Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 121842
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
142 PM CDT Thu Oct 12 2017

.UPDATE...
Issued at 647 AM CDT Thu Oct 12 2017

GOES-16 Nighttime Microphysics imagery shows extensive clouds over
central Iowa a bit farther west than earlier forecast. Have thus
extended clouds across the entire forecast area at this time. The
other change to cloud cover is holding onto it a few hours longer
than the earlier forecast. There are also patches of fog west of
the Des Moines metro. Have been watching this overnight and until
recently, it was just a few surface observations sites and none of
the webcams were showing any fog. However, more surface
observations and webcams show that fog has formed and in some
spots it is dense. Therefore, have issued a Special Weather
Statement since the fog should be fleeting and visibilities
improve within a few hours after sunrise. Rest of the forecast
remains on track at this time.

&&

.SHORT TERM.../Today through Tonight/
Issued at 331 AM CDT Thu Oct 12 2017

Plenty of low clouds with bases below 1000 feet have prevailed over
the eastern forecast area so far tonight as viewed by GOES-16/GOES-
-13. Underneath these low clouds, patches of drizzle and fog have
occurred at times. These clouds have been slowly retrograding west
of I-35 where the bases have been a bit higher. Where skies are
clear, there may be a few areas of fog noting KCSQ (Creston) being
down under a mile near 3pm. In the last half hour or so, GOES-16
Nighttime Microphysics is beginning to detect a fog/low cloud signal
from KMWM (Windom, MN) due south to near (KEST) Estherville and east
of KSPW (Spencer). A webcam near Jackson, MN at 3pm showed more low
clouds vs fog at this point, but will continue to monitor for fog
development over the western part of the forecast area.

By daybreak, expect most of the forecast area to have quite a bit of
cloud cover with the exception of the far western forecast area say
near Highway 71. By 18z, expect broken/clear skies nearing the I-35
corridor with clouds lingering through mid to late afternoon over
the eastern forecast area. With these clouds, highs were trimmed
back a few degrees more in line of a blend of the GFS and ECMWF.
However, where clouds are expected to hang on the longest, lowered a
degree or two further. Winds will also be rather brisk from the
south with sustained winds of 10 to 15 mph, though higher over the
northwest forecast area.

Heading into tonight, skies should become partly to mostly clear
this evening. However, this will be short lived as warm air
advection continues ahead of an approaching cold front. NAM/GFS
cross sections show increasing amounts of low level moisture and
therefore have brought clouds back in by late evening and overnight.
At this point, expect the forecast to remain dry tonight.

.LONG TERM.../Friday through Wednesday/
Issued at 331 AM CDT Thu Oct 12 2017

./Friday through Sunday/...Confidence Medium

Overall confidence in the near periods of the extended is slightly
lower than the out periods. Tonights H850 subjective analysis shows
little in terms of deep moisture over the Midwest/Central Plains. An
area of leftover moisture is feeding the cloud cover/drizzle over
the eastern portion of our area now...but deeper moisture is confined
to the Southern Plains for now with a ribbon of 5 to 15C dewpoints
beginning to edge northwest along the western reaches of the
Texas/OK Panhandles. The approaching trough/front/lead wave is
still on track to move into our area Thursday night/Friday with
frontal passage during the daytime hours. Initially overrunning
showers will develop northwest sections of forecast area overnight
with the front dropping south during the day Friday...skies will
quickly become cloudy regionwide and showers more widespread
during the afternoon and evening hours over central and southern
areas. Highs will vary quite a bit with lower 60s north/mid 70s
far southeast before rainfall takes over. Instability remains
rather limited as the trough settles into northern Missouri
during the day Friday and have left only a small area of potential
thunder in the far south during the afternoon hours. Later in the
evening with return southwest flow aloft over the boundary...
elevated convection should develop with instability increasing
during the overnight hours. The GFS/Euro are still at odds with
the overall evolution of the main wave which is expected to deepen
across Iowa Saturday into Saturday night before departing. Though
both models show the potential for some strong/severe storms
Saturday...the Euro currently has more ongoing convection
throughout the day while the GFS/NAM are both more focused on
morning warm frontal convection moving into the northeast followed
by trailing cold frontal convection in our southeast. The Euro is
trending to develop the stronger convection just south/east of
our forecast area. As per yesterdays model runs...the models
suggest sufficient collocated strong wind fields aloft at all
levels. So both speed and rotational shear is ample for severe
storms. Given the remaining uncertainty...it still appears the
that better chances for stronger convection would be in our
east/southeast and areas east and southeast of our entire forecast
area. This is also borne out in both GFS/NAM Bufr soundings. SPC
has included about the southeast half of our forecast area in a
slight risk with a marginal surrounding the slight for Saturday/
Saturday evening. Wind fields and instability favor hail and wind
though there may end up being a small risk for an isolated
tornado as well in the far southeast...though that is more
uncertain at this time. Temperature-wise...after a mild Friday
night... temperatures Saturday will warm to the lower to mid 70s
in the warm sector across the south and in the upper 50s to lower
60s north of the low over northern areas. Once again the models
are also pointing toward the potential of moderate to heavy
rainfall in the storms on Saturday. The GFS precipitable water
values rise from 1.5 inches Friday night to 2 inches Saturday
afternoon along the cold front in the southeast half of the state.
Though storm movement should lessen the risk of any hydro
concerns... a quick couple of inches of rain is still possible.
The front and system will quickly depart Saturday night and should
be out of the area by 12-15z Sunday with sunshine and cooler
temperatures returning to the entire area. H850 temperatures by
00z Monday will range from 2.5C in the northeast to 4C in the
southwest yielding afternoon highs in the lower 50s northwest to
upper 50s southeast on the heels of stronger 15 to 25 mph
northwest winds.

./Sunday Night Through Wednesday/...Confidence High

Little sensible weather for the remainder of the period. There are
few timing differences between the GFS/Euro model at this time with
both showing increasing H500 heights/H850 temperatures from Sunday
through Wednesday. Highs should recover nicely with warm southwest
winds to the 60s to lower 70s Tuesday/Wednesday.  There is some hint
at a weak front arriving mid to late week...but with a moisture
starved atmosphere...rain chances are few and far between at this
point.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 143 PM CDT Thu Oct 12 2017

Low CIGS to expected to lift and break up through the afternoon
hours with a period of VFR expected into the evening. Low clouds
to then build back in late tonight as moisture increases ahead of
a weak wave that could bring some light showers/drizzle to the
state Friday. Winds to pick up a bit as the clouds clear, with a
frontal boundary dropping into the state tonight. This will allow
winds to go light and variable in vicinity of the front and shift
around to the north/northeast behind the front.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

UPDATE...Ansorge
SHORT TERM...Ansorge
LONG TERM...REV
AVIATION...Beerends



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