Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 160005

Area Forecast Discussion
National Weather Service Des Moines IA
705 PM CDT Tue Aug 15 2017

Issued at 705 PM CDT Tue Aug 15 2017

Have updated the forecast to reflect the latest HRRR for
pops/storm chances tonight. Will have to monitor the eastward
progression of the line of storms moving through western IA, but
otherwise trends suggest it should begin to diminish on the
eastern edge as it continues to move ESE dying out across the
western portion of the forecast area. Otherwise much of the
south/southeast should remain dry for the remainder of the night.


.SHORT TERM.../Tonight through Wednesday/
Issued at 336 PM CDT Tue Aug 15 2017

Convection over the southeast will be departing prior to 02z.
Newly developing convection over the northwest will continue to
increase this evening with the approaching short wave and zone of
warm air advection over that area. Models continue to have a
difficult time handling the northwest area of convection...though
there is a vague consensus of the area developing north/northeast
through the late evening/overnight while the main trough continues
to increase from the west. Temperatures did not recover fully
over most of the area today due to the persistent cloud
cover...though lows will manage to remain about the same as the
previous forecast due to clouds. The southwest has cleared off for
now...but should see more clouds late into the early morning.
There remains some consensus for increasing PoP/thunderstorms
during the 12z-15z timeframe...though some of the CAMS members and
NAM continue dry until mid to late morning. For now will increase
PoP to chance through 12z and gradually to likely over the
northwest/west during the morning. Confidence is somewhat low
tomorrow morning...but afternoon as the entire trough slides east
the likelihood of showers/thunderstorms will be higher...and PoPs
remain in the likely/definite category. Once again locally heavy
downpours will be possible as PWATs will increase to 1.75 to 2
inches Wednesday afternoon. Have also trimmed highs for Wednesday
across most of the area with the exception of the southeast where
afternoon highs may still make it into the mid 80s. Though
instability will not be excessive tomorrow...there is a potential
for an isolated severe storm with wind being the main concerns.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 336 PM CDT Tue Aug 15 2017

Surface cold front tied to the aforementioned trough will move
across central Iowa Wednesday night with showers and storms
preceding the front. Forecast soundings show precipitable water
(PWAT) values will be well above normal between 1.75 and just over
2 inches across the area with deep warm cloud depths. As the
front moves through, drier air will follow behind with most of the
rainfall ending from west to east by sunrise Thursday as forecast
soundings show dry air in the middle and upper levels of the
atmosphere. As surface high pressure slides eastward over the
Plains, skies will become mostly sunny for Thursday with highs a
few degrees either side of 80.

By Friday, winds from the northwest will turn and blow from the
southwest as a shortwave trough drops out of central Canada through
the Dakotas toward Iowa. This will allow moisture to return back to
the state with PWATs rising from under an inch to around 1.5 inches.
This should allow for some showers and isolated storms to form and
move through later Friday into early Saturday. As the shortwave
exits the area on Saturday, very weak ridging will arrive overhead.
Kept PoPs mainly out of the area until late Sunday when moisture
will begin to move northward as a front approaches from the
northwest. This front looks to be a slow mover as the the
forcing/parent low will be over Canada. Sensible weatherwise, this
means that a mixture of clouds and sunshine with intermittent shower
and storm chances will be possible through early next week. Bottom
line for Monday`s eclipse, it`s still too early to tell one way or
the other on sky cover as timing of the front may change between now
and then.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 705 PM CDT Tue Aug 15 2017

Area of showers/isolated storms may impact KFOD and KMCW through
the evening, with expectations of the activity across western IA
to die off before it would impact KDSM. Some fog possible again
late tonight with low CIGS possible too, could see IFR VSBYS/CIGS.
Otherwise should dry out during the morning hours Wednesday, with
storms developing and possibly impacting KFOD and KDSM by late
Wednesday afternoon.





LONG TERM...Ansorge
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