917
FXUS63 KOAX 300849
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
349 AM CDT Thu May 30 2024

.KEY MESSAGES...

- Scattered strong thunderstorms are expected to develop across
  the region this afternoon. Small hail, gusty winds, a brief
  weak tornado and flooding will be possible with these storms.

- It is becoming increasingly likely that scattered strong to
  severe thunderstorms will develop and move across the area,
  particularly northeast Nebraska, Sunday evening and night.

- Another round of thunderstorms, some potentially on the
  stronger side, appears possible Tuesday afternoon and evening.


&&

.DISCUSSION...
Issued at 226 AM CDT Thu May 30 2024

...Today and Tonight...

Isolated showers and thunderstorms are expected to move into
eastern Nebraska this morning from the west. As diurnal heating
kicks in, these storms are expected to gradually increase in
coverage and strength. While there is still substantial
variability between CAMs as to how widespread storms will be and
their exact placement, two favored areas for strong
thunderstorms and heavy rain by this afternoon stand out. The
first is northeast Nebraska, primarily north of US-30 and west
of US-77. A weak cold front will be sagging south from South
Dakota, approaching the Nebraska-South Dakota border by noon
today. By around 1 PM, thunderstorms are expected to develop
along this front. Bulk shear of around 30 knots, mid-level lapse
rates of 6 to 7 C/km and CAPE as high as 2000 J/kg will support
a low-end hail and wind risk. Winds up to 60 mph and hail up to
quarter size will be possible in the strongest storms. Some
CAMs also suggest there will be sufficient low-level shear to
support a very low-end tornado risk. As such, a weak spin-up
tornado or two cannot be ruled out. There is some uncertainty
with how these storms evolve. CAMs are more uncertain with the
southward extent of storms on the front heading into the
afternoon. In addition to this, pre-frontal convection
associated with an MCV appears possible as well which may modify
environmental instability to work against storms tied to the
front.

Further south, a second MCV is expected to slowly traverse
across northern Kansas, near the Kansas-Nebraska state line. A
second area of higher thunderstorm coverage is expected to
develop in the vicinity of this disturbance this afternoon as
well. This will bring scattered thunderstorms and heavy rain
across portions of southeast Nebraska, primarily south of
Highway 2. In both of the above mentioned areas, rain chances
today will range from 80 to 90 percent. In between these two
areas in east-central Nebraska, and for portions of southwest
Iowa, isolated to scattered thunderstorms will also be possible
this afternoon, though the chance of precipitation will be
lower, ranging from 50 to 60 percent at best.

Lastly, storms today will pose a flash flooding risk,
particularly across northeast and southeast Nebraska. PWAT over
an inch is expected to advect into our region from the south
over the course of the day. Updrafts will be very efficient rain
producers as well with tall and skinny CAPE profiles seen in
forecast soundings across eastern Nebraska. Furthermore, weak
steering flow and the tendency for convection today to be tied
to the mesoscale features responsible for providing sufficient
forcing for ascent will support storms to linger and/or
redevelop over the same areas. As such, areas that see continued
thunderstorm development will have a flash flood risk, with
flooding possible into the overnight hours tonight. Rainfall
amounts could locally be as high as 2 inches.

...Friday and Saturday...

With weak zonal flow and embedded low-amplitude perturbations
ejecting into the plains, diurnally driven convection will be
possible both afternoons. As of now, it seems the highest rain
chances will be Friday afternoon, as CAMs suggest a tongue of
modest instability (on the order of around 1000 J/kg) will be in
place across southeastern Nebraska into Iowa. This will result
in thunderstorm chances of 50 to 70 percent. Further to the
north and west across east-central and northeast Nebraska, lapse
rates aloft and surface moisture will support isolated
thunderstorm development in the afternoon during peak heating.
No severe weather is expected with these storms at this time.

...Sunday and Sunday Night...

Medium range guidance continues to suggest the threat for strong
to severe thunderstorms Sunday evening into the overnight hours.
A cold front will approach the area from the northwest Sunday
afternoon and will provide the lifting mechanism at the surface
for thunderstorms to initiate during peak diurnal heating. Ahead
of this front, surface dew points will be in the upper 60s and
low 70s. This in conjunction with mid-level lapse rates of 7 to
9 C/km will yield SBCAPE values ranging from 2000 to 3500 J/kg.
Bulk shear will also be on the order of 30 to 40 knots across
northeast Nebraska and west-central Iowa. Aloft, a trough
ejecting out into the northern Plains will provide synoptic
scale forcing for ascent and associated height falls across our
area heading into Sunday night. This will be a favorable
environment for organized convection to develop along the cold
front north and west of our area Sunday afternoon. With shear
vectors roughly paralleling the frontal boundary, quick upscale
growth would be expected, resulting in an MCS or several bowing
segments entering northeast Nebraska by the late evening/early
overnight hours. While it`s still too early to pinpoint exact
severe hazards, and their timing and location, in general this
set-up favors a damaging wind threat for portions of eastern
Nebraska and western Iowa north of I-80. If these storms can
persist through the overnight hours as the low-level jet kicks
in, there will also be potential for a brief spin-up tornado or
two.

...Monday through Wednesday...

Models the last 24 hours have trended towards more progressive
solutions with regards to cold front movement Sunday night into
Monday. As such, much of the region, if not all of eastern
Nebraska and western Iowa will reside in the post-frontal
airmass by peak diurnal heating on Monday afternoon. As such,
there will be a low chance (20 to 30 percent) for an isolated
shower or thunderstorm to develop across southeast Nebraska and
southwest Iowa, primarily south of I-80. No severe weather is
expected at this time.

On Tuesday, another upper-level disturbance is expected to dive
southeast across the northern Plains with a cold front at the
surface. Scattered thunderstorms will be possible across this
front as it moves across the region Tuesday afternoon. Though
there is still substantial model spread with how fast this front
will progress through the area, nearly all medium range model
solutions depict sufficient pre-frontal airmass destabilization
to support strong to severe thunderstorms.

Behind the front that would move through on Tuesday, a high
pressure is expected to build into the central Plains as
northwesterly flow prevails aloft. As such, warm and dry weather
is expected for the middle and end of next week with highs in
the 80s. Surface dew points in the 60s will result in heat
indices potentially reaching well into the 90s by the end of
next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1141 PM CDT Wed May 29 2024

VFR conditions across the area this evening with winds out of
the southeast. Showers and storms are approaching from the west
and will move in overnight. Expect scattered showers and storms
to move into northeast Nebraska overnight and spread across our
area Thursday morning. They will be very scattered in nature and
any thunderstorm activity will only impact one area for a brief
period in time. Model guidance is all over the place with timing
and location of these storms due to their scattered nature, so
confidence is low in trying to forecast timing for when they
will impact the terminals. Will likely need to just amened when
we see them approach the terminals through the day on Thursday.
Scattered showers and storms continue into Thursday evening,
with some models showing cigs starting to lower to MVFR or even
IFR after 00Z. Right now looking like only about a 20% chance,
with greatest chance at KOFK where showers start to wrap up
earlier (40% chance).

&&

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

&&

$$

DISCUSSION...Darrah
AVIATION...McCoy