Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251142
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 AM CDT Tue Apr 25 2017

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

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Tue Apr 25 2017

Message of the Day: Cloudy and Rainy
Hazards: Lightning (This Evening-Early Wed AM)
Confidence: High

08z Tue sfc analysis depicts a weak sfc low over north-central
Minnesota, along with an attendant boundary draping southward
through far western Iowa. Though sfc dewpoint depressions are very
low near to the boundary, BUFKIT sounding analysis shows rapid
drying of profile/an inversion starting above 500 ft. This, coupled
with lack of any reasonable forcing, will keep this fropa dry early
on. Hi-res models have this boundary slowly meandering across Iowa
during the day today... centered near Interstate 35 by 00z Wed, and
nearing the eastern edge of the DMX CWA by 12z Wed. With WAA, plus
much thinner cloud cover south and east of the boundary versus CAA
and thicker/lower cover behind the boundary, differential heating
will amplify during the day today...leading to a notable temperature
gradient across the DMX CWA. Ottumwa should be in the mid 70s,
whereas Estherville may struggle to reach 50 degrees.

Phased with the aforementioned boundary, a sfc low will be pushing
off the Rockies this morning over Kansas. This low will rapidly
propagate northeastward, making it into SW Iowa by 18z-21z Tue. AS
was the case with yesterday`s 00z runs, 305K sfc beautifully picking
up on a band of very positive isentropic adiabatic lift making it
into SW Iowa around 12z Tue. Support for ascent continues aloft, as
a band of strong 650mb frontogenetical forcing oriented NW to SE
will be accompanying this forcing. By 18z Tue, this band will be
located across northern Iowa. In a slight change from yesterday,
there does not appear to be continual reinforcement of support for
precip behind this band, and thus have dried out our CWA south of I-
80 by 18z Tue.

Overnight tonight, strong frontogenetical forcing (oriented N-S),
especially at the 950-900mb level will push through Iowa with the
boundary from west to east. Have widespread pops following this
boundary. Enough instability to justify thunder mention...though
severe chances are effectively nil.

.LONG TERM.../Wednesday through Monday/
Issued at 340 AM CDT Tue Apr 25 2017

Active period setting up through the week into the weekend. Broad
upper level trough across the western US to shift eastward Wednesday
with a surface trough axis stretched across the eastern portion of
the state. This sfc low to shove eastward Wednesday night, with the
upper level wave lifting northeastward into Thursday. This should
allow for precipitation to move out early Thursday and mainly dry
and cool conditions for Thursday into Thursday evening as high
pressure briefly builds into the state. GFS fastest at bringing
precip chances back into the state late Thursday night into early
Friday, with the EC/Canadian a little slower. Have trended the
guidance grids down a bit for late Thursday and trimmed some into
early Friday.

Broad upper level trough remains in place across the west stretching
into the central US into Friday and the weekend. A developing
surface low is expected to be situated across the western high
plains by mid/late Friday, with southerly flow/moisture advection
northward into MO/IA. GFS brings the moisture furthest northward
with the baroclinic zone staying across southern IA/MO. Canadian/EC
continue to keep precipitation chances more in line with the
baroclinic zone, while the GFS continues to bring it further north
into the state. Still feel pops are somewhat overdone across IA for
Friday into Friday night. As additional energy digs into the back of
the upper level trough, it reinforces the sfc low across the
southern plains, with focus for precipitation moves further south
closer to the sfc low. This system to begins to eject northeastward
into late Saturday/Sunday, with the sfc low expected to be somewhere
in the vicinity of SE IA/NE MO/central IL by Sunday morning. Models
showing better agreement in track and timing, but still some
disagreement leading to some uncertainty. However all three
solutions still place IA with some precipitation Sunday as the
deformation zone moves across the state. Colder air in place on the
north side of the system may lead to some rain/snow mix across the
north early Sunday. Sfc low to lift due north into late Sunday/early
Monday with continued chances for lingering def zone precipitation.

Overall pattern a cooler pattern with lots of clouds/precipitation
chances also helping keep temps down. Colder air to move in behind
system late tonight/Wednesday, with a brief warmup ahead of the next
system Friday with colder air moving back in Saturday/Sunday.
However even the warmer periods still below average for this time of
year.

&&

.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 639 AM CDT Tue Apr 25 2017

Medium confidence TAFs. All sites starting this TAF period in VFR
and ending in at least IFR. Cannot completely rule out
LIFR late tomorrow morning...especially at KMCW and KALO.
Mentioned thunder at KDSM and KOTM and left out at other sites.
May need to add thunder this evening at KMCW and KALO...but
confidence not high enough to do so at this time. Precip should be
light enough to not significant degrade vsbsys, but will monitor.

&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Beerends
AVIATION...Kotenberg


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