Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 111728
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1228 PM CDT Fri Aug 11 2017

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Fri Aug 11 2017

A few remnant light showers/sprinkles still lingering over far
southern Iowa. This activity will diminish shortly as much drier mid-
level air arrives as high pressure continues to settle in from the
north. The primary forecast challenge through the reminder of the
morning is the potential for fog development. Despite drier air
arriving, light winds and relative humidity values approaching 100
percent are occurring. Expect areas of fog to develop with the most
favored areas over north central Iowa and through the Nishnabotna
valley. The fog could become locally dense before burning off
quickly by 8 am.

The remainder of the forecast period will feature the area of high
pressure bringing beautiful mid August weather to the state with
highs in the mid-70s to low 80s today and overnight lows in the 50s.
The only downside of the great weather today is many areas could use
precipitation.

.LONG TERM.../Saturday through Thursday/
Issued at 340 AM CDT Fri Aug 11 2017

Bottom Line up Front...
Slightly below normal temperatures expected through the weekend,
with high-s warming back up to near-normal levels by Wednesday,
and then above normal for Thursday into next weekend.

Slight precipitation chances exist across western to NW Iowa Sunday
into Monday, otherwise conditions should continue to be dry and
rain- free until the middle to later stages of next week, when
more widespread thunderstorms are possible.

Sunday - Monday PM...
Models continue to track a weak shortwave dropping S/SE through
Saskatchewan towards the upper Midwest. The sfc high currently
propagating across the upper Midwest should be near the Great
Lakes region by 00z Mon...close enough to continue to pump dry
air into the low levels of the atmos. As mentioned in the past few
morning long-term AFDs, low- level dry air will continue to be a
problem for precip amounts that will be tough to overcome.
Rainfall amounts will likely be relegated to a few hundredths of
an inch at best, as much of the precip looks to be virga/light.

850mb temperatures will be a few degrees cooler than the past few
days, suggesting highs in the mid 70s north to the low 80s south
for Saturday and Sunday. Warmed up temperatures an additional
degree or two across central to southern Iowa as dry conditions
impacting albedo & sfc moisture "sinks" enough to justify addt`l
warm up.

Tuesday AM and Beyond...
Models remain unchanged in terms of synoptic features. Broad
anticyclonic flow begins to set up over the Deep South, shutting
of the Gulf for us in Iowa. Aloft, we should be in westerlyish
flow, and at the sfc, Iowa should be in SE flow as the
aforementioned sfc high remains locked in over the Ohio Valley
region. After Wednesday, broad thermal ridging builds over the
intermountain west. With the thermal ridge gradually pushing
eastward, temperatures should warm up each day... with Thursday
into the weekend looking notably warmer as the ridge may come
crashing down over Iowa. For the past 4 nights, the GFS has had
850mb temperatures of +20C to +25C making it into Iowa by Friday.
If this pans out, low to mid 90s are looking likely across the
state and with sfc dwpts slated to near the 70s, heat index values
may near and exceed 100 degrees Thursday through possibly
Saturday.

Models try to push a few shortwaves through the zonal flow on
Wednesday through Thursday. Given the source of these waves is well
into the Pacific, confidence is very low in any timing/location of
rainfall. The 00z Fri GFS is very bullish on a sfc low coming off
the Rockies and making it into Nebraska by Friday PM. low-level
frontogenetical forcing/instability is off the charts, so if this
pans out (which is a big "if") severe weather potential will be
high for Friday PM.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1228 PM CDT Fri Aug 11 2017

Few concerns this period. Mainly VFR conditions with potential
for early am fog at KFOD. Otherwise gradual increase in mid to
high level clouds expected in advance of next upstream wave
moving east by late Sunday. Winds to remain light through the
period with the exception of some mixing yet this afternoon at
KMCW. /rev

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Kotenberg
AVIATION...REV



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