Norwalk, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

916
FXUS63 KDMX 300830
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
230 AM CST Thu Jan 30 2025

.KEY MESSAGES...

- Clouds and scattered light showers spreading in from the south
  today.

- Higher rain chances, heavier amounts (1"+), and a low chance
  (20-30%) of thunder mainly across southern and southeastern
  iowa tonight into Friday morning.

- Dry and mild this weekend, then turning cooler again with
  precipitation chances returning around the middle of next
  week.

&&

.DISCUSSION...
Issued at 229 AM CST Thu Jan 30 2025

Iowa is currently experiencing a very quiet night as a surface
high pressure ridge gradually slides away to the east, yielding
light winds and clear skies. However, a large and seasonally
strong mid-level low pressure system is emerging from the
Rockies over southeastern Colorado. There are two vorticity
maxima dancing around the gyre, one on the leading flank and the
other further behind over northwestern Mexico. The trailing
vortmax will swing rapidly northeastward this morning, actually
pulling the 500 mb low center southeastward into the Oklahoma
panhandle by this evening, with the cyclone then reconstituting
its center in association with the faster vortmax as it
continues northeastward into Missouri on Thursday night. This
evolution makes for a tricky forecast regarding timing of
precipitation in our area.

Broad mid/upper-level diffluence ahead of the approaching
system is generating a large cloud field over the central and
southern plains, blanketing Kansas, Oklahoma, and most of
Missouri at the time of this writing and quickly approaching
southwestern Iowa. However, it has been somewhat eroded on its
northern edge by lingering dry air from the aforementioned
departing ridge. Gradual saturation of the column is expected
today and the clouds should continue gradually shifting
northward, with light rain becoming a possibility during the
day. However, forcing for ascent is initially very weak because
of the interaction between the two vorticity maxima discussed
above, pulling most of the lift further southward initially. The
result is wide disparities in high-resolution model depictions
of precipitation later this morning, with some solutions showing
rain showers as far north as Highway 20 by around 10 AM, while
others have almost no precipitation in our forecast area until
late afternoon or this evening. Have thus maintained a northward
progression of POPs during the day today, but with only slight
chances of 20% during the morning and then 30-40% chances in
central and southern Iowa this afternoon. At any rate, with the
forcing being weaker during this time and temperatures generally
in the 40s, only light rain is anticipated with no real impact.

Things get more interesting tonight into Friday morning as the
more powerful, reconstituted cyclone moves just southeast of
our forecast area, over northeastern Missouri. All model output
depicts a band of seasonally heavy precipitation developing in a
TROWAL signature over our southern and southeastern counties
late tonight. Precipitable water is off the charts in the 99th
percentile for the date, thanks to the long fetch off the Gulf
coming up with the system. QPF of around an inch is currently
forecast in this part of the state, roughly from around Mount
Ayr and Lamoni up to Ottumwa or so, and some model output
exceeds those amounts in part of the area. For the last few days
there has been some potential for temperatures to cool enough
on the back side of the system, especially with the heavy
precipitation moving through our area during the overnight/early
morning hours, that the p-type could change over to snow within
the band. While the probability of this has been low, if the
changeover were to occur then snowfall amounts could be
substantial given the QPF and strength of forcing. However,
recent model runs have come in somewhat warmer, and now some NAM
members appear to be the only ones depicting this scenario,
with the rest of the guidance all maintaining rain as the
precipitation type. Also encouraging is that even the colder NAM
solutions continue to show a deep isothermal layer above the
surface just above freezing, indicating it would be quite
difficult to cool that entire layer enough for snow. We will
certainly be keeping an eye on this possibility today and
tonight, but for now the probability of snow is even lower than
before (around 10% or less). Also of note is that modest but
sufficient MLCAPE of around 50-80 J/kg will support a low chance
(20-30%) of some rumbles of thunder in southern Iowa tonight as
the band moves through, marking (potentially) the first
thunderstorms of the year in Iowa.

The storm system will depart quickly to the east on Friday,
pulling the lingering precipitation out with it and yielding a
dry and fairly mild Friday afternoon with temperatures climbing
back into the mid to upper 40s in most areas. From Friday night
onward a zonal 500 mb flow regime will set up across the
central U.S. through around the middle of next week, with
several shortwaves moving quickly through this flow. The first
will approach on Saturday but track to our north over Minnesota
and Wisconsin, resulting in south southeasterly low-level flow
in Iowa and another fairly pleasant and mild day. The surface
trough trailing from this northern system will move through
Saturday night, but with no real baroclinicity and in fact,
Sunday will be even milder with highs in the low to mid 50s
across Iowa. After that a large surface high pressure area will
build across the Upper Midwest, bringing northeasterly flow and
cooler temperatures to our service area with highs only in the
20s and 30s by Tuesday. The next system will then approach
around the middle of next week bringing renewed POPs to the
forecast, but details are unclear at this range.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1132 PM CST Wed Jan 29 2025

VFR conditions will prevail overnight into Thursday morning
across central Iowa with a period of low level wind shear
passing over far northern Iowa around and shortly after 12z. Clouds
will be increasing from south to north through the day Thursday
with restrictions down to IFR likely at OTM and DSM as rain
showers arrive in the afternoon and evening. Have held off any
PROB30 mention north of I-80 for now due to low confidence in
any impact.

&&

.HYDROLOGY...
Issued at 241 PM CST Wed Jan 29 2025

Concerns have increased about flooding in response to the
potentially seasonally moderate to heavy rainfall in the forecast
over the next couple days. Main concern would be mainly south of I-
80 especially south of IA 92.

The combination of QPF trending higher along with deep frost depths
may very well lead to a larger hydrologic response than what is
indicated by some of our river forecast guidance. One potential
mitigating factor is that soil moisture values are relatively low.
Thus, despite deep frost depths there is some infiltration potential
for the rainfall.

Present conceptual thinking is that if the higher-trending QPF does
verify and it falls as rain then significant within-bank rises would
be expected at some locations across the southern CWA. A couple
locations may even reach or exceed flood stage. The latest GFS-
forced National Water Model (NWM) guidance does support 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...None.

&&

$$

DISCUSSION...Lee
AVIATION...Ansorge
HYDROLOGY...Zogg