Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 010000
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
700 PM CDT Sun Apr 30 2017

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Monday/
Issued at 326 PM CDT Sun Apr 30 2017

Low pressure continues to lift north through northeast Kansas and
central Missouri this afternoon as a deep upper level low moves
north into southeast Kansas. A broad area of intense theta-e
advection is arriving over Iowa ahead of the system. Another wave of
showers and a few embedded thunderstorms has developed over the
central portion of the state in response to the theta-e advection.
This region will continue to lift north through mid evening. The
overall instability and mid level lapse rates have remained modest
enough that any severe weather potential is near NIL. Gusty
northeast winds with a few gusts over 45 mph have also developed
this afternoon over mainly west central Iowa and in the region of
strongest pressure gradient. The gradient will begin to weaken as
the low pressure arrives and the wind should drop off substantially
in vicinity of the surface low overnight.

The main region of any convection will be closely tied to the theta-
e advection surge through mid evening. The dry slot will arrive
though there will be pockets of enough moisture for a few showers
within this area mixed with drizzle. Areas of fog are possible and
will be most favored in the regions where the wind diminishes the
most, likely just east of the low. A period of light snow is
possible over the far northwest late tonight into Monday morning
though not expecting any accumulations as profiles battle dry air
intrusion throughout this period. The wind will switch from the
northwest during the day and may approach wind advisory criteria by
the afternoon as mixing deepens and modest cold advection develops.
The precipitation will end from west to east through the day.

.LONG TERM.../Monday night through Sunday/
Issued at 326 PM CDT Sun Apr 30 2017

Tue AM-Wed AM...
Confidence: High

Sunshine and blue skies are expected to return to the DMX CWA on
Tuesday. By 12z Tue, the system largely responsible for our cloudy
and rainy past weekend will have pushed towards the Great Lakes.
Strong pressure rises and a still somewhat tightened pressure
gradient should keep up gusty winds at least during early Tuesday
morning. By 18z-21z Tue, pressure gradient notably relaxes which
should help decrease winds. Models show large-scale subsidence and
dry air entrainment to help clear up atmos...which is further
verified by looking at current vissat that shows distinct clearing
behind this low. 850mb temps look to range within a few degrees of
+0C (-1 to -2 std dev) within a weak WAA regime...so though there
will be more blue skies than clouds, am not optimistic on sfc temps
making it much into the 60s, if at all.

Wed PM-Thu PM...
Confidence: Medium

Our sunshine and rain-free conditions are interrupted by another
wave slated to pass through the upper Midwest on Wednesday. Models
are coming into good agreement with ejecting an impulse off the
upper low spinning near the Gulf of Alaska. By 18z Wed, the trough
axis of this wave should be extending southward near the IA/NE
border...propagating to the east as the wave makes a southeasterly
progression. The 12z Sun GFS develops a sfc cyclone over west-
central MN to accompany this wave...and shows weak low-level
frontogenetical forcing to accompany this boundary passage.
Timing of this boundary will be key, as available energy goes away
with daytime heating. Largely uncapped with 0-6km MUCape values
near 500 J/kg (very minimal) and 0-6km shear approaches 20 to 25
kts (very minimal). In all, appears enough to get thunderstorms
over western to NW CWA, but as of now, will be pulse thunderstorms
at the very best. Models are trying to push through another band
of precip on Thursday, but under strong NW flow, with 20+ degree
sfc dwpt depressions, could just see 4000ft-based cu with virga.

Fri & Beyond...
Confidence: Medium

We are still on track for a major pattern change, with an omega
block trying to set up across the CONUS by next weekend. By
Saturday, the low discussed above should be closed and deepened,
located over the southeastern CONUS. Meanwhile, another upper low
should be coming onshore near California...with the ridging in
between these two lows. As of now, it appears the DMX CWA should be
in the dry section of the block...setting us up for a sunny & warm
weekend. At this point, tough to justify modifying temps from
guidance in either direction to where value can be added...so have
stuck with guidance. Instincts are that once the models settle on
when we will be settled under the ridge, temps will need to be
increased.

&&

.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 639 PM CDT Sun Apr 30 2017

IFR conditions for visibility due to the large rain shield moving
north across the state. DSM likely to be in the dry slot by around
01z or sooner, but fog and mist looks to continue to provide IFR
to MVFR visibility. Confidence is high for the IFR to lower MVFR
stratus to stick around for much of the TAF period. Lower
confidence in timing the IFR visibility this evening with the
rain. Winds shift and increase Monday morning and continue through
the afternoon hours.

&&

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

&&

$$

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


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