607
FXUS63 KOAX 211744
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1244 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Rain and thunderstorm chances increase this afternoon,
  particularly for southeast Nebraska and southwest Iowa. There
  is a 5-15% chance of severe storms Saturday afternoon or
  evening.

- Widespread rain is expected Saturday night and persist Sunday,
  with the heaviest rainfall anticipated from midnight Sunday
  through noon Sunday (50-90%). Rainfall totals through Sunday
  of 1 to 2 inches are expected south of Harlan Iowa to
  Omaha/Council Bluffs to Fairbury Nebraska line. Isolated
  totals of up to 3 inches are possible across extreme southeast
  Nebraska into southwest Iowa.


- One more warm day, with 90 degree readings along and south of
  I-80.Cooler conditions will arrive Sunday and persist through
  the following week, with highs dropping into the 60s and 70s.

&&

.DISCUSSION...
Issued at 400 AM CDT Sat Sep 21 2024

Today:

Early this morning isolated showers persist across portions of
the forecast area, with coverage expected to increase slightly
as 25-45kt LLJ increases moisture transport into the region,with
isentropic ascent and weak disturbance moving through periphery
of H5 ridge axis providing kinematic forcing. There`s a a slim
chance (5%) of a stronger storm developing with MUCAPE 1-1.5K
J/kg and bulk shear values 35-40kt, but relative warm H7 and H5
temperatures will limit hail production, thus wind primary
threat. Convection will wane in coverage during the morning and
early afternoon hours as LLJ veers and forcing diminishes. But,
expected uptick in convective development by 19-21z. By then
dynamic and kinematic forcing increases due to interaction of
upper level trough moving across central Canadian provinces and
upper low moving out of southwest U.S. A 80-100kt ULJ moves
along periphery of H5 ridge as several disturbances advect into
central plains in advance of southwest U.S. upper low. H5 trough
moving across Canada drives cold front through the forecast
area, and will be located generally along a Logan IA to Hebron
NE line by 18-19z. Convective temperatures met or exceeded by
19-21z, with deterministic soundings indicating MU CAPE 2-3K.
Limiting factor will weaker bulk shear values in the 25-35kt
range, but there`s enough to provide some storm organization.
Nudged temperatures toward 75% NBM, that are closer to
deterministic convective temperatures in the lower 90s across
southeast Nebraska and southwest Iowa.

Tonight:

Severe weather threat expected to end by early evening as front
moves south of the forecast area, but risk of moderate to heavy
rainfall will continue. However, southwest U.S. upper trough
slow to lift into the plains, thus large scale moist ascent will
continue across the region through the overnight hours.
Isentropic convergence along the 305-310K surfaces will allow
convection to train across southeast Nebraska and southwest Iowa
overnight, tapping into precipitable water values up to 2
inches.

Evaporative cooling and low level cold advection behind the
front will allow overnight temperatures to fall into the 50s.

Sunday and Sunday night:

Rain with embedded thunderstorms will continue into Sunday as
upper trough moves across the region. Each successive model run
places band of heavier rainfall to the south of forecast area,
however, the past couple issuances of WPC QPF has been
consistent in broad-brushing 1 to 2 inch along and south of
I-80, which will help reduce current soil moisture deficits of 1
to 3 inches. The likelihood of any flash flooding or flooding
appears low based on current rainfall forecast.

Upper low and ULJ jet core 70-80kt will move across forecast
area Monday evening, with better moisture transport shifting
into lower Missouri Valley. As such expect precipitation
coverage and intensity to wane by early evening,and effectively
ending during the overnight hours.

Lack of any appreciable sun, evaporative cooling and sustained low
level cold advection will keep temperatures fall-like, with
highs in the 60s. Cloud cover will keep temperatures for cooling too
much Sunday night, though mid 40 readings will be possible over
northeast Nebraska, with upper 40 and lower 50 degree readings
elsewhere.

Monday through Friday:

More seasonable temperatures are expected through week, with
high temperatures in the 70s and lows in the 40s and 50s.
Currently the forecast looks dry. There`s a slim chance of
showers Tuesday night as an open H5 trough digs in the central
plains. There is variance on the strength of the low, with the
operational GFS trying to create a closed low. It appears the
outlier based on the EC/CMC/GEFS, and NBM keeping PoPs below
15%. Another slim chance occurs Thursday into Friday as EC/CMC
and LREF depicting a compact closed low moving across the
southern plains. LLJ and PIVA may be enough to generated
isolated showers. Once again NBM keeping PoPs below 15%, but if
low tracks a little further north, southern parts of forecast
area could see PoP and thus shower coverage increase.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1232 PM CDT Sat Sep 21 2024

VFR conditions are expected through much of the period at all
three TAF sites with the exception of a few showers and
thunderstorms, mainly at KOMA and KLNK. Latest runs of CAMs
indicate that a few thunderstorms could develop near KOMA and
KLNK around 21z and continue through 00z before pushing
southeast of the sites. A few strong wind gusts and small hail
are possible with this initial development in addition to heavy
downpours. Widespread showers with a few embedded non-severe
thunderstorms will spread over much of southeast Nebraska and
southwest Iowa after 04-06z and continue through much of Sunday.
Although VFR conditions are expected for much of the time, we
could see some deteriorating condition down to MVFR or even IFR
in stronger storms.

&&

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

&&

$$

DISCUSSION...Fortin
AVIATION...Kern