National Weather Service Des Moines IA
541 AM CST Wed Dec 15 2021
...Updated for the 12z Aviation Discussion...
.DISCUSSION.../Today through Tuesday/
Issued at 423 AM CST Wed Dec 15 2021
- Historical Weather Today!
- Unprecedented Moderate Risk for severe weather later this
afternoon and early evening.
- Extreme non-convective wind gusts over 70 mph possible late this
afternoon and evening.
- Record high temperatures WILL occur and some locations may have
all time December record highs.
- Locally dense fog early over northern Iowa.
Surface cyclogenesis is ongoing on the Lee side of the Wyoming and
Colorado Rockies this morning as an intense PV anomaly is moving
eastward over Utah and northern Arizona. The cyclogenesis will
become more focused over eastern Colorado this morning and deepen
quickly as the upper system moves eastward. Strong southerly flow
has developed this morning with a large area of high pressure over
the eastern CONUS and the developing low to the west. This has
led to an impressive surge of surface dew points northward. Dew
points in the 60s is now near the Kansas City area and dew points
in the 50s are now north of Highway 20 in Iowa which is north of a
Waterloo to Fort Dodge line. Temperatures are following suit and
are already approaching records highs for this date with records
in the mid to upper 50s for much of the area.
Synoptic High Wind Event...
Mixed layer wind will gradually increase through the day,
however, the mixing efficiency of the mixed layer wind to the
surface will be negated some by the vertical ascent created by the
ongoing warm advection today. The pressure gradient will not be a
big factor through the early afternoon hours. A few wind gusts
over 50 mph will be possible during the early afternoon before the
stronger mixed layer winds and pressure gradient arrives for mid
to late afternoon ahead of the dry line and expected thunderstorm
development. Expect some wind gusts to 60 mph ahead of the storms.
A brief reprieve from the higher gusts is possible in the wake of
the thunderstorms but the strongest of the non- convective wind
gusts is expected to arrive this evening. An extremely intense
pressure gradient of over 100 microbars/km will arrive over
central and northern Iowa with 6hr pressure height rises of 24
millibars or greater. Differential cold advection will increase
low level lapse rates to in excess of 9C/km and several other
indicators of strong subsidence following the dry line including
strong mid-level drying and subsidence, will all further promote
strong mixing and momentum transfer of mixed layer winds. Concerns
continue to increase regarding the potential for a sting jet
driving extremely strong winds to the ground this evening. Mixed
layer winds in the 70 to 80 kt range are extreme certainly and if
that does indeed mix down, widespread damage and power outages is
possible. The wind will gradually diminish after 06z from
southwest to northeast as the low pressure departs.
Severe Thunderstorm Threat...
SPC has placed a Moderate Risk for severe storms across a large
portion of central and northern Iowa for later this afternoon and
into the early evening. Storms are expected to fire along the dry
line by mid to late afternoon then race east/northeast across much
of the area into the early evening. Storm motions of 70 to 80 mph
are expected. Damaging wind gusts are certainly a primary threat
with the potential for cold pool development high with the 0-3 km
theta-e differences near 15K or greater. It will not take much for
a cold pool to help produce 80+ mph winds with any storms given
the very strong winds just of the surface. Numerous storms along
the line could produce long yet narrow sustained areas with these
80+ mph winds. Many severe weather parameters would suggest a
tornadic threat as well with the 0-1 km SRH values near 400 m2/s2
and very low LCLs and much of the instability will be contained
in the lowest 0-3 km which will promote vortex tube stretching.
Should any tornadoes develop, it would not take a very strong
rotation to push the wind on the right side of the tornado
movement into the EF2 range considering an storm motion that would
already be 80 mph. A 40 mph rotation would push these wind to 120
mph. Given that speed movement, the damage would more represent
an enhanced area of straight line damage while the wind on the
left side would be nerfed so to speak due to the high storm
Finally, record high temperatures WILL occur today. The question is
will some locations reach all time records for December. The all
time record for Des Moines is 69 degrees, which has occurred several
times. The forecast for Des Moines is 71 and with any sun at all
today, that record should be broken. The dew points today in the 50s
to low 60s is also near records for December.
Mesoscale Discussion 2027
NWS Storm Prediction Center Norman OK
1228 PM CST Wed Dec 15 2021
Areas affected...Portions of eastern NE...northeastern KS...northern
MO...western/central IA...far southeastern SD...and southern MN
Concerning...Severe potential...Tornado Watch likely
Valid 151828Z - 152030Z
Probability of Watch Issuance...95 percent
SUMMARY...Storms are expected to develop and rapidly intensify as
they race northeastward this afternoon. Widespread severe winds of
60-75 mph are likely, with gusts to 80-100 mph possible. A few
tornadoes, including the potential for a strong tornado or two, may
also occur. Tornado Watch issuance is likely by 2 PM CDT.
DISCUSSION...A compact and intense shortwave trough evident on water
vapor satellite imagery over the central High Plains will eject
quickly northeastward across KS/NE this afternoon, reaching the
upper MS Valley by this evening. A strong low-level mass response is
occurring ahead of this feature, with surface pressure falls
generally ranging from 3 to 5.5 mb per 2 hours based on latest
observations across western/central KS/NE. Meanwhile, over eastern
KS/NE into much of MO/IA, low-level moisture continues to rapidly
advect northward in tandem with a pronounced, 50+ kt
south-southwesterly low-level jet. Surface dewpoints have generally
increased into the mid 50s to low 60s across this region. As the
pronounced large-scale ascent associated with the shortwave trough
overspreads the warm sector, the sharp low-level inversion noted on
the 18Z sounding from TOP will quickly erode.
Based on linear extrapolation of the band of clouds/showers
attendant to the lift with the approaching shortwave trough
currently over western KS, convective initiation across parts of
central/eastern NE and adjacent northern KS to the east of a surface
dryline appears increasingly likely by 20-21Z (2-3 PM CST). These
storms are forecast to rapidly strengthen and race northeastward as
they encounter at least a weakly unstable airmass and greater
low-level moisture with eastward extent over KS/NE. MLCAPE generally
ranging from 500-1000 J/kg will be more than sufficient to support
Very strong flow at both low and mid-levels is expected to
overspread the warm sector as the shortwave trough ejects
northeastward through this evening. The potential for widespread
severe wind of 60-75 mph appears likely, with stronger gusts up to
80-100 mph possible, as these anomalously strong winds aloft reach
the surface through mixing and convective downdraft processes. With
the deep-layer shear vectors oriented at least partially orthogonal
to the surface boundary, some potential for supercells may exist.
Given the strength of the low-level shear, a few tornadoes appear
likely, and a strong tornado or two may also occur. Otherwise, a
broken band of low-topped storms will likely develop as convection
shifts into western/central I, southern MN, and vicinity later this
afternoon and early evening, with a continued widespread
severe/damaging wind threat and potential for a few tornadoes. The
rapidly increasing severe risk across this region will warrant
Tornado Watch issuance by 20Z (2 PM CST).