448
FXUS63 KOAX 010859
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
359 AM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Severe thunderstorms and flash flooding will be possible late
  this evening and overnight tonight. The primary hazard looks
  to be damaging winds at this time, though all modes of severe
  weather will be possible.

- Another round of strong to severe thunderstorms will be
  possible tomorrow afternoon, primarily across southeast
  Nebraska and southwest Iowa. All severe hazards will again be
  possible with the primary risk being damaging straight-line
  winds.

- Unsettled weather continues through the week and into the
  weekend, with almost daily chances for thunderstorms. Another
  system may severe thunderstorms on the 4th of July.

&&

.DISCUSSION...
Issued at 245 AM CDT Mon Jul 1 2024

...Today and Tonight...

Scattered thunderstorms are already developing across portions
of northeast Kansas and south-central Nebraska. This activity
is expected to expand in coverage and shift northeastward over
the course of the morning. Though expected to be non-severe,
PWAT values between 1.0 and 1.5 inches in addition to strong
moisture transport from the south will bring the potential for
very heavy rain in the stronger storms. Widespread amounts of
up to an inch will be possible before storms dissipate and move
east of our area. The bulk of this activity will have dissipated
by noon, though a lingering shower or two will be possible
through 2 PM, particularly across portions of western Iowa. This
convection should remain elevated and non-severe, though small
hail will also be possible in the strongest storms.

At the surface, a warm front is expected to stretch from central
Nebraska, southeastward into east-central Kansas. Along this
boundary, a narrow corridor of ample moisture return will result
in a tongue of moderate instability (MLCAPE 2000-3000 J/kg) into
south-central and central Nebraska by this afternoon. The
prevailing upper-level pattern aloft will sustain troughing over
the northern Rockies, with an embedded shortwave rounding the
base expected to eject out into the northern Plains by this
evening. This system will provide the forcing for ascent for
thunderstorms to develop west of our area this
afternoon/evening in the vicinity of southwest
Nebraska/northwest Kansas. As these storms move northeast,
upscale growth into an MCS is expected. Though there is still
uncertainty with how far north or south this MCS will track,
there is confidence in a swath of strong to severe winds
stretching from central Nebraska into east-central Nebraska. The
strongest segment of this MCS will be anchored to the northward
extent of the instability axis, most likely to be near the
US-30 corridor, though severe storms could be as far south as
I-80 or as far north as US-20. Further north than this in our
area, instability will likely be insufficient for severe
convection. Though there is a non-zero chance for strong to
severe storms to build south of I-80, the potential is very low
(~10 percent) as a very strong capping inversion will be in
place during the evening and overnight hours. Wherever this MCS
tracks, damaging straight line winds will be the primary hazard.
Some wind gusts greater than 75 mph will be possible,
particularly between US-30 and I-80, and west of US-75. Just
after dark, a strong low-level jet is expected to develop and
enlarge boundary layer hodographs. Some CAMS suggest 0-1km SRH
may exceed 500 m2/s2 by 10 pm. Where these storms are able to
remain surface based, there will be the potential for spin-up
tornadoes on the leading edge. Isolated instances of severe hail
may also be possible as well though this will likely be a
secondary hazard with mid-level lapse rates largely remaining
below 7 C/km.

One uncertainty that has yet to be ironed out is the eastward
extent in which these storms will be severe. Storm propagation
will almost certainly exceed the speed at which the surface
front lifts to the north and east. As such, at some point late
tomorrow evening or night storms should ride over the front and
become elevated. This will likely happen somewhere near or just
west of the US-75 corridor. When this happens, the tornado
threat will likely be reduced to near zero, with the damaging
wind potential reducing but remaining for a few hours after. All
in all, the bulk of tomorrows severe weather should happen
before 1 AM. Behind this line of storms, training thunderstorms
may develop on the south side of the cold pool that is laid out,
likely near the I-80 corridor. This will be a favored area for
additional heavy rain through the night tomorrow night, with
flooding becoming the primary concern after midnight. Storms may
linger until 7 AM Tuesday morning. As such, some CAMs suggest a
swath of 1-2 inches of rain somewhere in east-central Nebraska.
Depending on how long storms are able to persist over any given
area, some local amounts may be as high as 4 to 5 inches.

A very low probability (5 to 10 percent) but potentially high
impact scenario would be if a discrete supercell is able to
develop ahead and/or southeast of this MCS tomorrow
afternoon/evening. Low-level shear profiles and buoyancy
southwest of the warm front will be more than sufficient to
support intense supercells should one develop. Strong capping is
expected to remain across this portion of the area through the
afternoon and evening likely impeding storm development ahead of
the MCS. Nonetheless, due to the strong forcing for ascent
associated with the shortwave ejecting out into the Plains,
it is within the realm of possibility that a storm or two may be
able to break through the cap. The most likely area for these
to develop will be west of US-77 and south of US-30.

...Tomorrow and Wednesday...

By tomorrow morning, an advancing cold front will enter the
region from the northwest. This front is expected to push
southeast over the course of the day. South of this front,
airmass destabilization will be universal and happen much
earlier in the day than today. As such, scattered showers and
thunderstorms will develop along this front as early as noon. By
this time, the front will likely extend along a line from Hebron
to Onawa. The earliness of convective development will likely
temper the magnitude of destabilization, at least across
southeast Nebraska and southwest Iowa. Most CAMs suggest MLCAPE
will likely be below 1500 J/kg. Nonetheless, through the
afternoon hours, widespread development along the front should
consolidate into a line, or a series of bowing segments. Again,
damaging winds will be the primary hazard, increasing in
probability with south and eastward extent. A brief tornado or
two and isolated instances of severe hail will also be possible.
Individual storm elements will track to the east-southeast,
with the front pushing more squarely to the southeast. The
severe weather potential will likely be south and east of our
area by 7 PM.

A more widespread concern tomorrow will be heavy rain yet again.
An extremely moist airmass will be positioned over our area,
with PWATs likely exceeding 2 inches. The prolonged period of
potential showers and thunderstorms will yield again large
swaths of precip up to an inch, with isolated instances up to 3
inches. The greatest concern for flooding will be along and
north of I-80 where soils will already be saturated from the
several rounds of thunderstorms expected today. Isolated
instances of flash flooding cannot be ruled out south of I-80
tomorrow evening however.

Wednesday is expected to be dry during the day, with a high
pressure expected to develop and move across the central Plains
from west to east. Wednesday night, convection from the High
Plains may reach eastern Nebraska after midnight in the form of
a decaying MCS. Severe weather is not expected at this time,
though gusty winds and brief heavy downpours will be possible.

...Independence Day...

Medium range guidance is in agreement of an amplifying shortwave
trough deepening and ejecting out into the central and northern
Plains on Thursday. This system looks to be a stronger piece of
energy than the one responsible for today and tomorrow`s storms,
with stronger height falls and PVA overspreading our region
Thursday afternoon. At the surface, a warm front will likely
extend across central Iowa into northeast Nebraska. Further
west, a cold front will approach the region by the afternoon,
with the most likely position of the front being between US-77
and US-81 by the early afternoon hours. Moisture in the warm
sector will be abundant, with guidance universally depicting
boundary layer dewpoints above 70 degrees. This will result in
moderate to extreme instability, with SBCAPE likely around 3000
J/kg. Some guidance suggests SBCAPE may exceed 5000 J/kg across
southeast Nebraska and southwest Iowa. The capping inversion
will likely inhibit convection through mid afternoon with CIN
values exceeding 50 J/kg. By the early evening, strong forcing
for ascent and height falls will help erode the cap resulting
in convection developing across portions of eastern Nebraska and
southwest Iowa by early evening. Though the details on exact
timing, extent of coverage and exact location of frontal
boundaries is unclear at this time, current guidance favors the
development of strong to severe thunderstorms capable of all
modes of severe weather. This system has the potential to be a
more significant severe weather outbreak, especially with it
coinciding with July 4th, so keep a very careful eye on the
forecast for this day as the details get ironed out over the
next few forecast cycles.

...Friday through Sunday...

Behind the Fourth of July system, northwesterly flow seems
likely aloft over the Plains with a high pressure system
establishing itself at the surface by the end of the week. The
quiet weather may only last a couple of days, as medium range
guidance has fairly persistently shown a deepening trough diving
southeast across the northern Plains sometime over the weekend
or early next week. Differences in timing are too great at this
time to pin down any given day as most likely to see
thunderstorms. Regardless, the general trend is to have another
storm system move through our region by the end of this
forecast period.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1124 PM CDT Sun Jun 30 2024

Storms remain on track to impact all TAF sites early Monday
morning, but trends have been slightly later than previously
forecast. Expect 2-3 hours of TSRA at each site with perhaps
some lingering -SHRA after. Following storms, expect IFR to low
MVFR ceilings for at least a few hours. Another round of storms
is expected to move through late in the period, with potential
for some of these storms to be severe. There is also a small
chance (20%) some severe storms develop ahead of this line,
from around 22Z-01Z, with LNK seeing the highest threat, but
confidence in these developing, much less hitting a TAF site are
quite low. Otherwise, generally expect southeasterly to
southerly winds outside of storms, with gusts of 20 to 30 kts
developing by Monday morning. Can`t completely rule out some low
level wind shear Monday evening ahead of the storms, but for
now, thinking surface winds will stay strong enough to preclude
mention.

&&

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

&&

$$

DISCUSSION...Darrah
AVIATION...CA