Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141133

Area Forecast Discussion
National Weather Service Des Moines IA
533 AM CST Wed Nov 14 2018

.SHORT TERM.../Today through Tonight/
Issued at 338 AM CST Wed Nov 14 2018

Confidence: High

Quiet weather is again anticipated this period. Low pressure
trough extends from southern Canada into the lee of the Rockies
at 07z with an attendant warm front southeast into northern Iowa.
A patch of midlevel stratus has formed upstream earlier today and
is now moving southeast in the H700 flow. The area should continue
to dissipate through 15z this morning as it moves away from the
upper level support that produced the deck. There will be some
clouds over the region at sunrise unless the dissipation process
occurs faster. Meanwhile, the ridge of high pressure over the area
Tuesday will linger over the southern Great Lakes southwestward
into southeast Iowa tonight. Even with warming this afternoon and
overnight tonight, the better warm air advection will be confined
to about the northwest half of the forecast area. This will
promote highs in the lower to mid 40s northwest with upper 30s to
lower 40s in the east. Tonight may see a repeat of some mid to
high level clouds over the northwest while generally clear skies
remain over the southeast/east closer to the old ridge axis. For
portions of the Cedar and Iowa River Valleys east of I35, have
lowered mins tonight back into the mid to upper teens where light
and variable winds are also anticipated. Over the remainder of the
area temperatures will remain in the 20 to about 25 degree range.

.LONG TERM.../Thursday through Tuesday/
Issued at 338 AM CST Wed Nov 14 2018

Key Points: The main focus of the extended period was on the
light snow event Friday night into Saturday over much of the
forecast area. The GFS and ECMWF have come into slightly better
agreement wrt to location, while the NAM remains the outlier
attm. Otherwise, dry and warmer by next week.

Thursday into Saturday...A clipper system cuts across Minnesota
and northern Iowa Thursday night providing a subtle wind change
to the northwest by around 12z Friday. Any precip with this
clipper remains well north of the state and temperatures remain
mild on Friday.

A second shortwave and associated cold front begin to enter the
northwest portions of the state past 00z Saturday. The ECMWF and
GFS are in much better agreement with the location of the QPF, but
still differ on timing and QPF amounts. The ECMWF is slightly
faster with the progression of the cold air, but is much less than
the GFS wrt to QPF amounts. The bulk of the forcing, albeit very
weak, moves north to south across the CWA b/t 03z and 15z

The problem period is b/t 06-12z Saturday when the rain to snow
transition looks to occur. Highly confident the precipitation will
be either rain or snow as there is no significant warm layer
aloft and no very cold near surface layer to result in any
refreeze. Have generally a widespread inch going for much of the
forecast area by Saturday morning, but cannot rule out some higher
amounts in the event the change over occurs a bit sooner than
anticipated. Thinking it`ll be all snow by around 08z Saturday
coinciding with the best forcing withing the dendritic layer.
There is some negative EPV present by 12z Saturday between the
Highway 20 to Interstate 80 corridors and the equivalent potential
temperature lines are folded backwards over the same location in
a cross section, resulting in the possibility of some CSI
release. The frontogenetical forcing remains weak, but there are
some hints of a maxima b/t 850mb to 700mb within the
aforementioned corridor. All this may lead to the potential for a
small "higher" band of snow developing Saturday morning. Hence,
confident in a few locations seeing around 2-3 inches within this
small band and will have to adjust snow forecast if models
continue to trend in this manner.

Northwest winds develops by Saturday morning and could have some
visibility restrictions through 18z. Winds remains between 15-20
knots attm, so not anticipating significant reductions.

Sunday into Tuesday...quiet weather returns back to the forecast
area during this time frame and a warming trend begins on Monday.
Tuesday and even Wednesday look to be fairly warm with southwest
flow and dry air mass in place.


.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 533 AM CST Wed Nov 14 2018

Few concerns this period. Patch of mid level stratus exiting area
this morning. South winds increase to near 12kts north aft 15z
with VFR conditions through period. Winds diminish aft 00z with
southwest flow generally under 10 kts. Some sct-bkn250 may brush
areas along/north of US 30 aft 23z through 12z Thursday. /rev




LONG TERM...Podrazik
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