Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212110
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
410 PM CDT Fri Jul 21 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 409 PM CDT Fri Jul 21 2017

Synopsis...
Giant ridge across the heart of the country to the Ozarks to
gradually propagate southward. Meanwhile, two large-scale upper lows
positioned in Canada...one east of the Hudson Bay, and the other
over the Gulf of Alaska. These lows complete the setup for zonal
flow across the northern third of the country. Meanwhile, 20z water
vapor imagery picking up on a shortwave pushing through the Rockies
on the Canadian side. This wave should be near northern
Minnesota by 00z Sunday and will help nudge the boundary that
has largely been supportive of thunderstorms to the south of Iowa.
Further this low will support a surge of cooler air over Iowa for
the beginning of next week.

Strong to Severe Thunderstorm/Heavy Rainfall Potential...

Bottom line up front, the environment this evening (as of now) is
more supportive for strong to severe wx...with damaging winds the
main threat and possibly a short-lived EF0-1 tornado or two. Like
last night, there is somewhat high disparity among hi-res models as
they are struggling with the location of the boundary...which will
be a key trigger to support convection. Big, big, big
limiting factor is the strong cap of +14 to +15C. So, any storms
that are able to push through the cap may rapidly intensify.

A good starting point would be to compare mesoscale env for this
evening to yesterday`s env. Theta-e differential and 0-3km shear are
both more impressive this evening than yesterday evening. Shear
vectors not purely line-normal to MUCAPE. This suggest potential for
deeper convection and a stronger cold pool to support wind/QLCS
potential...when paired with LCLs below 1000m and SRH over 200 m2/s2
behind/ the boundary.

Will have to monitor heavy rainfall potential as well across
eastern/NE CWA. Much of this area, especially Bremer County, has
received upwards of 1 to 3 inches of rain in the past 24-36 hours.
PWATs are near record levels, nearing 2.25 to 2.5 inches. With
training possible, some locations may receive several hours of
moderate to heavy rainfall which may add up to 1 to 2 inches through
the night...especially in the Flood Watch locations.


Excessive Heat...

With this boundary sagging south as discussed earlier, relatively
lower dewpoints to spread from north to south across Iowa. 850mb
temperatures drop a few degrees compared to Friday. In all, this
will yield apparent temperatures in the 100 to 110 range along and
south of I-80 for Saturday...as opposed to the 105 to 115 degree
range experienced Thursday and Friday afternoon. This is the fourth
consecutive day of apparent temperatures at or above Heat Adv
criteria, so have stuck with Excessive Heat Warning along and south
of Interstate 80.


.LONG TERM.../Saturday night through Friday/
Issued at 409 PM CDT Fri Jul 21 2017


Sunday...
Our boundary should be into northern Missouri by Sunday
afternoon/evening. There is a chance the southern tier or two of
counties may be clipped by the boundary Sunday PM. With lapse rates
high and DCAPEs moderate to high, expecting damaging winds and large
hail to be the primary threats. Elsewhere, temperatures should still
be above normal, in the low to mid 90s along and south of I-80...but
dewpoints should be into the 60s across Iowa...meaning heat index
values will stay in the 90s Sunday.


For Monday into Tuesday...
A true break from this heat wave. 850mb temperatures will fall from
the +23C to +25C range of today down to the +13C to +17C range. This
will support near-normal temperatures in the 80s.


Wednesday and Beyond...
The thermal ridge continues over the intermountain west throughout
the week. As the wave discussed earlier pulls away, return flow will
make it back into Iowa. This will bring a return chance for
thunderstorms and warmer temperatures. Slight difference it the
temperatures for this period look to be some cooler than they have
been for this heat wave. So while it will likely warm back up into
the 90s by the next of next week, the excessive heat should
remain to the south of Iowa.


&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1250 PM CDT Fri Jul 21 2017

Aviation forecast challenging at KMCW and KALO in terms of timing
thunder arrival and subsequent lowering of vsbys. Seems likely
that both those sites would be impacted by TSRA at some point
during the evening. KDSM may miss out, though cannot rule out need
for VCTS later in the evening as well. Confidence high in VFR for
Saturday.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM CDT Saturday for IAZ057>062-
070>075-081>086-092>097.

Flash Flood Watch through Saturday morning for IAZ027-028-038-
039-050.

Heat Advisory until 8 PM CDT Saturday for IAZ044>050.

&&

$$

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Kotenberg


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