Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 222338
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
638 PM CDT Sat Jul 22 2017

...Updated for 00z Aviation Discussion...

.SHORT TERM.../Tonight through Sunday/
Issued at 300 PM CDT Sat Jul 22 2017

Synopsis...
Giant ridge impacting the central to southern half of
the country continuing to propagate southward and slowly break down.
Meanwhile, 18z water vapor imagery shows two large-scale upper lows
positioned in Canada...one east of the Hudson Bay, and the other
over just off British Columbia. These lows complete the setup for
zonal flow across the northern third of the country.
Additionally, 18z water vapor imagery picking up on a smaller low
propagating along the Minnesota/Canada border. There is westward
tilt with this low, as a sfc reflection will be located just ahead
of the core of this upper low. Attendant to the sfc low is a
boundary oriented NE- SW and stretching into northern Iowa by 12z
Sun and SE Iowa near 21z Sun-00z Mon.

Saturday PM... Hi-Res models starting to come into agreement with
placement of the boundary...In fact, are handling location and
magnitude of boundary much better than Friday. 18z RAP sounding
analysis correctly had boundary south of KDSM and +15C to 750mb.
In short, best chance for initiation south and east of Iowa, and
have removed POPs for the remainder of this evening in Iowa. Hi-
res models picking up outflow from the above discussed low to
possibly clip our northeastern counties overnight, but severe
threat would be effectively nil if it manages to reach this far.

Sunday...
Timing of this boundary will impact max temps... a
slightly faster progression would imply cooler temps across the
central to southern portion of the CWA... slower progression would
imply warmer temps. With 850mb temps still expected to be in the
+20C to +22C range for Sunday along and south of I-80, still expect
highs in the upper 80s to mid 90s. Drier air being advected into
Iowa will keep dewpoints in the 60s...serving to keep heat index
values *only* in the low to mid 90s. The location of the above
boundary during peak heating would also have an impact for
thunderstorm potential. At this time, however, it appears the
boundary will be into northern Missouri, relegating the severe
weather potential to northern/NE Missouri.

.LONG TERM.../Sunday night through Saturday/
Issued at 300 PM CDT Sat Jul 22 2017

Monday through Wednesday...
Trough continues to dip east allowing for cooler and drier air to be
funneled in from the north. Comfortable highs and dewpoints will
persist into Monday. However, the building ridge behind the trough
will return us to southerly flow moving into Tuesday bringing with
it added moisture and heat. A surface boundary will move east
Tuesday into Wednesday bringing us the next chance for rain and
thunderstorms. Confidence is low at this time on the chances across
the state for rainfall. Sufficient instability and influence from
the upper level jet looks to be in place for Wednesday which
could support more storm chances across the area. Worley

Wednesday night through Saturday...
Surface front pushes through the state through Thursday morning,
continuing chances for storms across the far south Wednesday
night. On Thursday there is some model discontinuity as the Euro
stalls the front across southern Iowa keeping storm chances going
while the GFS has a stronger surface high build in and push the
quasi-stationary boundary further south...into Missouri. The
Canadian appears out to lunch so I did not consider in the range
of solutions. Beyond Thursday, a big ridge of high pressure builds
into the west and extends into the Upper Midwest for an extended
period of dry conditions. The GFS drops a ridge riding shortwave
across the northeast into the east and want to create some
spurious pop`s with that. It is the most robust of all the models
and appears overdone so for now, I discounted any pops northeast.
Otherwise temps will be generally in the mid 80`s with lows in the
60`s to lower 70`s which is about normal and humidities will be
tolerable.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 638 PM CDT Sat Jul 22 2017

Main concern this TAF period is possible fog development later
tonight. The factor that may limit fog is drier air that will
slowly work into the state tonight. Nearing 0z, dewpoints were at
least in the low 70s with dewpoints in the 60s upstream. Cross
sections show higher relative humidity after midnight, especially
over the eastern TAF sites. Not surprisingly, this is also where
forecast soundings show an inversion with saturation near the
surface. The blend of much of the short term, high res models have
the best signal for fog development across the eastern portion of
the forecast area, including KALO and KOTM. Will introduce fog
into these locations with MVFR visibilities, though patchy fog is
also possible farther west and will reevaluate at 6z TAF issuance.
Otherwise, VFR conditions will prevail at the TAF sites through
the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for IAZ057-058-070.

Excessive Heat Warning until 8 PM CDT this evening for
IAZ059>062-071>075-081>086-092>097.

&&

$$

SHORT TERM...Kotenberg
LONG TERM...Worley/FAB
AVIATION...Ansorge


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