Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 050917
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
317 AM CST Fri Mar 5 2021

.DISCUSSION.../Today through Thursday/
Issued at 317 AM CST Fri Mar 5 2021

Today Through Sunday:

Positively tilted short-wave trough is moving into the lower
Mississippi River Valley this morning with greatly amplified ridge
axis across the Rockies. The short-wave trough and associated
surface cyclone is well south of Iowa. There are some high clouds
that have arrived in far southwestern Iowa that were advected from
that system, but largely no sensible impacts. The amplified ridge
has been largely in control of weather for Iowa over the past 24
hours and that will continue to be the case. Subsidence has allowed
surface high pressure to develop across the upper Mississippi River
Valley and continues to be reinforced by the AVA, as the area
remains in northwesterly mid to upper-level flow downstream from the
ridge axis. Even with the subsidence, there have been pockets of
moisture that has allowed for some cloud cover, as seen on satellite
imagery. However the cloud cover has not been overly thick and not
widespread. Today, the ridge axis will slowly move into the central
Plains. The surface high pressure will shift slightly southward as
the mid-level flow steers it in this direction. Near surface flow
will shift from easterly to east-southeasterly this afternoon. This
will result in continued weak CAA, especially for eastern and
northeastern Iowa. As a result, temperatures this afternoon will be
slightly cooler than yesterday, but will not be a drastic change.
The thin passing cloud cover may also limit insolation at times, but
it does not appear that today will be completely overcast for
extended periods of time. Saturday, a short-wave trough will begin
to lift across the northwest CONUS into Canada, resulting in lee
cyclogenesis in the northern Rockies. This system will miss Iowa as
it rides on the backside of the amplified ridge, but this will
enhance the flow enough to pick up the speed of the ridge`s
propagation across the central Plains into the upper Midwest.
Further, surface to 850mb flow turns south-southwesterly in response
to the cyclogenesis in the far northern Rockies which will begin
another period of WAA. On Saturday, the stronger WAA will be
confined to western and southwestern portions of the forecast area.
Locations east of Interstate 35 will not see overly robust WAA on
Saturday. Sunday, ridge axis continues to propagate eastward.
Southwesterly flow between the surface and 850mb ramps up quickly,
and so does the WAA. This will spread across most of the state on
Sunday. In addition to strong WAA, model soundings are in favor of
rather vigorous boundary layer mixing. In this setup, temperatures
will be able to reach to the mid 60s. NBM starting guidance
continues to catch up to the warmer temperatures, and the spread
among its inputs has decreased over the past 24 hours. For Sunday
though, still bumped the temperatures up toward the 75th percentile
given the continued signal for strong WAA and strong mixing. Some
reporting stations may break daily record highs March 7th. While
this warm up is ongoing, the ridge will block most of the short-wave
perturbations this weekend. Therefore, the forecast will be dry. One
other note for Sunday will be gusty winds, especially across the
west-central and northwestern portions of the forecast area. At this
time, winds still look to be below advisory criteria, but this will
need to be monitored closely as Sunday approaches.

Extended:

Warm conditions will continue through Monday and Tuesday. Tuesday
though will start the mark of a more active weather period across
the forecast area. 00z deterministic GFS and ECMWF are starting to
come into somewhat better agreement with the large trough and deep
surface cyclone Tuesday through Wednesday. Forcing will be
widespread across Iowa. Among the ensemble members, there is still
some discrepancy over QPF amounts and placement. GEFS members still
try to depict a dry slot that works through at some point. Thus it
is difficult to pinpoint the timing of potentially impactful
weather. Wednesday night into Thursday, Iowa looks to be in the warm
sector and some instability is being depicted in several model
solutions. Thunderstorm development is possible. With this appearing
to be a highly dynamic system with stronger jet streak, the severe
threat is non-zero. However, the signal is not overly strong, thus
at this time will call the severe threat low Wednesday into
Thursday. After the passage of this first system, another low
pressure system is forecast to move across the upper Midwest Friday.
Once again, model consensus is still rather weak. The track of this
cyclone will likely have a large dependence on the preceding system.
The second system though does look to perhaps bring in colder air,
which could eventually lead to snow or some kind of wintry
precipitation. This will then be followed by seasonably cool weather
conditions.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1131 PM CST Thu Mar 4 2021

VFR conditions prevail through the current TAF period.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

DISCUSSION...Krull
AVIATION...Martin


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