924
FXUS63 KDMX 310829
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
229 AM CST Fri Jan 31 2025

.KEY MESSAGES...

- Rain and fog south and east early this morning, but gradually
  moving away during the day and yielding to a fairly pleasant
  afternoon.

- Temperatures will remain well above seasonal normals for the
  next few days, with the highest readings on Sunday when highs
  will range in the upper 40s to mid 50s across Iowa.

- A somewhat more active weather pattern may set in around the
  second half of next week, but details are uncertain at this
  range.

&&

.DISCUSSION...
Issued at 229 AM CST Fri Jan 31 2025

A cyclone passing over Missouri early this morning is producing
a band of rain pivoting near the Iowa border and encroaching on
our southern counties at this hour. Thus far it has not made as
much northward progress as anticipated and CAM solutions have
trended a bit further south with it as the system progresses
into today, but we should still see the northern fringes of
light rain extend into our central and eastern counties several
hours from now and have maintained chance (30-50%) POPs in those
areas. The higher POPs (80-100%) and amounts, however, are in
our southern and southeastern counties. Short-term guidance has
backed off a bit on QPF, however with the system still maturing
have not made significant changes to forecast/messaging in terms
of amounts, with QPF of around an inch still advertised around
the Lamoni/Ottumwa/Bloomfield area. Hydrological impacts are
expected to be minimal, as described in the HYDROLOGY section
below.

In addition to the rain areas of dense fog have developed
overnight, mainly over the southeastern half or third of Iowa
where surface winds are lighter and near-surface moisture
content higher. This prompted the issuance of a Dense Fog
Advisory through 6 AM, which has not been altered as of this
writing. Some cancellation/extension may be possible in the next
few hours as we monitor short-term observational trends, but
expect the dense fog to gradually lighten/move off to the east
as rain fills in further north and surface winds gradually tick
up a bit toward sunrise.

As the low pressure system moves away across Illinois later
today the band of rain will exit our forecast area with it,
yielding a fairly pleasant day to close out January. Although
moderate northerly breezes would typically result in cool
weather at this time of year, cold air advection appears minimal
and the clouds should move off and yield sunny skies for much of
the afternoon, allowing temperatures to climb into the mid to
upper 40s or so, and perhaps even hit 50 degrees in a few areas.
While this is cooler than yesterday`s near-record warmth, it is
still far above normal highs for the date which are generally
in the mid 20s to lower 30s throughout the area.

A surface high pressure ridge will slide quickly across Iowa
tonight, then retreat eastward on Saturday with low-level winds
coming around to south southeast. Concurrently a shortwave
moving across the Upper Midwest will spread clouds and
precipitation across much of the Dakotas, Minnesota, and
Wisconsin during the day. The clouds will likely spread into at
least the northern half of Iowa if not further, and light
precipitation may as well but this is less certain. Many global
model solutions are painting patchy, light QPF across parts of
the area Saturday afternoon and evening however forecast
soundings are very dry and most CAMS seem to be picking up on
this, with only a few stripes of light precipitation scraping
our northeastern counties. Have maintained a forecast more in
line with the CAMs and dry across most of the area, but this
bears watching for updates later today and tonight. Even if
precipitation does become somewhat more widespread late
Saturday, it would be light and of little to no impact given
expected high temperatures in the upper 30s northeast to mid 40s
central/southwest.

From Sunday through Tuesday a brisk, zonal 500 mb steering flow
regime will set up across the north/central U.S. and Iowa.
Initially on Sunday temperatures will climb well into the 50s
across much of the state making for another very nice day,
however this will be short-lived as a modest cool front moves
through the area late in the day, moderating temperatures early
next week. Some low POPs (20-30%) pepper the forecast over
northern Iowa at times during this period, however, most
precipitation should remain north of our area over Minnesota and
even there be relatively light in nature. Overall the early part
of next week appears quiet for our service area, with daily
highs generally in the mid 20s north to around 40 south.

The outer periods of the forecast, in the mid to late part of
next week, are somewhat more active as a 500 mb trough carves
out over the northwest U.S., turning our flow to more
west southwesterly with several energetic shortwaves moving
overhead or nearby. This will result in more unsettled weather
and a return of POPs at times, however, with the timing and
location of impulses and thermal fields highly uncertain at this
range, no further details are apparent at this time.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1144 PM CST Thu Jan 30 2025

Upper system passing south of Iowa will produce rain into the
southern third of the state overnight into Friday morning. MVFR
to IFR conditions are expected from fog and rainfall with
restrictions to both ceilings and visibilities. Across northern
Iowa conditions remain VFR overnight. The system departs by
midday Friday with widespread VFR conditions over the entire
area on Friday afternoon through the end of the period. Surface
winds will be light overnight but increase from the north toward
daybreak into Friday.

&&

.HYDROLOGY...
Issued at 251 PM CST Thu Jan 30 2025

Flooding concerns in response to the forecast rainfall through
Friday are similar to or less than those on Wednesday. The area of
main concern is roughly the southeast third of the CWA.

Seasonally moderate to high QPF along with deep frost depths may
lead to a larger hydrologic response than what is indicated by some
of our river forecast guidance. One potential mitigating factor is
low soil saturation values which would allow for more infiltration
vs. frozen ground with high soil saturation. Still we expect a
decent amount of runoff.

If the QPF does verify and it falls as rain then confidence would be
moderate to high (50-80%) in significant within-bank rises at some
locations. A couple locations may approach or exceed flood stage
although confidence is lower (30-50%) in that happening. The latest
GFS-forced National Water Model (NWM) guidance supports these
possibilities.

Although complications from break-up ice jams are not expected to be
a significant factor they cannot be totally ruled out. If the ice
jams were to be a problem we would expect them to be localized at
best. That is because (1) southern Iowa has a relatively low
climatological risk of flooding from break-up ice jams, (2) river
levels were relatively low during the freeze-up and (3) the ice has
not had enough time to grow significantly in thickness. Still,
depending on the evolution of the hydrologic response there may be
some localized, higher rises from the ice activity.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 6 AM CST early this morning for
IAZ038-039-049-050-061-062-073>075-083>086-094>097.

&&

$$

UPDATE...Cogil
DISCUSSION...Lee
AVIATION...Cogil
HYDROLOGY...Zogg