Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230451

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Mon May 22 2017

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Tuesday/
Issued at 346 PM CDT Mon May 22 2017

Showers have lingered in far southern Iowa this afternoon near low
level moisture transport axis. This precipitation will continue to
pull away from the south this evening as the axis shifts farther
southeast. Meanwhile, surface front extends from near Omaha to just
north of Des Moines to near Dubuque.  Short term models suggest some
brief development along this boundary late this afternoon/evening
during peak heating, although much of this will likely die off
quickly.  Otherwise, the more organized area of precipitation will
be associated with the upper low across South Dakota which is quite
evident on Water Vapor imagery.  Showers and storms will become more
numerous late this afternoon into the evening across northwest Iowa
into eastern Nebraska as mid level cooling arrives.  There remains
some threat of marginal severe weather with these storms with low
freezing levels helping hail formation and inverted-V soundings
suggesting some wind potential.  However, instability remains
relatively low for May and best bulk shear is over eastern Iowa.
Therefore, sustained organization will be somewhat difficult with a
severe storm or two but nothing widespread.  This activity will
slide east across the forecast area this evening but gradually
weaken and dissipate as instability subsides.

Upper low will cross Iowa on Tuesday with any clearing that occurred
overnight quickly becoming filed with cumulus as daytime heating
increases.  Scattered showers are expected to develop by later
morning across much of the area and persist into the afternoon with
the cool air aloft.  Some isolated thunder will be possible as well
but quite limited in scope given the lack of overall instability.
Temperatures are expected to remain well below normal with cold
advection persisting.

.LONG TERM.../Tuesday night through Monday/
Issued at 346 PM CDT Mon May 22 2017

Cold core upper low will continues to traverse the state Tuesday
night.  However by this time we lose our instability and surface
winds are rather brisk so while I cannot rule out a continuation of
the funnel cloud potential early will definitely be waning
into 00Z. Cold air will be in place over the forecast area but cloud
cover will keep temps in check Tuesday night. Some precip should
linger in the east during the evening but by and large, most of the
precip will be done in the early to mid evening.

On Wednesday models differ as to the location of the low but in
general, the next system will push out of the northwest shifting an
upper ridge into the Plains and pinching it between the low
northwest and the Tuesday`s low now in IL.  This will keep at least
some chance for showers across the east or southeast depending on
which model you buy into.  Meager instability means little potential
for thunderstorms so I did keep just a shower mention in the

On Thursday the upper ridge is over Iowa so we will be dry with
warmer temps.  Thursday night a lead shortwave pushes across the
state flattening the ridge and bringing a chance for showers across
the state.  Instability will work it`s way back into Southern Iowa
during the overnight hours so some thunderstorms are possible
Thursday night, mainly southwest.

Friday through the rest of the forecast period we will be dealing
with the northern Plains upper low that will now be over the Upper
Midwest.  There will be weak shortwaves dropping through the flow
and kicking off periods of showers/storms through the period. At
this point it`s hard to say whether a few storms would be severe but
it is not looking likely with a weak inversion in place on Friday
and generally weaker flow aloft through the weekend, and storms that
do develop do not look like they would get out of hand.  One thing
that is disturbing is the GFS is painting some very high qpf across
southern into northeast Iowa Saturday into Saturday night. This is
tied to a surface low and warm front which the other models either
do not have or it`s further south and east.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1152 PM CDT Mon May 22 2017

VFR ceilings will prevail overnight as a cold front slowly slides
across the state. Ceilings will lower with low VFR ceilings
expected by later Tuesday morning as winds from the north and
northwest average 10 to 15 knots. Scattered showers will develop
toward midday across the central Iowa terminals and continue to
handle this with VCSH through the afternoon. It is also possible
that ceilings may drop into MVFR for a short time as a shower
passes over a terminal, though this isn`t specifically mentioned.
Late in the TAF period, may have some ceilings drop toward high-
end MVFR over the northeastern TAF sites.




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