Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 292344
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
644 PM CDT Sat Apr 29 2017

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Sunday/
Issued at 343 PM CDT Sat Apr 29 2017

Area of low pressure over eastern Texas will lift north/northeast
tonight and will be over far eastern Kansas by early Sunday morning
then continue north into southwest Iowa by Sunday afternoon. Current
band of precipitation moving across the state is beginning to move
east and will result in a mostly dry period into this evening.
Precipitation chances overnight will be mostly focused over the
western forecast as a ribbon of strong 750 mb theta-e advection
lifts into that area ahead of the approaching low. Most of this
precipitation will be in the form of light rain though could be
locally moderate. Should be able to evade any snow flakes as warm
advection arrives and finally removes any residual near isotherm
profiles around 0C aloft. Could be a few wind gusts this evening
above 40 mph over the west.

Another wave of strong theta-e advection will arrive late tonight on
the nose of 60 kt low level jet. This will produce MUCAPES of 500 to
1000 J/KG over the southeast third of the area including Ottumwa.
This instability along with increasing mid level lapse rates may be
conducive enough for a few updrafts strong enough to support
marginally severe hail. The dry slot will lift into Iowa behind the
convection though a few showers and isolated thunderstorms near the
surface low by the afternoon. Big range in temperatures with upper
30s northwest to upper 50s southeast. Not expecting enough
precipitation to cause any stream flooding concerns with fast moving
storms on Sunday and less than previously expected precipitation
occurring through tonight.

.LONG TERM.../Sunday night through Saturday/
Issued at 343 PM CDT Sat Apr 29 2017

Monday-Tuesday Morning...
Confidence: High

Models are in great agreement with continuing to push our weekend
rain-maker discussed in the short term...thus have gone with the
consensus blend. Consensus has the sfc low making it into NE Iowa by
12z Mon and to the U.P. of Michigan by 06z Tue. Profile should
remain decently saturated in the low-levels through the morning.
Pressure rises at the rate of +3 to 4mb/3 hrs is expected behind
this low...and by 18z Mon, weak large-scale subsidence infiltrates
Iowa. The pressure gradient is just above 3mb/100km. With winds
atop the mixed layer winds nearing 35kt to 40kt, expecting a windy
afternoon Monday...especially west of Interstate-35. In their
current state, 12z Mon-00z Tue fcst grids probably have too much
precip going, and may be significantly lowered in the next update.
Have nudged temps upward versus guidance. As of now, it looks
like winds should stay below Wind Advisory criteria, but will need
to monitor.

Wednesday-Thursday...
Confidence: Medium

Long-range models have an impulse ejecting off the Aleutian Low near
Alaska...propagating through Canada and making off the Rockies
into the Plains states by 12z Wed. Meanwhile, a substantial 500mb
ridge comes onshore across the Intermountain West. This ridge
looks to nudge this upper wave to the south...and it becomes a
closed low over the Ozarks by Thursday afternoon. Overall, it
appears the best moisture transport and forcing will remain just
south of Iowa...possibly clipping our southern counties Wednesday.
By Thursday morning, a very weak trough axis associated with the
longwave trough originating over Canada passes through Iowa. This
boundary should be very moisture starved, so chances for
widespread, well-sustained precip are minimal.

Friday & Beyond...
Confidence: Medium

Since the 12z Tuesday runs, at least the long-range ensembles have
had at least some inkling of an Omega Block pattern setting up over
the CONUS. By Saturday, the low discussed above should be over the
southeastern CONUS and another upper low should be coming onshore
near California...with the ridging in between. As of now, it appears
the DMX CWA should be in the dry NE section of the ridge of the
block...setting us up for a sunny weekend with slightly above
normal temperatures.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 636 PM CDT Sat Apr 29 2017

MVFR ceilings likely to persist at OTM and DSM through the evening
with a break possible later tonight into tomorrow morning. Delayed
precip onset until tomorrow morning, at least the MVFR to IFR
visibility restrictions due to the rain. Winds continue through
much of the forecast period.

&&

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

&&

$$

SHORT TERM...Donavon
LONG TERM...Kotenberg
AVIATION...Podrazik


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