Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 040519

Area Forecast Discussion
National Weather Service Des Moines IA
1119 PM CST Sun Dec 3 2017

.SHORT TERM.../Tonight through Monday/
Issued at 342 PM CST Sun Dec 3 2017

Confidence: Medium to High

Overall only a few modifications from previous package for tomorrow.
Expecting gradual increase in moisture as moisture plume aloft over
Southern Plains this morning begins to accelerate north with time
tonight and begin to saturate mid levels across the north this
afternoon and evening...then backfilling across the region tonight.
Cross sections again show most of the moisture confined to the near
surface over the south while slightly better saturation through the
column will take place northeast/east tonight and Monday. Overall
not too excited about prospects for widespread precipitation. Best
lift appears to be between 700 and 500mb around 06z.  Models still
hinting at some instability northeast around 06z-12z with secondary
push of slightly unstable air over the east between 12-16z. In each
case looking for isolated thunder at best. As the sfc low tracks
northeast from western Nebraska at 06z tonight deepening along the
way as it moves through northwest Iowa then toward western Lake
Superior as a 979mb low by 06z Tuesday. The trajectory of the system
will bring the sfc cold front into northwest sections by 13z moving
southeast to near Ottumwa 22z. Strong subsidence and cold air
advection will quickly follow the boundary. Prior to that rather
warm air will remain over the area tonight into Monday noon...with
lows tonight in the upper 40s north to the mid 50s south.  Highs
Monday will reach the upper 50s/lower 60s northeast to the mid 60s
south central/southeast. Colder air will already be moving southeast
into northwest Iowa during the mid morning hours. During the
afternoon following the front most of the precipitation will be
coming to an end with some prefrontal rain/iso thunder southeast and
some def zone light rain mixing with light snow northwest prior to
the deformation zone lifting northeast and out of the forecast area.
Due to the track... no accumulation of any snowfall will occur over
northwest Iowa. Bufr soundings and H850 winds strongly support a
wind advisory over the north half of the area by 21z Monday...
extending into the evening hours. Will have headlines out for that
area with this package. There is some uncertainty as to how far
south to extend the wind advisory but for now will bring as far
southwest as Atlantic northeast to Waterloo/Cedar Falls.  Sustained
winds will reach 30 to 35 mph at times with higher values north and
gusts may approach 45 to 50 mph at times for a brief period of time
tomorrow afternoon and evening.

.LONG TERM.../Monday night through Sunday/
Issued at 342 PM CST Sun Dec 3 2017

Bottom Line Up Front...
Potentially hazardous winds start the long term forecast period,
signaling the arrival of much colder arctic air to Iowa for the
entire long-term forecast period...dropping wind chill values to
the single digits during the morning hours in many locations. At
this time, it appears that any precipitation that falls should be
light and of little to no impact.

Overall setup...
Pattern described in yesterday afternoon`s long-term AFD section
mostly remains unchanged... serving to boost confidence in this
forecast. 21z Sun water vapor imagery picking up on a well-defined
upper low spinning east of Oregon towards the Rocky Mountains.
Subsequently, an impulse will eject off the semi-permanent Hudson
Bay/Arctic Circle low and rapidly propagate southward through
Canada, phasing with our Pacific NW upper low over Lake Superior
by around 12z Tue. The impact this will all have on Iowa is to
serve to aide and accelerate CAA into the upper Midwest and Iowa.
A huge blocking high, rex block sets up over the Pacific NW
through midweek, keeping us solidly in NW flow throughout this
time period.

There may be a few pockets of brief, periodic flurries throughout
the week as minor waves of energy move through the Midwest.
However, not worth delving into details as any flurries that fall
appear to have no sensible impacts at this time.

Carrying discussion for winds from the short-term...parameters in
place for strongest winds to remain through the 06z Tue to 09z
Tue timeframe. If anything, this is a slightly accelerated
departure from yesterday afternoon`s update, where it appeared the
strongest winds would last through 09z Tue. By 09z to 12z Tue,
3-hour 4 to 6 mb pressure rises well east of DMX CWA, along with
best low- level/near-sfc subsidence. Pressure gradient does remain
in place throughout Tuesday, along with mixed-layer winds in the
30 to 40 kt range... which should be able to be transported down
to the sfc.

Winds then look to decouple after sunset Tuesday, only to pick up
again during the day Wednesday... This pattern more or less repeats
itself each day for the remainder of the week... meaning this will
likely be a breezy week... and have bumped up winds accordingly.

850mb temps fall from around +10C or so at 12z Mon, to around
-10C at 12z Tue. This will yield about a 30-degree temperature
drop in 24 hours... With the given wind speeds, wind chill values
will be around 10 degrees Tuesday morning. As mentioned above, not
much pattern change Tue, Wed, and Thu... meaning persistence will
be a somewhat good starting point for temperatures during this
time. Worth noting... Forecast soundings and 925mb-850mb RH field
plots picking up on what could be a potent, prolonged stratus
deck. If this holds true through tonight`s 00z run... diurnal
temperature range will need to be tightened up. Would highly
indicate value added vs guidance by shaving about 5 degrees off of
max temps, and adding a few degrees to overnight min temps.

For next weekend, respective long-term models still holding up their
respective differences in timing of another surge of cold air.
The GFS has been further west and colder with this potential cold
surge, and the ECMWF has been further east and not as cold overall
with this potential cold surge. Too early to reliably trace this
air to its source region and moderate its changes as it makes it
into the upper Midwest. GEFS plumes spread from around 30 degrees
to 5 degrees for a max temp on Fri and Saturday. The scary thing
is the GFS would have a climo influence still...meaning it could
actually still be too warm. Ultimately, have begun to knock
guidance temperatures down towards GFS. Hopefully future shifts
will follow this and be ready to drop expeditiously temps if the
ECMWF shifts towards the GFS solution.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1119 PM CST Sun Dec 3 2017

Moisture to increase overnight which should allow VFR CIGS to
lower to MVFR range and possibly into IFR in spots. Still a chance
for a rain shower at mainly the eastern TAF sites overnight into
Monday. Gusty winds out of the south into Monday, then winds
increasing behind a cold front passing through late Monday
afternoon into early Monday evening. Winds to become very strong
and gusty for Monday night.


Wind Advisory from 3 PM Monday to midnight CST Monday night for



LONG TERM...Kotenberg
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