-- Discussion containing changed information from previous version are highlighted. --
898
FXUS63 KOAX 181035
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
535 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

- There is a 20-30% chance of storms late this afternoon into
  this evening, with a 5-15% chance of severe storms. Hail and
  damaging winds will be the primary threats, but a tornado
  cannot be ruled out (2% chance).

- Additional isolated severe storms will be possible (5% chance)
  in far southeast NE and far southwest IA Thursday afternoon
  and evening.

- Widespread rain chances arrive Friday evening and last into
  Monday, peaking Saturday into Sunday morning at 50-80%. Expect
  a widespread 1.5 to 2.5 inches of rainfall with this system.

- Temperatures remain above average through the remainder of
  this week, with highs in the 80s to lower 90s. Cooler
  conditions are anticipated with and after the rain this
  weekend, with highs in the 60s and 70s from Sunday into next
  week.

&&

.DISCUSSION...
Issued at 348 AM CDT Wed Sep 18 2024

The main story early this morning was a band of showers and
storms associated with low level moisture transport and some
shortwave energy pushing east toward the area. Additionally, an
associated outflow boundary was out in front firing some more
storms in northeast NE. These could produce some small hail and
gusty winds through the early morning hours, but severe weather
is not expected. Guidance is in good agreement that the main
band of storms will weaken as it pushes into our area, as
moisture transport weakens through the morning. Still, could see
a few light showers push toward and maybe slightly east of the
MO River by late morning/early afternoon. Otherwise, despite
having associated clouds to contend with, we`ll still warm up
into the mid to upper 80s and possibly even 90 with decently
strong southerly flow allowing us to do some good mixing.

Attention then turns to strong to isolated severe storm chances
late this afternoon through the evening. A cutoff low currently
over the MT/WY border will continue pushing northeast today
with a surface boundary sliding east into the area this
afternoon. Ahead of this, dewpoints are progged to be in the mid
to upper 60s with HREF mean SBCAPE approaching 2000 J/kg. Shear
will be somewhat limited, but latest RAP guidance suggests 25
to 35 kts of 0-6 km shear edging into northeast NE. As a low
level jet ramps up late this afternoon into this evening, low
level shear will also increase, and resulting moisture transport
should allow for storm development in vicinity of the boundary.
There is some degree of low level hodograph curvature that
could yield a small (2%) tornado chance, but the primary severe
weather concerns will be hail and damaging winds, especially in
northeast NE where deep layer shear will be strongest.

The severe weather threat should taper off by 10 PM or so, but
like past nights, expect at least spotty showers and storms to
persist into the early morning hours Thursday with moisture
transport continuing to point into the area. Clouds should then
clear out by mid to late morning while a surface cool front
starts to push through the area. Areas that remain ahead of the
front through much of the day (likely southeast NE and far
southwest IA) will see continued southwesterly flow, allowing
temperatures to climb into the lower 90s, while areas behind the
front "only" top out in the mid 80s. So quite warm for this
time of year, but still several degrees short of record highs
(97 for Omaha, 100 for Lincoln). Once again, expect some storm
development in vicinity of the passing front by late
afternoon/evening with decent instability progged to be in place
ahead of it. Strongest shear looks to remain behind the front,
but still could be just enough for storm organization, yielding
another strong to isolated severe storm threat. The big question
will be how far west/east storms go up, as a few pieces of
guidance suggest they could go up east of us and we end up with
nothing. As it stands, we`re looking at a 15-20% chance of
storms in far southeast NE into southwest IA and a 5% chance of
severe storms in those areas. Hail and damaging winds will once
again be the primary threats, but a tornado cannot be completely
ruled out.

Then heading into Friday and the weekend, guidance continues to
trend slower with a potent shortwave trough ejecting out of the
Desert Southwest with an attendant surface low spinning up,
pushing through KS, and bringing us widespread rain. Latest
consensus only gives about a 15-25% chance of storms Friday
evening, and anything that does develop would likely hold off
until 10 PM or after when moisture transport starts to ramp up.
Shower and storm chances quickly climb after that with 40 to 70%
chances in place across the area through the day on Saturday
and Sunday, highest at any given time near and south of I-80.
Still some questions on exact track and timing of the system,
but given its strength, will need to keep an eye on potential
for severe weather. Instability will likely be quite limited
with persistent rainfall/cloudcover, but if we get any breaks we
could see a bit of a threat as the low pushes through (likely
on Sunday). Also of note, current forecast calls for a
widespread 1.5" to 2.5" of rainfall Friday evening through
Sunday night. Locally higher amounts will certainly be possible,
especially if we`re able to develop some instability. For what
it`s worth, guidance suggests a 60 to 80% chance of at least 1"
and a 40 to 60% chance of at least 2". So overall, relatively
high confidence we`ll get some decent rain out of this system.

Some precip could linger into Monday, especially if the slowing
trend continues. As the system passes through and on the back
side, we`ll see much cooler and more fall-like temperatures,
with highs in the 60s and 70s Sunday through at least the middle
of next week. We could also see additional spotty light precip
through Wednesday as we remain under the influence of cyclonic
flow aloft, though surface high pressure should limit that
potential quite a bit.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
-- Changed Discussion --
Issued at 528 AM CDT Wed Sep 18 2024

Southerly winds at 10-15 knots, strengthening to 15-20 knots by
17Z will prevail at all TAF sites for the duration of the
forecast period. Wind gusts of 25 to 30 knots will be possible,
particularly across northeast Nebraska (KOFK) between 17Z and
00Z. There is a conditional threat for isolated thunderstorms
across portions of northeast Nebraska (KOFK, 20 to 30 percent
chance of rain). Have put in a VCSH TEMPO group to highlight the
time of greatest storm potential. A few storms may be strong to
severe with wind gusts up to 50 knots and hail to the size of
quarters. A stray shower or thunderstorm will be possible across
the rest of the forecast are this evening, primarily after 23Z.
Chances are too low to include in TAFs at this time (10 to 20
percent chance of rain).
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...CA
AVIATION...Darrah