Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221748

Area Forecast Discussion
National Weather Service Des Moines IA
1248 PM CDT Mon May 22 2017

Issued at 655 AM CDT Mon May 22 2017

Have sent out an update to match small pops this morning to the
isolated to scattered high-based showers across the north this
morning. Had some sprinkles in there previously, but actually went
with some pops with radar echoes strengthening a bit to produce a
greater likelihood of rain reaching the ground. Otherwise
forecast remains on track.


.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Mon May 22 2017

Area currently beneath cyclonic flow aloft with lobes of energy
rotating south/southeastward down through the region around the main
upper trough spinning over western Ontario. At the surface have the
main sfc low up to the north across SW Ontario, and an area of high
pressure off to the south and east. This area of high pressure will
shift southeastward as a weak developing area of low pressure across
the Dakotas slides SE. This is a lobe extending westward from the
main sfc low further NE, associated with the shortwave trough
digging in the backside of the main upper low dropping from
Saskatchewan through MT and expected to drop through IA tonight. WAA
ahead of the shortwave currently responsible for some mid/high
clouds across SE SD/NE Nebraska and NW IA. A few light echoes
showing up on radar which could result in some sprinkles across the
western/central areas of the state through sunrise.

Have another shortwave trough moving through CO which will push
eastward today. Theta-e advection in advance of this shortwave will
bring the chance for some showers/thunderstorms across the far south
this morning into the afternoon hours. CAMS suggest majority of
activity will hug the IA/MO border or be south of the border across
MO. Therefore have confined pops to the southern third of less of
the CWA through the day.

With the other shortwave approaching tonight, the sfc low to drop
into SW MN with the sfc trough axis extending SW into central NE.
May see some isolated storms develop out ahead of the sfc trough
axis, but overall expect majority of storms associated with this
energy to develop along the sfc trough axis as it drops
southeastward through the state this evening. Another lobe of energy
to dig into the backside and merger with the other shortwave toward
Tuesday morning allowing continued chances for showers across
northern IA late tonight. CAPE values expected to approach 1000 J/kg
to 1500 J/kg by late afternoon ahead of the sfc trough, with
somewhat marginal shear. Therefore agree with the SPC marginal
outlook, with a few strong to possibly severe storms possible.
Instability to wane into the late evening/overnight hours so expect
mainly showers after 06Z.

.LONG TERM.../Tuesday through Sunday/
Issued at 340 AM CDT Mon May 22 2017

On Tuesday and Tuesday night a mid/upper level low pressure system
will move from north to south across Iowa and surrounding states,
while at the surface modest north northwesterly flow will bring
weak cold air advection. The mid/upper low will bring a large
shield of thick clouds and light showers, but a lack of
instability will preclude any thunderstorms. Overall this period
will be cold, gray and damp across the forecast area. On
Wednesday the mid/upper low will move away to the southeast,
replaced by modest deep-layer ridging and a gradual return of
southerly low- level flow through Thursday. The result is a mostly
dry forecast and slowly moderating temperatures.

By late Thursday a large gyre will have set up across southern
Saskatchewan and Manitoba, where it will remain nearly stationary
through Saturday. Around the southern half of this gyre a nearly
zonal steering pattern will prevail, with a number of shortwave
impulses moving through this flow and resulting in several rounds
of precipitation across portions of the Midwest. The first of
these will come to Iowa around Thursday night/Friday morning as
the leading shortwave ejecting from the gyre pushes the preceding
ridge eastward. There will likely be a focused vorticity maximum
associated with this feature that may result in a cluster of
thunderstorms moving somewhere across the region, however, at this
range prognostic models are showing a wide variance in the
location and timing of any such storms. Elsewhere nebulous forcing
will probably result in scattered showers and perhaps a few
thunderstorms. Given the degree of uncertainty have maintained
generally 30 to 50 POPs for now and will continue to assess this
period as solutions gradually converge throughout the week. After
the leading Thursday night/Friday morning impulse the long-range
models diverge rapidly, both between each other and from run to
run, and forecast confidence for next weekend decreases
accordingly. With it being a holiday weekend and the traditional
kickoff to outdoor summer activities, this will bear watching over
the next several days. While temperatures and precipitation
chances are murky for next weekend, the general synoptic pattern
is not favorable for organized severe weather events, so we do
have that going for us.


.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1248 PM CDT Mon May 22 2017

Upper system dropping toward Iowa will produce some scattered
showers and a few thunderstorms into the evening. While conditions
will generally be VFR, ceilings and visibilities near the
precipitation will likely be MVFR to IFR late this afternoon into
the evening. Much of the precipitation will diminish overnight
although ceiling will continue across the state. Southwest winds
will become north to northwest overnight and into Tuesday morning.





SHORT TERM...Beerends
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