Area Forecast Discussion
Issued by NWS Des Moines, IA

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347
FXUS63 KDMX 211041
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
441 AM CST Sun Jan 21 2018

.SHORT TERM.../Today/
Issued at 441 AM CST Sun Jan 21 2018

A well-advertised area of low pressure is coming off the Oklahoma
panhandle and is beginning to churn towards Iowa. In advance of
this area of low pressure, a well-defined boundary has been
pushing northward through Iowa. The 290K surface picks up on this
boundary well, with a decent slug of isentropic lift accompanying
it. At 09z Sun, the boundary was near I-80. Temperatures along and
south of the boundary were in the upper 30s to low 40s. North of
this boundary, skies cleared for a brief period early this
morning, allowing temperatures to fall to the mid to upper 20s.

Hi-res models have been a little over-aggressive with low ceilings
this morning, calling for CIGs near 500 ft, where in reality, they
have struggle to reach much below 2500 ft. This lack of shallow
moisture has kept vsbys from plummeting as much as advertised behind
the boundary.

for the rest of today, expect forcing to persist behind the
boundary, aided by weak, but sufficient sfc convergence. Fcst
soundings have cloud thicknesses under 5000ft. In all, this
supports widespread drizzle across Iowa as this boundary slowly
pushes northward. Across northern Iowa, primarily along and north
of Highway 30, freezing drizzle is possible. With any ice accums
expected to be little to none, icing should be a very minimal
impact today. Visibility may be reduced to the 1 to 3 mile range
throughout today.

.LONG TERM.../Tonight through Saturday/
Issued at 441 AM CST Sun Jan 21 2018

Surface low pressure will be lifting north into eastern Kansas by
this evening then will continue to lift slowly northeast into
Iowa by Monday morning. Areas of drizzle and some fog will be
ongoing across much of central Iowa to start the period and
temperatures should be at or above freezing for most locations
except for the far northwest through the night. A region of
convection should lift into central and southeast Iowa this
evening as the short wave approaches from the southwest and the
low level jet and steeping mid level lapse rates help generate
MUCAPEs to near 500 J/kg. This convection likely will produce
graupel and/or small hail as it quickly moves through.

The precipitation will transition to snow over the northwest once
the residual mid level dry air erodes and the deformation band
begins to form. Convective elements withing the deformation band
look possible as the steepening lapse rates to near moist
adiabatic, modest MUCAPES and good negative EPV are all present.
This could produce very heavy snow fall rates of 1 to 2 inches per
hour for a period. Model proximity soundings show a nearly
isothermal layer between 0C and -2C from near the surface to
around 725 mb. This profiles setup suggests the initial snowfall
could be very wet and heavy and would be very sticky. Past events
with similar profiles have had instances with power outages with
snowfall of 3 inches or more with these properties due to tree
damage. Should this wet and heavy snow occur, the increasing winds
would increase the potential for tree damage and subsequent power
outages would be possible. Snow ratios will increase through the
day over the northwest as colder air begins to move into the area
leading to a fluffier snowfall. Only minor adjustments to the
forecast snowfall with a band of 6 to 12 inches still possible
with the heaviest amounts confined to Emmet county and adjacent
areas to the west and north. Still expect subtle adjustments to
the exact track of the snow band but even a 20 nm shift would have
big impacts especially along the southern edge of the snowfall.
In addition, strong and gusty northwest winds will develop as the
surface low pressure begins to move into eastern Iowa and northern
Illinois causing the pressure gradient to increase and the cold
advection and good mixing arrive. Wind gusts in excess of 40 mph
are likely. Considering the forecast snowfall amounts and these
winds, will be upgrading portions of the northwest to a Blizzard
Warning with the primary blizzard impacts during the day on Monday
and into the evening. A winter storm warning has been issued for
a few counties just south of the blizzard headlines. This area had
4 inches or more of snowfall forecast in the northwest corners of
these counties along with the strong winds to produce blowing
snow.

Warm air will encompass much of the remainder of the forecast area.
A deformation snow band will move east across the forecast area
by late Monday and into the evening. Any snowfall amounts would
generally be one inch or less but could have some travel impacts.

Little change to the remainder of the forecast as the cold advection
will not last long with a transition back to ridging and later
southwest flow by Thursday into Friday. The next precipitation
chances will be Friday night into early Saturday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1134 PM CST Sat Jan 20 2018

Brief clearing/lifting of cigs is allowing br to form and this will
bring MVFR vsbys to TAF`s.  Moisture will deepen and low cigs will
develop overnight.  If we can get drizzle to develop across northern
TAF locations aft 09Z FZDZ would be possible for a few hours this
A.M. but confidence in this is not high enough to include in TAF`s.
After 15Z DZ will begin to develop further reducing cigs/vsbys to
IFR with rain developing late in the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Blizzard Warning from midnight tonight to midnight CST Monday
night for IAZ006-023-033.

Winter Storm Warning from midnight tonight to midnight CST
Monday night for IAZ007-016-024-034.

Blizzard Warning from 9 PM this evening to midnight CST Monday
night for IAZ004-005-015.

&&

$$

SHORT TERM...Kotenberg
LONG TERM...Donavon
AVIATION...FAB



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