Area Forecast Discussion
Issued by NWS Des Moines, IA

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586
FXUS63 KDMX 181044
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
544 AM CDT Fri Oct 18 2019

.DISCUSSION.../Today through Thursday/
Issued at 311 AM CDT Fri Oct 18 2019

We have much to discuss.

A deep layer ridge is moving eastward away from our area early
this morning and will continue to do so today as a trough
approaches from the west. Between these two systems, a tight
pressure gradient and good low-level mixing during the day will
result in surface winds turning from southeast to south and
increasing substantially by late morning and through the
afternoon. High-resolution model forecast soundings almost
unanimously indicate top of mixed layer winds around 40 to 45 KT,
and it is likely we will see peak gusts reaching wind advisory
criteria at least across the northwest half or so of the forecast
area. In addition, temperatures will rise into the lower 70s in
many areas making for a mild, windy fall day. This will provide
conditions favorable for the spread of fires, though relatively
high dewpoints (with RH remaining in the 40s for minima) preclude
Red Flag Warning conditions. Given the forecast wind speeds and
gusts, will hoist a wind advisory for the bulk of the area and
advertise elevated fire danger as well.

As the aforementioned trough approaches tonight and moves across
the area Saturday morning, it will interact with a modest
nocturnal low-level jet beneath a region of broad but fairly weak
lift to generate increasing coverage of showers, mainly across
about the southeast half of the forecast area. QPF will generally
be under a quarter inches, but a few spots may receive upwards of
half an inch. Instability is very limited, and have removed
thunder from the forecast. Little to no impact is anticipated from
the convection late tonight/Saturday morning, with the trough and
any lingering showers clearing our southeastern counties around
early afternoon. Behind the trough any cold air advection will be
modest and we should see good sunshine for most of the day, so it
will remain pretty nice out with temperatures still rising well
into the 60s on Saturday afternoon and winds becoming light and
variable beneath mostly sunny skies.

The pattern will repeat itself to some extent from Saturday
afternoon into Sunday as a brief period of weak ridging ensues,
with light winds and dry/quiet weather giving way to southeasterly
breezes and increasing clouds by Sunday afternoon as yet another,
this time more robust trough approaches from the west. The mid-
level driver for this system will be notably more robust, with
various prognostic models in much better agreement tonight
depicting a deepening closed 500 mb low swinging from South Dakota
into Minnesota on Sunday/Sunday night, a trailing cold front
extending south/southwest from the associated deepening surface
cyclone, and a pronounced pre-frontal trough swinging across our
forecast area around Sunday evening. The ECMWF is glaringly slower
than the rest of the model suite with the passage of this feature,
and have trended POPs toward the relatively faster progression
depicted by the other models. Given the dynamic nature of the
system we are almost certain to see a swath of scattered/numerous
showers along and ahead of the pre-frontal trough, with a few
thunderstorms scattered in due to somewhat higher instability.
However, with the instability being limited and the trough coming
through after peak diurnal heating, such as it is, any severe
weather threat appears limited. We will continue to monitor this
hazard over the next couple days however. Overall Sunday should be
a decent day for outdoor interests with temperatures rising into
the low to mid 60s, increasing southeast breezes, and increasing
clouds but the bulk of any rainfall holding off until late in the
day or after dark.

After a relatively pleasant weekend, bookended by rain chances,
the early part of next week will hit like a slap in the face as
the aforementioned cold front, trailing from the powerful cyclone
moving east northeastward from Minnesota into the Great Lakes
region, sweeps across Iowa early Monday morning. Behind this front
winds will increase substantially, likely to levels even higher
than today, as strong cold air advection kicks in. There will also
likely be a large cloud shield wrapped around the back side of the
slowly departing cyclone, with light showers especially over the
northern half or so of Iowa. All in all this will make for a cold,
raw, damp, blustery day across much of the state on Monday. From
Monday night onward confidence in forecast evolution decreases,
initially due to uncertainty regarding the timing of the departing
low and extent of wraparound clouds/precipitation, meaning how
long these effects may linger especially across our
north/northeast. For now have maintained POPs into Tuesday in this
area, but some solutions, most notably the GFS, are faster at
moving the clouds and rain out. Also, on Tuesday morning
temperatures will drop to the mid 30s up near the Minnesota
border, but between the marginal forecast temperatures and
uncertainty in whether precipitation will still be ongoing at that
time, have opted not to mention any low chance of light snow for
now.

Looking further into the extended forecast period, from around
Wednesday onward, it appears we will see a brief period of brisk
zonal flow aloft in wake of the departing early week trough,
followed quickly by another broad, deep trough moving through
sometime late next week. Details are murky at this point, but
overall the synoptic scenario supports continued cool and active
weather in the outer periods of the forecast and essentially for
all of next week.

&&

.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 544 AM CDT Fri Oct 18 2019

Winds will turn from southeast to south and increase considerably
today, resulting in crosswinds of 20 KT or greater at times on
east-west runways and also in periods of LLWS 35-40 KT. Winds and
LLWS will then abate somewhat after dark tonight, but showers and
associated low MVFR ceilings and visibility will develop after
06Z toward the end of the TAF period. Expect the timing of this
wind decrease and lowering cigs/vsbys to be refined in TAF
issuances later today/tonight.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 10 AM this morning to 6 PM CDT this evening
for IAZ004>007-015>017-023>028-033>039-044>049-057>060-070>072-
081-082-092-093.

&&

$$

DISCUSSION...Lee
AVIATION...Lee



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