National Weather Service Raw Text Product

935 
FXUS63 KDMX 150003
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
603 PM CST Tue Dec 14 2021

...Updated for the 00z Aviation Discussion...

.SHORT TERM.../Tonight through Wednesday/
Issued at 200 PM CST Tue Dec 14 2021

Key Messages:

- Significant, possibly historic, high wind event possible late
  Wednesday afternoon into Wednesday night. 
- Line of quick moving storms expected late tomorrow afternoon
  and evening. Enhanced wind gusts and a few tornadoes possible. 

At 20z GOES water vapor imagery depicted a pronounced shortwave
trough moving onshore to the western conus. Pieces are already in
motion well downstream that will set the stage for a very active 
Wednesday, evidenced by fog/stratus over KS/MO on the leading each
of southerly moisture return. Our weather conditions remain quiet
through the remainder of the day into the tonight. Pressure falls
in response to the approaching wave will ramp up local advection 
patterns overnight, pushing a highly anomalous warm/moist air 
mass northward. Temperatures may briefly dip this evening before 
holding steady or even rising a bit through the overnight. Some 
fog or drizzle will be possible as moisture continues to increase,
especially over the remnant snow pack in northern Iowa. 

The powerful upper low ejects from the four corners region
tomorrow. Lee cyclogenesis fosters a rapidly deepening surface 
low in southeast Colorado that quickly pushes northeast. The 
response out ahead will be continued pressure falls and unabated 
theta-e advection. Daily record highs are a near certain, but 
whether we push toward monthly records will hinge on stratus 
coverage. Kinematic forcing overspreading the moist boundary layer
may trigger some scattered shower activity during the afternoon. 
Much more important is the developing high wind event which can be
broken down into three periods: 

1 - Gradual strengthening of southerly winds during the morning 
and afternoon. It is less likely that we will see the advertised
higher end gusts during this period as a warm air advection 
regime is less likely to efficiently mix down higher velocity air
aloft down through the boundary layer. However, still at least 
advisory level winds developing by the afternoon with gusts of 40 
to 50+ mph likely. Some stronger gusts are possible within any 
shower activity. 

2 - Exceptional wind gusts possible late afternoon and early 
evening as a dry line pushes through. Model soundings all show 
deep BL mixing and rapid drying/subsidence, promoting a favorable
environment for transporting higher momentum air aloft to the 
surface. SW gusts of 55-70 mph look likely, possibly even higher 
in some locations as model soundings indicate 70-80+ kt winds at 
the top of the boundary layer at times. Very concerning should
those be realized at this surface. 

3 - Hi-res models are picking up on a brief lull in the strongest
gusts as the local pressure gradient relaxes a bit before a cold
front surges through later Wednesday evening. An intense 
isallobaric contribution will be the main driver through this 
period owing to pressure rises on the order of 10-15mb/3hr. Gusts 
of 50-60+ mph will remain in play,especially over northern Iowa, 
until the system quickly pushes off the northeast late Wednesday 
night into early Thursday morning. 

Not mentioned yet is the severe weather concern as the dry line 
swings through, most likely during a 4pm-9pm window. Warmer air 
aloft will keep us capped most of the day, but model soundings 
show the cap eroding by late afternoon as the dry line approaches 
western Iowa. Nearly all hi-res guidance shows rapid convective 
initiation with exceptionally fast storm motions to the ENE. 
Ensemble max sbcape does not exceed 1000 J/kg, but low topped 
supercells are still in play in the low cape/high shear 
environment. HREF shows this signal with updraft helicity paints a
few tracks over northern Iowa into Minnesota, highlighting where 
low level directional shear is more favorable. A few tornadoes 
are possible in addition to the locally enhanced wind gusts as 
downdrafts transport higher velocity winds down to the surface.

It is important to note that the comprehensive severity of this
event is still a bit uncertain given the highly anomalous setup 
with few historical analogs that compare. However, it is becoming 
increasingly likely that at least portions of our area will 
experience an impactful high wind event with the potential to 
create power outages, disruption to travel, and trees and 
structural damage.

&&

.LONG TERM.../Wednesday night through Tuesday/
Issued at 1249 PM CST Tue Dec 14 2021

Key Messages:

- Largely quiet and cooler, but still near to above normal 

Synoptic scale flow will remain predominantly zonal to southwesterly 
through the remainder of the week. While it will be cooler, 
temperatures will remain above normal with highs in the mid 30s 
to mid 40s, continuing the warm fall and winter trend thus far. 
Leaning into the weekend, shortwave troughing will move across the
northern US and bring with it another push of CAA that will drop 
temperatures more in line with normals this time of the year, 
highs in the 20s to mid 30s and lows 10s to 20s. Just ahead of the
CAA, moisture will begin to move back northward into the Missouri
Valley, but will struggle to provide much influence in Iowa. A 
light shower or two may be possible Friday PM across the far SE, 
but inconsequential. The largely dry air mass in place overhead 
and better mid-upper level forcing into Minnesota should also keep
the remainder of the state dry. Alternating periods of surface 
high and low pressure Saturday onward will provide breezy 
conditions at times, but much more in line with what one would 
expect this time of the year. Temperatures too will continue to 
remain seasonable in the mid 20s to 30s for highs and low double 
digits to mid 20s into next week.

&&

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 600 PM CST Tue Dec 14 2021

VFR conditions still expected to erode this evening into the
overnight hours with MVFR and some IFR CIGs expected, the lowest
CIGs most likely to impact northern sites MCW and possibly ALO.
Some patchy fog is also possible in northern Iowa, but may not
directly impact TAF site MCW, so not in TAFs at this time. Winds 
become the other main factor overnight with LLWS concerns during 
the overnight hours before strong winds out of the south/southwest
continue to increase during the day Wednesday, becoming sustained
20-30 knots, gusting to 50+ towards the end of the TAF period.
Additionally, rain/thunderstorm chances increase into the 
afternoon and evening tomorrow with thunderstorms impacts most
likely starting around 23Z.  

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from noon Wednesday to midnight CST Wednesday 
night for IAZ023>025-033>037-044>049-057>061-070>075-081>086-
092>097.

High Wind Warning from 3 PM Wednesday to 3 AM CST Thursday for 
IAZ004>007-015>017-026>028-038-039-050-062.

&&

$$

SHORT TERM...Martin
LONG TERM...Curtis
AVIATION...KCM