National Weather Service Raw Text Product

Displaying AFOS PIL: AFDDMX Received: 2018-01-03 21:18 UTC


639 
FXUS63 KDMX 032118
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
318 PM CST Wed Jan 3 2018

.DISCUSSION.../Tonight through Thursday/ 
Issued at 318 PM CST Wed Jan 3 2018

Despite plenty of sunshine today, cold air advection has kept 
temperatures mainly in the single digits above zero. The winds from 
the northwest at 10 to 15 mph have accentuated the chill with wind 
chill values in the single digits to middle teens below zero. GOES-
16 snow/ice band and day cloud phase distinction RGB shows a small 
patch of generally supercooled clouds that are moving into north 
central and eastern Iowa this afternoon and evening. Otherwise, 
expect a good radiational cooling night, which will allow 
temperatures to fall below zero with upper single digits below zero 
over southern Iowa and middle teens below zero over northern Iowa. 
While Iowa will be under the influence of high pressure, the center 
will remain north of the area so the pressure gradient will support 
winds of 5 to 10 mph. This will result in wind chill values of -20 
to almost -30F over much of central Iowa. Based on timing of when 
the wind chills will rise above -20F and in collaboration with 
neighboring offices, will extend the advisory until 10am for an end 
time. 

As was the case today, high temperatures tomorrow will take another 
step downward several degrees with arctic air in place as northwest 
flow aloft persists. NAM and a few high resolution models have shown 
low model composite reflectivity across western Iowa tomorrow. NAM 
soundings at Creston and Carroll show a small layer of saturation 
near or just below the dendritic growth zone. While there is dry air 
between 900 to 950mb, moistening of this layer is expected 
throughout the day so have have inserted flurries over western part 
of the forecast area tomorrow. A better chance for flurries or a 
narrow band of light snow arrives Thursday night as a weak clipper 
drops into the area on the northwest flow. Overall, the system is 
weaker than yesterday and not everywhere will see flurries/light 
snow across our forecast area.

After this weak clipper clears the area early Friday morning, the 
upper pattern will transition as the trough over the eastern US 
pushes offshore and some ridging pushes toward our area. Meanwhile 
Friday, a shortwave trough will arrive over the Pacific Northwest 
dropping into the central Rockies by Saturday. As surface high 
pressure moves into the eastern US, will see our surface winds come 
from the south and eventually southwest allowing for warm air and 
moisture advection. Temperatures are poised to rise Saturday night 
into Sunday morning as winds from the southwest remain around 10 
mph. As the shortwave moves east from the central Rockies on Sunday, 
surface low pressure will remain south of the area along with the 
best forcing. Still, with weaker forcing over the state, will have 
light precipitation spreading into at least the southern half to 
southern 2/3rds. At this time frame, only rain and snow is mentioned 
in the forecast. Will need to monitor precipitation type forecast as 
there will be a transition zone that will creep northward into the 
state. While GFS and ECMWF soundings indicate all rain over southern 
Iowa part of Sunday, concern is how well are the models representing 
the boundary layer with snow cover on the ground and any 
precipitation cooling effects. Also of note are road temperatures 
that are currently in the upper teens to low 20s across the south. 
If road temperatures stay below freezing despite above freezing air 
temperature, could be an interesting day. That's too much detail to 
sort out at this juncture, but something to monitor in the days 
ahead.

Behind this shortwave trough, another round of cold air advection 
will occur that will bring temperatures a few to several degrees 
below climatological values with dry weather expected the first part 
of next week. 

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1127 AM CST Wed Jan 3 2018

VFR conditions are expected at all terminals this period. Several
METARS /e.g. KMCW/ are reporting occasional IFR CIGS associated 
with suspended ice crystals. However webcams and satellite imagery
continue to show clear skies across Iowa. Expect gusty northwest 
winds thru early evening...which could result in some minor 
drifting snow on runways. Winds will diminish in the late evening 
and overnight.

A suspected comms issue is leading to several missing elements at
KOTM. Have included AMD NOT SKED while maintenance/troubleshooting 
is ongoing.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory from 9 PM this evening to 10 AM CST Thursday 
for IAZ004>007-015>017-023>028-033>039-045>050-058>062-071>075-
082>086-093>097.

&&

$$

DISCUSSION...Ansorge
AVIATION...Fowle