Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251745

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT WED MAY 25 2016

.SHORT TERM.../Today/
Issued at 340 AM CDT Wed May 25 2016

Complex forecast once again today as uncertainty is present as to
the impacts of a couple different MCS`s that will move across the
forecast area this morning.  Current mesoscale analysis is showing
that the current storms to impact the area are moving into a little
less favorable environment for then to become severe though small
hail and some wind gusts to between 40 and 50 mph are likely with
the stronger storms.  The storms from these complexes will move
across the forecast area through the morning hours...coincident with
a shortwave passing across central Iowa and will lay an outflow
boundary out somewhere across Iowa...likely across eastern or
southeastern Iowa...but this will be the area of concern for
potential convection later.

The remainder of todays forecast is difficult.  Uncertainty is great
regarding how much clearing and resulting destabilization will occur
today and if we do get sunny and destabilize...where the old outflow
boundary lies will be a pretty good focus for convection.
However...the models are handling the convection poorly and seems to
be ignoring an area of drying/subsidence moving in to the west
and north, and with time...central Iowa. Much like the past
couple of days...once this MCS dies/moves out I believe much of
the day will be dry and we will likely even see some sun. I have
trended pops with this scenario in mind and have generally dry or
slight chance conditions going for the afternoon. The short term
updates will be the best tool to handle how things evolve in the
smaller scale today.

.LONG TERM.../Tonight through Tuesday/
Issued at 340 AM CDT Wed May 25 2016

Forecast focus in the extended range remains on convective trends,
including potential for severe weather and locally heavy rainfall.
As has been discussed for the past several days, forecast
confidence is rather low due to daily mesoscale uncertainty.

Tonight: Forecast Confidence = Low to Medium

Complex forecast again tonight, much of which will be predicated on
what occurs today.  Current thinking is that convective overturning,
and subsidence associated with todays convection will limit
convective potential overnight. A weak upper trough is forecast to
track ewd across ND/MN however weak forcing and upper level height
falls should remain well north of this area. In addition,
orientation of the nocturnal LLJ does not appear favorable for
ascent as the exit region is progged to be south and east of Iowa.
As such, have trimmed PoPs considerably over many areas tonight.
Several of the CAM solutions do indicate the potential for
isolated/scattered convection developing across northern Missouri,
or perhaps southern Iowa. Left a chance of thunder in these
locales. Lows tonight should remain mild in the 60s. Several
guidance products are suggesting the possibility of patchy fog or
haze as well /perhaps 1-3SM/ - but will leave out explicit mention
in the forecast for now.

Thursday and Thursday Night: Forecast Confidence = Low to Medium

Thursday afternoon and Thursday night appear to have the highest
"conditional" risk of severe weather. A surface low is expected to
deepen over western KS with a frontal boundary extending
northeastward into Iowa.  A warm and moist airmass is expected to be
in place over Iowa by Thursday afternoon with highs in the 80s and
sfc dewpoints in the mid/upper 60s.  Quick note - the GFS/NAM have
been overdoing dewpoints recently - so cut back their forecast
values which are currently in the in the low/mid 70s.  In any event,
expect moderate to strong instability to develop by afternoon with
MLCAPE values of 1500-3000+ J/kg/.  While the wind profiles will not
be extreme, expecting roughly 35-45kts of deep layer shear by
Thursday afternoon which would support multicellular and
supercellular storm structures. The combination of stronger upper
forcing associated with an approaching shortwave /PV anomaly/ and
convergence along the surface boundary should initiate storms
over the region by late afternoon/early evening. There is still
uncertainty exactly where storms will initiate, and this will need
to be fine tuned in the future. All modes of severe weather appear
possible into Thursday night. The current SPC Day 2 Outlook has
the entire region in a Slight Risk, which is reasonable for now.
PWAT values will also increase to 1-1.5 inches as moisture pools
over the area, and this will likely lead to areas of heavy
rainfall into Thursday night if an MCS does indeed form.

Friday through Tuesday: Forecast Confidence = Low

Unfortunately, the unsettled weather pattern appears to persist into
the Memorial Day holiday weekend.  Periods of showers and
thunderstorms are likely on Friday and Saturday as the stronger
upper shortwave slowly pivots northward across the region.  Details
become more nebulous on Sunday through Tuesday but at least a chance
for precip will continue.  Temperatures should remain mild, with
readings in the 70s and 80s.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1244 PM CDT Wed May 25 2016

Main concern once again is convection along with CIGS/VISBY where
convection develops this evening. For now weakening trends
continue. A warm front/sfc converegent area has developed across
northwest/southeast IA. Storms may fire again along this axis by
00z/02z and track east southeast with time. Most sites would see
some convection between 03-11z with storms less likely at KMCW.
Along the axis of better storm chances-have also lowered CIGS to
IFR/MVFR along with MVFR visby through 13z. Remainder of period
looks less active as MCS winds down./rev


.DMX Watches/Warnings/Advisories...


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