Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 192041

Area Forecast Discussion
National Weather Service Des Moines IA
341 PM CDT Sat May 19 2018

.SHORT TERM.../Tonight through Sunday/
Issued at 336 PM CDT Sat May 19 2018

The primary concern into tomorrow will be precip trends with the
most significant convection, if any, this evening. Currently water
vapor imagery notes the upper low over the High Plains with a noted
PV anomaly and dry slot pushing through the MO Valley. This suggests
the steep mid level lapse rates are further north and east than
depicted by recent RAP objective analysis. Our window to realize
this isn`t great however with the 20z surface analysis placing the
cold front from between Creston and Lamoni to just south of
Waterloo, and any convergence of substance over southern IA near
Osceola and Chariton. The flow is fairly weak however so it won`t be
too strong. MLCAPEs in this area appear to be uncapped around 1000
j/kg so there is still some potential for re-development in the
wedge of clearing ahead of the front and behind the ongoing central
IA convective cluster. The effective shear is weak and RAP hail
parameters are not impressive, but the drying aloft per the
aforementioned water vapor imagery suggests a brief hail potential
may be greater than objectively analyzed. Plenty of low level
moisture and limited shear as created an unfavorable wind
environment outside of isolated downbursts as cores collapse. The
weak low level wind and shear will diminish much of the tornado
potential as well, however 0-3km CAPEs are quite high so isolated
thermodynamically dominate/stretched funnel clouds/landspouts will
be possible near boundaries, synoptic or mesoscale.  Isolated
locally heavy rains are possible too with anomalously high
precipitable waters and slow cell movement. Isolated 1-3 inch rains
have already been report with a few storms.

Any appreciable instability and thermodynamic forcing should be
limited to this evening, however continued kinematic support ahead
of the upper low will linger into Sunday, especially the morning,
keeping chances for scattered showers and low end thunderstorms in
the forecast. There should be plenty of low clouds as well with
relatively light north to northeast winds doing little to scour
things out.

.LONG TERM.../Sunday night through Saturday/
Issued at 336 PM CDT Sat May 19 2018

Sunday night...A passing shortwave Sunday evening will keep shower
and thunderstorm chances going over the state but the instability
and shear will be tied to the boundary which will be well south
of the state so not looking at any kind of severe threat. It will
be a cool and wet period with non severe storms in the forecast.

Monday through Tuesday...An upper ridge will build over the
region but with a trough digging in the west, shortwaves will be
riding the backside of the ridge just to the west so it will be
mainly dry Monday with small chance PoPs creeping into mainly the
west Tuesday/Tuesday night.

Wednesday through Thursday, the upper ridge begins to shift east
as an upper trough passes into the Northern Plains. Weak waves of
vorticity move through ridge and across the state, increasing
PoPs across western/northern Iowa through this period but the
better instability still remains west until later Thursday/Friday
when the upper trough pushes into the Upper Midwest. We will see
more widespread showers/thunderstorms Thursday night into early
Saturday with the passage of the upper trough and surface
boundary, which should pass to the east of the state by mid-day
Saturday. Precip chances will then shift east later Saturday into
Sunday with cooler temps moving in for the latter part of the


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 128 PM CDT Sat May 19 2018

Small cluster of storms will affect KDSM early this afternoon and
likely affect other sites to the north in some form later in the
afternoon. Will try to highlight precip and thunder trends in the
short term, but default to VCSH wording later as confidence
decreases. A frontal boundary is also bisecting Iowa SW-NE at
18z. As this front sags south and east, confidence does increase
that MVFR to IFR ceilings will develop into Sunday morning.




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