Winter Storm in Iowa, December 11 to 12, 2020

Area Forecast Discussion
National Weather Service Quad Cities IA IL
312 PM CST Fri Dec 11 2020

.SYNOPSIS...
Issued at 310 PM CST Fri Dec 11 2020

18Z surface data has a weak low south of the Kansas City metro area.
A pseudo warm front ran east northeast from the low across central
Missouri and into northern Indiana. Dew points were in the 40s south
of the warm front with 20s and 30s from the Great Lakes into the
central and northern Plains.

&&

.SHORT TERM...(This Evening through Saturday Night)
ISSUED AT 310 PM CST Fri Dec 11 2020

No changes to the current headlines.

The lack of dry air aloft is helping to keep precipitation as all
rain across the area. KMKX radar shows a band of lower CC across
southeast Wisconsin that correlates fairly well with a rain/snow mix
being reported between observation points.

Real-time data shows the main low east southeast of Kansas City and
pressure falls are suggesting a track further north than what the
models indicate. IF correct this 'could' affect overall snowfall
amounts.

Thus through sunset light to moderate rain will continue across the
area.

After sunset the atmosphere will cool which will allow a rain/snow
mix to develop fairly quickly in the highway 20 corridor that moves
south to a Sigourney, IA to Milledgeville, IL line by midnight. By
mid to late evening the change over to all snow will have occurred
generally north of a Freeport, IL to Belle Plaine, IA line.

The rain/snow line will slowly move south through sunrise Saturday
as well as the change over to all snow. Areas south and east of a
Milledgeville, IL to a Memphis, MO line will likely remain all rain
through sunrise Saturday.

Saturday morning the rain/snow transition zone will quickly move
south and east through the remainder of the area as colder air is
pulled in behind the departing low. Across the northwest half of the
area all snow will be seen.

Saturday afternoon snow will end across the northwest half of the
area while the rain/snow mix changes to all snow across the
southeast half of the area. By Sunday evening the northwest half of
the area will be dry while lingering flurries continue across the
southeast half of the area. By midnight dry conditions will be seen
across the entire area.

Now the snow with this event will be a wet heavy snow. The bulk of
the snowfall looks to occur in a 6 to 9 hour time frame; from
between midnight and 3 AM to approximately 9-10 AM Saturday.
Snowfall rates of 0.5 to 1 inch per hour are the most likely. Where
heavier bands of snow develop, rates of 1 to 1.5 inches per hour
will be possible.

Now for amounts. The suggested further north track (based on real-
time data) is concerning which would suggest lower amounts. However
the potential banding of snow 'may' offset a more northerly track.

Taking this into account the heaviest snow looks to occur north of a
line from roughly Rockford, IL to Oskaloosa, IA. Here snowfall
amounts of 2 to 5 inches are expected. Due to the potential banding
nature of the snow amounts are expected to vary over short distances.

South of the Rockford, IL to Oskaloosa, IA line and down to a
Milledgeville, IL to Ottumwa, IA line 1-2 inches is expected. South
of the Milledgeville, IL to Ottumwa, IA line amounts under 1 inch
are expected with little or no snow south/east of a Sterling, IL to
Memphis, MO line.

.LONG TERM...(Sunday through Friday)
ISSUED AT 310 PM CST Fri Dec 11 2020

Sunday through Friday
Assessment...medium to high confidence

Near normal temperatures through the period.

The model consensus has dry conditions through the period with the
northern and southern jets well north and south of the area. However
the global models do show weak disturbances aloft moving through in
the flow. IF the weak forcing has enough moisture to work with then
I cannot fully rule out some isolated flurries or sprinkles
occurring depending upon the time of day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
ISSUED AT 1137 AM CST Fri Dec 11 2020

Incoming storm system will drop VFR conditions to MVFR/IFR
conditions by 00z with rain. After 00z/12 conditions will
deteriorate further to IFR/LIFR conditions. KCID/KDBQ will see
rain change to snow through 12z/12 with accumulations while
KBRL/KMLI remain rain. After 12z/12 IFR/LIFR conditions will
continue with snow across most of eastern Iowa and a rain/snow mix
developing across northern Illinois.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until 6 PM CST Saturday for Benton-
     Buchanan-Delaware-Dubuque-Iowa-Jackson-Johnson-Jones-Keokuk-
     Linn.

IL...Winter Weather Advisory until 6 PM CST Saturday for Jo Daviess-
     Stephenson.

MO...NONE.
&&

SYNOPSIS...08
SHORT TERM...08
LONG TERM...08
AVIATION...08

Hazardous Weather Outlook
National Weather Service Quad Cities IA IL
339 PM CST Fri Dec 11 2020

IAZ040>042-051>054-063>068-076>078-087>089-098-099-ILZ001-002-007-
009-015>018-024>026-034-035-MOZ009-010-122145-
Buchanan-Delaware-Dubuque-Benton-Linn-Jones-Jackson-Iowa-Johnson-
Cedar-Clinton-Muscatine-Scott-Keokuk-Washington-Louisa-Jefferson-
Henry IA-Des Moines-Van Buren-Lee-Jo Daviess-Stephenson-Carroll-
Whiteside-Rock Island-Henry IL-Bureau-Putnam-Mercer-Henderson-
Warren-Hancock-McDonough-Scotland-Clark-
339 PM CST Fri Dec 11 2020

This Hazardous Weather Outlook is for portions of north central
Illinois...northwest Illinois...west central Illinois...east
central Iowa...northeast Iowa...southeast Iowa and northeast
Missouri.

.DAY ONE...THIS AFTERNOON AND TONIGHT

Rain will change to snow mainly north of a line from Sigourney,
Iowa to Freeport, Illinois tonight. A winter weather advisory is
in effect for this area. A wet heavy snow is expected.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

Snow will continue on Saturday. Areas that are still raining will
see the rain turn to snow. Snowfall amounts of 2 to 5 inches are
expected in the advisory area. Outside of the advisory area little
to no snow accumulation is expected.

.SPOTTER INFORMATION STATEMENT...

Snowfall reports of one inch or more will be appreciated.