FXUS63 KDMX 111146

Area Forecast Discussion
National Weather Service Des Moines IA
546 AM CST Tue Dec 11 2018

...Updated for 12z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 431 AM CST Tue Dec 11 2018

09z sfc analysis shows a weak, ill-defined sfc high/anticyclone
crossing into northwestern Wisconsin and low pressure over the
plains. GOES-R IR imagery has shown widespread mid-upper level
clouds across Iowa overnight which has helped prevent a purely
radiational cooling night and subsequent widespread fog potential.
Today should be quiet across the state as our low shouldn`t have
influence in Iowa until this evening into the overnight hours. Given
that models have underdone temperatures in non-snow covered
locations and have overdone temperatures in snow-covered locations
the past few days, have adjusted temperatures appropriately.

09z Water vapor imagery picking up on shortwave propagating
east/southeastward through Wyoming. 1.5 PVU sfc excellent for
tracking the progression of this system. As hinted at above, this
track will continue overnight, with models in great agreement
placing the wave over western Iowa by 06z Wed, and into
Wisconsin/Illinois by 12z-15z Wed. Westward tilt with height,
supporting strengthening as sfc reflection crossing into Iowa
around 21z Tue-00z Wed, and departing the state around 09z-12z
Wed. Decent 950mb frontogenetical forcing phased with weak
moisture surge through the DGZ should support flurries as the
attendant boundary propagates through Iowa from west-east.

As sfc low strengthens over NE Iowa, cold air begins to advect into
Iowa on the backside of the comma-shaped low. Right now have light
freezing rain/drizzle. Wetbulb temperatures close to 25 degrees. Am
wondering if this cold air aloft makes it in sooner that precip type
will change to pure flurries. Expecting little to no impacts at this
time from this system, but will monitor and fine-tune forecast

.LONG TERM.../Wednesday through Monday/
Issued at 431 AM CST Tue Dec 11 2018

Tomorrow Night through Thursday Night...
Another system looks to impact Iowa tomorrow night into Thursday.
This system is slightly more concerning impact-wise. Models in
good agreement featuring the upper low that is currently around
500 miles off the Pacific NW rapidly propagating to the SE, making
it all the way down to the Oklahoma Panhandle region by around
06z Thu. Concurrently, a PV anomaly will eject off this low and
ride along the Canada/US border. These systems are generally
phased with the southern system being notably stronger than the
northern system. A deformation zone with strong 950mb
frontogenetical forcing extends off the southern system into
central-southern Iowa. 850mb RH analysis shows a moisture surge
pushing into Iowa from south to northeast roughly making into the
south-central and then eastern portion of Iowa... suggesting this
is where the more impactful QPF will be. At this time, it seems
the best thermodynamic forcing with this system will remain in
Illinois, but with run-to-run consistency yielding only medium
confidence, will watch for forecast changes.

For ptype, forecast soundings at KLWD (Lamoni, IA) have a huge warm
nose from roughly 3kft-6kft above the sfc, with temps falling below
freezing sub-3kft. This sounding strongly supports freezing
rain/freezing drizzle. Forcing/moisture may make it up to the Des
Moines metro/Ames area. Should this occur, the Thursday morning
commute could be notably impacted. At this time, confidence not high
enough for any headlines, but will monitor closely for need of

Friday PM and Beyond...
Thermal ridge building over the intermountain west crashes down
into Iowa on the heels of the Thursday system, leading to warmer
temperatures and dry conditions for this weekend. Highs should
make it into the 40s, near to slightly above normal.


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 542 AM CST Tue Dec 11 2018

Mainly VFR conditions throughout this TAF period. Only wx impact
likely to come via a band of flurries that pushes through the
state from west to east during the evening hours from roughly 00z
Wed through 06z Wed. At this time, it appears precip should be
light enough to where vsbys should stay at or above 6SM, but will
have to monitor.

KALO may have lingering precip to deal with from around 09z Wed
through 15z Wed or so which will be fine tuned in the next




SHORT TERM...Kotenberg
LONG TERM...Kotenberg

NWS DMX Office Area Forecast Discussion