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fxus63 kdmx 130855 

Area forecast discussion
National Weather Service Des Moines Iowa
255 am CST Wed Nov 13 2019

Discussion.../today through Tuesday/
issued at 255 am CST Wed Nov 13 2019


A period of mixed precipitation and snow will spread into parts of
western Iowa this morning and transition to all snow as it shifts
eastward across the state ending this evening. A light glaze or a
few hundredths of ice accumulations is possible out towards Carroll
and Denison with snow amounts over central Iowa generally under an
inch. Temperatures will generally be on the rise through the
remainder of the week into this weekend, but will be below normal
for the middle of November. Another chance of precipitation is
possible toward Saturday night and again Monday.


Low level warm air advection is ongoing early this morning and
surface temperatures have mostly leveled off and/or are beginning to
slowly rise as clouds stream over Iowa ahead of a shortwave trough
that will move quickly from the northern rockies into our area
today. 00z koax radiosonde observation showed a deep layer of dry air between 850mb
and 600mb and this was fairly well represented in the models. Models
did struggle with representing surface and low level temperatures
with the surface temperatures colder by 1c to 3c and the Max warm
nose just above 850mb a half a degree higher. While both could have
implications on precipitation type, the latter is more of a concern
as this may allow for a longer duration of mixed precipitation. And
as previous discussion noted about possible shift in the track of
the storm, there has been a slight shift back northward and have
tried to hone in on the precipitation band.

The dry air will likely delay precipitation start time to after
sunrise over our far western counties. Saturation will occur fairly
quickly and as it does, hydrometeors will melt as they fall through
the warm air aloft. With the surface layer well below freezing as
the precipitation starts, this will likely result in freezing rain
or perhaps sleet. There are differences in how quickly this warm air
aloft cools to below freezing with the NAM seeming the fastest on
the order of a few hours and the hrrr the longest at 4 to 6 hours.
Any freezing rain accumulation could have an impact on travel so
have issued a short duration Winter Weather Advisory more so for
potential impacts versus criteria. The precipitation will spread
eastward through the remainder of the morning and as the
atmospheric column cools below freezing, will have a transition to
all snow by mid-afternoon. The fast moving shortwave will begin
to pull away from the state tonight with snow ending this evening.
Ice accumulations of a glaze to few hundredths if that are
possible, mainly between Highway 20 and Interstate 80 and mainly west
of Highway 169. Snow ratios will generally be less than 10:1
except perhaps as the snow begins to end and the cooler air moves
in. Total liquid precipitation will be around a tenth of an inch
so this will bring snow totals in mainly under an inch.

While heights will fall and there will be cold air advection
following Wednesday night into Thursday, this will not be a surge of
deep polar air. High pressure will pass through the region on
Thursday into Friday and provide dry weather. Heights will slowly
rise Friday and a bit more on Saturday ahead of the next shortwave
trough that will bring a surface cold front through the state. There
may be some precipitation ahead of this front later Saturday into
early Sunday, but moisture will be limited with global models
hinting at this possible. This will be followed by another shortwave
trough that could bring another shot of precipitation on Monday.


Aviation.../for the 06z tafs through 06z Wednesday night/
issued at 1132 PM CST Tue Nov 12 2019

Few changes other than slowing down precip start and lowering of
cigs to MVFR by an hour or two. Still some uncertainty as to where
the main band of light snow will set up...with some hint at hires
models now along/north of US 30 to us20. A small band of -pl or
light -fzra may also develop early in the day between 15-16z
between kcin and kbnw; then turn back over to -sn through the day.
Due to the fast moving nature of the system, will probably need to
make some additional adjustments with 12z package. /Rev


Dmx watches/warnings/advisories...

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