Heavy Snow in Iowa, February 21, 2021

Area Forecast Discussion
National Weather Service Des Moines IA
358 AM CST Sun Feb 21 2021

.DISCUSSION.../Today through Saturday/
Issued at 358 AM CST Sun Feb 21 2021

Today:

Long-wave trough that been developing over the western CONUS is
beginning to split into two separate short-wave features with the
first jet streak ejecting into the central Plains, and the second
moving from the Four Corners Region into the Texas and Oklahoma
Panhandles. The northern jet streak and associated vorticity maxima
has resulted in surface lee cyclogenesis in the last 12-18 hours and
has phased well, allowing the surface cyclone to deepen. Ahead of
the cyclone, stronger WAA has been ongoing between the Missouri
River Valley and Mississippi River Valley, and isentropic ascent
will continue to strengthen this morning. With H5 height falls
ongoing across the forecast area, expect snow showers to begin
developing and expanding across western Iowa after 4am or so, being
aided by the isentropic ascent. As of 3am CST the OAX radar is
already showing snow shower activity in eastern Nebraska. Still
expecting the area with greatest snowfall accumulations to be along
the Interstate 80 corridor and northward, then eventually dropping
off north of Hwy. 18. There is a still a decent amount of
uncertainty for precipitation south of Interstate 80.

First, a discussion in the area with greater confidence in
accumulating snow. This area will be along a line from Atlantic
northeast to Ames to Waterloo. From a sounding perspective, snow
will be the dominant precipitation type as the DGZ remains saturated,
and the layer below maintains a RH above 85 percent with
temperatures below freezing. Along and north of Interstate 80, there
is no indication of a warm-nose above freezing. Further, there is
also decent lift through the DGZ in this area, with omega ranging
between -8 to -15. Interestingly, the 00z deterministic GFS/NAM do
not depict overly robust frontogenetical processes within the
deformation zone, as the surface cyclone center moves along the IA-
MO border. With the strength of this short-wave and synoptic
forcing, typically would expect there to be stronger frontogenesis.
00z deterministic ECMWF is similar. However, several of the 00z CAMs
do have a band of stronger frontogenesis around 750mb from Atlantic
to Waterloo. The HRRR for example produces multiple pockets of 6 to
7 inches within the wider swath of 4 to 6 inches along and north of
Interstate 80. Further, multiple members of the 00z GEFS also hint
at a wide swath of 4 to 6 inches, with pockets of 6 to 7 inches. The
00z HREF mean remains consistent with this swath of 4 to 6 and max
values of 6 to 7. Given the strength of the synoptic system, the
CAMs stronger frontogenesis bands within the deformation zone of the
cyclone are credible, and therefore a reasonable solution. However
while they have the general swath of heavier snow accumulations, the
HRRR, NSSL WRF, NAM Nest, and other CAMs are not in complete
agreement with where localized amounts of 6 to 7 inches may occur.
Because snowfall amounts of 6 to 7 inches will be isolated and
likely spaced apart by up to 30 miles in some instances, will
continue to just maintain the Winter Weather Advisory for today.
Trying to capture the locally higher amounts with a Winter Storm
Warning would result in an unnecessarily large warning with respect
to the areal extent of it. Instead, will simply highlight that the
areas forecast to have 3 to 5 inches could see a few pockets of 6 to
7 inches. 12z-18z will be the time of highest snowfall rates for
west-central and central Iowa, as HREF mean points to rates between
0.75-1.00 inch per hour. For eastern and northeastern Iowa, this
will be more in the 17z-21z time frame. This is when visibility may
drop to one mile or less. One positive though is that the winds
during the time of greatest snowfall rate will light, thus limiting
the potential for blowing to be a major issue. Snowfall rate is
expected to the biggest factor behind any travel impacts.

Now, to address the complexity for areas south of Interstate 80,
especially our far southeast portion of the CWA. There will still be
plenty of isentropic ascent associated with the WAA regime for
southern and southeast Iowa. This may be able to boost temperatures
into the upper 30s. Over the past two days, model soundings had
shown decent saturation in the low-levels, but was very questionable
on ice-introduction with RH less than 65 percent through the DGZ.
This would suggest a freezing drizzle or freezing rain solution.
However, over the past 12 hours, RAP and HRRR soundings are showing
much drier air close to the surface. Further, satellite trends have
not shown much in the way of cloud development across southern Iowa,
in agreement with these RAP and HRRR soundings. As of 3am,
conditions are not looking favorable for much precipitation
development in southeast Iowa for today. However, with the stronger
WAA and higher temperatures here, RAP and HRRR do have a warm-nose
around 900mb that is above freezing. Should any higher clouds move
in and provide ice introduction, this may be enough to melt and
create a freezing drizzle scenario. However, if this ends up being
the case, it will be a very short window. If thicker clouds to do
develop, the warming trend will hault quickly and eventually cool.
After this cool down, it would be a transition to light snow.
Overall, with uncertainty about how temperatures and low-level
saturation play out, there was not enough confidence to issue a
Winter Weather Advisory for the far southeast at this time. Will
allow the day shift to monitor radar and satellite trends, and
should a freezing rain/drizzle scenario begin to develop, or, if GFS
like solution brining snow showers comes to play out, the advisory
can be expanded. But at this time, confidence is too low in what the
impacts will be.

Extended:

Quieter, warmer weather is in the forecast for the majority of the
next work week. Monday and Tuesday will be marked by weak thermal
ridging. A short-wave trough moves through Wednesday but long term
guidance does not have any QPF associated with this. A strong system
may develop by next weekend but there is no strong consensus at this
time. With a period of warmer temperatures, the snow pack will
continue to melt through the week. Rivers and streams will need to
be monitored a little more closely, as run-off will likely result in
some elevated levels throughout central Iowa.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1151 PM CST Sat Feb 20 2021

Spotty MVFR conditions due to fog and haze were in place across
northern IA at 06z with mid and high cloudiness continuing to
stream into the state. There was some light snow aloft north, but
additional snow is expected to move into IA west to east later
this Sunday morning and persisting through much of the afternoon.
Mixed precip is also possible at KOTM. Mainly IFR conditions
should be in place during the precip, with LIFR at times as well.
The precip should end by either side of 00z, but at least MVFR
stratus may linger into the evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM CST this evening for
IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>085-092>096.

&&

DISCUSSION...Krull
AVIATION...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
308 AM CST Sun Feb 21 2021

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>085-092>096-221100-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Taylor-Ringgold-Decatur-Wayne-Appanoose-
308 AM CST Sun Feb 21 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Accumulating snow is expected across much of Iowa today. A Winter
Weather Advisory is in effect for the area today. Please visit
weather.gov/desmoines for more details.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotters will not be needed. Snowfall reports are always
appreciated.

IAZ086-097-221100-
Wapello-Davis-
308 AM CST Sun Feb 21 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Light snow, possibly mixed with freezing rain for brief periods is
expected today.  Light accumulations are expected.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotters will not be needed.