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811
FXUS63 KOAX 300748
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
248 AM CDT Tue Apr 30 2024

.KEY MESSAGES...

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- Showers and storms return this afternoon, with a 15-40% chance
  of severe storms, highest in southeast Nebraska and southwest
  Iowa. Large hail and damaging winds are the primary threats,
  but tornadoes cannot be ruled out.

- Storms redevelop Wednesday and linger into Thursday,
  with potential for a few severe storms and flash flooding.

- An active weather pattern will continue into the weekend,
  though severe weather chances appear low at this time.
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&&

.DISCUSSION...
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Issued at 248 AM CDT Tue Apr 30 2024

Fairly quiet early this morning, though radar was showing some
light returns over northern KS and western NE, associated with a
couple weak bits of shortwave energy sliding through. In
addition, a strengthening low level jet/moisture transport was
leading to increasing clouds over northwest KS, which eventually
should become some showers/storms, with several CAMs suggesting
they eventually move through southeast NE and southwest IA from
around 5 AM through 10 AM this morning. Shouldn`t amount to a
whole lot, but it may be worth keeping an eye out for any
remnant outflow boundaries that may have an impact on later
storms.

Regarding those storms, the forecast largely remains on track
as a shortwave trough is currently making its way across the
Rockies in MT/ID/WY and will eventually push into the forecast
area by this afternoon. At the surface, a low will slide east
through SD and drag an attendant cold front through the forecast
area this afternoon. Said front will be the primary focus for
storm initiation, though questions remain on exact timing/where
the front will be when the storms develop, as the forcing for
ascent associated with the shortwave appears to arrive in
separate "waves." The first will be in SD by mid morning
followed by the second in our area around mid afternoon. Still,
we may not have to wait for the second wave as guidance suggests
the ongoing moisture transport, while remaining modest, will be
persistent throughout the day. With HREF members in good
agreement of 1500-2000 J/kg of SBCAPE with very little CIN
across eastern NE by around 2 PM, thinking we`ll see at least
some storm development along the front around by then, which is
an hour or 2 earlier than several of those members` simulated
reflectivity fields would suggest.

Once storms do develop, storm mode, which remains somewhat of a
question, will play a large role in severe weather hazards.
While shear profiles are certainly sufficient for storm
organization and supercell development and 0-6 km shear vectors
crossing the boundary would suggest a more discrete storm mode,
convergence along the front and increasing forcing for ascent
through the afternoon would suggest more of a linear mode. The
most likely scenario would seem to be initial discrete storms
(some supercells) going up and producing large hail as mid-level
lapse rates look fairly steep, with some potential for
tornadoes as model hodographs suggest some decent curvature in
the low levels. That said, moisture return does look to be
fairly modest with resulting cloud bases remaining somewhat on
the high side, which should limit tornado potential a bit. The
discrete storms would then merge into a line with perhaps a few
breaks and transition to more of a damaging wind threat, with
models showing a somewhat deeply mixed boundary layer/inverted V
sounding with HREF mean DCAPE around 1200 J/kg in east-central
NE into southwest IA. As a result, some significant damaging
wind gusts over 70 mph would be possible, as well as embedded
quick spin up tornadoes. Again, this is the most likely
scenario. If storms remain more discrete for a longer time, the
larger hail threat would persist longer. And if everything
becomes more linear quickly, damaging wind would be the primary
threat. Finally, should also note that the timing of these
storms would impact the afternoon/evening commutes from
work/school, so make sure to check the weather prior to heading
home.

The front and storms should exit by 8-9 PM this evening with
the front eventually stalling out somewhere across east-central
KS toward the KS/OK border. A larger scale trough will then
begin to deepen over the western CONUS while a surface low spins
up over southeast CO. Southerly flow ahead of the low will
start to push the front back north as a warm front, with
moisture transport continuously pointing into it and leading to
shower and storm development through the day on Wednesday. Said
precip, depending how heavy, could work to keep the front to our
south, and while model consensus has trended toward keeping it
farther south, a few pieces of guidance suggest it still could
get to around the NE/KS border. Should this occur, we would see
another severe weather threat Wednesday afternoon into the
evening in the vicinity of the front with all hazards on the
table. Notably, strong moisture transport will continue to point
into the front overnight Wednesday and into early Thursday
before the surface lows slides through the area and brings a
cold front through. There would be some threat of strong to
isolated severe storms with hail and gusty winds near the cold
front, but instability looks to be rather limited with ongoing
precipitation ahead of the low. The larger threat may end up
being flooding with repeated rounds of potentially heavy rain,
owing to precipitable water values in the 1.25 to 1.5 inch range
and the aforementioned moisture transport pointing into the
warm front.

Beyond Thursday, the pattern remains fairly active with
guidance in pretty good agreement of another shortwave trough
moving through sometime Friday night into Saturday and bringing
more rain chances. It`s still early, but the severe weather
threat looks low with this one, with a general lack of
instability and moisture. There are signs of an additional
system sometime Sunday/Monday, but confidence is quite low with
a lot of spread in various ensembles. Otherwise, expect most
days over the next week to see high temperatures in the mid 60s
and/or mid 70s, outside of today, which will be a bit warmer
(mid 70s to mid 80s).
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&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1042 PM CDT Mon Apr 29 2024

VFR conditions are expected at all TAF sites through 11Z. After
11Z, isolated to widely scattered showers and thunderstorms may
develop across southeast Nebraska (KLNK and KOMA). As such have
included a TEMPO group for this morning convection. Morning
convection should weaken and move east of the region by 15Z. After
19Z, an additional round of thunderstorms, some of which may be
severe, will develop across northeast Nebraska and expand
southeastward through 04Z. Thunderstorm conditions are not
expected to last long enough to be prevailing, so will
necessitate the inclusion of a TEMPO group in the next TAF
issuance.

&&

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

&&

$$

DISCUSSION...CA
AVIATION...Darrah