-- Discussion containing changed information from previous version are highlighted. --
845
FXUS63 KOAX 210520
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1220 AM CDT Tue May 21 2024

.KEY MESSAGES...

- A stray thunderstorm or two will be possible (15 to 25
  percent) across southeast Nebraska and southwest Iowa. Small
  hail and gusty winds will be possible.

- Several rounds of strong to severe thunderstorms will be
  possible beginning tonight at 8 PM and lasting as late as 5
 PM tomorrow afternoon.

- There will be additional rain chances beginning Thursday night
  and lasting through this upcoming weekend.

&&

.DISCUSSION...
Issued at 157 PM CDT Mon May 20 2024

     Today through Tomorrow Night...

With much less cloud cover today compared to yesterday, high
temperatures across the region are expected to rise into the low
80s, with upper 70s for portions of northeast Nebraska. At the
surface, a frontal zone extends northeast from south-central
Nebraska into the Sioux City area. South and east of this area,
surface moisture has increased substantially from yesterday,
with dew points largely in the mid 60s. On the other side of
this boundary, dew points have hovered around or just under 60
degrees. As such, a broad moderately unstable airmass with
2000-2500 J/kg of SBCAPE is expected to develop south and east
of this boundary. However, due to a capping inversion and weak
forcing for ascent during the afternoon hours, the probability
for storm development is low, somewhere between 10 and 20
percent. For the stray thunderstorm that is able to develop, it
will be sometime between 4 and 8 PM, with small hail, lightning
and heavy rain being the main threat.

After 8 PM, stronger forcing for ascent will move into the area
as a longwave trough moves east out into the central Plains.
Scattered thunderstorms are expected to develop along the
frontal zone. The preferred zone for initial thunderstorm
development will be north of I-80 and south of US-20. As
the evening progresses, a speed max is expected to eject out
into the central Plains. This will allow bulk shear over our
area to increase from approximately 20 knots this evening to
near 70 knots by Tuesday morning. In response to this
disturbance moving into the Plains, a surface will develop
somewhere in southwest Kansas before tracking northeast into our
area. As this surface low deepens, strong advection out of the
south will transport boundary layer moisture and keep
temperatures relatively warm into the overnight hours south of
any ongoing convection. This will keep the airmass immediately
south of these storms modestly unstable, with MLCAPE likely
ranging from 1000 to 2000 J/kg. Furthermore, with the
development of the low-level jet, low-level hodographs are
expected to enlarge with 0-1 kilometer SRH on the order of 200
to 400 m2/s2. Height falls overspreading the Plains will also
help keep lapse rates relatively steep in mid-levels, with lapse
rates on the order of 7 to 9 C/km. The final piece of the
puzzle worth mentioning is a cold pool will develop in response
to convection that develops late this evening and tonight,
reinforcing and enhancing the frontal zone draped across our
area this afternoon. This frontal zone/outflow boundary will
act as the preferred area for convection to develop and track
across tonight.

Pulling all of this together, tonight`s set up favors training
strong to severe thunderstorms through 7 am tomorrow morning.
The individual thunderstorms themselves will pose a risk for all
modes of severe weather. The tornado risk will be greatest in
storms that are able to stay surface based along and/or just
south of wherever the outflow boundary sets up. It is also
important to note that the hail and damaging wind risk will
extend north of this frontal zone/outflow boundary area. The
greatest threat for severe weather will be between 10 PM and 5
AM tonight. Lastly, with the tendency for these storms to train
over the same areas, flash flooding will also be of concern
tonight. The time of greatest concern for flash flooding will be
between 12 AM and 7 AM.


Tomorrow morning, there will be a brief window of time where
rain/storm coverage will be minimal. The most likely possibility
will be lingering showers and isolated thunderstorms across
west-central Iowa and far northeast Nebraska. At the surface,
the primary surface low is expected to track northeast from
south-central Nebraska to northwest Iowa. South of this low
pressure, a cold front is expected to drape to the south-
southwest and move east across our area later in the morning and
through the early afternoon hours. The airmass ahead of this
cold front is expected to destabilize quickly through the late
morning and early afternoon hours, with MLCAPE potentially
exceeding 2500 J/kg by noon in southwest Iowa. Surface based
instability will remain south of the outflow boundary set by
storms the night prior. In response to strong surface
cyclogenesis, a strong southerly jet in low levels will help
advect this boundary to the north. Low-level hodographs are also
expected to enlarge in response, increasing 0-1km SRH up to 200
m2/s2. Bulk shear by noon tomorrow will likely be on the order
of 60 to 80 knots as the jet max aloft nudges over eastern
Nebraska. Strong forcing for ascent near and just south of the
low pressure center will likely initiate convection on the cold
front sometime between 10 AM and noon in central Nebraska. As
the cold front sweeps east, strong to severe thunderstorms will
track northeast into northeast Nebraska as additional
thunderstorms should develop south along the front to the I-80
corridor by 2 PM. All modes of severe weather will be possible
with this round of storms as well.

There are two points of uncertainty that have yet to be ironed
out. One is the extent at which convection develops and persists
tonight. This has contributed to additional uncertainty in
another area of the forecast which is the exact track of the
surface low on Tuesday. If storms are stronger and more
widespread tonight, that would tend to have the low center
track further south, likely down the I-80 corridor. Should the
storms be less widespread or weaker overnight tonight, the
surface low would likely track closer to a line from Hastings to
Sioux City. The greatest severe risk will track along and just
south of wherever the surface low is. The general consensus is
to have storms push east of our area no later than 5 PM
tomorrow evening. With high soil saturation from the storms
tonight, flooding will continue to be a risk with this second
round of storms on Tuesday, particularly across northeast
Nebraska and west-central Iowa.


     Wednesday through Sunday Night...

Warm and dry conditions are on tap for the region Wednesday,
with highs in the 70s, dew points in the 40s and clear skies.
Temperatures will warm up into the upper 70s and low 80s for
Thursday, still likely keeping things dry through the afternoon
hours. Our next chance for rain comes late Thursday night as a
disturbance initiates a cluster of storms in the northern High
Plains. These showers and storms should move east/southeast
across our area into Friday morning. The potential for severe
weather appears low at this time, though gusty winds and small
hail are not out of the question. Additional rain chances will
be possible through this upcoming weekend, though model
differences in the evolution of these disturbances precludes any
discussion on exact timing and spatial extent of these rain
chances.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1211 AM CDT Tue May 21 2024

Scattered showers and storms are present across the area and
will persist through the overnight hours. MVFR ceilings and
visibilities (with a possibility of IFR) will be possible in
areas with heavy showers. MVFR ceilings will be present at all
terminals through the overnight period. A secondary line of
showers and thunderstorms (with potential for a strong segment
of bowing winds) will go through the terminals in the 10-14Z
timeframe. Winds will remain northeasterly in northeast NE and
southeasterly in southeast NE tonight before shifting clockwise
to northwesterly through the day. Winds over 12 kts will be
possible in areas with strong thunderstorm outflow.

A lull in convection is expected in the 15-17Z timeframe before
another line of showers and thunderstorms moves through the
area. Confidence is low in these storms initiating far enough
west to reach the terminals, it will be monitored for future TAF
issuances. Any potentials storms will be in the 18-21Z
timeframe. VFR conditions will return in the afternoon with
strong northwesterly winds through the remainder of the forecast
period.
-- End Changed Discussion --

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
NE...Flood Watch through this evening for NEZ015-033-034-042>045-
     050>053-065>068-078.
IA...Flood Watch through this evening for IAZ043-055-056-069-079-
     080-090-091.
-- End Changed Discussion --

&&

$$

DISCUSSION...Darrah
AVIATION...Wood