Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182037

Area Forecast Discussion
National Weather Service Des Moines IA
337 PM CDT Wed Oct 18 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 337 PM CDT Wed Oct 18 2017

The weather this week has been beautiful by nearly all standards and
will continue through at least tomorrow. That isn`t to say there
aren`t some features moving through the region, just no support,
such as sufficient moisture or large air mass changes, to cause any
real sensible weather concerns.

As hinted at, moisture through the column is severely lacking
throughout the region. As a result , a pair of vorticity maxes
skirting the state to the north and south have and will continue to
struggle to produce anything but high cloud cover. Later this
evening an approaching weak cold front will traverse the state from
the northwest, but will also struggle to produce anything but scant
high clouds. With surface high pressure slated to move across
tonight and even drier air behind the front than what`s currently
here, have dropped overnight lows a couple degrees from guidance
across the northwest and Nishnabotna Valley areas where longest
duration of light winds and driest air should reside.

As surface high pressure overnight quickly exits Thursday morning, 5
to 10 kt southerly to southeasterly flow will return. Temperatures
will result in similar above normal highs as today, low to mid 70s
primarily form north to south, and some degree of moisture returning
as well, albeit still very dry.

.LONG TERM.../Thursday night through Wednesday/
Issued at 337 PM CDT Wed Oct 18 2017

West to northwest flow pattern aloft to become more southwesterly
into the end of the week as a large upper trough digs into the
intermountain west. This will allow for moisture return northward
into the region Friday into Saturday, with waa as well. The upper
low to be located near southern Saskatchewan by early Saturday
morning with the system closing off and lifting ENE through
Manitoba. The entire longwave trough to be more progressive through
the central US with a cold front expected to sweep through the state
Saturday afternoon into Saturday night. Pieces of energy will eject
northeastward through the region prior to the passage of the main
trough axis, this will result in some chances for showers and
thunderstorms across central Iowa Friday mainly late Friday night
into early Saturday. The best chance for precipitation will be into
Saturday afternoon into the evening hours as showers and storms are
expected to develop in vicinity of the cold front and track eastward
across the forecast area with the passage of the front. Still a
possibility of a stronger storm with the activity, but overall
timing should limit the overall instability and resultant sfc-based
activity. Therefore expect mainly elevated storms, and therefore
will limit the available shear as well thus keeping storm
organization lower. Dry conditions then expected for Sunday as the
trough pushes east into the OH valley. EC/GFS in good agreement with
this wave being more progressive and moving into more of a northwest
flow pattern into next week with a cooler trend and majority of the
precipitation remaining east of the forecast area. However the
Canadian in major disagreement trying to stall the weekend trough
across the central US resulting in a closed off upper low meandering
across the area through the middle of the week. Unfortunately
guidance grids then had broad-brushed pops through much of mid-week.
Did work with neighbors to drop pops across much of this time frame
to trend toward the EC/GFS, with the best chances for potential
precip coming Monday night as the GFS has a reinforcing cold frontal
passage across the state, and into mid-week as the GFS/EC have
another cold front dropping through the area with an upper trough
moving through Manitoba into Ontario.

Biggest discrepancies between the GFS/EC surrounds temperatures with
the GFS suggesting the western US ridge building a bit further
north/east with H85 temps returning to around +10C toward Wednesday
of next week, and the EC keeping the ridge dampened and bringing a
surge of cold air into the state dropping h85 temps to around -5C to
-10C. Therefore will monitor in the coming days as this could mean
 much of the area will see a frost/potential freeze toward the
 middle of next week if the EC is correct. For now cooler, but not
 to the extreme of the EC at this time.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1227 PM CDT Wed Oct 18 2017

VFR prevailing through yet another TAF period. Winds to continue
out of the S/SW during the daylight hours with gusts around 20kts
at times. Sustained winds and gusts will subside quickly at
sunset and eventually turn out of the northwest, but light, as a
weak front moves through. Winds will return back out of the S/SE
tomorrow morning as surface high slides by overnight as well.





LONG TERM...Beerends
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