988
FXUS63 KOAX 210859
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
359 AM 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 uptic 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:

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 broadbrushing 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.

Lack of any appreciable sun, evaporative cooling and sustained low
level cold advection will keep temperatures fall-like, with
highs in the 60s.





&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1124 PM CDT Fri Sep 20 2024

VFR conditions are favored with the caveat of potential
scattered showers and storms tonight into tomorrow bringing
periods of reduced visibility and ceilings. Due to the low
confidence in coverage of these storms (20% chance), have opted
to withhold them from the TAFs at this time. Amendments will be
made if storms develop within the vicinity of a terminal.

Winds will remain southerly and below 12 kts before a cold front
shifts winds clockwise to northerly. The front is expected to
pass KOFK 14-16Z, KOMA 19-20Z and KLNK 18-20Z. Gusts above 12
kts will be possible after the frontal passage. More widespread
showers and thunderstorms will move in from south to north
tomorrow afternoon and continue into Sunday. This will likely
impact KLNK by 18-19Z and KOMA by 19-20Z. Confidence in
widespread showers reaching KOFK is low at this time.

&&

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

&&

$$

DISCUSSION...Fortin
AVIATION...Wood