Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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309
FXUS63 KOAX 202357
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
657 PM CDT Sun Aug 20 2017

.SHORT TERM...(Tonight through Wednesday)
Issued at 310 PM CDT Sun Aug 20 2017

Main forecast issues are thunderstorm chances through Monday night
and eclipse forecast specifics, especially sky cover.

Persistent focus of lower level convergence with low level jet and
thunderstorm activity in southeast Nebraska into southwest Iowa
was decreasing at mid afternoon. Scattered storms continued to
develop and push east across parts of western IA. 3 pm surface
analysis indicated a cold front curving southwest from near the
WI/MN border area into northwest IA, then the boundary was nearly
stationary and extended back into eastern CO. A secondary front, a
warm front, seemed to stretch east from eastern CO into northeast
KS and that was at least partially associated with outflow from
storms last night into this morning. South of that boundary,
temperatures had warmed well into the 90s across parts of central
and western KS. The question becomes where will convection be most
prevalent tonight. One other thing of note was the moisture feed
from Mexico that extended up into CO and parts of KS, seem in
water vapor satellite imagery.

Thermal ridge at 700 mb was to our west, with 12Z plot showing
+15C at KDEN. Weak height rises were occurring across the region
and there did not seem to be any strong impulses tracking toward
our area.

The strongest signal for convection tonight is from southeast SD,
northeast NE and southwest MN into IA. That seems to make sense
based on where the nose of the low level jet will provide the
strongest forcing, especially if the surface pattern turns out to
verify closed to the NAM. Used a blend of the convection allowing
models (CAMs) to draw POPs for tonight. Storms that occur will
have some potential to be severe. Raised lows slightly from the
previous forecast.

--Eclipse Outlook for Monday--

Precipitation chances in the totality area (and even for areas
farther to the north) look to be mainly 20 percent or less (but
more like 25-30 percent at the SD border). We have the sky cover
percent forecast mainly 55 to 70 percent. It appears this would be
mostly high level moisture (but some mid clouds too). It is often
difficult to forecast just how dense the high clouds will be. For
now, let`s say that conditions are far from ideal for viewing in
our area, but not to the point where the sun would not be visible
at all. Highs should reach mainly lower 80s (possibly mid 80s)
north, and mid to upper 80s south. If the warmer air is able to
mix out a bit more, areas near the KS border could hit lower 90s.

Highs should be held down slightly due to less insolation for the
day, so reduced highs a bit from MOS guidance. By late afternoon,
there could be two areas of showers and storms. One from northern
IA back into northern NE and another starting from KS toward
southeast NE. Have POPs going up into the 50 to 70 percent range
for most of the area Monday night. Strongest focus for excessive
rain is from northeast KS into northern MO and southern IA, but
we could see some locally heavy rain in parts of our area. This
would be shifting out of our area by mid morning Tuesday.

Cooler and mainly dry conditions should occur from Tuesday
afternoon into Wednesday. There are weak signals that there could
be some light precipitation with return flow and warm air
advection Wednesday for parts of northeast NE. Look for highs
Tuesday 75 to 80 north and 80 to 85 south.

.LONG TERM...(Wednesday night through Sunday)
Issued at 310 PM CDT Sun Aug 20 2017

At the start of this period, a 500 mb ridge should extend from NM
into Alberta and Saskatchewan, with a trough off the coast of
southwest Canada and the Pacific Northwest. With time, the pattern
deamplifies and then on the weekend a 500 mb anticyclone
strengthens from the Four Corners region back into NV and CA.

Will have some low POPs Wednesday evening, then look for mainly
dry weather later Wednesday night into Friday morning. Pattern
looks a bit more unsettled then with slight chance to low end
chance type POPs. Temperatures should average a little below
normal.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 619 PM CDT Sun Aug 20 2017

Main concern is possible TSRA developing later this evening and
carrying over into late tonight. Latest models are indicating
convective development over NERN Neb shortly before midnight on
the nose of increasing low level jet...then spread into w-cntrl IA
after midnight. Believe KOFK will be more at risk to see TSRA
activity...with a slight chance at KOMA. Otherwise...VFR
conditions are expected thru the remainder of the fcst pd.

&&

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

$$

SHORT TERM...Miller
LONG TERM...Miller
AVIATION...DEE



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