844
FXUS63 KOAX 202116
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
416 PM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Rain and thunderstorm chances return late this evening,
  particularly for southeast Nebraska and southwest Iowa.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 (50-90%). Rainfall amounts of 1 to 2
  inches are expected for southern counties.

- End of the 90s? Cooler conditions will arrive over the
  weekend and into next week, with highs dropping into the 60s
  and 70s.

&&

.DISCUSSION...
Issued at 415 PM CDT Fri Sep 20 2024

The cool air that built into the area last night behind a cold
front was quickly mixed out. Most of the area is enjoying sunny
skies and high temperatures in the lower 90s. Those 90s don`t
come around that often in late September. In fact, over the past
30 years the average last date of 90 degree temps is September
19th. Today or tomorrow may be the final 90 degree day of 2024
for the Corn Belt. RH values are pleasantly low with numbers
under 30%.

Moisture transport ramps up this evening on the developing LLJ.
Within 12 hours, PWAT doubles from less than an inch to nearly
2" which is climatologically in the 99th percentile for
September`s second half. CAMs aren`t producing a lot for
convection tonight, but with LLJ in place, expect CAMs to under
forecast. Elevated convection will have steep mid-level lapse
rates available and sufficient instability to produce some
stronger storms. The elevated storms won`t have all the shear
available to them, so that may be the missing ingredient to
produce damaging wind or hail. Still PoPs of 20% seem reasonable
as does the marginal severe weather outlook with the best
chance of strong storms between 11pm and 3am tonight.

.SATURDAY...

As the LLJ shifts east, the nose of the warm front pushes
northeast toward the Great Lakes and a strong cold front will
push through this CWA mid-day. Forcing for ascent along the
front will increase as the upper level low spinning across the
Four Corners approaches and throws shortwave energy through the
Corn Belt. Again, the instability sufficient if not
considerable at about 1500 J/kg in areas south of I-80. Shear
values of 30-40 knots may produce an isolated thunderstorm after
lunch south of the interstate. It won`t be a rain-out for most
outdoor activities... not yet... as the convective nature of the
precip will leave at least as many locations dry as wet.

Heaviest and most widespread rain waits until after midnight as
the upper low pushes into Wyoming and increases the isentropic
lift along the plunging cold front and produces widespread
forcing for ascent. PoPs grow through the evening until peaking
at 50-95% for the southern half of the CWA overnight and early
Sunday morning. Latest HREF produces a swath of 1" in far
southeast Nebraska and southwest Iowa... notably farther south
than runs from earlier this week. PMM QPF maps tend to keep the
precip bullseyes in central Iowa, western Missouri, and
southwest Kansas, keeping the best drought-busters away from
our farmers and ranchers. WPC QPF now keeps the 1-2" QPF totals
south of I-80.

High temps on Saturday may range from near 90 at Omaha, Lincoln,
Beatrice, and points south to shy of 80 along the SD state line.

.SUNDAY...

Much cooler air will be in place with the spinning upper low
passing over head and relentless CAA at the surface. Expect
highs mostly in the 60s. It`ll be the coolest day since early
May. Rain showers - heavy in the morning - will taper and come
to an end by Monday morning.

.NEXT WEEK...

The GFS pushes an upper low through the area on Tuesday and
tries to produce some showers but other guidance, including the
NBM, aren`t on board. Have kept the forecast dry with
temperature moderation forecast over the course of the work week
leaving high temperatures close to late September norms of
upper 70s by the end of the week.


&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1228 PM CDT Fri Sep 20 2024

VFR conditions persist through the TAF period with the caveat
noted that scattered showers are increasingly possible in the
southern half of the forecast area over the next 24 hours.
Those showers are not included in the TAF as of now with chances
of precip just below 30% until after 18Z. Both LNK and OMA could
see some TSRA or SHRA toward the end of the TAF period. They may
be added to future TAFs should confidence increase.

Otherwise south-southeasterly flow of about 10kt continues
until a cold front pushes through mid-day Saturday. It may pass
through OFK around 15Z and around 20Z Saturday at OMA and LNK.

&&

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

&&

$$

DISCUSSION...Nicolaisen
AVIATION...Nicolaisen