Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271127
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
627 AM CDT Tue Jun 27 2017

.SHORT TERM.../Today through Tonight/
Issued at 357 AM CDT Tue Jun 27 2017

High pressure is moving across Iowa this morning. This system has
very dry air associated with it with low relative humidity values
through the column leading to clear skies and PWAT values below one
half inch. The high pressure system will move quickly east today
and be over the upper Ohio Valley by late this afternoon. This
in conjunction with low pressure moving into the Western Plains
will lead to good return flow over Iowa and warm advection,
especially by the afternoon. Breezy southerly winds and good
mixing will warm temperatures into the mid 70s to lower 80s. While
these readings are still several degrees below average, these
temperatures will be warmer than the past three days.

The low level jet will increase this evening west of the Missouri
river with strong theta-e advection advancing northeast ahead of
the system this evening. The very dry air across Iowa will take
some time to saturate but given the degree of deep layer moisture
advection, saturation should occur quickly after 06z as the low
level jet increases to 65 kts and veers into Iowa as a short wave
reaches the region. Thunderstorms will begin to push east as this
occurs and may transition into a linear system, especially across
northern Iowa into southern Minnesota. Strong to a few severe
storms may occur and will depend on if a damaging wind threat can
materialize due to the elevated nature of the storms. Layer winds
near 2 kft will be 50 kts or greater and it may not take much
downward momentum to drive these to the surface if a sufficient
cold pool develops. An attendant heavy rain threat will develop as
well as PWATS drastically increase and will be near 2 inches by
09z. This deep layer saturation should diminish any large hail
threat.

.LONG TERM.../Wednesday through Monday/
Issued at 357 AM CDT Tue Jun 27 2017

The main forecast concern was focused on severe weather potential
on Wednesday across much of the forecast area. Other time periods
of concern were Thursday night into Friday and Sunday night into
Monday for additional thunderstorm chances. Utilized a blend of
the NAM/GFS and hires models Wednesday/Wednesday night and then
the GFS/ECMWF through the remainder of the forecast.

Wednesday into Wednesday night...The ongoing convection Wednesday
morning, mainly east of Interstate 35, should be diminishing with
the LLJ. Timing of this convection diminishing/moving east is
critical to the severe weather potential later in the day. The
NAM12 as well as the ARW/NMM and NSSL WRF have have the first
round in eastern portions of the forecast area by around 15z
Wednesday. This will leave a decent window for destabilization
prior the trough moving through the forecast area by the
afternoon. Even if the cloud cover doesn`t completely clear out,
this system possesses strong enough forcing for ascent to break
through, if any, weak cap Wednesday afternoon. According to the
GFS/NAM, LCL heights lower to around 500-600 meters by the
afternoon with steep low to mid level lapse rates present ahead
of the trough. The 27.06z NAM is much more bullish with the
instability (MLCAPE values increase to ~3000-4000 J/kg), but
regardless of the model, enough instability looks to build into
the state behind the morning convection to cause for concern. Good
veering winds up to about 800mb b/t 18-21z Wednesday with 0-1km
and 0-3km shear values ranging from 25-30 knots and the deep layer
shear topping 50 knots by the early afternoon Wednesday. Fairly
high confidence in a second round of convection Wednesday with the
trough moving across the state, but how severe and widespread the
severity still remains in question as this is somewhat dependent
on rapidly the atmosphere recovers from the morning convection.
Models are trending toward a second round of strong to severe line
of storms. Certainly a hail and wind threat, and confidence is
increasing somewhat with the tornadic threat due to the good shear
profiles Wednesday afternoon.

Thursday through Friday...A pair of shortwaves look to cut across
the region during this time frame with the first one Thursday
afternoon/evening and then a second stronger wave quickly
following by Friday morning. The better forcing remains over the
southern half of the forecast area from both shortwaves and thus
have a bit higher pops going Thursday night into Friday across the
south.

Saturday through Monday...surface high pressure brings a brief
period of dry conditions Friday night into Saturday. The GFS and
ECMWF are started to show signs of agreement towards the end of the
weekend with a decent upper level vort max ejecting out of the
central Rockies Sunday night into Monday. GFS remains the faster
and stronger solution, but both agree on the relative track if
the system across the state.

&&

.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 627 AM CDT Tue Jun 27 2017

VFR conditions will persist today. Light winds this morning will
become breezy from the south. Thunderstorm chances will increase
overnight especially over the northern half of Iowa. In
addition...strong winds in the 1-2 kft layer will lead to low
level wind shear to develop after 06z.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Donavon


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