magnitude of
forcing along the front suggests that storms will quickly form
into a solid line with updraft-downdraft interference limiting
overall severe threat. The timing of the front and convective
initiation will be key in identifying locations at risk for severe
storms, but currently the focus appears to be southeast of a line
from Beatrice to Nebraska City to Red Oak. While the front will
quickly surge southeast this evening, have maintained fairly high
pops in southeast NE and southwest IA well into the evening as it
appears uncapped elevated parcels will exist for parcels
immediately above the elevated front and if enough convergence
develops in this area could see another band of thunderstorms with
some low-end potential for hail. All precip will end during the
early morning hours.

Temperatures behind the front will be MUCH cooler today with highs
only in the 60s for much of the forecast area and winds gusting
out of the north at 20-30 mph at times. Lows then fall into the
40s over northeast NE by Sunday morning.

On Sunday night, the front lifts back north in advance of an
incoming short wave trough with widespread isentropic ascent and
LLJ convergence helping to force widespread thunderstorms. Could
see localized heavy rain as the LLJ veers given strong positive
moisture flux into the region and potential for some training.

.LONG TERM...(Monday night through Friday)
Issued at 337 AM CDT Sat Sep 16 2017

The weather pattern remains active through the coming week with
one strong short wave trough expected to become negatively tilted
as it crosses the Dakotas on Tuesday. Flow aloft will be quite
strong, and depending on cap strength and frontal timing, could
see a severe weather threat develop especially in northern parts
of the forecast area late Tuesday into Tuesday night. A long wave
trough will then deepen over the western CONUS by mid to late week
with substantial poleward moisture flux across the Plains. This
will lead to additional chances for storms in an unstable and
increasingly moist environment by the end of the week into next
weekend. Throughout this period, temperatures will be near normal,
although a few days will push warmer into the 80s with warm
advection in advance of the stronger troughs.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1212 PM CDT Sat Sep 16 2017

17Z Surface observations and radar data indicate the surface cold
front from just west of KHNR to south of a KOMA to KLNK line.
While the majority of convective precipitation has temporarily
shifted to the southeast of the TAF sites, a band of IFR ceilings
present immediately behind the front will impact KOMA and KLNK
before lifting to MVFR this afternoon. Winds will steadily
increase from the north this afternoon before diminishing by
evening. Latest short-term model guidance suggests that shower and
thunderstorm development will become increasingly likely by about
00z along and south of a KOMA to KLNK line with this activity
persisting through the evening hours. Expect the MVFR ceiling to
move to the southeast of the area tonight with prevailing VFR
conditions thereafter.

&&

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

$$

SHORT TERM...Barjenbruch
LONG TERM...Barjenbruch
AVIATION...Mead



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