448 
FXUS63 KDMX 072103
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
303 PM CST Sat Dec 7 2019

.SHORT TERM.../Tonight through Sunday Night/
Issued at 303 PM CST Sat Dec 7 2019

Confidence: Medium to High

Few major concerns through at least Sunday night. Warm air advection 
continues to stream into Iowa today with cirrus moving in from 
the west overspreading the area this afternoon. Tonight the
approaching trough will continue to near Iowa with a thermal 
ridge advancing over the area. This will keep overnight lows quite
mild with some additional clouds expected. Sunday will see an 
increase in thicker/lower clouds especially in the afternoon over 
the northwest as the trough nears and in the southeast as another
weak system drifts north toward the region. Despite the cloud 
cover tomorrow, southwest flow at the surface and aloft should 
allow for at least 10 degrees of recovery into the afternoon over 
most of the region and will maintain highs in the upper 40s/lower 
50s south with mid to upper 30s likely in the north. There is a 
trend this morning for some delay in onset of precipitation in the
NAM and short term models due to a slower arrival of the main 
Arctic cold front over northern Iowa to just about 12z Monday. 
This may have a slightly greater impact on overnight low temperatures
into Monday morning. The current forecast is near the lower end 
of the spread of guidance at around the 25th percentile of the 
output. The latest guidance is warmer overall and even at the 50th
percentile values, overnight mins should be some 3 to 4 degrees 
warmer. With some adjustments in timing of precipitation onset, 
the main area of light snow is being slowed by about 3 to 4 hours 
northwest and then progresses southeast through 12z as prior 
forecast. 

.LONG TERM.../Monday through Saturday/
Issued at 303 PM CST Sat Dec 7 2019

/Monday through Tuesday Night/...

Confidence: Medium

As the boundary moves southeast during the day Monday, a general 
light rain or light snow event is expected with a small chance of a 
light freezing mix along the transition zone Monday morning which 
should not cause any significant impacts as soundings continue to 
show the column quickly saturating. The balance of the precipitation 
will end by late afternoon. Snow totals continue to be light and 
generally similar to the previous forecast with around a half inch 
in the far north and little to any accumulation south of Highway 20. 
One concern in the current set of HiRes models and the Euro is a 
secondary band of snow developing and moving along the Arctic front 
from 14 to about 19z over west Central to Central Iowa in the
Highway 20 to I80 corridor, then weakening quickly in the early
afternoon. This appears tied to stronger mechanical forcing aloft
and also the main frontal boundary aloft between H850 and H700. 
Though the present risk appears large enough to mention, but with
some time to evaluate the chances, the risk is not enough to add 
any additional snow to the forecast. If the area of snow did 
develop, it could add about an inch across western to central 
Iowa during the morning hours Monday. 

As previously mentioned, the onset of the cold air is likely 
beginning around 12z in the northwest. Though a minor delay, the 
impact will be greater on highs Monday as little recovery is 
anticipated over much of the area once the boundary settles south. 
Winds will pick up along and behind the boundary during the late 
morning and afternoon hours though values of 15 to 25 mph are at 
most expected. Once the first boundary moves south of the area, a 
secondary boundary will arrive by 12z Tuesday with a stronger push 
of cold air and scattered flurries. H850 temperatures quickly drop 
to 15 to 18 below from 18 to 00z Wednesday. Model spread in highs 
have actually increased for Tuesday. Currently our high is near the 
mean, though given the uncertainty temperatures might be colder. 
Temperatures Tuesday night will be quite cold in the single digits 
above and below zero over the area. 

/Wednesday through Saturday/...

Confidence: Low to Medium

The remainder of the week shows more uncertainty as the models begin 
to diverge in both strength and placement of systems.  Wednesday 
warm air advection returns to the region by afternoon with both 
the GFS/Euro indicating some lift along a warm front and the 
potential for a light precipitation event. Beyond Wednesday, the 
GFS is stronger and faster with a northern stream wave by Thursday
with subsequent energy arriving Friday night and again late 
Saturday. The Euro holds onto the Great Lakes ridge into later 
Thursday with a weaker northern stream wave Thursday. The next 
significant system arrives on Saturday. Overall, the general 
trends in both models that are at least consistent from yesterday 
include overall milder temperatures and at least one or two 
chances for precipitation. Highs are still expected to be in the 
20s to 30s north and in the 30s to 40s south where any 
precipitation that falls will be warm enough for either a mix or 
all rain. 

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1136 AM CST Sat Dec 7 2019

Main concern through the period will be increase in MVFR cigs. 
Approaching weak system southeast and return of boundary layer 
moisture will increase risk of MVFR cigs south aft 16z Sunday far 
southeast. In the far north, an increase in MVFR cigs associated
with the approaching trough/wind shift will also aft 17z Sunday.
Otherwise LLWS increases aft 23z and continues through 10-11z as
stronger southwest winds arrive. Main widespread challenges with 
cigs will arrive aft 18z. /rev


&&

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

$$

SHORT TERM...REV
LONG TERM...REV
AVIATION...REV