Hail and Damaging Winds in Nebraska, May 12, 2022

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1217 PM CDT Thu May 12 2022

...Updated Aviation Forecast Discussion...

.DISCUSSION...
Issued at 332 AM CDT Thu May 12 2022

Fairly quiet in the forecast area as of 3 AM as earlier storms
that formed on the nose of the low level jet in north-
central/northeast NE have pushed into SD. Southerly flow persisted
across the area, keeping temperatures in the lower to mid 70s
with dewpoints ranging from the mid 60s to lower 70s. Overall,
another muggy night. Otherwise, features of note included the
well-advertised negatively tilted upper level trough centered over
ID/UT and a surface low spinning roughly along the WY/CO/NE
border.

Said trough and low will push northeastward into the Dakotas
today, eventually dragging a cold front from west to east across
the forecast area this afternoon through the evening and overnight
hours. Given the warm, moist air in place, ample instability
(2500- 3500 J/kg of MLCAPE) will be present for storm development
along/ahead of the front this afternoon. While the strongest deep
layer shear still looks to be confined to areas behind the front,
there should be enough overlap of instability and said shear for
organized storms and a severe weather. Given the instability,
large hail will certainly be a threat, especially as storms first
go up. However, the threat should rather quickly transition to a
damaging wind threat as storms congeal into a line owing to
boundary-parallel deep layer shear. There may also be a window for
a quick spinup tornado threat in the area as low level shear
strengthens when the low level jet ramps up this evening. However,
modest 0-3 km shear (largely sub-30 kts) suggests the line may
become quite outflow dominant early. In fact, several CAM
simulated reflectivity fields show hints of an outflow boundary
racing out ahead of the line of storms by 7 or 8 PM, just as the
low level jet is strengthening. While confidence is relatively
high that we'll see severe weather in the forecast area this
evening, a few questions do remain:

1. How far east/west is the cold front when storms go up, i.e.
will we have a large hail threat?

2. How far south/east will the damaging wind threat extend into
the forecast area as the storms become more outflow dominant and
likely eventually elevated this evening?

3. Will the line of storms become too outflow dominant prior to
the low level shear strengthening to preclude a tornado threat?

Current most-likely scenario would say storms go up just west of
the forecast area in central NE around 3-4 PM, congeal into a line
over our northeast NE counties and somewhat quickly gust out as
they push east of Highway 81. Some signs they hold together longer
farther north in the area where 0-3 km shear could be a bit
stronger and more perpendicular to the line. So would say the
greatest risk for severe weather in the area would be north of the
Platte River, though areas to the south certainly have a threat
as well. Just another day with lots of small-scale details to work
out.

In general, the damaging wind, hail, and tornado threat should
gradually decrease as we go through the evening, but storms will
continue as the front progresses eastward overnight. With
precipitable water values over 1.5" and warm cloud depths near 3.5
km, could see some efficient rain production through the night.
In addition, latest CAMs do show some signs of periodic training
of thunderstorms, especially in parts of southeast NE, with some
solutions giving pockets of 1-2" of rain in these areas. That
said, with moisture transport not overly strong, nor pointed
directly into the boundary, and the boundary itself looking to be
at least somewhat progressive, thinking any flooding threat will
be on the lower side, but not zero.

The front should linger in far southeast NE/far southwest IA into
Friday morning though with precip generally on a dissipating
trend. Expect some redevelopment of storms along the boundary by
the afternoon hours, though most CAMs/short term guidance have the
front well east of the area by then. That said, enough larger
scale models/ensemble members keep the front farther west and
in/near the area to warrant some 20 to 30 percent storm chances in
far southeast NE/far southwest IA Friday afternoon.

Otherwise, expect at least some relief from the heat and humidity
behind the front, though still should have warmer-than-average
temperatures into next week, with highs in the mid 70s to mid 80s.
We'll also see more on and off shower and storm chances into at
least the middle of next week a surface boundary sags south into
the area Saturday and hangs around somewhere in the region for a
few days. Still some model spread on exact timing of precip
chances in the area, owing to timing differences in various bits
of shortwave energy sliding through, as well as placement of the
boundary, but there's starting to be a little better agreement on
a couple time periods. First would be late Saturday afternoon into
early Sunday morning. Second would be sometime Monday evening
into Tuesday, though there is a little more timing disagreement in
that period. Currently not expecting any severe weather in either
of those time periods, as appreciable instability looks to remain
south of the forecast area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1143 AM CDT Thu May 12 2022

Deteriorating conditions expected at all TAF sites this evening.
Line of strong TSRA is expected to develop over central NE
sometime around 21Z this afternoon and intensify as it moves east.
Severe winds will likely accompany line of storms with G50KT or
greater by 02Z-03Z. Expect TSRA activity to move out of the area
then around 07Z. Expect very strong non-convective LLWS threat at
all terminals leading up to storm arrival.

&&

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

DISCUSSION...CA
AVIATION...DEE

Hazardous Weather Outlook
National Weather Service Omaha/Valley NE
311 PM CDT Thu May 12 2022

IAZ043-055-056-069-079-080-090-091-NEZ011-012-015>018-030>034-
042>045-050>053-065>068-078-088>093-131015-
Monona-Harrison-Shelby-Pottawattamie-Mills-Montgomery-Fremont-
Page-Knox-Cedar-Thurston-Antelope-Pierce-Wayne-Boone-Madison-
Stanton-Cuming-Burt-Platte-Colfax-Dodge-Washington-Butler-
Saunders-Douglas-Sarpy-Seward-Lancaster-Cass-Otoe-Saline-
Jefferson-Gage-Johnson-Nemaha-Pawnee-Richardson-
311 PM CDT Thu May 12 2022

This Hazardous Weather Outlook is for portions of southwest
Iowa...west central Iowa...east central Nebraska...northeast
Nebraska and southeast Nebraska.

.DAY ONE...Tonight and Friday

A line of severe thunderstorms will move through northeast
Nebraska this afternoon. The primary hazard will be winds to 80
mph or stronger. These damaging winds may be accompanied by hail
up to the size of golfballs in the most intense storms.

This evening, an additional line of strong to severe storms will
move across much of eastern Nebraska and western Iowa. The
strongest parts of this line of storms will be capable of
producing winds to 70 mph or stronger, as well as hail up to the
size of quarters. Storms should move out of the area by 2 AM
Friday.

.DAYS TWO THROUGH SEVEN...Friday night through Thursday

Additional thunderstorms will be possible Saturday night into
Sunday, though they are not expected to be severe.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.