Area Forecast Discussion
Issued by NWS Des Moines, IA

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595
FXUS63 KDMX 191725
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1225 PM CDT Wed Jul 19 2017

...Update for severe weather potential arriving this afternoon
over northern Iowa, discussion on latest heat headlines, and 18z
aviation discussion...

.UPDATE...
Issued at 1224 PM CDT Wed Jul 19 2017

.Severe weather potential arriving this afternoon over northern
Iowa...

Closely monitoring the thunderstorms over eastern South Dakota
early this afternoon, which have produced numerous measured wind
gusts of 60 to 80 mph and even a 100 mph wind gust at an RWIS
north of Highmore, SD. It is increasingly likely that these
storms will be able to maintain themselves as they travel east-
southeast early this afternoon and reach the northwest part of our
forecast area (e.g. Emmet, Palo Alto Counties) between 2 and 3pm.
This assumes a steady state of speed, which is not guaranteed as
a strong cold pool may develop accelerating the storm speed.
Arrival of storms this afternoon is several hours earlier than
previous forecast thinking.

Overall, convective allowing models (CAMs) have not had a good
handle on these storms. A previous version of the ESRL HRRR did
represent the current thunderstorms fairly well and had storms
reaching northern Iowa, including north central Iowa mid-
afternoon. Other CAMs such as the WRF ARW and NMM cores would have
storms reaching us early to mid-evening. The NCAR ensemble
updraft helicity tracks show storms more into northern Iowa late
this afternoon and early evening. The 12z GFS supports this idea
of more into northern Iowa. The 12z NAM, 00z ECMWF, and 00z CMC
have storms not as far south. The most plausible outcome given
current trends and poor performance of most of the CAMs seems to
be storms farther south into northern Iowa by mid- afternoon.
Therefore, have updated PoPs and weather to reflect this thinking.

.Heat headlines...

Added Tama County to the heat advisory in coordination with WFO
Quad Cities and given forecast heat index values. The I-80
counties were considered for addition to the Excessive Heat
Warning, but confidence in dewpoints is low. In addition, current
storms over South Dakota are farther south and this brings further
credence to long term forecasters thinking last night on where
the boundary may set up. Therefore, will maintain current spatial
area of the Excessive Heat Warning.

&&

.SHORT TERM.../Today through Tonight/
Issued at 353 AM CDT Wed Jul 19 2017

Thunderstorms have been festering across the southern portion of the
forecast area during the overnight hours.  Initially they developed
along/south of the highway 30 corridor where the instability axis
had little CIN.  However the storm motion took these into an area of
stronger CIN and the storms fell apart.  Most recently storms have
developed in southwest IA in an area of deep moisture convergence
and theta-e advection.  These storms may drop some small hail and
may be a little better organized, lasting into the early morning
before falling apart but they are in an area of weak shear and as
such, I don`t expect them to evolve into much...unless the shear
improves.

As for today and tonight...the upper ridge tries to rebuild across
the southwest while an upper trough slides across the Northern
Plains/far northern Upper Midwest.  A couple of shortwaves will ride
across the flow further south buckling the flow somewhat and
creating more of a northwest component across northern/northeast IA.
At the surface, a frontal boundary which currently resides
along/just south of the highway 20 corridor will push back to the
north into the evening hours as a warm front.  Warmer temps and
higher dewpoints will work into the area this afternoon...especially
south.  Most the of area will be dry today with the exception of the
far east to east central where some better convergence along the
boundary could exist.  The hi-res models all develop a storm complex
over southern or southeast MN and drop it southeast into northern IA
overnight.  Shear will be pretty good along/near the boundary so
storms this evening could produce some large hail and with DCAPES
approaching 1500 J/Kg late afternoon and evening, damaging wind
gusts would also be possible.  Heavy rainfall is also a
consideration tonight as PWAT`s soar to over 2 inches in the
evening.

Heat indices this afternoon get to 107 or 108 across the southwest
into south central IA which fall in the current headline areas so
I`m not planning any changes at this point.

.LONG TERM.../Thursday through Tuesday/
Issued at 353 AM CDT Wed Jul 19 2017

Much of the Wednesday night thunderstorm activity should be well
east of the area by Thursday morning. The question is how far
south the surface boundary will sink into the state. Both the NAM
and ECMWF have the boundary dropping into the Highway 30 to
Interstate 80 vicinity by late morning. The NAM is the most
aggressive and continues to drop the boundary into far southern
Iowa by the afternoon. Have leaned towards the ECMWF solution
which is a compromise of the NAM/GFS solutions. The first impact
is with heat potential. The compromised solution fits the current
headlines well. A few storms could bubble along the boundary
during the day but best potential for storms will be during peak
heating and more so Thursday night as stronger thermodynamics
arrive with another round of strong theta-e advection arrives.
Best storm chances Thursday night will be across the north.

Friday looks to be the warmest day as the boundary retreats back
to the north. Good mixing potential south of the boundary should
dry dew points into the 60s. This drier air along with recent dry
conditions could help push high temperatures to near 100 for Des
Moines and south. Farther north a complex of storms is likely to
be ongoing in the vicinity of the IA/MN border with the main focus
to the north of this line. These storms will carry an attendant
severe weather and locally heavy rainfall threat. A strong short
wave will move into the far Northern Plains Friday night and
Saturday causing a degradation of the sub- tropical ridge and
allow a cold front to move through central Iowa during the day
Saturday. Not overly optimistic on precipitation chances with the
boundary passage as the elevated mixed layer remains intact while
the cooler air undercuts the warmer air aloft. Sunday will be
cooler and less humid followed by even cooler conditions on Monday
and Tuesday with highs in the upper 70s to mid 80s. Precipitation
chances beyond Saturday are looking scarce therefore if some
areas miss out on precipitation prior to then, the extended dry
spell will continue.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1224 PM CDT Wed Jul 19 2017

Main focus is on thunderstorm chances at KMCW with complex of
thunderstorms moving southeastward through SE SD likely to move
toward KMCW by mid afternoon. Highlighted potential timing with a
VCTS and will refine as storm track becomes more apparent. High
winds and hail potential with the complex of storms. Could see MVFR
CIGs at KMCW with the storm potential and low level CU field
developing currently as well. Otherwise overall threat of
thunderstorms limited at the TAF sites through the period, with
southerly winds becoming more light and variable to southwesterly
into Thursday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT Saturday for IAZ033>035-044>050-
057>062.

Excessive Heat Warning until 8 PM CDT Saturday for IAZ070>075-
081>086-092>097.

&&

$$

UPDATE...Ansorge
SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Beerends



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