National Weather Service Raw Text Product

348 
FXUS63 KDMX 141742
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1142 AM CST Tue Dec 14 2021

...Updated for the 18z Aviation Discussion...

.DISCUSSION.../Today through Monday/
Issued at 517 AM CST Tue Dec 14 2021

Key Messages:

- Locally Dense Fog Possible Tonight North
- High Wind Warning in effect Wednesday and Wednesday Night. Power
  outages likely.
- A Few Severe Storms with Damaging Winds and Isolated Tornadoes
  possible late Wed Afternoon and Evening
- Record High Temperatures Wednesday. May reach all time December
  record High at Des Moines.

A complex forecast the next few days as an intense upper low 
pressure system moves over California today then towards the Rocky 
Mountains tonight before emerging in the Four Corners region by 
early Wednesday. Lee side cyclogenesis will be ongoing over eastern 
Colorado during this time before quickly deepening on Wednesday and 
racing northeast towards northwest Iowa. 

For today, a warm front is already becoming established south of the 
state as good Gulf flow northward is developing with a large high 
pressure system over the southeast CONUS and the developing low to 
the west. A broad area of stratus has already developed within this 
flow and is just now reaching far southwest Missouri then extending 
all the way south to the Gulf Coast. This moisture and higher dew 
points will move north today and reach far southern Iowa during the 
afternoon with dew points in the 50s already arriving. High 
temperatures today will be in the mid 40s north to mid to upper 50 
south. The moisture surge will continue north and reach northern 
Iowa overnight. The rising dew points will help drive slowly rising 
temperatures through much of the night for the entire area. Areas of 
advection fog may develop overnight, especially over northern Iowa 
where any residual snow cover exists. Will need to monitor for a 
period of dense fog as well. Drizzle is also expected to develop 
over mainly northern Iowa tonight into Wednesday morning as the warm 
advection with in a sufficiently deep low level saturated layer will 
support droplet collision and lead to drizzle development. 

Extremely strong dynamics are still setting up for Wednesday. The 
surface low will continue to deepen through the day and into 
Wednesday night as it lifts northeast through Nebraska and towards 
northwest Iowa by late afternoon then across Minnesota and Wisconsin 
overnight. Several very impressive features with this system. 

First, the winds speeds at very low levels in the atmosphere are 
extreme with this event. Only limited mixing should easily pull 50 
kts into the mixed layer and in many deterministic solutions, 60+ 
kts reaches 900 mb or below and is easily obtainable in the mixed 
layer, especially as the dry line moves through the state. Stratus 
will limit the mixed layer wind early in the day but at least 
partial clearing and better mixing should occur within the warm 
sector by the afternoon from south to north. Not expecting 
widespread high winds in the warm sector as rising motion associated 
with the warm advection will minimize downward momentum transfer of 
the mixed layer winds. Still, the wind likely will climb to above 
advisory criteria in the warm section and could have gusts above 50 
mph. The strongest wind should arrive right behind the dry line 
which will lead to increased mixing to tap into the 60+ kts within 
the mixed layer. The cold advection is not maximized behind the dry 
line but low level lapse rates of 9C plus will be maximized along 
with mid level drying and subsidence. Wind gusts approaching 70 mph 
or higher will be possible. The strongest pressure gradient, cold 
advection and pressure height rises will be over far northwest 
Iowa Wednesday evening. While the mixed layer wind is not quite 
high in this region, those aspects will likely lead to 60 mph or 
greater gusts. This has the potential to be the strongest non-
convective wind event in Iowa in quite some time. Many recall 
November 10, 1998 which was a long duration high wind event over 
Iowa but did not have the max winds this storm may have. The wind 
will gradually subside after midnight as the system lifts away to 
the northeast but still it will still remain windy through that 
time. Despite the lack of leaves on the trees, there remains a lot
of weak areas on trees from the 2020 Derecho that could be 
further damaged during this event. Widespread power outages could 
become a problem. Once the wind switches to westerly behind the 
dry line, expect semi traffic to have very difficult travel on 
north/south bound roads including Interstate 35.

Still expecting thunderstorm potential immediately ahead of the 
dry line from late Wednesday afternoon and racing east during the 
evening. MLCAPE values of 500-1000 J/kg are still expected which 
appears will be sufficient for updrafts to withstand the strong 
shear, much of which will be in the low levels. The obvious 
question is what will the tornado potential be with any storms? 
Past climatology given the upper and surface low locations would 
suggest a tornadic threat across the state. The 0-1 km shear 
values of 45 to 50 kts and 0-1 km SRH values around 400 m2/s2 are 
quite high for Iowa but are in the range of past low topped 
tornadic supercell events. LCL heights will be extremely low. Do 
expect the potential for tornadoes in this scenario should storms 
develop and survive the extreme shear. Mesocyclones very well 
could be shallow and difficult to detect beyond 60 nm from the 
radar in addition with storm motions near 70 kts. It would not 
take much of a cold pool to further enhance any local winds so 
with that, winds of 80 mph plus are not out of the question.

Finally, record warm weather is forecast for Wednesday with some 
areas topping all time December records. The daily records are 
certain to fall, the all time records of 69 at Des Moines is 
reachable is the clouds break. Per the Iowa Environmental Mesonet, 
the all time record December dew point at Des Moines is 60 which was 
reached on December 28, 1984. That may also be attainable.

Did not change much to the forecast beyond Wednesday night. High 
pressure will dominate large parts of the forecast into the weekend 
with cooler and mostly dry conditions expected. The focus will 
remain on the Wed/Wed night event until it passes.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1139 AM CST Tue Dec 14 2021

VFR conditions will erode later this afternoon across the TAF
sites as MVFR ceilings push into the area. Moving into the
overnight, MVFR and IFR conditions will continue to prevail across
KMCW/KALO with less certainty at sites KFOD/KDSM/KOTM as they may
reside near the western edge of the MVFR/IFR cloud cover. Winds
will also quickly pick up towards the end of the period and beyond
with S/SW winds gusting into the 30s kts by 18z and in excess of
45 to 50 kts in the hours thereafter. 


&&

.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.

&&

$$

DISCUSSION...Donavon
AVIATION...Curtis