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494
FXUS63 KOAX 182321
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
621 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Several rounds of strong to severe thunderstorms will be
  possible across our region tomorrow through Tuesday afternoon.
  All modes of severe weather including flash flooding will be
  possible.

- There will be another chance for thunderstorms Thursday night.
  Overall risk of severe weather appears low at this time.

&&

.DISCUSSION...
-- Changed Discussion --
Issued at 159 PM CDT Sat May 18 2024

     Today through Tomorrow Night...

A warm and dry air mass will be in place this afternoon in
association with a high pressure that has taken residence across
our area. Clear skies and high temperatures in the upper 70s to
mid 80s will be on tap for us before temperatures start cooling
after sundown this evening.

Tonight will begin a prolonged period of rain and thunderstorm
chances, lasting through Tuesday afternoon. Aloft, models have
consistently resolved a deepening longwave trough over the
western CONUS. While this main upper-level disturbance is not
expected to eject out into the Plains until later Monday night,
a series of subtle shortwaves are anticipated to eject out into
our area downstream of this feature in the days prior.

The first in this series of disturbances is going to be a low-amplitude
negatively tilted shortwave that should kick out into the
central High Plains by 7 PM this evening. Scattered showers and
thunderstorms should develop in response to the forcing
associated with this disturbance, and generally grow upscale as
the system propagates towards the northeast overnight tonight.
With surface dew points in the 40s, this convection is expected
to remain elevated. However, short term CAMs depict a cold pool
maturing under this cluster of storms owing to evaporative
cooling from the deep and dry low-levels. As such these storms
will pose a low-end wind threat through the early morning hours
tomorrow morning. Storms should moved into portions of east-
central Nebraska by 3 AM in the morning, with the US-30 corridor
an approximate center line for the region of highest rain and
strong wind chances. Wind gusts of 45 to 55 mph are possible,
with a severe wind gust up to 60 mph not out of the question.
These storms should move into central Iowa by 9 AM tomorrow
morning.

Additional showers and thunderstorms are expected to trail this
initial round of storms, bringing cloud cover and rain to much
of eastern Nebraska and western Iowa through the morning hours
tomorrow. This will act to delay recovery and destabilization of
the atmosphere until the afternoon hours. By early afternoon
tomorrow, winds are expected to become south/southeasterly as a
warm front slowly advances north across the area. This will
help advect gulf moisture back into our region from the south,
with short term guidance bringing dew points in the low 60s up
to the I-80 corridor by 7 PM tomorrow. This front will be the
focus for a conditional threat for isolated severe thunderstorms
tomorrow. All CAMs have trended towards eroding the capping
inversion by 4 to 5 PM, with MLCAPE ranging from 1500 to 2250
J/kg. Aloft, a 500 mb jet is expected to be in place over our
area, bringing 45 to 60 knots of bulk shear over portions of
southeast Nebraska and southwest Iowa. 0-1 km shear will be on
the weaker side with shear magnitudes under 20 knots and 0-1km
SRH around or under 100 m2/s2. This will support a large hail
and damaging wind threat with any storm that can develop. The
tornado risk appears to be much lower with these storms, but
non-zero. Further to our southwest, scattered thunderstorms are
expected to develop along a dryline in western Kansas and
Nebraska. These storms will quickly grow upscale into an MCS and
track east across the central Plains. These storms are not
expected to reach our area until after 7 PM tomorrow evening.
The primary threat for these storms will be strong damaging
winds. With lowering LCLs and a strengthening low-level jet
tomorrow after sundown, a spin-up tornado or two cannot be ruled
out. The highest risk for severe weather will be south of I-80
sometime between 9 PM and midnight.

There will be a more conditional threat for strong winds with
another potential cluster of storms that may track across
northeast Nebraska tomorrow night. The pre-storm environment in
this region will be less favorable as the warm front will
struggle to get this far north in addition to the environment being
more worked over by convection tonight into tomorrow morning.
Nonetheless, a few CAM solutions depict a second cluster of
storms that will weaken as they traverse east across northeast
Nebraska. These will also pose a risk for strong winds tomorrow
night, with the time frame of highest risk being between 10 pm
and 1 am. The risk for severe hail and tornadoes with this
particular cluster of storms appears low at this time.

     Monday through Tuesday Night...

The aforementioned longwave trough out west will approach our
area on Monday. Synoptic scale forcing for ascent and height
falls will aid in surface cyclogenesis in the central Plains.
The favored area for surface low development seems to be
southwest Kansas at this point in time. Extending northeastward
out of this developing low pressure, a warm front will arc
across central and northeast Nebraska, with a moderately
unstable airmass to the south and east of the boundary. Surface
dew points by Monday evening will likely range from the mid 60s
across east-central Nebraska and western Iowa, to around 70
degrees in southeast Nebraska. Model trends over the last day
have been to weaken the capping inversion over our area Monday
afternoon as well. As such, with periods of substantial forcing
as low-amplitude shortwaves eject into the Plains downstream
from the main disturbance, the potential for scattered strong to
severe thunderstorms Monday evening is increasing (30 to 50
percent at this time). Uncertainty with regards to the magnitude
of destabilization and exact strength of the capping inversion
will remain until Monday becomes in range for short term CAMs.
This will preclude any further discussion on coverage details.
Nonetheless, GFS and EURO both show an airmass with 2000 to 2500
J/kg of surface based CAPE under 10 to 20 J/kg of CIN across
much of eastern Nebraska and western Iowa. For what it`s worth,
the 18Z run of the HRRR leaves southeast Nebraska uncapped by 1
PM on Monday. Furthermore, SREF probabilities for CAPE at or
greater than 3000 J/kg range from 50 to 75 percent south of the
I-80 corridor. The limiting factor to organized convection, at
least early Monday afternoon will be weak shear over our area.
Bulk shear values will remain under 30 knots until around mid-
afternoon. As the evening progresses and flow aloft strengthens
with the approach of the trough out west, the dynamic
environment should improve to better support organized
convection. All in all, this will all support an isolated
threat for strong to severe thunderstorms, with large hail being
the primary hazard Monday late afternoon and evening.

By Monday night, the environment is expected to look very
different than the one in place over our region Monday
afternoon. Aloft, a speed max is expected to eject out into the
Plains as the axis of the main trough moves to the east of the
Rockies. With much stronger flow aloft, bulk shear across our
region will range anywhere from 40 to 60 knots by midnight
Monday night. At the surface, the aforementioned surface low
will track north-northeast, likely in the vicinity of south-
central Nebraska/north-central Kansas by Tuesday morning. With
the development of a strong low-level jet (flow by 4 AM Monday
at 850 mb may exceed 50 knots), all modes of severe weather will
become increasingly likely for any convection that can persist
into the overnight hours. The one complicating factor to this
set up is there is still substantial uncertainty with how much
the nocturnal boundary layer will cool across our region. Strong
mixing from the low-level jet in conjunction with strong
advection of warm air and moisture at the surface from the
south will mitigate nocturnal cooling (and by correlation
minimizing CIN that can develop ahead of any pre-existing
storms). On the other hand, strong southwesterly flow in mid-
levels may advect much warmer temperatures that may develop a
capping inversion above the boundary layer, helping weaken
storms into the early morning hours. Regardless, there may be
sufficient forcing for ascent driven by the strong dynamics at
play Monday night to overcome this. These uncertainties should
be ironed out over the next few forecast issuances. Until then,
will refrain from any further discussion on timing, coverage,
and threat details.

Tuesday morning. A surface low is expected to be somewhere in
northeast Nebraska or southeast South Dakota. A cold front will
extend to the south-southwest of this front through central
Nebraska into central Kansas. The airmass ahead of this front
will destabilize very quickly, with convection initiating on the
front as early as the late morning hours. While there is still
uncertainty with the exact placement and timing of the system at
this point, odds are increasing for rapid development of
thunderstorms along the cold front in eastern Nebraska by around
noon, with strong to severe thunderstorms exiting our region by
mid afternoon. Surface based CAPE may exceed 3000 J/kg by noon,
particularly across southwest Iowa. Bulk shear will also be very
strong, with magnitudes 50 to 65 knots possible. Hodographs
Tuesday appear to be fairly straight line at this time which
would limit the tornado risk, at least until these storms move
east of our region. As such, large hail and damaging winds will
be the primary risk, with a low but non-zero tornado risk slowly
increasing as the afternoon progresses. The highest risk for
severe weather at this time appears to be southwest Iowa in the
early to mid afternoon hours.

Lastly, due to multiple rounds of heavy rain traversing our
region, there will be a threat for flooding across all of
eastern Nebraska and western Iowa through Tuesday afternoon.
This risk appears to be maximized across northeast Nebraska and
west-central Iowa where storm total rainfall amounts will range
from 1 to 2 inches, with local amounts of 3 to 4 inches
possible.

     Wednesday through Friday...

The pattern should quiet down after our main system moves east
on Tuesday. Another round of thunderstorms appears possible
Thursday night, however the potential for severe weather is low
at this time.
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&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
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Issued at 621 PM CDT Sat May 18 2024

VFR conditions will likely prevail through the forecast period
with increasing shower and thunderstorm chances late tonight
into Sunday morning. Measurable precipitation appears most
probable at KOFK where prevailing -SHRA will be indicated Sunday
morning. At KOMA and KLNK, TEMPO -TSRA groups will be included
for a 2-3 hr period Sunday morning. Otherwise, southeast winds
are expected to increase to 12+ kt by mid-morning at KOFK, and
midday or early afternoon at KOMA and KLNK.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Darrah
AVIATION...Mead