833
FXUS63 KOAX 221138
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
638 AM CDT Sun Sep 22 2024

.KEY MESSAGES...

- Cooler and breezy with widespread (50-75%) showers for the
  first day of Fall. Diminishing by Sunday afternoon and
  evening.

- More seasonable temperatures for the work/school week with
  very low chance (10-15%) chance of showers Tuesday; otherwise
  mostly dry conditions through Thursday.

- Complex weather pattern late in the week, as upper low
  interacts with tropical system, could affect longer range
  precipitation chances.

&&

.DISCUSSION...
Issued at 357 AM CDT Sun Sep 22 2024

Today:

Elongated area of showers is stretched across portions of the
forecast area this morning due to moist convergence 305-310K and
forcing for ascent associated with right entrance area of
70-80kt 250dm upper level jet and PIVA from southwest U.S. 500dm
trough. Forecast models are in consensus in slowly moving trough
eastward, and shearing the system as it syncs with another 500
dm trough that will move into Manitoba and Ontario. Southern
portions of the forecast area remain on the cyclonic shear side
of U.S. trough through 18z which will provide modest dynamic
forcing for ascent; however, moist convergence 305-310K
surfaces moves south of the area, with isentropic downglide
occurring during the afternoon. Thus expect showers, and
possible isolated storms to vary between scattered to numerous
coverage through midday, then wane during the afternoon hours.
There remains 5-15% potential for heavier rainfall, as HREF
precipitable water values 1.4-1.8 (75-99 percentile)persist
through late morning; however, average rainfall will range from
trace to 0.75 inches.

Breaks in cloud cover are expected across northeast Nebraska and
northwest Iowa, with skies remaining mostly cloudy further
south. Low level cold advection will keep temperatures 15-20
degrees cooler than Saturday, with highs ranging from the lower
60s along the Kansas and Missouri borders, to to around 70 along
the South Dakota border.

Tonight:

A few showers may develop this evening as upper level low moves
across the area. With isentropic fields moist convergence again
developing 305-310K. Coverage at best will be isolated so kept
PoPs < 20%. Cloud cover is expected to linger overnight, which
will mitigate potential cooling, and NBM temperatures appear
reasonable with a small adjustment above mean.

Monday:

Upper low slowly moves generally along KS-NE border during the
day and weak, moist isentropic convergence 305-315 persists, so
isolated showers possible through mid-day southeast Nebraska and
southwest Iowa. By afternoon, forecast area on anticyclonic
shear side of upper trough, along with large scale isentropic
descent, thus any showers will come to an end.

800dm thermal trough will remain over area through midday,
resulting in continued low level cold advection. By mid-
afternoon, southerly flow returns. Net result is that
temperatures may be a little warmer on Monday, but still near
seasonal norms in the 70s. Clearing skies and radiational
cooling may allow patchy fog to develop overnight, especially
where ground is moist from rainfall.

Tuesday:

A vigorous upper trough dives southward from Alberta into the
central plains on Tuesday. The deterministic models have been
somewhat inconsistent in development of upper low centers, but
are in better agreement each successive run. Passage of trough
and any associated upper low will result in very low chance
(10-15%) of very light rain or sprinkles, but that`s about it
given moisture is limited to mid-levels of the atmosphere.
Downslope winds will allow for warming max temperatures in the
middle to upper 70s.

Wednesday through Saturday:

Generally dry conditions are expected. Friday into Saturday
somewhat in question. EC and CMC now depicting a similar
evolution of the large scale pattern similar to earlier runs of
GFS. Both develop closed low near ArklaTex, with fujiwhara
effect between upper low and a tropical system. LREF also
depicting a similar scenario. Not sure how much it will affect
the area, but may result in isolated to scattered tropical-like
convection advecting into southern parts of the forecast area.
Temperatures remain seasonable with highs in the 70s and lows in
the upper 40s to middle 50s.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 636 AM CDT Sun Sep 22 2024

The main story of the TAF period continues to be the light
shower coverage that has started redeveloping near the
KOMA/KLNK terminals, and should gradually dissipate by 14z or
so. No lightning is expected with any of the showers, and
northerly winds will carry us into the afternoon and overnight
hours, with winds dropping to 5 kts or lower after 01z or so.

&&

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

&&

$$

DISCUSSION...Fortin
AVIATION...Petersen