Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

986
FXUS63 KDMX 222317
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
517 PM CST Mon Dec 22 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Mild, above normal temperatures this week. Low potential for
  fog over snowpack tonight.

- Cloudy through Christmas Eve and Christmas day, with low
  chance (less than 20%) for precipitation in the south and east.

&&

.DISCUSSION...
Issued at 202 PM CST Mon Dec 22 2025

Warm southerly flow today has boosted temperatures into the upper
40s to near 50 in southern and central Iowa, with slightly cooler
temperatures in the mid to upper 30s over snow pack in north central
and northern Iowa. With this warmer flow blowing directly into the
snowpack and wetbulb temperatures in the upper 30s to low 40s
advecting into the area, snow should melt nicely through the
afternoon. This will be especially true as the gulf moisture stream
pushes eastward and cloud cover diminishes. Tonight, a weak surface
low will pass over the state, flipping flow to westerly, then
northerly by tomorrow. With the diminishing winds and melting
snow tonight, fog may develop over snowpack as temperatures
cool through tomorrow morning. However, winds struggle to go
calm until the surface low fully departs and high pressure moves
in, which may limit radiational cooling and fog potential.
Therefore, while there is a chance for fog tonight and tomorrow
morning, it`s not a high confidence scenario.

Iowa will generally be placed along the periphery of a 500 mb ridge
through mid-week, resulting in mild, above normal temperatures
through much of the week. Subtle waves in the broadly zonal
pattern will bring variations in the surface pattern through the
week but, to most, conditions will appear fairly similar
through mid- week. Temperatures are only forecast to vary by a
couple of degrees each day, all of which will have above normal
highs. Winds also remain light through this period with partly
to mostly cloudy skies.

The main change from previous forecast cycles has been a
considerable drop in forecasted highs from the national blend of
models (NBM) for Christmas Day. As was alluded to in previous
discussions, NBM had been running a bit warm with temperatures for
Thursday, resulting in highs in the 60s over central and southern
Iowa, despite expectations of cloud cover. Today, NBM has
rubber- banded back the other direction, slashing Thursday
(Christmas Day) highs by almost 10 degrees, now generally in the
40s to mid 50s. This is in part due to the expected cloud cover
through the day, but also because of an easterly surface flow
pattern which isn`t as conducive for strong WAA. NBM may be
slightly over-reacting with these cooler temperatures, so have
bumped forecasted highs back toward the 75th percentile of NBM.
However, given the expected setup, colder highs is certainly a
trend in the right direction. There also remains a signal for
some light rain/drizzle Wednesday (Christmas Eve), then again
Thursday (Christmas Day) afternoon as better low level moisture
rotates up into southern and southeastern Iowa. Lift is quite
weak, and saturation marginal, so confidence is fairly low in
this occurring, but it does show up to varying degrees in the
guidance. Fortunately, temperatures should be warm enough to
negate any freezing concerns or potential impacts beyond a moist
holiday.

A change in the pattern comes this weekend as the ridge begins to
break down and a slightly more amplified wave passes through
the area. This wave will bring colder temperatures down into
the state Saturday night into Sunday. However, there are still
some significant discrepancies between guidance on the
evolution and track of this wave, as well as any associated
precipitation, so will continue to monitor in the coming days.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 517 PM CST Mon Dec 22 2025

Mainly VFR conditions are forecast through the TAF period, with
some high clouds around FL150-250 and light winds turning from
SSW to NW overnight but at less than 10 KT throughout. There is
some potential for fog formation overnight, mainly around MCW
and ALO, however with calm winds not expected and high clouds
overhead this potential is too low to advertise in the 00Z TAFs.
Will monitor short-term trends overnight in case changes
become warranted.

&&

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

&&

$$

DISCUSSION...Dodson
AVIATION...Lee