Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141738
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1138 AM CST Tue Nov 14 2017

...Updated for 18z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 349 AM CST Tue Nov 14 2017

Good southerly flow has developed across Iowa with high pressure
to the east and trough of low pressure approaching from the
west. A strong elevated mixed layer(EML)remains in place creating
a strong inversion around 850 mb. The southerly flow has led to
low level moisture advection north from the Gulf and into Iowa.
This moisture is being trapped under the inversion. This moisture
is leading to widespread stratus with ceilings dropping below 500
ft over many areas across Kansas and Nebraska and shifting east
into Iowa. A few patches of drizzle have developed as well in
addition to areas of fog with visibilities below 2sm over
northwest Iowa.

The low level moisture advection will strengthen through the day
with dew points rising into the mid 40s to lower 50s by mid to
late afternoon. With dense cloud cover in place, temperatures
will not stray too far from the dew point values and should mainly
stay within 2-3 degrees of the dew points. A few patches of
drizzle and fog will persist into this morning. The inversion will
lower a bit by this afternoon and compress the moist layer depth
and should result in a reduction in coverage. An upper level
short wave arrives by this evening along with Pacific mid-level
moisture. This moisture will not completely saturate the EML aloft
and likely will result in developing showers above this layer.
Due to this, have continued to keep thunder mention out of the
forecast as this will limit overall instability. Best potential
for widespread precipitation will be between 03-09z before a cold
front and drier air move into central Iowa.

.LONG TERM.../Wednesday through Monday/
Issued at 349 AM CST Tue Nov 14 2017

The main forecast concern was focused on pops and temperatures
late in the week. The ECMWF and GFS are in fairly good agreement
with the timing of the surface low and associated cold front to
push across the region. The ECMWF is slower with the fropa and a
bit warmer behind the front compared to the colder and faster GFS.

Wendesday into Thursday...Upper low over the Great Lakes looks to
pull a cold front across the state overnight tonight into
Wednesday morning. Leaned toward the cooler NAM/ECMWF for highs
as well as timing of highs late in the morning. Have mention of a
non-diurnal temperature trend with the strong CAA developing
throughout the day. Nudged up winds and gusts as well to coincide
with the fairly deep mixing by the afternoon. Surface high centers
itself directly over the state late Wednesday night into Thursday
morning providing well below normal temperatures across the
forecast area. By Thursday afternoon, the surface high will have
shifted to over the Ohio Valley allowing for some return flow to
develop late in the day. Low confidence this WAA makes much of an
impact on temperatures with the cold start, the easterly component
and also the late development of the southeasterly winds.

Thursday night into Friday...the WAA continues and finally becomes
true WAA past 06z Friday across the state. It also increases
during this time and remains strong before the cold front sweeps
through past 21z Friday. Much of the forecast area remains in the
warm sector on Friday and nudged up maximum temperatures. The
other concern is storm chances Friday across the central to
southeast portions of the forecast area ahead of the front. Decent
speed and directional shear during the morning into the early
afternoon hours per DSM to OTM forecast soundings. However,
instability remains limited and thus the thunderstorm chances.

Saturday through Monday...Dry and cool weather is likely over the
weekend into early next week as temperatures remain roughly 5 to
10 degrees below normal.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1138 AM CST Tue Nov 14 2017

Confidence is high that widespread restrictions will continue at
all terminals for the remainder of the afternoon into the night.
With the exception of KOTM through part of this afternoon,
restrictions will be IFR or LIFR. As low pressure moves from
central South Dakota to Lake Superior overnight, a cold front
will sweep across the state and be accompanied by scattered rain
showers. Behind the front, winds will become gusty from the
northwest with conditions returning to VFR by mid-morning. Some
mid to high level VFR clouds may wrap around the low pressure
over the northern terminals.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Ansorge



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