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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KOAX 150919
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
419 AM CDT Fri Sep 15 2017

.SHORT TERM...
-- Changed Discussion --
(Today through Saturday)
Issued at 351 AM CDT Fri Sep 15 2017

Early Friday morning, a complex of thunderstorms over north
central Nebraska was associated with a well-defined mesoscale
circulation. This was one of several small-scale embedded short
wave circulations in the region with others in far north Texas,
eastern Colorado, and a compact but well-organized short wave
trough spinning into southern Wyoming this morning and helping to
steer the other weaker impulses. Additionally this morning, a zone
of broad isentropic ascent was focusing vertical motion into a
weakly unstable elevated layer over western Iowa, supporting
showers and a few thunderstorms early this morning. The focus for
the western Iowa precipitation is expected to shift northeast
early this morning while a more focused zone of isentropic ascent
and convergence along the nose of the LLJ is expected to shift
into eastern Nebraska in association with the aforementioned short
wave trough by sunrise. This feature should be supportive of
scattered shower and thunderstorm development mainly between 4-10
AM, although some convective precip could linger into the early
afternoon hours. There is also some potential for the energy
coming out of north Texas to provide a zone of focused ascent as
it moves across the forecast area early this afternoon. It seems
any precip in this time frame would also be rooted aloft with
minimal effective instability to work with.

By mid to late afternoon, diurnal heating in advance of a cold
front over far northern Nebraska will contribute to a moderately
unstable airmass with effective wind shear in the 25-35 kt range.
Model guidance also shows (and the environment would support) a
surge of deep mixing from southwest to northeast just ahead of
the front by late afternoon and expect thunderstorms to develop on
the interface of enhanced low level convergence, strong heating,
and deep mixing probably in the 3-5 PM time window. The
combination of shear and instability may be sufficient to support
some storm organization, which in combination with moderate lapse
rates could support a large hail threat. Perhaps a bit more
interesting is the potential for a few locally strong downbursts
given the expectation for a rather dry sub-cloud layer with high
LCL/LFC combinations, steep low-level lapse rates, and a rather
moist profile above the LFC. In the end, the overall severe threat
will be low, but have at least some potential for a few severe
storms mainly in northeast Nebraska. Storms may gradually spread
southeast but will probably struggle until another weak short wave
trough tracks into southeast Nebraska overnight, probably
providing enough additional lift to increase storm coverage.

Aside from storm potential, it will be hot and windy today.
Temperatures will depend somewhat on the extent of elevated
convective cloud cover during the day, and expect to be a few
degrees cooler than Thursday, but still in the upper 80s to low
90s for most of the area. The cold front will likely stall
overnight in northeast Nebraska, keeping a majority of the
forecast area warm and breezy overnight.

On Saturday, the front will again begin making its way across the
area and expect another round of strong storm development mainly
over southeast Nebraska and southwest Iowa by peak heating on
Saturday afternoon. Shear will once again be on the weak side but
storms should have more access to better boundary layer moisture,
and could again see a few strong to severe storms on Saturday
afternoon and early evening before the front pushes all the way
through.


.LONG TERM...(Saturday night through Thursday)
Issued at 351 AM CDT Fri Sep 15 2017

Sunday will be mostly dry and pleasant in the post-frontal
airmass, but the active weather pattern will remain in place
through the rest of the forecast period with fairly fast southwest
flow aloft and occasional short wave energy crossing the Plains.
There is strong model agreement that one particular short wave
trough will interact with the retreating surface front Sunday
night into early Monday, bringing a good chance for widespread
storms to the area. Model guidance diverges a bit beyond Tuesday,
mainly lending uncertainty to the timing of specific precipitation
chances, but at times with indications neutral to negatively
tilted trough passages suggestive of thunderstorms with potential
for a round or two of strong thunderstorms through next week.
Expect fluctuating temperatures with a pair of frontal passages,
while generally windier conditions can be expected as well given
the likelihood of a persistent surface pressure gradient across
the central Plains.
-- End Changed Discussion --

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday evening)
Issued at 1158 PM CDT Thu Sep 14 2017

VFR conditions through the period. LLWS at all TAF sites 04-14z,
with winds up around 2000` around 35-40 knots. Could also see a
few thundershowers in the vicinity of KOFK 09-14z as well.
Otherwise, gusty south winds redevelop at all sites 14z and
beyond. Could see additional convection at all TAF sites later in
the TAF period, but too far out to predict with certainty.


&&

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

$$

SHORT TERM...Barjenbruch
LONG TERM...Barjenbruch
AVIATION...DeWald