Blowing Dust Advisory 1, NWS Omaha, NE

Area Forecast Discussion
National Weather Service Omaha/Valley NE
200 PM CST Wed Dec 15 2021

...Updated Mesoscale Forecast Discussion...

.MESOSCALE DISCUSSION...
Issued at 116 PM CST Wed Dec 15 2021

A Tornado Watch has been issued for much of eastern Nebraska and
western Iowa. Temperatures have soared into the 70s and dew point
temperatures are in the upper 50s. Storms are currently ongoing
in south central Nebraska where tornado and thunderstorm warnings
have been issued. The storms have a forward motion to the
northeast at approximately 80 mph. Widespread damaging winds are
expected with peak wind gusts up to 90 mph possible. Impressive
shear and storm relative helicity values will aid smaller scale
circulations within the squall line leading to a risk for
tornadoes.

After the squall line has passed through, there may be a lull in
wind gusts, but it will only be temporary. Sustained wind speeds
of 30 to 40 mph are expected through the afternoon and into the
evening. Peak wind gusts of up to 70 mph are forecast between 4 PM
and 8 PM. This will be several hours of severe criteria winds
which could lead to damage to trees, power lines, and some
structures. Power outages are likely. Winds are expected to begin
to decrease after 8 PM, but 40 mph gusts are forecast up until
midnight, after which the winds gusts should drop to lower than 20
mph.

Along the NE/SD border, light snow with accumulations under an
inch are possible. Blowing snow leading to reduced visibilities
due to strong winds and snowfall could result in hazardous
travel.

&&

.DISCUSSION...
Issued at 401 AM CST Wed Dec 15 2021

.Synopsis:

A highly impactful weather system is set to affect the entire
forecast area today, with prolonged damaging winds gusts, areas of
extreme fire danger, severe weather, and brief whiteout conditions
all being possible hazards.

Today:

Water vapor imagery this morning places a very progressive,
neutrally-tilted upper trough over the Intermountain West, with
broad southwesterly flow at the all levels over the central CONUS.
This trough is expected progress to the lee of the Rockies by late
morning, beginning a rapid deepening process throughout the day. A
surface center of low pressure is expected to develop and travel
from the Nebraska Panhandle across northern Nebraska/souther
South Dakota, dragging a very strong dryline ahead of a cold front
to the west of it. As the system deepens, the mid/upper flow is
expected to become negatively-tilted, inducing strong moisture
flux into the area and general warm air advection as extremely
strong wind fields in the lower levels develop as the system
deepens. Deep mixing and extremely impressive low-level winds are
going to allow for efficient momentum transfer to the surface.
Wind gusts values are expected to land in the 60 to 75 mph range
this afternoon, with the strongest winds being associated with a
southwest to westerly sting jet that wraps around the dry conveyor
belt. Power outages, damage to trees/power poles/potentially
structures, and hazardous travel conditions for any vehicles are
expected. To put this system in context, the large majority of
fields available (such as surface temperatures, sustained winds,
and gust) are at the very top of the ECMWF model reanalysis for
this time of year; going back 20 years.

The next piece of the hazard puzzle comes in the form of extreme
fire danger, associated with the strong winds and low relative
humidity values behind the dryline. As the dryline passes through
the area, portions of southeast Nebraska are going to see dewpoints
fall by as much as 44 degrees over the course of an hour or two.
Arcing power lines and sparks from vehicles or machinery could serve
as a source of ignition while winds gust to over 70 mph in spots
collocated with minimum relative humidity values as low as 12%. The
main are of concern for extreme and uncontrollable fire behavior is
generally south of a line from Columbus to Harlan, and also has Red
Flag Warning in place beginning at noon.

With the strong moisture flux ahead of the dry line, dewpoints ahead
of the surface low and dryline top out in the low-to-upper 50's
assisting MLCAPE values reach values as high as 900 J/kg in far
northeastern Nebraska, with bulk shear values well upwards of 60
kts. Current CAM runs indicate that the main convective mode will
take the form of a line of showers and storms along the main
boundary, reinforcing the primary wind threat. Sufficient
curvature exists in the low-level hodographs to support a tornado
threat embedded in the main line of storms as well, with the main
area of concern being in northeastern Nebraska into north-central
Iowa. A lower-probability scenario is the possibility of a more
discrete, quick-moving tornadic supercell or two where the main
pressure falls are expected with the advancement of the surface
low. The main failure mode of the latter scenario will be the
difficulty that a consistent updraft will be sustained despite the
very strong shear and entrainment. The main timeframe for severe
weather in the CWA will be from 2 to 7 PM.

A complicating factor to the forecast associated with the wraparound
moisture will come in the form of snow along the backside of the
surface low, where wind speeds will still be strong as much cooler
temperatures push south; introducing the possibility of brief
whiteout conditions. The chances for this happening currently look
to stay north of a line from Albion to Pender. These rapid-onset
whiteout conditions are often accompanied by a flash freeze of road
temperatures, creating extremely hazardous travel conditions if
they manifest. The main timeframe for this hazard looks to be from
5 to 9 PM.

The overall weather even is expected to wind down late this evening
into the overnight hours, with wind gusts returning to sub-30 mph
speeds by sunrise.

.Thursday into Early Next Week:

Going forward into the forecast period quiet weather is expected as
the mid/upper pattern modulates between zonal to southwesterly flow.
Little chance for meaningful precipitation are forecast, as the
main weather makers miss our neck of the woods to the
north/south/east. Highs will generally stay above-average, and in
the low-to mid 40's for much of the area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1134 AM CST Wed Dec 15 2021

Extremely strong wind gusts will occur this TAF period with peak
wind gusts of 50 to 60 kts. The strongest winds are expected to
occur between 22Z and 02Z. There are signals from the HRRR/RAP/FV3
that wind gusts could be stronger than forecast. A line of
thunderstorms are also forecast to move through the area this
afternoon. That line of storms will be moving quickly with speeds
up to 80 mph.

The strong winds will not end with the storms. A wind shift is
expected from the south to the southwest, and eventually to the
northwest. Severe criteria winds are expected to persist for
several hours. Ceilings may briefly drop down MVFR criteria with
the passage of the thunderstorms. Skies will quickly clear up
afterward.

There is likely not to be a typical updated afternoon general AFD.
Next update will be on the overnight. There will be an updated
aviation discussion for 00Z TAFs.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...High Wind Warning until 9 PM CST this evening for NEZ011-012-
     015>018-030>034-042>045-050>053-065>068-078-088>093.

     Red Flag Warning until 9 PM CST this evening for NEZ050>053-
     065>068-078-088>093.

IA...High Wind Warning until 9 PM CST this evening for IAZ043-055-056-
     069-079-080-090-091.

     Red Flag Warning until 9 PM CST this evening for IAZ069-079-080-
     090-091.

&&

MESOSCALE...Fajman
DISCUSSION...Petersen
AVIATION...Fajman

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