Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202021

Area Forecast Discussion
National Weather Service Des Moines IA
321 PM CDT Fri Jul 20 2018

.DISCUSSION.../Tonight through Friday/
Issued at 318 PM CDT Fri Jul 20 2018

Low pressure system and short-wave trough has continued to lift
into the upper Midwest and across the Great Lakes, sweeping a cold
front through. Northeast and eastern Iowa are still on the backside
of the low pressure system in the cyclonic flow, and thus clouds
will remain throughout the day. Gradual clearing will occur with
heating during the afternoon. Western Iowa has been under an area
of subsidence, with sinking motion clearing out skies there. The cold
front passage has brought northwesterly flow into the region, slightly
cooling temperatures this afternoon into the low to mid 80s. A few
sprinkles may still develop in the northeastern part of the CWA later
this afternoon on the back side of the low, but POPs
continue to decrease. Across most of the CWA this evening clear skies
will allow for a decent amount of radiational cooling, which will send
overnight low temperatures into the mid 60s. Although areas further
northeast under cloud cover will not have as much radiational cooling,
the lack of daytime insolation this afternoon will lead to slightly
cooler overnight temperatures compared to rest of the CWA. Gusty winds
will continue through the afternoon and overnight. Although relative
humidity will be high in some locations, the gusty winds will promote
enough mixing to prevent any heavy mist or fog development.

Heading into the weekend, conditions will continue to be mild with high
temperatures into the mid 80s and low humidity. Upper-level ridge will
continue to amplify across the Rockies and Great Plains. AVA into the
region will aid in developing surface high pressure for tomorrow and
into the rest of the weekend, keeping conditions dry. Winds into
Saturday should be calmer with a weaker pressure gradient once the low
completely moves out of the region.

In the extended, the upper-level ridge will continue to dominate for
much of next week. Surface high pressure will remain in place for a
good amount of the time, with temperatures remaining in the mid 80s.
GFS/ECMWF keep a trough lingering over the east coast. With ridging
over the Plains, this may cause a few short-waves to move through
bringing in some isolated POPs for a short period of time on Monday
across the northern CWA, does not look to be anything substantial. As
surface high pressure retrogrades Tuesday, long term guidance in decent
agreement on a closed-low at H500 moving across the Canadian Prairie
Provinces which will develop zonal flow across Iowa. Depending on how
far south the upper-level low comes, a short-wave could develop within
the zonal flow which will introduce low POPs for Wednesday across the
CWA. Overall no highly impactful weather is expected next week, with
the stronger upper-level flow staying to the north.


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1215 PM CDT Fri Jul 20 2018

Northeastern Iowa remains on the backside of the low pressure
system in the cyclonic flow, which is allowing for the widespread
cloud cover. Ceilings will gradually rise this afternoon across
the northern terminals, and will fluctuate between MVFR and VFR.
This afternoon terminals across central and southern Iowa are
forecast to remain VFR. Winds gusts up to 20 knots possible across
Iowa today. Overnight, low MVFR and possibly some IFR ceilings
may occur across the north.




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