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000
FXUS63 KOAX 151143
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
643 AM CDT Mon Apr 15 2024

.KEY MESSAGES...

- Very high to extreme fire danger is expected this afternoon
  for portions of northeast Nebraska and west-central Iowa.

- Several rounds of strong to sever thunderstorms are possible
  beginning this afternoon through Tuesday afternoon. All modes
  of severe weather are possible at this time.

- Much cooler temperatures work their way into the region
  beginning Wednesday and are expected to last through the
  weekend.

&&

.DISCUSSION...
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Issued at 257 AM CDT Mon Apr 15 2024

     Today Through Tomorrow Night...

Upper air analysis continues to show a potent trough over the
Great Basin with ridging over the central CONUS. At the surface,
a stationary front is draped across central Kansas with dew
points in the 50s and 60s to the south and dew points in the 20s
and 30s to the north. As the aforementioned upper-level
disturbance approaches the central Plains over the course of
today, strong surface cyclogenesis will develop a surface low in
the central High Plains this afternoon. A strong isallobaric
response in the surface winds in conjunction with diurnal mixing
of the boundary layer will allow gusty winds to develop by late
morning across the area. Winds may gust up to 40 miles an hour,
especially along and north of Highway 30. As the surface low out
west continue to deepen, the stationary front across central
Kansas is expected to become a warm front and move north across
our area, stretching from northeast Nebraska into southwest
Iowa by this afternoon. Areas north of the warm front this
afternoon are expected to have very high to extreme fire
danger, as relative humidities are expected to drop into the 20
to 30 percent range. Moisture advection from the south will help
RH values increase through the evening hours limiting the
period of highest fire danger to early/mid afternoon.

South of the warm front, dew points in the 60s and temperatures
warming into the 80s will bring surface based CAPE of 1000 to
2000 J/kg. The environment will be somewhat capped through 6 PM
with HREF mean CIN values over 100 J/kg across the region
(particularly near the warm front). This capping inversion
should erode through the afternoon and evening hours, allowing
for the potential (30 to 50 percent chance) of isolated to
widely scattered thunderstorms to develop along or near the warm
front. Any storms that are able to develop along the warm front
will have the potential to become super-cellular with bulk wind
shear values likely ranging from 35 to 45 knots. The primary
risk will be large hail, though damaging winds or a tornado
cannot be ruled out. Some hail stones may be larger than golf
ball sized.

Further south into southern Nebraska and Kansas, the capping
inversion is expected to erode faster than in areas further
north. This will leave the warm sector largely uncapped by mid
afternoon (3 to 5 pm). Storms however may struggle to develop at
first with the absence of any focused areas of surface
convergence (unlike the warm front to the north and the dry line
further west into the High Plains). Nonetheless, there is some
signal amongst short term guidance that weak perturbations in
upper-level flow will round the eastern side of the main trough
bringing periods of subtle yet enhanced forcing for ascent. This
may result in a few strong to severe thunderstorms developing
and tracking northeast across northern Kansas and southern
Nebraska into the evening and early overnight hours. However,
any convection that is able to develop in this region may
struggle to persist in the absence for persistent forcing for
ascent. Any established severe thunderstorm will bring the risk
for all modes of severe weather, especially large hail.

Further west in the High Plains along the dryline, more
widespread convective development is expected by this evening (7
to 9 PM). As storms track to the northeast, they are expected to
persist through the nighttime hours. The surface dryline
feature, instead of its normal diurnal pattern of retrograding
to the west after dark, should continue to be advected east
supplying a persistent forcing mechanism for surface based
convection well into the night. Aloft, a jet max is expected to
round the base of the main trough bringing much stronger wind
shear into the region by early tomorrow morning. Closer to the
surface, a strong low-level jet is expected to develop bringing
winds as strong as 50-55 knots at 850 hPa. Considering all
these factors, the dynamic environment will become increasingly
favorable for organized convection over night tonight as storms
track northeast into eastern Nebraska. Early tomorrow morning,
in the 4 to 9 am time frame, an uncommon period of enhanced
severe potential may unfold for portions of eastern Nebraska and
western Iowa. A round of showers and thunderstorms (70 to 90%
chance of rain) is expected to move through southeast Nebraska
and southwest Iowa. Hodographs enlarged by the low level jet may
feature 0 to 1 kilometer SRH values ranging from 100 to 200
m2/s2. The aforementioned jet max will be more squarely over
the region bringing bulk shear values of 40 to 50+ knots.
Height falls and steepening lapse rates in the mid-levels may
allow surface based CAPE to remain near 1500 J/kg even by
sunrise. This is especially possible as boundary layer mixing
throughout the night in addition to strong WAA is expected to
negate the effects of nocturnal radiative cooling. This may
bring a period of enhanced severe weather potential in
southeast Nebraska and southwest Iowa with all modes of severe
weather possible Tuesday morning. The highest chance for severe
weather will be along and south of I-80 during this time frame,
though HREF members suggest severe storms may be possible into
northeast Nebraska (primarily east of Norfolk) and west-central
Iowa. With the return of diurnal heating tomorrow morning,
storms may reinvigorate and strengthen as they move through
southwest Iowa. All mode of severe weather will be possible
including tornadoes, hail up to 2 inches in diameter, and
damaging winds.

A final round of strong to severe storms appears possible (40 to
60 percent chance of rain, 10 to 20% chance of storms being
severe) Tuesday afternoon (2 PM through 7 PM) across eastern
Nebraska and western Iowa. Hail up to the size of quarters will
be the primary threat with these storms. With enhanced surface
vorticity near the surface low, strong deep layer shear and
sufficient buoyancy from steep mid-level lapse rates, a tornado
cannot be ruled out with this last round of storms. By tomorrow
evening, a cold front is expected to push southeast across the
area bringing an end to the potential for severe weather by 9
PM. This cold front will usher in a period of cooler and
quieter weather for the rest of the forecast period.


     Wednesday through Sunday Night...

Temperatures are expected to return near normal for Wednesday
with highs in the 60s regionwide. Wednesday night a reinforcing
cold front should move south across the region as scattered rain
showers (50 to 70 percent chance of rain) will move into the
area from the west by midnight Wednesday night. The bulk of the
rain should move east of the region by noon on Thursday, but
light rain showers could linger (particularly across southeast
Nebraska) into Friday morning. Temperatures will also be cooler
for the end of the week into the weekend with highs in the low
to mid 50s and lows near freezing.
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&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
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Issued at 639 AM CDT Mon Apr 15 2024

VFR conditions will hold through this morning and much of this
afternoon ahead of an approaching low pressure system. Low-level
winds shear currently occurring as winds are picking up at KOFK
and KLNK will dissipate as surface winds start to increase here
by 14-15Z. Expect winds at the terminals to ramp up to around 20
to 30 kt where they will hold out of the southeast through this
afternoon and evening. Storms will develop and approach the
terminals from the southwest this evening starting around 00Z.
Confidence is lower in storms impacting the terminals during the
evening hours so have left out of the TAFs for now, but have
put storms in the TAFs for the overnight hours which we have
higher confidence in. Storms this evening through Tuesday could
potentially be severe with large hail, downburst winds, and
tornadoes all possible.
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&&

.OAX WATCHES/WARNINGS/ADVISORIES...
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NE...Red Flag Warning from 11 AM this morning to 7 PM CDT this
     evening for NEZ011-012-015-018-032>034.
IA...Red Flag Warning from 11 AM this morning to 7 PM CDT this
     evening for IAZ043.
-- End Changed Discussion --

&&

$$

DISCUSSION...Darrah
AVIATION...McCoy