879
FXUS63 KOAX 172344
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
644 PM CDT Fri May 17 2024

.KEY MESSAGES...

- A weak cold front will bring slightly cooler temperatures to
  the region tomorrow afternoon with a low chance of
  sprinkles/light rain in the morning hours.

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

- There will be additional chances for storms (40-60%) next
  Thursday into next weekend.

&&

.DISCUSSION...
Issued at 127 PM CDT Fri May 17 2024

     Today and Tomorrow...

Above average temperatures are expected today, with highs well
into the 80s region wide. A few areas across northeast Nebraska
may hit 90 degrees this afternoon. Deep mixing of the boundary
layer is also aiding in bringing gusty winds down to the surface
across portions of northeast Nebraska, primarily north of US-30
and west of US-75. Several wind gusts up to 35 mph have been
observed in this area already this afternoon. As such, isolated
wind gusts up to 40 mph will be possible through 7 PM this
evening.

Early tomorrow morning, a weakening cold front will enter the
region from the northwest. This will bring another round of
gusty, but now northwesterly winds to northeast Nebraska, with
the potential of gusts up to 40 mph. As the front moves
southeast across eastern Nebraska and western Iowa tomorrow
morning, a few sprinkles or stray light showers will be
possible. Temperatures will be more moderate behind this front,
with highs in the upper 70s in northeast Nebraska and in the
low to mid 80s elsewhere.

     Sunday through Tuesday...

A series of disturbances will bring an extended, yet complex setup
for severe potential Sunday through Tuesday.

Sunday morning, a warm front will lift north through the region,
allowing gulf moisture to return to much of eastern Nebraska and
southwestern Iowa. Dewpoints by Sunday afternoon will likely be
in the low to mid 60s with surface based CAPE near the Kansas
border approaching 3000 J/kg. Bulk shear is expected to increase
across southeast Nebraska and southwest Iowa over the course of
the day, on the order of 35 to 45 knots by Sunday evening.
These environmental parameters at face value will create a
favorable environment for organized severe convection. However,
there are some complicating factors to the scenario on Sunday.

First, guidance has consistently resolved scattered showers and
thunderstorms developing in the south-central Nebraska/north-
central Kansas area early Sunday morning. These storms will
track to the northeast over the course of the morning and into
Sunday afternoon. While an increasing number of model solutions
keep these storms in central and northeast Nebraska, if these
storms were to track further south, instability available for
afternoon convection will be reduced, with the most favorable
environment pushing south into Kansas and western Missouri.
Another complicating factor is the capping inversion that is
expected to be in place to the south and east of this morning
convection. A fairly robust cap, with CIN ranging anywhere from
50 to 100+ J/kg is expected to develop across portions of the
central Plains and Midwest. Nonetheless, where heating is not
inhibited by cloud cover from ongoing morning convection,
diurnal heating of the boundary layer will erode the majority of
this capping inversion by the late afternoon. At the surface,
there is also an increasingly strong signal for a surface
trough/boundary to linger across southeast Nebraska and
southwest Iowa. While the placement of this boundary is still
somewhat uncertain depending on how morning convection plays
out, this will be the primary point of interest for initial
convective development Sunday afternoon.

All this being said, the most likely scenario for Sunday (60-70%
chance) is as follows. A round of scattered showers and
thunderstorms will develop in south-central Nebraska Sunday
morning, tracking northeast into northeast Nebraska by Sunday
early afternoon. Sometime between 3 and 6 PM, a second round of
thunderstorms will initiate between US-30 and I-70, west of
US-77. These storms will track east over the course of the
evening, with the primary hazard being large hail initially, and
transitioning more into a damaging wind threat as general model
consensus resolves upscale growth of convection into the early
overnight hours. A few tornadoes will also be possible,
especially if thunderstorms can take advantage of enhanced
surface vorticity on any pre-existing surface boundaries. 0-1
km SRH across southeast Nebraska will be on the order of 100-200
m2/s2 by the early overnight hours as the low-level jet
develops. This will also contribute to the tornado potential
for storms that persist in our area through the early overnight
hours. Thunderstorms are expected to push southeast of the
region by midnight.

The severe risk for Monday afternoon appears to be the most
conditional of the three days. Southerly winds will persist
behind the round of storms on Sunday, bringing afternoon dewpoints
into the upper 60s and low 70s. This will result in surface
based CAPE ranging from 2500 to 3500 J/kg, particularly across
southeast Nebraska and southwest Iowa, though moderate
instability will likely be in place across the entire forecast
area. The capping inversion Monday appears to be much more
robust than Sunday, making the severe potential more conditional
on whether storms are able to break through or not. The
strongest synoptic forcing for ascent is also expected to remain
west of the area Monday afternoon and evening, as an amplifying
longwave trough over the western CONUS will likely stay west of
the Rockies until Monday night. There is some signal however
for another subtle shortwave trough to eject out into the
southern Plains Monday evening, potentially bringing sufficient
forcing for a few strong to severe thunderstorms to develop
along a frontal boundary extending from south-central Nebraska
into west-central Iowa. Any storm that is able to develop Monday
afternoon/evening will primarily pose a large hail and damaging
wind risk, as weak low-level shear will limit the potential
with regards to tornadoes.

Monday night, as the longwave trough out west shifts into the
central CONUS, flow at 500 hPa is expected to strengthen up to
60 knots. This will greatly increase bulk shear across eastern
Nebraska and western Iowa, with bulk shear values increasing
from around 20 knots to as high as 60 knots. Just above the
boundary layer, a low-level jet is also expected to develop,
increasing 0-1 km shear up to 30 knots by midnight, particularly
for areas south of I-80. Stronger forcing for ascent with the
approach of the main upper level disturbance will bring a more
widespread potential for thunderstorms during the overnight
hours, with chances of rain 60 to 80 percent. For severe storms
that are able to develop and persist into Monday night, all
modes of severe weather will be possible. Uncertainties with
regards to timing and convective coverage preclude any
discussion on specifics with regards to timing and location at
this time.

On Tuesday, the primary upper-level disturbance is expected to
eject out into the Great Plains. At the surface, a low pressure should
deepen somewhere in the central Plains, with a warm front
extending northeast into the upper Midwest, and a cold front
extending southwest into southwest Kansas. Ongoing morning
convection is expected, with highest chances (70 to 80 percent)
along and north of the warm front. This includes portions of
northeast Nebraska and west-central Iowa. The surface low is
expected to track northeast, likely somewhere in east-central
or northeast Nebraska by noon Tuesday. Ahead of the cold front,
surface dew points are expected to remain as high as 70 degrees.
This in conjunction with steep mid-level lapse rates and
cooling aloft with the approach of our upper-level disturbance
will bring moderate instability to the region. Surface based
CAPE may be as high as 3000 J/kg by early afternoon. The
dynamic environment will also be very favorable for organized
convection, with bulk shear ranging anywhere from 45 to 60
knots. With uncertainties regarding morning convection coverage
and timing of this front, will refrain at this time from any
discussion on timing and exact location for severe potential in
our region Tuesday afternoon. Regardless of where this front
is, convection will likely begin developing along the front by
early afternoon. With this in mind, current model trends favor
a slower progression of this cold front, potentially bringing a
third day of strong to severe weather for portions of east-central
Nebraska and western Iowa. While all modes of severe weather
will be possible, damaging winds appear to be the preferred
hazard with shear vectors largely parallel to the advancing cold
front. The forecast for Tuesday afternoon will likely change as
details with regards to the speed at which this system
progresses will become more clear by subsequent AFD issuances.

     Wednesday and Thursday...

A high pressure is expected to build into the region Wednesday,
bringing seasonal temperatures and dry conditions. There is some
signal in medium range guidance for another potential round of
thunderstorms Thursday evening and night.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 636 PM CDT Fri May 17 2024

VFR conditions are expected through this TAF period with a low
probability of a few showers and thunderstorms (<20%) overnight
in association with a veered nocturnal low-level jet. This will
also create low-level wind shear overnight. Shortly after
sunrise, winds will begin to mix out ending the low-level wind
shear. A cold front will move south through the sites turning
winds to the northwest and gust up to near 20-25 kts through
Saturday afternoon.

&&

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

&&

$$

DISCUSSION...Darrah
AVIATION...Kern