Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 222351
AFDDMX

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

...Forecast Update and 00z Aviation Discussion...

.UPDATE...
Issued at 652 PM CDT Mon May 22 2017

Updated forecast for current trends as the line of showers and
thunderstorms moves through central Iowa this evening. Latest SPC
mesoanalysis shows this line of storms in an area characterized by
weak MLCAPE less than 500 J/kg and bulk shear less than 30 knots.
However, afternoon NUCAPS pass showed downdraft CAPE between 600
and 700 J/kg, which lines up well with current SPC mesoanalysis.
These storms will continue to pose a gusty wind potential as has
been observed at several ASOS/AWOS/RWIS sites recently. In
addition, with low freezing and wet bulb zero levels, may have
some sub-severe hail as well. Hence, an SPS was recently issued
for these possibilities as these storms move toward I-35.

&&

.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 on...it 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
forecast.

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 00Z TAFS through 00Z Tuesday evening/
Issued at 652 PM CDT Mon May 22 2017

A cold front accompanied by scattered showers and thunderstorms
is moving through the state this evening with prevailing groups
for timing of showers/storms and TEMPOs for expected impacts if
necessary. Otherwise, should have VFR ceilings through much of the
night. MVFR ceilings may develop well after midnight or tomorrow
morning over the eastern terminals behind the cold front. In
addition, isolated showers are expected to develop across Iowa
later Tuesday morning and persist into the afternoon hours. For
now, have covered this with VCSH starting around midday at all the
terminals.

&&

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

$$

UPDATE...Ansorge
SHORT TERM...Cogil
LONG TERM...FAB
AVIATION...Ansorge


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