Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230455

Area Forecast Discussion
National Weather Service Des Moines IA
1155 PM CDT Thu Mar 22 2018

.SHORT TERM.../Tonight through Friday/
Issued at 347 PM CDT Thu Mar 22 2018

Bottom Line Up Front...

Confidence remains high in winter system to impact Iowa Friday
through Saturday. Accumulating snow and strong winds likely in
northern Iowa, and moderate to heavy rainfall in central to southern

20z GOES-R Water vapor imagery picking up on huge upper low still
spinning off the British Columbia coastline. On the southern side
of this low, an impulse is being ejected onshore in California.
Models have been in good agreement with propagating this upper low
through the Four Corners tonight, and into the Plains States by
Friday afternoon. Models also continue strong agreement with
developing a sfc low in response... placing the low over western
KS by 00z Sat and northern Missouri by 12z Sat.

Overall, there have been little changes to the overall mesoscale
features of this event. There is still a huge slug of low-level
frontogenetical forcing, especially evident at 900mb that sets up
over southern to south-central IA. With this system tilting
northwestward with height, there is good response with 700mb
frontogenetical forcing further northward along the I-80 corridor.
Confidence increasing slightly in being able to pin down a slug of
strong isentropic adiabatic ascent propagating through southern
Iowa at some point during the 06z Sat to 12z Sat timeframe. Thus,
may see some higher localized rainfall rates and even some

Precip type across northern Iowa will be very touchy. Very
temperature-dependent. Trend is to be slightly cooler with sfc
temps, which would drop accumulating snow slightly further south,
closer to the Highway 30 corridor. Snow ratios also touchy with
this event. Obviously, the cooler temps would support higher snow
ratios... but if the track shifts 40 miles northward, snow ratios
could drop to 4:1 range... which would hammer snow amounts. For
this reason, have held current Winter Storm Watch as is,
anticipating overnight shift making final adjustments. Our far
northeastern counties may need to be upgraded to a Winter Storm
Warning, while the rest of the Watch area could be transitioned to
a Winter Weather Advisory. Factoring into impacts will be
easterly winds in the 15 to 25 mph range. This would reduce
visibility to one mile or less... possibly even 1/4 mile during
periods of more heavy snow.

Going backwards in time, slight concern event start time may need to
be pushed earlier. The GFS is especially picking up on band of
isentropic ascent phasing with deep enough low-level moisture for
cloud thicknesses to support light precip by 15z Fri. Will watch in
next shift, but cannot rule out earlier start time of any headlines.

.LONG TERM.../Saturday night through Thursday/
Issued at 347 PM CDT Thu Mar 22 2018

System will have departed by Saturday evening with weak surface
ridging sliding into the state.  Model sounding continue to indicate
plenty of low level moisture lingering, especially the south and
west, therefore cloudiness is expected to persist into the overnight
and Sunday for much of the area.  Exception may be northeast areas
where drying will extend a bit deeper into the boundary layer.  Warm
advection begins into Sunday as next large trof moves onto the west
coast and into the intermountain region.  Increasing isentropic lift
into the afternoon produces some deeper saturation across the far
west by later in the afternoon with some sprinkles/light rain

Strong theta-e advection and moisture transport arrive late Sunday
night into Monday which should spur shower development across
central Iowa during the early morning hours on Monday.  Elevated
instability will increase across the area on Monday with the strong
southerly flow with some isolated thunderstorms expected with the
shower activity.  By late Monday afternoon, low pressure will be
approaching southwest Iowa near the Omaha area with surface
instability edging into the southwest CWA.  Models would indicate
some potential for stronger deep convection, nearly surface based in
eastern Nebraska into Kansas by late afternoon.  Some of this may be
able to reach the far southwest portions of the forecast area by
Monday evening, although this time of year, weakening usually occurs
after sunset limiting overall severe threat.  Much of the rest of
the forecast area will see widespread showers and some
thunderstorms as the surface front passes through the state Monday
night. Lingering showers will persist into the southeast on
Tuesday before the system departs and subsidence increases. Some
quieter weather is expected into the middle of next week with
seasonable temperatures expected.


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1154 PM CDT Thu Mar 22 2018

VFR conditions are in place across the forecast area at 06z with
varied levels of cloud coverage and height. These conditions should
not change much early this morning, however clouds should gradually
lower with light rain spreading NW to SE toward midday Friday. The
combination of lowering ceilings, precipitation and possibly some
fog will drop conditions to at least MVFR and likely IFR due to
visibilities as the precipitation and/or fog increase. IFR
conditions will become more likely as ceilings lower further and
precipitation intensity increases into the evening with snow also
developing north near KMCW.


Winter Storm Watch from Friday evening through Saturday
afternoon for IAZ006-007-016-017-026>028-038-039-050.



SHORT TERM...Kotenberg
AVIATION...Small is the U.S. government's official web portal to all federal, state and local government web resources and services.