Ankeny, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

317
FXUS63 KDMX 112106
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
306 PM CST Wed Dec 11 2024

.KEY MESSAGES...

- Cold weather expected for tonight through Friday morning.
  High confidence (80%) in lows below 0 degrees in the north
  tonight.

- 30% chance of light snow in mainly the southern half of the
  state Thursday morning. Amounts up to 1 inch expected.

- Wintry mix, primarily of freezing rain and rain, expected
  Friday night into Saturday morning (90% confidence). Expect a
  gradual transition to rain, especially after midnight Friday
  night. Impactful ice accumulations likely in the northern half
  of the state (80% confidence).

&&

.DISCUSSION...
Issued at 300 PM CST Wed Dec 11 2024

Efficient mixing allowed for a train of HCR to move across the
northeast this afternoon, bringing with it a few areas of
flurries/snow showers. Winds potential was also almost maximized
with the subsidence and CAA, leading to 40 mph gusts in the north.
Expect winds and showers to continue as long as the sun is out. Some
localized visibility restrictions are possible due to the snowfall
blowing around, but confidence in this is low (<20% chance) as
available moisture for snowfall production is very low, reducing
concentration. Additional accumulations are not expected. Skies will
clear in the north tonight and drop temperatures below 0
degrees Fahrenheit tonight in the north. Add to it the light
(but present) winds and the wind chills values will drop to near
-20 degrees by morning, especially for places like Mason City.
The cold regime with northwest flow will stay Thursday, limiting
highs to the teens.

The other story for Thursday will be the quick shot of snowfall
across the southern half of the area in the morning. There remains a
displacement of the low and mid-level moisture. Optimal ice
introduction and lift are collocated across the south, but
cross sections illustrate the 2-3 kft dry layer beneath.
Northeasterly flow will reinforce this dry layer and work to
keep snowfall amounts generally below 1 inch, but some slightly
higher amounts will be possible in the southeast where moisture
is more uniform in the lower atmospheric profile.

Regarding the wintry system that moves in Friday and Saturday: the
main change since the previous issuance is the wind profile
near and at the surface (this is due to changes in the
forecasted position of the surface high to our east Friday).
Some drizzle can`t be ruled out over the morning hours Friday,
but the moisture return has trended slower. Model consensus
places Iowa in southeast flow by midday Friday. This means that
the near surface temperatures will be allowed to gradually warm
through the event, but not as quickly as the warm nose aloft.
The system overall has slowed, delaying its arrival to Friday
night into Saturday. What this translates to is the following:

1) Increasing confidence (75%) in seeing no impacts during the
daytime Friday.

2) Decreasing confidence (30%) of seeing freezing rain in far
southern Iowa.

3) A decreased window of freezing rain, focused primarily through
early Saturday morning.

4) Decreasing chances for snow in the far north (~40%
confidence of snow mixing with freezing rain)

There remain discrepancies in model ice output largely due to the
colder, wetter biases of models such as the NAM and Canadian. Warmer
solutions such as the GFS and ECMWF deterministics raise wet bulb
temperatures above freezing for Des Moines by midnight Saturday,
leaving the higher chances of freezing rain north of I-80 for the
remainder of the night. Poor ice introduction keeps precipitation
more as a freezing drizzle/light rain, transitioning to rain as the
WAA increases. A questionable factor at this point will be the
delayed onset of warming surface temperatures above freezing.
Factors inhibiting warming include the cold temperatures in the days
prior as well as the cloud cover covering the state Friday. This
would lead to extended impacts from ice, even when air temperatures
rise above freezing.

PWAT values will approach 0.9" in the southeast for this event, a
more rare feat for mid December and one that will lead to a healthy
dose of liquid water for the warm sector of this system. Rainfall
amounts over a half an inch could be seen in the southeast.

At least the southern half of the state will remain in WAA with
highs in the 40s through the weekend before another frontal passage
arrives Monday, cooling us back to seasonal norms for a short time.
The moisture axis looks to be displaced primarily south and east,
keeping the highest precipitation amounts out of the area.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1145 AM CST Wed Dec 11 2024

Afternoon heating has allowed for some low level cumulus to
form, bringing with it some scattered snow showers, especially
northeast. Localized vsby restrictions on the magnitude of MVFR
will be possible in the afternoon. Otherwise, cigs will bounce
between VFR and MVFR through sunset, becoming prevailing VFR.
Winds will also ease after sunset and veer to northeasterly
after 12z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jimenez
AVIATION...Jimenez