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fxus63 kdmx 292027 
afddmx

Area forecast discussion
National Weather Service Des Moines Iowa
327 PM CDT Mon may 29 2017

Short term.../tonight through Tuesday/
issued at 327 PM CDT Mon may 29 2017

Shortwave trof / pv anomaly will continue sliding eastward from
the County Warning Area this evening. Loss of daytime heating will bring an end to
the scattered rain showers and isolated thunderstorms and rain across the northern cwa, as
well as the elevated wind speeds as the boundary layer decouples.
A cooler night is expected tonight with lows generally in the
middle 40s to lower 50s.

Tuesday looks to be similar to today weather-wise as another
shortwave trof / pv anomaly drops through the upper level flow across
the region. Elevated wind speeds and gusts are again expected as the
boundary layer mixes considerably, along with renewed scattered rain showers
and isolated thunderstorms and rain across the northern County Warning Area. Some scattered showers
and isolated thunderstorms and rain are also possible across the far southeastern cwa,
where more instability and thermodynamic forcing will exist.

Long term.../Tuesday night through Monday/
issued at 327 PM CDT Mon may 29 2017

Little has changed in overall forecast thinking today. The period
from Tuesday night through Wednesday will be characterized by
dry, quiet and relatively cool weather associated with gradually
weakening cyclonic flow aloft and the passage of a surface ridge
of high pressure. From Wednesday night through Friday a large but
fairly week mid/upper level ridge will build over The Rockies and
very slowly progress into the northern High Plains. A series of
fairly subtle shortwave impulses will travel over the ridge and
down the eastern flank, moving across Iowa at intervals and
inciting multiple rounds of convection in the latter half of the
week. There has been a consistent signal for the leading impulse
moving overhead late Wednesday night and Thursday morning, but
after that timing becomes more uncertain. With the initial wave
have maintained likely pops across about the southern half of the
area, but after that have limited pops to chance category or lower
for the most part.

In the lower levels, long range models have continued to depict
the development of a modest baroclinic zone, acting as an
effective warm or stationary front, developing somewhere over
northern Iowa or southern Minnesota on Thursday. The ec, GFS and
Gem all now depict convective development along this boundary as
the nocturnal low-level jet develops on Thursday evening/night,
and perhaps again on Friday evening/night. The Thursday signal is
now consistent enough that some likely pops have been included in
our northern counties and severe weather potential can be
somewhat diagnosed for that period. It appears likely that
sufficient instability will be realized for robust updrafts, but
vertical shear and flow fields are somewhat limited, with the GFS
for example only predicting 0-6km bulk shear of around 20 knots.
Still, given the expected instability and enhancement from the low level jet
some severe weather risk is apparent, and there may also be heavy
rainfall if the boundary and attendant thunderstorms persist
across the same area for multiple hours. This will certainly bear
watching in the coming days. On Friday evening/night the threat is
more uncertain due to diverging model solutions, but if
significant convective overturning does not prohibit
destabilization once again, then there may be some possibility of
severe weather, especially with surface dewpoints continuing to
increase.

The forecast for next weekend remains quite uncertain. By Friday
evening, at 500 mb, the aforementioned northern High Plains ridge
will be pinched between a gyre over eastern Canada and a trough
swinging ashore over the Pacific northwest. A robust shortwave
swinging rapidly around the western periphery of the eastern
Canada gyre will interact with the two systems to the west, but
long range models are both divergent and inconsistent in their
solutions for this evolution. Some show the shortwave completely
overpowering and disintegrating the northern plains ridge, then
barreling down over the Midwest and near Iowa with an associated
surface cyclone and prolonged precipitation chances. However, the
bulk of the solutions have depicted the ridge muting the
longitudinal extent of the shortwave and forcing it further to the
east, passing over the Great Lakes region and pushing a backdoor
cool front across our region, resulting in sunny and dry weather.
Needless to say, these discrepancies have a significant effect on
the sensible weather forecast for next weekend. For now have gone
closer to a persistence forecast, which is in agreement with the
bulk of long range solutions, specifically the drier weather
regime with very limited pops from Saturday night Onward. However,
this may certainly change in the coming days as additional data
becomes available.

&&

Aviation.../for the 18z tafs through 18z Tuesday afternoon/
issued at 1213 PM CDT Mon may 29 2017

Mainly VFR conditions are expected during the valid taf period.
Mixing of the boundary layer will result in surface wind gusts of 25
to 35 kt this afternoon. Scattered to isolated showers are psbl this
afternoon mainly along and north of US Hwy 20, which may contain
enhanced gusty winds and perhaps a brief period of MVFR cigs. Any
lingering showers will dissipate and winds will abate this evening.



&&

Dmx watches/warnings/advisories...
none.
&&

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