Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200531

Area Forecast Discussion
National Weather Service Des Moines IA
1131 PM CST Mon Feb 19 2018

...Updated for 06z Aviation Discussion...

Issued at 939 PM CST Mon Feb 19 2018

Have extended Winter WX Advisory another tier of counties
southeastward. In short, it seems the hi-res models, now backed by
observation data, have favored the GFS solution, versus the ECMWF
solution. This has lead to 2 forecast changes...

1.) QPF has been slightly lowered.
2.) The temperatures have been cooled down as the boundary
placement has been shifted eastward. What this means is that
south-central Iowa will now likely have temperatures a degree or
two cooler than previous forecast, which will yield more freezing
rain/freezing drizzle than rain.

Confidence has grown that south-central Iowa will likely see the
highest ice accumulation totals. Also, want to reiterate messaging
that though precip may be over with for much of the area by
Tuesday afternoon, winds will increase to the 15 to 20 mph range,
which would exacerbate impacts to power lines and tree branches
that have decent ice/water loading. Thus, there may still be
adverse impacts occurring after precip has ended.

Dense Fog Advisory for northeastern counties very touchy. Black
Hawk County Law Enforcement reported visibility of around 100
yards, with slippery conditions developing. Am thinking dense fog
criteria to remain in the eastern parts of Bremer and Black Hawk
counties through tonight, but the western parts of these counties
may have visibility of 1 mile or so.


.SHORT TERM.../Tonight through Tuesday/
Issued at 312 PM CST Mon Feb 19 2018

Confidence: Medium

Challenges remain regarding storm evolution and qpf output. Vast
differences between GFS/Euro with regard to storm track and extent
of moisture pulling due north late tonight into Tuesday morning.
This challenge has been ongoing the past 3 days...with the GFS and
Euro diverging.  Due to the differences in the models...a blended
approach to precipitation amounts and accumulations to
get a general average...has been taken for this event.  The first
portion ending this morning has been more well forecast by most of
the models. This morning`s weak low has pulled the first surface
front into eastern Iowa with the Arctic boundary just behind
it...and should catch up and overtake most of that airmass tonight
except for the southeast where temperatures are expected to remain
above freezing.

Looking at current observations over the Southern Plains and GOES-16
mid-level water vapor imagery, the large scale upper level trough
continues to show energy digging a bit more south as a wave over
western Mexico aloft is tracking nearly due north. A ribbon of mid
level moisture has been arching northward most of the day as well.
As wave energy ejects north/northeast tonight isentropic lift above
the boundary will quickly lead to shallow overrunning across the
region.  With the European solution being farther west...a general
area of .20 to .30 inches of water would fall in the corridor where
freezing rain is expected while the GFS with its more southeast
solution projects about 0.10 to 0.15. Precipitation amounts farther
north are also higher with the Euro. Though no model is absolutely
preferred at this time...there is slightly more credence in
selecting the Euro solution. Having said this...the axis of heavier
ice accumulations for tonight`s event has not changed...but the
amounts may end up slightly higher/lower than currently forecast.
The best lift will occur between 06-12z with a push of moisture at
that time of between 0.15 to up to a quarter inch over much of
central Iowa. The other factor that might lessen ice accumulation is
convective potential overnight. With the farther north track of the
Euro...instability is forecast all the way up through Des Moines
overnight. Even GFS bufr soundings show weakening mid-level
instability over the area. Have added into the forecast between 06-
12z. Depending on the precipitation rates...we may end up with some
sleet mixed in with the freezing rain over central Iowa which would
lessen ice totals. If the rainfall rates aren`t enhanced enough and
the convective activity results in just higher qpf...then ice
accumulation may occur much faster from 06 to 12z. So adjustments may
be needed for those processes. Most of the ice that occurs overnight
should arrive prior to the morning commute...resulting in difficult
to hazardous travel tomorrow morning.  Once the surface low tracks
farther east by 18z isentropic lift above the boundary will decrease
over Iowa and precipitation will end in our east between 18-00z. The
other factor to address is wind. Winds will remain between 10 to 15
mph overnight for most of the area but once the low pulls east
tomorrow will increase to 15 to 25 mph during the afternoon on
Tuesday. Depending on final ice totals and loading...this may cause
significant problems by afternoon/evening with tree damage or power
outages. It is likely that some problems will occur even with the
current forecast.  The overnight shift will further refine the
potential for tomorrow.

Headlines for the event will remain as is for now.  There may be a
need for the advisory to be expanded into the southeast for the
afternoon well as upgrades to a warning for some of
the central counties overnight tonight.  Lows tonight will not fall
much from current values northwest and settle into the lower 30s in
central areas to near 40 southeast. Highs Tuesday will vary little
from overnight lows.

.LONG TERM.../Tuesday night through Monday/
Issued at 312 PM CST Mon Feb 19 2018

Tuesday night...High pressure noses into the state eventually
pushing the precip east. At 00Z we should still have some
lingering precip in the far east/southeast but the bulk of the
precip will be done. Sounding show the precip to be freezing
drizzle or freezing rain initially but switching over to snow
before ending.

Wednesday and Wednesday evening...High pressure will remain in
place over the state on Wednesday and push off to the east
Wednesday evening. We will be dry and cool for Wednesday into
Wednesday evening. Theta-e advection pushes across the state late
Wednesday night through mid-day Thursday along with some weak
forcing. Soundings show some dry air to contend with initially
with warm air aloft. We should see snow over northern portions of
the forecast area with freezing drizzle further south where we
have saturation issues. Across the southeast where it will be
warmest we may see drizzle going to light rain when we saturate.
By late day Thursday, soundings saturate and enough cold air comes
in for some light snow Thursday night.

That system then pushes east for Friday and we have another dry
and cool day. Friday night the deep upper trough that has been
bringing us these periodic systems finally lifts into the Southern
Plains. As it does this precip will spread across the Southern
Plains and clip southeast Iowa with an initial shot of snow or a
rain/snow mix.

Models vary with this system for Saturday with the Euro spreading
precip across the entire state and the during the day and the GFS
confining precip to the southeast third. I kept the model blended
solution which leans heavier to the Euro than the GFS. Precip will
be mixed again with rain southeast, snow northwest and a wintery
mix in between. Given model uncertainty, confidence is low as to
placement and duration of precip at this point, which will be fine
tuned in the coming days.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1127 PM CST Mon Feb 19 2018

Freezing precipitation obviously biggest concern. Will have to
keep an eye on OTM. At this time have -RA, but cannot completely
rule out -fzra/-fzdz potential. KALO touchy with fog... vsbys
likely 1/4 mile or less in eastern Black Hawk county, but around 2
miles in western Black Hawk county. KALO sited in the middle, so
will likely oscillate between 1/4 mile and 2 miles this morning.
Once precip moves east of area generally by noon except for KOTM,
winds will begin to increase and will persist in the 10kt to 20kt
range overnight.


Winter Weather Advisory until noon CST Tuesday for IAZ004>007-

Dense Fog Advisory until 6 AM CST Tuesday for IAZ028-039.



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