Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 122136

Area Forecast Discussion
National Weather Service Des Moines IA
336 PM CST Thu Dec 12 2019


Tonight through Friday Night: Forecast Confidence: Medium

High Impact Weather Highlights:

Expecting a light wintry mix of snow and freezing rain/drizzle to
impact portions of central and northern Iowa Friday morning
through Friday night. Although snowfall totals will only range
from 1-2" and ice accumulations of just a couple hundreth are
expected, hazardous travel conditions are possible especially on
any untreated roads.

Technical Details:

Initial low pressure system continues to track newd into WI late
this afternoon. To the south/east of the low, mild December temps
are present with readings in the mid/upper 40s across southern
Iowa. However, behind the associated cold front, readings have
fallen off into 10s and 20s and NW Iowa. Besides some flurries or
snow showers behind cold front, expecting rather benign
conditions this evening and into the overnight. Lows will be much
colder than last night with readings in the lower teens north to
the middle 20s south.

Friday...another upper shortwave trough will move across the
Midwest. Increasing SW flow ahead of this system will induce
another round of moderate WAA/isentropic lift peaking from mid-
morning through afternoon. This combined with moisture advection
/specific humidity 3-4 g/kg on 285-290K/ will lead to another round
of light precipitation. There is still some uncertainty with
exact QPF amounts, with the CAM /HREF/ solutions showing higher
values than the global ensemble counterparts. Even the higher
/90th percentile/ solutions are only showing around 0.20" and
would expect 0.02" to 0.10" to be more common. The bigger
challenge is likely to be thermal profiles and resultant p-type.
Deep saturation through the ice nucleation layer will likely yield
mostly snow with the initiation wave of precipitation although
the warm layer aloft tries to push above zero, and PBL temps also
approach 0C near and south of US30. Thus, a messy mix of light
snow, freezing, rain, sleet, or even rain are possible. As
mentioned, only light snow and ice accums are expected, but travel
problems could occur. After the initial wave of forcing pushes
north and east after 21-00z, model soundings indicate that a
fairly deep saturated layer may persist in the lowest 1-2KM. So, a
continuation of at least patchy drizzle/freezing drizzle is
possible into Friday night.

.LONG TERM.../Saturday through Thursday/
Issued at 211 PM CST Thu Dec 12 2019

Forecast Confidence: Medium

Sfc high pressure is forecast to move across the Midwest on
Saturday and Sunday with a predominantly zonal flow aloft. While
conditions are expected to be dry, temperatures will once again
trend below mid December averages with highs only in the teens
and 20s.

Still watching the potential system for Sunday night into Monday
night as additional shortwave energy ejects out of the mean
longwave trough over the western and central CONUS. The various
deterministic and ensemble guidance continues to show lee
cyclogenesis over the central plains with a sfc low eventually
ejecting e/newd. Still quite a bit of uncertainty with the
evolution of this system due to timing and phasing differences.
At this point, impactful weather is still possible over southern
Iowa and south - but will need to watch the evolution over the
next few days. Temperatures still look to remain below normal
early next week, with some indications of a modest warming trend
from mid to late week.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1132 AM CST Thu Dec 12 2019

Low pressure system and associated cold front continues to move
across Iowa. Expecting dry conditions the remainder of the
afternoon into the overnight across all TAF sites. CIGs will be
rather variable...with mostly VFR at KDSM/KOTM. Further north at
KFOD/KALO/KMCW expecting mostly MVFR to transition to IFR for a
few hours just behind front...then back to MVFR/VFR overnight.

Another weather system will move into the area late in the TAF
period. This will likely produce a wintry mix of precipitation
after 15Z at KDSM/KFOD...a bit later at the other terminals.




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