Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 160446
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1146 PM CDT Tue Aug 15 2017

.SHORT TERM...(Tonight through Friday)
Issued at 323 PM CDT Tue Aug 15 2017

Showers and storms are affecting northeast NE, while the rest of the
area waits its turn. Subjective 12Z upper-air analysis indicates
continued rather weak upper-level flow and features, with a ridge
axis from IA through MN, upper-level low centered in northern ON,
and weaker upper-level low over AB, with trough axis extending
through MT/ID/NV/CA. 850mb flow and features also were weak and
poorly defined, especially in the central US, but a pool of 12-16C+
dewpoints was in place from NE/southwest IA southward. Surface low
at 19Z was centered in eastern CO, with something like a warm front
extending from southwest to east central NE/northwest IA, as well as
an inverted trough extending through western SD into western ND.

Main forecast concerns are convective/rain potential tonight and
Wednesday, then after a short respite, active pattern beginning
again on Friday. For the rest of today and this evening, the band of
thunderstorms across northeast NE will continue to lift
northeastward. The band may generate some locally heavy rainfall as
it goes across, with some backbuilding into central NE prolonging
the event. The area is quite dry and sandy, with full summer
vegetation, and should be able to take a lot of rainfall before
flooding becomes a widespread or long-lasting concern. That said,
some local street flooding (especially in towns) and rises on small
streams are possible with this heavy rain, especially during or just
after the rain falls. Environment is supportive of at least some
hail threat, as well, with a smaller risk of gusty winds.

Convection should lift out of the area this evening, with a lull
becoming more likely this evening into tonight before showers/storms
return. Models have trended much slower with bringing the next round
of precipitation into the area, with the eastern CWA possibly not
seeing showers/storms until sometime Wednesday afternoon. Northeast
NE and southeast NE are the two areas that are more likely to see a
return of showers/storms earlier, with the main boundary lingering
near southeast NE and a warm sector wing of precipitation more
closely tied to the surface low crossing KS. Have toned down POPs
this evening and tonight, especially in the east.  Potential for
rain still exists on Wednesday, though instability and thus
strong/severe thunderstorm risk will be lower. Showers/storms could
fester for much of the day across the area. That said, without the
punch of instability aiding convection, the very heavy rainfall
amoutns may be limited, and perhaps the total rainfall will be a
gentler 0.5-1" in many locations.

Rain finally should exit the area by Thursday afternoon/evening,
leaving a break in weather as surface to mid-level high pressure
slides across the central US, with moderating temperatures
continuing into Friday. Chances of showers/storms will return,
though, as a shortwave slides through the upper Midwest and
encounters rich moisture in the central Plains.

.LONG TERM...(Friday night through Tuesday)
Issued at 323 PM CDT Tue Aug 15 2017

Active pattern will continue through the period, as a series of
fairly weak and transient upper-level troughs brings the potential
for showers/storms on just about every day. Timing and location of
any one precipitation event will be challenging, given the low-
confidence, fast-moving upper-level pattern, but we can at least say
with confidence that the period is unlikely to be dry and sunny
across the area. Temperatures should remain a few degrees on either
side of normal (which is mid 80s at this time of August), with daily
temperatures probably depending pretty strongly on effects of
convective cloud debris or lingering showers.

ECLIPSE DAY FORECAST (MONDAY)...

With a generally active pattern in place, the potential remains for
showers/storms to impact the eclipse path. Right now, it still looks
like the better chances are for overnight convection to move across
eastern Nebraska on Sunday night, heading into Iowa on Monday
morning. There is still uncertainty with the timing amd location of
that round of convection, of course, as well as with how long clouds
would linger in its wake. Have again shaved a couple of degrees off
of model guidance and included temperatures dipping downward in the
couple of hours on either side of the eclipse peak, consistent with
research on the effect of an eclipse. The amount of temperature dip
in our forecast remains on the conservative side based on the
potential for clouds and general uncertainty of hourly temperatures
6 days from now. If the day is sunny, the amount of dip (and
lowering of temperatures away from guidance) would be several
degrees more. Also, based on research, have dropped about 2kt from
winds near the peak of the eclipse.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1144 PM CDT Tue Aug 15 2017

Scattered showers and thunderstorms will continue to move across
the TAF sites overnight and even into much of Wednesday morning.
Variable conditions will exist however prevailing VFR to MVFR is
expected.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Flash Flood Watch until 7 AM CDT Wednesday for NEZ011-012-
     016>018-030>032.

IA...None.
&&

$$

SHORT TERM...Mayes
LONG TERM...Mayes
AVIATION...Kern



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