FXUS63 KDMX 111126

Area Forecast Discussion
National Weather Service Des Moines IA
526 AM CST Mon Dec 11 2023

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

.DISCUSSION.../Today through Sunday/
Issued at 154 AM CST Mon Dec 11 2023

Key Messages:

- Low clouds and fog across northern Iowa early this morning,
  dissipating slowly after sunrise.
- Daily highs generally in the 30s to mid-40s today through
  Wednesday, then mid-40s to lower 50s Thursday and Friday.
- Very little chance of any precipitation within the next week
  and a half or so.

The large field of low stratus clouds that has plagued parts of
Iowa for the last couple days continues to complicate things
overnight. While these clouds have cleared most of the state, they
remain entrenched over the far northeast, and the southwestern
edge of the stratus has been very slowly retrograding over the
last several hours, creeping southwestward toward Waterloo and
Mason City where it is likely to reach in the next few hours.
Beneath the stratus temperatures remain generally in the lower
20s, but over the clear areas of north central Iowa temperatures
have plummeted in ideal radiational cooling conditions, reaching
single digits at Mason City and Forest City, and fog is also
forming in these areas with visibility falling below a mile at
times. Meanwhile over southwestern Minnesota a more extensive area
of dense fog has formed, and while it currently remains just
north of the Iowa border, it should spread into our far
northwestern counties around Estherville before sunrise. These
various factors make the sky cover, temperature, and visibility
forecasts very challenging this morning, but at this time believe
any dense fog reducing visibility to less than a quarter mile will
be relatively small in areal extent and do not plan to issue an
advisory, but rather cover with special weather statements as
needed. Aviation impacts are also expected at MCW and ALO during
the morning.

The surface ridge stubbornly clinging to northwestern Iowa, and
primarily responsible for the persistent stratus as well as the
light winds inducing radiational cooling in clear areas, will
finally move off to the east today as a trough sweeps down across
the Northern High Plains. This will allow the stratus to finally
creep eastward away from our area later this morning, and surface
winds to come around to the southwest by the end of the day but
remain fairly light. The aforementioned trough will then surge
southeastward through Iowa tonight, accompanied by a band of
mid/high clouds that should somewhat limit overnight cooling
initially, but then with modest cold air advection kicking in well
behind the boundary and skies clearing late, allow temperatures
to fall again by sunrise Tuesday, though likely only into the
mid/upper teens northwest with a persistent northwest breeze

By Tuesday a large surface high pressure area will settle over
Iowa behind the trough, keeping daytime temperatures relatively
cool and persisting through Wednesday. This high will finally
move eastward away from Iowa on Thursday, with light but organizes
southerly surface flow returning to our area through Friday and
allowing for modest warming at the end of the week. High
temperatures both Thursday and Friday are expected to reach the
mid-40s to lower 50s. A 500 mb trough will then move overhead
sometime around Friday night and Saturday and should bring more
cloud cover during that time, but any signal for precipitation
associated with this feature remains nebulous and weak. We are
unlikely to see any meaningful moisture from this system, with
POPs currently only around 10 percent across the service area. The
far extended forecast, from Sunday into the first half of the
following week, continues to look dry and quiet with benign west
northwesterly steering flow dominating the region.


.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 526 AM CST Mon Dec 11 2023

The expansive deck of low stratus clouds that affected portions
of the area the past couple of days and nights persists in
northeast Iowa early this morning, is producing LIFR or VLIFR
ceilings at MCW and ALO, and may reach FOD in the next couple of
hours. Areas of fog have also formed in the north, and especially
at MCW are reducing visibility to below 1SM at times. Have
reflected these conditions at the aforementioned terminals and
made best estimate of timing improvement later this morning and
afternoon, as the stratus is expected to slowly clear eastward
during the day. Amendments are likely, however. At DSM and OTM
only VFR conditions are forecast.