Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180511

Area Forecast Discussion
National Weather Service Des Moines IA
1211 AM CDT Tue Jul 18 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 337 PM CDT Mon Jul 17 2017

Main challenges/concerns will be extent of heat and convection.
Early morning subjective sfc analysis shows diffuse warm front
continues draped across far western Iowa with an attendant low
stratus deck at H850 in central Iowa. Where sun is shining over the
west...temperatures are responding again quickly this morning. Under
the cloud deck not so much.  By late this afternoon and tonight the
front will show less definition. Instability remains rather high
over the area just to the west of the decaying boundary. This may
lead to a few isolated thunderstorms from 4 pm to 7 pm this
evening...if the H700 cap can`t hold. Currently temps are over 11C
and this is likely to inhibit convection.  Despite low confidence
given any real forcing...convective clouds beginning to develop
southwest of Des Moines...and have added isolated thunder over a
small area west/southwest of KDSM from 20-00z. Low pressure over the
Northern Plains and associated cool fronts drop southeast and become
more stationary by 12z Tuesday over southern Minnesota. This trend
will keep Iowa in the warm sector through tomorrow with increasing
moisture again advecting into the area in the next 24 hours. Aloft
at H850 a ridge of high pressure extends from the southeast to
central Iowa. The H850 moisture channel remains just west of our
area at 12z with a dewpoint ribbon of 15C extending from the Gulf
north into northwest Wisconsin. The models are in modest agreement
with the progression of synoptic features through 00z
Wednesday...but some questions regarding the extent of convection
remain. The 12z GFS increases thetae advection over northwest
Iowa/eastern Nebraska from 18-00z tomorrow while gradually building
an H700 cap aloft. Any diurnal convection that develops tonight
should generally remain west of our area...then track into northern
Iowa tomorrow closer to the surface boundary...somewhat bounded by
the H700 thermal cap.

The Euro model is showing a more pronounced surface boundary farther
south into the state by Tuesday afternoon...suggesting convection
would also be farther south...developing across the north during the
afternoon hours. This reflects at least one hires member...the NMM
and somewhat similar to the NAM. The ARW is more in line with the
GFS...and farther northwest/north with the convection through 00z.
With that in mind...will keep higher PoP in the afternoon hours over
the north with more cloud cover regionwide by late day. Overall this
will tend to cap highs north with cooler temperatures once
thunderstorms have occurred there.  Over the south clouds will
overspread the region by late day but probably not quickly enough to
keep highs from reach the lower/mid 90s.  Though just shy of
advisory criteria...south of I80 will likely see heat indices
between the upper 90s and the lower 100s. Similar to previous
afternoons...precautions for heat overexposure should again be
emphasized.  If values are trending higher with next run of model
data...overnight crew may add a headline for tomorrow.

.LONG TERM.../Tuesday night through Monday/
Issued at 337 PM CDT Mon Jul 17 2017

At 00z Wed, core of well-advertised ridge should be centered near
the moves towards the Ozarks/Oklahoma region... serving to push
the DMX CWA into return flow. Long-range model guidance trying to
suggest core of high pressure to push into the Deep South by
Saturday. As suggested since Saturday afternoon`s AFD... models
will likely and indeed have slowed this progression. The
operational 12z Mon ECMWF in particular slows the departure of the
ridge... keeping both Friday and now Saturday in warmer
temperatures. Will probably continue to see Saturday`s
temperatures bumped up as this feature`s departure may stall an
additional day.

Above Normal Temperatures/Excessive Heat Potential...
Confidence: High

With 850mb temperatures around +18C to +22C under scattered clouds,
much of the DMX CWA reached into the upper 80s and low 90s for
Monday afternoon. By Wednesday, 850mb temps jump in a range of +21C
to +24C across the DMX CWA...and by Thursday and Friday, 850mb temps
flirt with +27C. Tracking dewpoints, no reason to see much departure
from persistence forecast from the past several days... meaning sfc
dwpts in the low to mid 70s seems plausible. Combining
temperature and dewpoints, apparent temperatures should be well
into the triple digits each afternoon Wednesday through Saturday.
Excessive Heat Watch/Warning criteria includes Heat Advisory
criteria being reached four consecutive days. Confidence high
enough to issue Excessive Heat watch with this criteria in mind.

For Saturday, long-range models are trying to zero in on a sfc low
with an attendant "cold" front passing through Iowa from NW to SE.
As mentioned earlier, with the ECMWF already trending slower,
coupled with historical trends for systems such as this to have a
delayed progression, have already nudged up temperatures for
Saturday. If this low and boundary does not make it southward
through Iowa by peak heating Saturday, would expect Saturday
temperatures to be closer to persistence from Friday.

Strong to Severe Thunderstorm Potential...
Confidence: Medium

Somewhat similar to last week, instability/CAPE off the charts each
afternoon with sfc dwpts into the 70s. For sustained convection to
generate, location of trigger/focusing mechanism key. With steep
low- level lapse rates, large hail will be primary threat.
Damaging winds also a primary threat, as DCAPE reaches 2000 J/kg.
At this time, it appears that each day Wed-Fri, will feature a
weak shortwave racing eastward through and near to the IA/MN
border. Given the relatively small size of these shortwaves,
expect them to be highly variable in terms of timing and location.

With run-to-run consistency slowly continuing to build, can
highlight northern Iowa for Tuesday night. Reinforcing rounds of
theta-e advection coincide with a slug of 1000mb-850mb moisture
transport. Depending on flavor of model, PWATs in this region jump
to over 2 to 2.25 inches. The GFS has a pocket over 2.5 inches,
which would be near to a daily record amount. Have bumped up QPF vs
guidance and if this setup holds, next shifts may have to bump
notably further. Interesting in the 12z Mon GFS is a SE track of
semi-well phased forcing supporting MCS/outflow boundary
development. The best slug of theta-e advection continues its
generally easterly track and becomes not as well phased as the
system nears central Iowa, so precip may die out by the time it
reaches the Metro. Timing will have to be monitored.

Similar setup for Wednesday evening into Thursday. However, today`s
model runs have the boundary establishing itself comparatively
further south... closer to Highway 20. Expect run-to-run variability
to continue, though northern Iowa still looks to be the focus area
for this moderate to heavy rainfall potential. Reports of corn
starting to tassel in that area suggests ground will be able to
support higher rainfall amounts...hindering flash flood potential.
Parameters in place to continue severe threat of large hail and
damaging winds during the evening...gradually transitioning to
damaging wind and heavy rainfall during the overnight period. For
Friday and Saturday, this boundary may shift southward towards I-
80...bringing the same strong to severe thunderstorm/moderate
rainfall potential to this area.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1207 AM CDT Tue Jul 18 2017

Mainly VFR conditions through much of the period. Low chance that
current convection over far NW IA reaches KFOD. Better chances for
storms over northern Iowa by Tuesday evening. Expect south to
southeast winds at less than 10 kts overnight then becoming breezy
from the south to southwest on Tuesday.


Excessive Heat Watch from Wednesday afternoon through Saturday
evening for IAZ033-034-044>050-057>062-070>075-081>086-092>097.



LONG TERM...Kotenberg
AVIATION...Donavon is the U.S. government's official web portal to all federal, state and local government web resources and services.