Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 181136
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
536 AM CST Mon Dec 18 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 330 AM CST Mon Dec 18 2017

Sfc trough axis has been slowing moving from west to east through
E Iowa/NW Illinois overnight. Ahead of and within the trough,
winds were light and dewpoints were in the 30s which caused areas
of fog to form.

The lowest visibilities as of 3 AM, down to 1/4 mile in some
cases, were in the counties along and east of the Mississippi
River (north of Monmouth, IL). Immediately west of the trough,
sfc winds have increased out of the SW and visibilities have
improved to better than 3 miles for most locations.

Have a graphicast out to highlight most likely area for dense fog
through early this morning, but do not think a Dense Fog Advisory
will be necessary because the lowest visibilities at any one
location will only last for 1-2 hrs.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 330 AM CST Mon Dec 18 2017

Today and Tonight

Low stratus is forecast to thin and eventually clear out form west
to east through the day. GOES-16 imagery is already depicting a
large area of clear skies expanding over parts of the NW forecast
area.

For this afternoon, SW winds and some sun will allow temps to
reach into the lower 40s N/NE to the lower 50s south. For the
overnight period, temps are only forecast to fall into the lower
30s north to upper 30s far south due to steady SW winds
maintaining a mixed boundary layer. Uttech

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 330 AM CST Mon Dec 18 2017

Mild December weather continues until late this week, then a large
scale pattern change will bring much colder weather and periodic
snow chances from the weekend into next week. In the transition, the
late week storm system continues to trend on a more northwesterly
track, setting up the forecast area for primarily a rain event, with
a possible transition to light snow as it exits late Friday. From
there, temperatures will get progressively colder with some models
suggesting highs in the teens or much colder by Christmas Day.

Tuesday through Thursday, a mostly zonal flow, tracking storm
systems well north and south of the area will continue above to
above normal temperatures. Thursday, will continue to carry low
chance pops across the northwest, where models continue to suggest
isentropic lift in an inverted trough developing ahead of low
pressure lifting out of the southern plains. While the timing and
thermal profiles are low confidence, there would be a potential for
mixed precipitation or possible sleet at the early morning onset
over our far northwest. Developing warm advection at the surface and
aloft would support possible light rain north of I-80 during the day
with the south remaining dry.

While there remains some model timing differences with the storm
system lifting out of the southwest Thursday night through Friday
night, the general consensus has the surface low tracking across
east central Iowa into northwest Illinois. This would favor the bulk
of accumulating snow occurring north of the forecast area across
northern IA, southern MN into WI. Looking at timing in the 00z runs,
the 00z GFS is the fastest, followed by the ECMWF, then the slower
and much more dynamic system shown by the Canadian, which a notable
outlier as it nearly bombs a secondary low from the southern Plains
into IL and the Great Lakes by Saturday morning. Precipitation is
likely to transition from rain Thursday to snow in the northwest
Thursday night, with this rain to snow translation in the wrap
around cold air then taking place from northwest to southeast Friday
into Friday night, when the faster solutions have it exiting the
region. Also, during this period, there is likely to be a dry slot,
into at least the south sometime early Friday. Thus, overall
confidence in the depiction of rain vs snow is low, but the synoptic
setup points toward any measurable light snow mainly northwest of a
Cedar Rapids to Freeport axis.

Saturday and Sunday, a longwave trough enveloping an arctic airmass
sweeps down into the central U.S. This will result in sharply colder
temperatures, with forecast highs in the mid 20s to mid 30s Saturday
and 20s Sunday possibly too optimistic as 850 mb temperatures drop
to the teens below, to possibly 20s below zero Celsius. Low
confidence chances for snow arrive Sunday as the setup is favorable
for waves of at least modest lift interacting with the arctic air.

Beyond, it is worth noting an anomalously strong arctic high shown
by the GFS and Canadian models over the northern plains spilling
into the local area from Christmas Day into the middle of next week.
Depending on the snow cover, this could limit highs to the single
digits and teens with overnight lows below zero.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday Morning)
ISSUED AT 523 AM CST Mon Dec 18 2017

IFR ceilings will improve through this morning as a clearing line
approaches from the west. No other significant impacts are
forecast through the remainder of this TAF period. Expect steady
SW winds today and tonight of 10-15 kts with mainly clear skies
from mid to late morning on. Uttech

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...Uttech
SHORT TERM...Uttech
LONG TERM...Sheets
AVIATION...Uttech



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