Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 111820

Area Forecast Discussion
National Weather Service Des Moines IA
120 PM CDT Wed Oct 11 2017

...Updated Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 328 AM CDT Wed Oct 11 2017

Yesterdays storm system is moving away well to our east, but has
left behind a large shield of stratus clouds that has been
covering most of our forecast area overnight. Our far northwest
counties have cleared out, allowing temperatures to fall toward
freezing, and at this hour satellite imagery depicts holes eroding
in the cloud field across other portions of the forecast area, so
temperatures will be funky in the pre-dawn hours, dropping
several degrees as holes open up and then rising as they close
again. However, frost should remain confined to our northwest
where the advisory is in effect until 8 am.

During the day today a surface ridge will move slowly across Iowa,
allowing the clouds to hang around and even fill back in at times,
especially over about the eastern half of our area. There is low
confidence in how far west the edge of this cloud field will be
maintained, and where it will reside at sunset. However, with the
ridge anchored over eastern Iowa tonight, wherever the stratus
field does remain will be susceptible to fog and light drizzle
development late tonight, and fog is probable further west under
clearer skies in the central and western portions of the forecast
area. Given the uncertainty in location/amount of cloud cover and
also in the degree to which winds will decouple, have maintained
only patchy fog wording for tonight. However, in areas where skies
clear out, if the winds to go calm then given these conditions
and the wet ground, fog may become more widespread and locally
dense. This will bear monitoring through the day and tonight.

.LONG TERM.../Thursday through Tuesday/
Issued at 328 AM CDT Wed Oct 11 2017

./Thursday through Friday Night/...Confidence Medium to High

Main challenge will be recovery of temperatures for the next 48
hours along with the onset of next round of precipitation as well
as coverage through 12z Saturday. GFS/Euro continue to advertise
clear conditions Thursday with increasing southwest winds aloft
and at the surface. Overall thermal characteristics of both models
are similar with H850 temperatures generally 11C to 13C by 00z
Friday. This will lead to a nice afternoon recovery in
temperatures with 15 to 20kts mixing during the day. Highs are
likely to range from lower 60s northeast to the lower 70s across
the southwest. Though GFS Bufr soundings are showing some cumulus
development during the afternoon hours...likely that plenty of
sunshine will be had across the region. As a strong H500 upper
level trough enters the western CONUS a leading shortwave will
eject toward the northern/central plains. A surface front will
move just south of the area between 18-00z Saturday with
isentropic lift into an upper level H850-H700 frontal boundary.
This will result in an area of showers in the zone of isentropic
lift across northern/central Iowa. Despite GFS forecast
precipitable water increasing to 1.25 to 1.50 the H850 wind vector
is not favorable for tremendous lift into the frontal zone.
Showers are increasingly likely along and just to the north of the
boundary but the better lift across the boundary will be over
Minnesota and Wisconsin. Overall rain amounts should remain under
a quarter inch with the weak forcing and transient nature of the
boundary. By 00z Saturday the front will stall over southern Iowa
or northern Missouri. There is some uncertainty between the
GFS/Euro as to where this will occur. Between 00 and 12z Saturday
the H850 low level jet will increase into the boundary resulting
in increased coverage of showers and possibly isolated
thunderstorms through 12z Saturday. Both the Euro/GFS MUCAPE
values are rather meager...generally 300 to 500 J/kg overnight.

./Saturday Through Tuesday/...Confidence Medium

Confidence regarding the weekend system is increasing. Both the
GFS and the 00z Euro now have the system tracking to near
central Iowa by 00z Sunday with a solid shield of overrunning
thunderstorms from 12z Saturday through about 00z and both have
the system exiting by early Sunday. Cape/shear are ample enough to
promote some severe storms across the boundary with elevated
convection along and north of the retreating warm front through
the morning hours/afternoon. With the southeast half in the warm
sector during the afternoon...there is a possibility of another
round of strong to severe thunderstorms. There are some
differences as to how quickly the trailing cool front will pass
through the area as the system evolves with time. This along with
any chance for solar insolation will affect the strength of the
thunderstorms during the afternoon hours in our forecast area
there. Both the GFS and Euro have strong coincident wind fields to
sufficient depth to support deep convection with the GFS wind
fields at H850/H700/H500/H300 of 47/53/54/76kts and the Euro with
H850/H700/H500/H300 wind fields of 41/47/53/68 kts. Overall the
placement of the greater risk of storms may be over our southeast
or just east/southeast of the forecast area during the 20-01z time
frame. With a portion of our area in the warm sector during the
afternoon...highs should reach the lower/mid 70s in the south and
in the upper 50s/60s north of the trailing cool front.
Precipitable water again rises into Saturday afternoon/evening
especially along the boundary as convergence takes over late in
the afternoon with values of 1.75 to nearly 2 inches by 00z
Sunday. Warm cloud depths across the south are forecast to rise to
about +11 kft by 00z Sunday which should help promote efficient
rainfall. Rainfall totals of a couple of inches in the heavier/or
training storms is very possible both near the warm frontal
boundary lifting northeast and the trailing cool front over the
southeast. Once the system sweeps east late Saturday night into
Sunday much cooler air will be in place by Sunday afternoon with
H850 temperatures falling to about 0 to 3C. Highs Sunday should
once again be fall- like with afternoon readings in the lower 50s
to lower 60s. Monday morning may also see some frost north...if
wind fields relax enough. Currently the Euro model suggest return
southwest flow by 00z Monday while the GFS is holding onto a ridge
axis a bit longer. In either case...lows in the 30s are expected
north to the 40s in the south. The remainder of the extended is
quiet into Tuesday. Gradually increasing southwest winds will lift
highs back into the 50s/60s Monday with Tuesday in the 60s to
near 70.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 109 PM CDT Wed Oct 11 2017

MVFR stratus looks to persist throughout the afternoon hours with
some periodic increases to just above 3000 feet at times.
Confident the prevailing ceilings will be MVFR through the
evening before IFR ceilings set in late tonight into Thursday
morning. Drizzle/mist remains possible across the east and likely
only affecting ALO/MCW with MVFR visibility. Winds begin to
increase late the TAF period and should aid in eroding the
stratus by midday Thursday.





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