121
FXUS63 KOAX 201045
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
545 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Rain and thunderstorm chances return late this evening,
  particularly for southeast Nebraska and southwest Iowa.
  through Saturday with it still warm. There is a low chance of
  a few severe storms early Saturday morning and again Saturday
  afternoon or evening.

- Widespread rain will start Saturday night and persist Sunday,
  with the heaviest rainfall anticipated from midnight Sunday
  through noon Sunday (60-90%). Rainfall amounts of 1 to 2
  inches are expected, with local amounts up to 3 inches.

- Above average temperatures, with highs in the 80s and low 90s,
  are expected to persist through today. Cooler conditions will
  arrive over the weekend and into next week, with highs
  dropping into the 60s and 70s.

&&

.DISCUSSION...
Issued at 435 AM CDT Fri Sep 20 2024

Today:

Surface cold front moved through the area, and currently is
located across the lower Missouri Valley area. Shallowness of
frontal zone will be overcome quickly, thus expect return to
southerly flow by afternoon. Upper level ridge anchored over
Texas, will dominate sensible weather. Perusal of deterministic
soundings indicated sufficient dry mixing to allow temperatures
to rise toward 75-percentile of NBM, with areas south of I-80
in Nebraska rising into the lower 90s. No precipitation is
expected as moisture transport will be limited until later in
the evening.

Tonight:

Slight eastward shift of upper level ridge axis expected.
Simultaneously, an upper trough moves Alberta into Saskatchewan,
compressing heights and allowing upper level jet streak to
develop that will traverse the the northern periphery of upper
ridge. Unsure how much forcing for ascent ULJ will provide
given forecast area is located on the anticyclonic shear side.
Better forcing likely will be due 40-50kt LLJ impinging on a
warm front draped across southern Nebraska and northeast Kansas,
aided by a diffuse disturbance moving along northern periphery
of ridge. LLJ will gradually enhance atmospheric moisture as
well, sufficiently to allow ML CAPE to increase into the 1-2K
range after 21/06z. There is variance in bulk shear magnitude
ranging from 30-50kts, with the upper range indicating the
potential for isolated severe storms. The storms should be
elevated in nature, thus primary threats hail and wind. CAMS
providing different scenarios for initiation and coverage, thus
kept highest PoPs in the 30-50% range.

Saturday:

Any convection that develops during the overnight hours is
expected to move east of the area as LLJ veers. The
aforementioned upper trough moving across Canadian plains will
drive a more substantial cold front across the area as the warm
front moves into Iowa and Minnesota. Ahead of the cold front,
trended slightly toward 75% NBM, with 50% NBM values appearing
sufficient behind the front. Simultaneously, a series of upper
level disturbances will move into the central plains ahead of an
upper level low that begins to eject out of the desert
southwest. The net result is increasing dynamic forcing aloft
and low level thermodynamic forcing. HREF indicating SBCAPE
around 1500 J/Kg, with MLCAPE up to 2K J/kg. HREF denoting
increasing bulk shear values in the 30-40kt range as afternoon
progresses. Weak (10-20kt) 0-1km shear producing hodographs with
some low level curvature capped by straight-line hodograph
above. Thus expect isolated weakly rotating storms to grow
upscale into MCS during the late afternoon into the early
evening hours.

Saturday Night and Sunday:

Southwest upper low moves into the central plains Saturday
night. Surface boundary likely will move into the southern
periphery of forecast area due to convection; however large
scale moist isentropic ascent will allow precipitation to
persist north of the front and increased PoPs across the region
into the 60-90% range across all but extreme northeast Nebraska.
Expect some training of convection across extreme southeast
Nebraska into southwest Iowa due to persistent H8 jet 25-30kt,
where WPC QPF indicates storm total QPF 1.5 to 2 inch range.
HREF precipitable water values in the 1.6 to 1.8 inch range are
in 90 percentile for mid-September, so wouldn`t be surprised to
see localized 3 inch totals. Given soil moisture deficits are in
the 1 to 3 inch range, not expecting widespread flash flooding
or flooding; however, cannot discount if isolated rain totals
exceed 3 inches. Precipitation begins to wane by late Sunday
afternoon as upper system lifts into northern plains and jet
streak moves from southwest Kansas into western Missouri Sunday
evening. Scattered shower will linger Sunday night into early
Monday as upper low moves across the area. Temperatures on
Sunday will be markedly cooler due to evaporative cooling and
low level cold advection, with most areas remaining in the low
to middle 60s.

Monday through Thursday:

Cyclonic flow aloft and low level cold advection will keep
temperatures cooler than normal for the early part of the week,
ranging from the 60s on Monday to the middle 70s by Wednesday.
Currently, the forecast remains dry. There`s a possibility of
scattered showers Tuesday night into Wednesday as a short wave
drops southward into the mid-Missouri valley. Upper level
ridging increases Thursday into Friday which will allow
temperatures to increase to climatological normals.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 534 AM CDT Fri Sep 20 2024

Weak northerly winds under 5 knots will quickly become southerly
at 5 to 10 knots at all TAF sites by 14Z. Isolated showers and
thunderstorms will be possible across southeast Nebraska and
southwest Iowa after 04Z tonight. Convection should initially
develop south of KLNK and KOMA, but will have a tendency to
build to the north. While there are chances of rain at these two
TAF sites the last few hours of the forecast period,
probabilities are too low to include in TAFs at this time. just
beyond the TAF period, there will be the potential for scattered
thunderstorms, primarily near KOMA and after 18Z tomorrow. The
stronger storms will bring the risk for hail up to the size of
quarters and wind gusts up to 50 knots.

&&

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

&&

$$

DISCUSSION...Fortin
AVIATION...Darrah