Pleasant Hill, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

542
FXUS63 KDMX 111750
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1150 AM CST Wed Dec 11 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Snow bursts remain possible this morning with slick roads and
  reduced visibility the main impacts. Impacts could remain
  beyond the falling snow. Plan for extra driving time today.

- Turning windy and colder today with the cold lasting through
  Thursday.

- Light snow tracks across portions of central and southern Iowa on
  Thursday. Accumulations under an inch.

- Impactful winter storm with rain, freezing rain, sleet, and
  snow developing later Friday into Saturday. Uncertainty
  remains on temperature profiles, which impacts confidence in
  exact precipitation types, transition zones, and
  accumulations.

&&

.DISCUSSION...
Issued at 355 AM CST Wed Dec 11 2024

Our weather pattern will remain active over the next 7 days with
plenty of wintry systems passing over and through the region. GOES-
East upper level water vapor imagery shows the longwave trough
over the central part of the US with Iowa residing beneath its
axis. There is a lot of kinematic energy along with plenty of
vorticity, which in conjunction with the surface cold front has
generated snow showers this morning. These have been most
prevalent over central and northern Iowa, but have seen radar
echos and surface observations expanding into southern Iowa.
Winds gusting 35 to around 40 mph have occurred along and
immediately behind the front. This along with falling snow has
resulted in brief bouts of reduced visibility with the worst
conditions being report at Sheldon a little after 1am with a
visibility of a half mile with a wind gust of 43 mph. Compared
to last night, so far the intensity of these has not lived up to
expectations, but short range models continue to show potential
for small areas to have visibility under a mile with gusty
winds and falling snow. With the morning rush hour approaching,
this could be quite impactful and will continue to monitor and
issue Special Weather Statements or if the need arises a Snow
Squall Warning, which is seeming less likely. Snow rates still
look to be a half inch per hour or less with the exception being
in those bursts. While most will see snowfall of less than an
inch, the latest HREF continues to paint streaks of 1 to 2
inches with a few embedded higher amounts. Much of this round of
snow showers along with the cold front will clear the forecast
area by mid-morning. However, clouds look to linger over north
central into eastern Iowa and latest HRRR has indicated elevated
instability with a few HCR streaks this morning into early this
afternoon. Have included low PoPs for this potential and things
look to dry out by later this afternoon. Beyond snow, winds
will be increasing, though compared to this time yesterday they
do not look to be as strong. Colder air will be delivered on
these blustery winds with highs today lowered to the 25th
percentile of the National Blend of Models, which is also below
the MAV and MET guidance. Overall, the coldest air is lagging a
bit so highs today are a bit higher than yesterday morning`s
forecast.

Into tonight, winds will decrease with the pressure gradient
relaxing and the influence of an upstream high pressure nosing
into Iowa. Skies will begin to clear over much of the state,
but low level warm air advection (WAA) will cause clouds to
increase from southwest to northeast through the night.
Temperatures were lowered to account for the good radiational
cooling setup over northern Iowa where confidence is highest in
this outcome. Lowering low temperatures over central and
southern Iowa is possible and would result in the current low
forecast being about 3 to 4 degrees too high. Fortunately,
winds will be less and so wind chills fall into the teens below
zero over northern Iowa and single digits above zero over
southern Iowa by Thursday morning. Clouds will thicken into
Thursday with light snow expected as the best QG convergence and
WAA passes over southern Iowa and Missouri with departing
kinematics. Cross sections and soundings do show that the low
levels will remain quite dry with high pressure funneling in dry
air on winds from the east. Thus, the snow will be fighting to
get through this layer, but deterministic models continue to
show light QPF with the CMC and ECMWF ensemble means showing
probabilities up to around 50% of 0.01" over central into
southern Iowa. Snow amounts continue to look to be under an
inch.

What looks to be the main winter weather event of the 7 day period
is set to develop later on Friday into Saturday. By this point, the
longwave trough will have lifted over northeastern North America
with the flow over Iowa having a passing ridge with an approaching
shortwave trough. Moisture advection ahead of weak surface low
pressure associated with this shortwave will be occurring.
Isentropic lift on the 295K surface will have a long fetch from
the Gulf of Mexico into Iowa by Friday evening that wanes into
Saturday. This will cause a warm nose to develop as the low
levels warm to between +2 to 4C. With strong QG convergence
moving in and with modest kinematics in the form of a 75 knot
jet, expect precipitation to develop later Friday that will
persist into a portion of Saturday. That warm nose will result
in melting hydrometeors and mixed phase precipitation types over
a good portion of central Iowa as surface and low levels are
below freezing at the start of this event. Thus, precipitation
looks to start as freezing rain over southern into central Iowa
that will gradually transition to rain Friday night as warmer
air arrives, but not before some ice accumulation and possible
related impacts. Over northern Iowa, snow will be how the
precipitation starts with a transition to sleet and eventually
freezing also possible into Friday night. Not surprising, there
remain difference in the thermal profiles between the NAM, GFS
and ECMWF ensemble soundings so refinements to this will
continue over the next few days. Finally, we will need to
monitor for possible drizzle of freezing drizzle Friday morning,
which is not in the forecast at the moment, as saturation looks
to be deep enough for just rain. However, these types of setups
can often result in this scenario so something we will be
monitoring in the forecast cycles.

As this system departs late Saturday, ridging will follow behind it
with another shortwave trough approaching from the western US.
Differences abound with the ECMWF developing precipitation over the
state by Monday morning with ensemble support of 30 to 40% of 0.01"
of liquid precipitation. The GFS and CMC and their ensemble means
develop this farther east. Profiles at this time horizon point to
warm enough for mostly liquid, but given the difference in timing
will continue to assess in the coming days.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1145 AM CST Wed Dec 11 2024

Afternoon heating has allowed for some low level cumulus to
form, bringing with it some scattered snow showers, especially
northeast. Localized vsby restrictions on the magnitude of MVFR
will be possible in the afternoon. Otherwise, cigs will bounce
between VFR and MVFR through sunset, becoming prevailing VFR.
Winds will also ease after sunset and veer to northeasterly
after 12z.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Jimenez