Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 110829
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
329 AM CDT Wed Oct 11 2017

.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 06Z TAFS through 06Z Wednesday night/
Issued at 1152 PM CDT Tue Oct 10 2017

high pressure undercutting the low level moisture far nw creating
clearing and potential frost mainly near EST.  Elsewhere VFR to MVFR
cigs expected over central Iowa with IFR over eastern Iowa becoming
MVFR around daybreak.  Sfc wind becomes SE aft 12/00Z which should
produce some fog and/or lower clouds again.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Frost Advisory until 8 AM CDT this morning for IAZ004-005-015-
023-033-034-044.

&&

$$

SHORT TERM...Lee
LONG TERM...REV
AVIATION...FAB



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