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fxus63 kdmx 131141 
afddmx

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

..updated for 12z aviation discussion...

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

Overview...

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.

Details...

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 12z tafs through 12z Thursday morning/
issued at 541 am CST Wed Nov 13 2019

Models have been trending northward with band of light wintry
precipitation and so there is the potential for greater impacts
now at mcw along with continued risk of impacts at fod and alo.
Kept light snow in forecast at dsm, though if trends continue dsm
may miss out on much of the wintry precipitation. While there may
be a brief period of ice pellets or freezing rain over parts of
western Iowa, currently do not see that occurring at the official
taf sites so have left out. Restrictions will primarily be IFR or
MVFR through the core of the precipitation. As precipitation ends
later this afternoon and evening, low clouds will persist, but
most if not all terminals will become VFR by 6z.

&&

Dmx watches/warnings/advisories...
Winter Weather Advisory until noon CST today for iaz023-033-034-
044>046.

&&

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