Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211122
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
622 AM CDT Thu Mar 21 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Varied precipitation late tonight into Friday morning. Wintry
  mix and accumulating snow north and northeast. Confidence in
  accumulation details is low.

- Large, prolonged system this weekend into early next week.
  Accumulating snow likely north.

&&

.DISCUSSION...
Issued at 400 AM CDT Thu Mar 21 2024

It will be an extended period of active weather, starting
tonight and lasting into the next work week with only a small
break to end the week. Two systems will bring appreciable
precipitation, especially the second. In the mean time, the
radar has looked ominous north and northeast overnight with
banded convective elements along and ahead of a subtle short
wave/PV anomaly which shows up nicely along the 310-320K theta
surface. The precip is rooted in a low static stability, and at
times negative EPV layer 600/500mb. However, little to none of
this precip has reached the ground tonight with plenty of dry
air in the lowest 1km, and temp/dewpoint depressions atypically
high for this time of night, still ~25 degrees. This mechanism
is expected to persist later today, with a few flurries or
sprinkles possible north and east as the dry low levels slowly
erode.

Later tonight our attention will turn to increasing lift along
and ahead of the current strong WA potential vorticity anomaly
as its remnants reach the Dakotas by 00z. This will spread
strong mid level forcing into MN, and equally strong
thermodynamic support farther south into IA with precip
expanding overnight. With the thermal profile changing fairly
quickly with time, this can best be described as a wintry mix
mess with persistent elevated instability continuing to
introduce convection, and even some thunder into the forecast.
Much of the precip should be all rain south half, however north
everything is on the table with light freezing rain,
sleet/snow pellets, and snow all possible at times as the
thermal profile changes hour to hour.

Once the precip becomes more prevalent a convective wintry mix
is anticipated north. Although not as evident as previous runs,
a minor +0C warm layer still looks to occur with the mix before
cooling a bit and flipping to mainly snow northeast toward 12z
Friday. On the back side, the moisture depth decreases reducing
ice introduction, with a change back to light freezing rain
possible northwest into Friday morning.

The low static stability, convective nature, and strong
frontogenesis depicted all make for a challenging snow amount
forecast. Some recent hi res model guidance shows stripes of
west/east heavy snow on the order of a county or less. Have
opted to ignore this output and proceed with more generalized
amounts for two reasons. 1) Pinpointing where this occurs, if it
does, is very difficult. 2) The hourly QPF amounts generating
these high amounts seem unrealistic, with some hi res guidance
suggesting QPF of several tenths in an hour of all snow, which
seems unrealistic and would likely be more mixy with convection
involved. Thus the snow forecast is a more conservative 1-4
inches across northern and northeast IA. Higher amounts are
certainly possible, but confidence in occurrence, and especially
location, is too low to introduce what would be a considerable
narrow snow gradient. Just keep in mind that isolated amounts
in excess of these snow projections could still occur. Have
opted for a first volley Winter Weather Advisory northeast
overnight into Friday morning with some snow accumulations, and
possibly light glazing toward the morning commute in play. The
system should exit later Friday leading to a brief break from
precip into the night as high pressure settles into the MO
Valley.

The respite won`t last long however with another larger and
prolonged system expected later in the weekend, and bringing
waves of precip for several days. Models are in a bit better
agreement, however differences still exist with the northern
extent of warm air and timing. While details are still in
question, confidence is high in widespread precipitation with
NBM QPF >=1” probabilities 50-80% southeast-northwest over the
course of the event (72 hrs). This should be driven by waves of
mid level forcing starting Saturday, and continuing Sunday into
Monday in the persistent SW flow aloft. Strong warm/theta-e
advection follows Saturday night with noted frontogenesis
clipping far northwest sections in the deformation zone Monday.
Confidence is fairly high that moderate to heavy snow will occur
with this system, but the question is how far south into
northern IA. 00Z EPS and GEFS 24hr probabilities of 3+” of snow,
and using conservative snow ratios, are fairly similar at
50-70% far north with better potential in MN. Thunderstorms
will also be possible at times, especially around Monday as the
associated surface features finally move through the state. A
break from active weather finally arrives by the middle of next
week with a period of brisk winds and continued seasonal
temperatures.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 622 AM CDT Thu Mar 21 2024

Confidence is high that VFR conditions should continue through
at least early evening with varied degrees of mid cloudiness.
Some precipitation will be evident on radar, but much of that
should remain virga/aloft. Ceilings should decrease to MVFR
overnight however as precipitation chances increase. Thunder is
possible overnight too, but any mention has been omitted at
this lead time with low confidence in timing and location.
Confidence in precipitation type is low at KFOD, KMCW, and KALO
as well with all forms of wintry precipitation possible at
times.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 1 AM to 10 AM CDT Friday for
IAZ006-007-016-017-026>028-039.

&&

$$

DISCUSSION...Small
AVIATION...Small


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