Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

016
FXUS63 KDMX 221115
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
515 AM CST Sun Dec 22 2024

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

.KEY MESSAGES...

- Low chance (20%) of freezing drizzle north and east late
  tonight into early Monday. Little to no accumulation
  expected, but if any ice accrues on roads could have some
  travel impact.

- Generally quiet weather from Monday afternoon through
  Christmas Day, with light rain passing just southeast of our
  area over Missouri and Illinois around Christmas Eve.

- A more active weather pattern will bring prolonged
  precipitation chances from Thursday into next weekend,
  generally a 20-40% chance in any given period but the overall
  chance for rain during this time is higher (likely/60% or
  higher). Most or all of the precipitation is expected to fall
  as rain.

&&

.DISCUSSION...
Issued at 206 AM CST Sun Dec 22 2024

A 500 mb low is currently moving from southwestern Canada into
Montana, and downstream surface cyclogenesis is occurring near
the Black Hills region. Ahead of this system winds across Iowa
are from the southeast to south at around 10 mph, helping keep
temperatures from falling much despite clearing skies. During
the day today the aforementioned system will translate eastward
over the Dakotas, and by sunset the 500 mb low will reside
roughly over Bismarck while the surface trough approaches
northwestern Iowa. That means nearly southerly surface flow is
here to stay today and will help boost temperatures, with highs
in the low to mid 50s across about the southwest half of Iowa
but still in the 30s further northeast where there is some snow
on the ground and clouds may linger for parts of the day.

Tonight as the 500 mb slowly deepens and moves into central
Minnesota, the surface trough will also close off and slowly
deepen, pivoting northeastward as it wraps into the mid-level
circulation. This will induce increasing near-surface moisture
mainly across northeastern Iowa overnight, nearer to the parent
system. However, forecast soundings indicate the resulting low
cloud layer will be quite shallow, with depths generally 1-2
thousand feet insufficient to generate precipitation. Even so,
there are a few solutions that indicate a little deeper cloud
layer with pockets of modest forcing moving through, so some
light freezing drizzle remains possible in our northeastern
counties late tonight into Monday morning. Have maintained only
a slight chance (~20%) with no real accumulation/QPF.

On Monday the low over Minnesota will move across Wisconsin and
into the Great Lakes, followed by a high pressure ridge spilling
into Iowa and Minnesota from the northwest. On Monday night and
Tuesday morning this ridge will be entrenched across Wisconsin
and neighboring portions of Minnesota and Iowa, leading to
light/calm winds across much of our area. Given what occurred
with a similar set-up 24 hours ago, late Friday night/early
Saturday morning when temperatures fell rapidly in clear and
calm conditions, and the remaining snow cover in parts of
northeast Iowa, there is potential for another bout of
substantial radiational cooling late Monday night/early Tuesday
morning. However, this is mitigated somewhat by the potential
for low stratus development, as indicated by some forecast
soundings. Still, lowered the Tuesday morning forecast low
temperatures across our northern counties by several degrees and
this potential will bear monitoring over the next couple of
days.

A more active weather pattern is still expected to develop over
much of the region later this week, with a series of 500 mb
troughs crossing the central U.S. in rapid succession. The first
wave is still on track to move through on Christmas Eve/morning,
but has trended slightly southward since last night and its
associated precipitation shield is now expected to remain just
outside our area over Missouri and Illinois, and our forecast
is now dry over Christmas. The second, stronger wave will then
approach on Thursday but long-range model solutions are highly
disparate and varying from run to run, with some showing a more
open/further south wave and others showing a stronger/further
north low. Obviously such details may have a significant impact
on sensible weather in our forecast area, however, temperatures
in the latter part of the week are generally warmer in the 30s
and 40s and is it likely that most or all of the precipitation
we receive between Thursday and next weekend will come in the
form of rain. Will maintain a prolonged period of widespread
20-40% POPs during this time until details of precipitation
timing and placement become clearer.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 515 AM CST Sun Dec 22 2024

VFR conditions are forecast today, with SSE winds becoming gusty
at times. Late tonight, toward the end of the 12Z TAF period, a
weak front will move into the area causing winds to veer to
southwest and eventually northwest later Monday morning. The
passage of the front will also bring low clouds to the area,
mainly in the northeast where MCW and ALO are more likely to see
low ceilings. However, this will not occur until close to 12Z
Monday so with low confidence in precise timing and ceiling
heights, have not yet advertised MVFR conditions in the TAFs.
This potential will be addressed in subsequent updates today.

&&

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

&&

$$

DISCUSSION...Lee
AVIATION...Lee