Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 212059
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
359 PM CDT Thu Sep 21 2017

.SHORT TERM...(Tonight through Saturday evening)
Issued at 242 PM CDT Thu Sep 21 2017

The short term through Saturday will be very warm, windy, humid, and
precipitation-free. Temperatures across the area this afternoon
were in the upper 80s to lower 90s with dewpoints generally in
the low 70s while winds gusted to 30 mph. A long wave trough axis
from near San Fransisco into western Canada will progress slowly
but steadily east over the next few days while multiple nodes of
enhanced short wavelength energy and associated jet streaks wrap
around and through the long wave pattern. Model guidance also
indicates that some elements of eastern Pacific/Baja moisture will
be pulled into the plains over the next few days. However,
through Saturday, the result will be persistent surface low
pressure over the high plains. This will maintain southerly low
level flow into the local area with a persistent fetch of Gulf of
Mexico moisture keeping dewpoints around the 70 degree mark.
Overnight winds will aid in boundary layer mixing, keeping
overnight lows quite warm (possibly near record warm values),
while mostly sunny skies and continued warm advection through
Saturday will allow for temperatures up into the 90s once again
while the heat index approaches 100. By late in the day on
Saturday, expect moderate instability to develop in the frontal
zone over north central Nebraska. Afternoon heating in conjunction
with a stream of weak short wave troughs will likely be
sufficient to force thunderstorms along the front by late in the
day. The question will be how far east these storms progress as
the front is expected to move very little and front- parallel flow
will maintain a predominantly northeast storm motion. Have
included low end pops into far NW parts of the forecast area for
the potential for a few storms to clip the area during the
evening.

.LONG TERM...(Sunday through Thursday)
Issued at 242 PM CDT Thu Sep 21 2017

From Saturday night through Monday, the surface front and forcing
aloft will make a very slow progression across the forecast area.
In fact, the surface cold front is unlikely to push all the way
through the forecast area until late on Tuesday. This slow
progression is largely due to the slow eastward motion of the
long-wave trough which maintains deep front-parallel flow in the
mid and upper levels. The general frontal zone is also likely to
remain in the vicinity of a steady stream of short wave troughs
moving through the fast flow aloft. These individual waves, with
their enhanced convergence, ascent, and cooling aloft, will
support periods of enhanced precipitation coverage and rates.
Precipitable water values will frequently be in the 1.5-1.8 range
which is quite high for this time of year, and the warm cloud
depth and freezing levels are climatologically quite high as well.
This, along with the rather long duration of the event and
presence of modest deep instability through much of the period
suggest that heavy rainfall will be likely in parts of the area.
Exact amounts will depend highly on periods of convective
enhancement and training, but the most favored area for heavy
rain is likely to be focused in north central Nebraska where some
locations will likely receive more than 3 inches and possibly a
fair amount more. Farther southeast, the majority of rain in far
southeast NE and southwest IA may hold off until Sunday night or
Monday while north central NE will likely experience off-and-on
thunderstorms for a majority of the period Saturday into Monday.

All told, severe storm potential is quite low. There are a few
periods for which the instability/shear combination may be
sufficient to support storm organization, but more frequently
expect heavy rain to be the primary focus through this event.

Late Tuesday through the remainder of the week looks to be
predominantly dry with a return to split flow aloft and near-
normal temperatures

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1218 PM CDT Thu Sep 21 2017

VFR conditions will prevail thru the fcst pd. However, significant
LLWS is likely at all terminals later tonight.


&&

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

$$

SHORT TERM...Barjenbruch
LONG TERM...Barjenbruch
AVIATION...DEE


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