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fxus63 kdmx 191143 
afddmx

Area forecast discussion
National Weather Service Des Moines Iowa
543 am CST Tue Feb 19 2019

..updated for 12z aviation discussion...

Short term.../today through Wednesday/
issued at 431 am CST Tue Feb 19 2019

Short term...
short term focus on major winter wx system to impact Iowa beginning
this afternoon/evening and lasting through most of Wednesday.

Synopsis...
09z Tue water vapor imagery picking up on two distinct systems...
an upper low located over Montana, and another upper low located
over southern Arizona. These systems phase together through this
morning, as the Montana low barely propagates eastward through
today, with the southern upper low riding northeastward through
the positive vorticity advection side of the longwave trough, making it to the Great Lakes
region by 00z Thu.

Overall, models coming into very good agreement with the track, qpf,
and timing of this system to where a consensus blend of short-
term/hi-res models is very reasonable guidance. By 12z Wed,
attendant sfc low should be in eastern KS, and by 00z Thu, sfc low
into northwestern WI. -12c to -18c dry slot should be well east of
the dmx cwa, with additional moisture wrapping in on the backside of
this low to keep light snow/flurries going along and north of I-80
into Wednesday afternoon. By 06z Thu, dry air entrainment and
large- scale subsidence win out and there should be a brief
period of sunshine before the pending weekend system.



Timing...
noon Tuesday - afternoon/evening commute:
hi-res models picking up on a small band of saturation/forcing
through the dgz propagating from south to north that may yield
flurries. No accumulations expected. Light snow will begin to
infiltrate southwestern Iowa late afternoon.

7 PM Tuesday - 7 am wednesday:
this is the time period where most snow will fall. From 7 PM Wed-
10 PM Wed, there is a band of strong low-level moisture flux
convergence that is perfectly phased with a strong band of Theta-E
advection that will be pushing through Iowa from south to north.
Aloft, this is phased with the right entrance region of a 195 kt
250mb jet. During this narrow window, there may be more intense
snowfall rates of 1 to 2 inches per hour... which could yield very
poor visibility and get roads snow-covered and slippery in due
haste.

Around the midnight hour, the more widespread slug of moisture,
Theta-E advection, moisture flux convergence, etc. Will arrive from
south to north. Forecast soundings slam the low-levels, carrying
saturation from the sfc well into the dgz. Using roebber snow
ratios, have increased snow ratios vs guidance to the 14:1 to 16:1
range through 12z from kdsm through Kalo. Have really increased snow
ratios from 12z Onward, especially in northern Iowa, as 20:1 seems
very plausible. Wind speeds will be in the "slightly breezy" range
of 10 to 15 mph through the event... so not quite the "snow globe"
snow of the previous event, but not incredibly gusty to where
blowing and drifting snow will be a significant/major issue.



Wednesday am commute...
snow will likely be ongoing, especially along and north of
Highway 20... where an additional 2 to 4 inches may fall from 7 am
through noon. Around the Des Moines Metro, and additional 1 to 2
inches of snow may fall from 7 am Wed through noon Wed.

Similar to our Sunday event, flurries/light snow will likely linger
through Wednesday afternoon, though very little to no accumulations
are expected.



Impacts... roadways will likely be snow-covered and slippery Tuesday
night through Wednesday morning. This may make travel very hazardous
to impossible, especially on unplowed/untreated roadways. If you
must travel during this time, keep an extra flashlight, food and
water in your vehicle in case of an emergency.

The latest Road conditions for the state you are calling from can be
obtained by calling 5 1 1.



Long term.../Thursday through Monday/
issued at 431 am CST Tue Feb 19 2019

Attention shifts to yet another winter storm that may impact most to
all of Iowa Saturday into Sunday. At this time, the track is still
very touchy. There appears to be strong warm air advection ahead of the system, and
strong cold air advection behind this system. Further, in the warm air advection Wing, long-range
models show a 60kt 850mb jet that may transport moisture into this
system from the Gulf of Mexico. This would certainly support
thunderstorms... possibly strong thunderstorms. On the backside, the
cold air advection may support decent snowfall totals.

If this system pans out as is currently shown, winds will likely be
very strong on the backside of this system on Sunday, which would
lead to blowing and drifting of the fallen snow. At this time,
too far out to get into fine mesoscale details, but the track of
this potential system absolutely bears monitoring... especially
for those with travel plans over the weekend. This could have
significant impacts on travel across Iowa.



&&

Aviation.../for the 12z tafs through 12z Wednesday morning/
issued at 539 am CST Tue Feb 19 2019

Expect linger fog impacting taf sites, especially kfod, to lift by
late morning. Snow to arrive in Iowa from SW to NE beginning late
this afternoon and lasting through Wednesday. As per usual during
snow events, visibility will be incredibly challenging. There
could be some high snowfall rates and large flakes, supporting
brief periods of vsbys less than 1 mile. At this time, not
confident enough in the location or timing to lower vsbys beneath
1 sm.

&&

Dmx watches/warnings/advisories...
Winter Storm Warning from 6 PM this evening to noon CST
Wednesday for iaz033-034-044>048-058>062-072>075-084>086.

Winter Storm Warning from 6 PM this evening to 9 am CST
Wednesday for iaz057-070-071-081>083-092>097.

Winter Storm Warning from midnight tonight to 6 PM CST Wednesday
for iaz004>007-015>017-023>028-037>039-050.

Winter Storm Warning from midnight tonight to noon CST Wednesday
for iaz035-036-049.

&&

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