Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201140

Area Forecast Discussion
National Weather Service Des Moines IA
540 AM CST Sat Jan 20 2018

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

.SHORT TERM.../Today through Sunday/
Issued at 431 AM CST Sat Jan 20 2018

Lee side surface cyclone over eastern Colorado will continue to
organize today. Gulf moisture will continue to lift north as
moisture lifts north on the west side of a large high pressure
system situated over the southeast CONUS. This moisture is well
represented by low clouds and fog over much of eastern Oklahoma and
eastern Texas. Farther north, dew points in the mid 30s have reached
much of eastern Kansas and western Missouri. The area of low clouds
and fog will reach far southern Iowa by this afternoon the expand
northward overnight. As the moisture deepens and warm advection
increases, collision coalescense will occur for drizzle to develop.
The freezing drizzle is expected over portions of central and
northern Iowa where temperatures remain below freezing and/or ground
temperatures remain below freezing, resulting in some icing late
Saturday night into Sunday morning. Any icing amounts should remain
at one tenth inch or less with mainly travel impacts. Temperatures
by Sunday afternoon should be at or above freezing with little to
no winter weather expected until late afternoon or evening.

.LONG TERM.../Sunday night through Friday/
Issued at 428 AM CST Sat Jan 20 2018

Deeper moisture will arrive Sunday night as the low pressure
organizes to the west and the pressure gradient increases. Several
challenges remain with this system. Overall a good consensus on the
500 mb closed low track from northern Missouri into northern
Illinois late Sunday night and Monday, though the GFS remains a bit
faster and farther north. The deformation snow band is well north
of the surface low which moves into central and southeast Iowa and
more in the vicinity of a strong thermal gradient near 700-750 mb
and a region of good isentropic lift. The deformation snow band will
be mainly focused over southeast South Dakota and southern Minnesota
but may clip into northeast Iowa. The snow band will have a tight
gradient and any subtle shifts in location will have a big impact on
snow amounts. As of now, a wintry mix of precipitation remains
likely over the far north along with the aforementioned ice
potential. In addition some wrap around snow remains likely even if
the primary deformation snow misses to the north. Therefore, have
maintained the winter storm watch area as is but adjusted to add 6
hours into Monday evening to include slower timing and the potential
for blowing snow as winds increase late Monday and Monday night.
Blowing snow could become an issue is several inches of snow does
occur with wind gusts over 40 mph expected.

Farther south, strong kinematic and thermal advection arriving
Sunday evening should generate a region of elevated thunderstorms
over central and southeast Iowa. These storms likely would contain
graupel or small hail. This activity will rapidly lift northeast
ahead of the mid level dry slot. The warm sector will reach a the
south during this time and could have temperatures near 50 degrees
at some locations mainly south of Interstate 80 Sunday night before
the cold advection arrives Monday morning. Much of the area will
have the potential for some light snow Monday and Monday night as
the wrap around light snow band moves through and cold advection
arrives with the surface low departing to the northeast. The gusty
winds to the north will move across the entire area during this time
and may require wind or winter weather headlines pending how much
snow falls.

Little change to the remainder of the forecast with mostly dry
conditions expected through much of next week with high pressure
arriving Tuesday night and Wednesday the a return to warm advection
by Thursday into Friday. Falling temperatures on Monday then
remaining seasonably cool into Wednesday before temperatures rebound
back into the 40s to end the period.


.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 536 AM CST Sat Jan 20 2018

Significant changes made to this TAF fcst from previous issuance.
Notably increased magnitude and made earlier arrival for low
CIGS. IFR CIGs now expected at KOTM by 20z... falling to LIFR by
00z Sun. At KDSM... MVFR to arrive by 00z Sun, with IFR by 03z
Sun. Kept LIFR out of this package, but LIFR cannot be ruled out
around 12z Sun. Remaining TAF Sites likely to hit IFR from low
CIGs towards end of this TAF period.


Winter Storm Watch from Sunday evening through Monday evening
for IAZ004>007-015.



SHORT TERM...Donavon
LONG TERM...Donavon
AVIATION...Kotenberg is the U.S. government's official web portal to all federal, state and local government web resources and services.