Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 172356
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
556 PM CST Sun Feb 17 2019

.UPDATE...
Issued at 547 PM CST Sun Feb 17 2019

Snow continues to be quite dendritic along and north of I-80,
where the trof convergence is combining with the upper low
forcing. That lines up well with the afternoon update to bump up
snow amounts over the highway 30 corridor, while amounts in the
south are rather light. It appears the south has converted over to
light snow, rather than freezing drizzle, and the advisory there
will be allowed to expire at 6 PM.

&&

.SYNOPSIS...
Issued at 307 PM CST Sun Feb 17 2019

Waves of enhanced lift ahead of a slow moving upper level low over
northwest Iowa continued to produce widespread snow over most of the
forecast area this afternoon. At the surface, low pressure was
centered over the IN/KY border area with a trough axis reaching
northwest across IL, through our far southern counties into south
central Iowa. Along and south of this surface trough, the
precipitation has been falling as light freezing drizzle, mixed with
snow at times. Widespread 4 to 6 inch snowfall totals have been
reported roughly northwest of a line from Sigourney through Iowa
City to Dubuque, with lighter amounts to the south and east. The
snow has been mostly of a dry nature, with liquid to snow ratios of
15:1 to 20:1 and higher observed so far.

&&

.SHORT TERM...(This Evening and Tonight)
ISSUED AT 307 PM CST Sun Feb 17 2019

Snow will continue into this evening and the Winter Weather Advisory
will remain in place as mostly light snow diminishes over the area.
The upper low over western Iowa is progged to advance slowly
eastward and slowly fill as it moves across the forecast area
tonight. At the low levels, the trough axis will remain over the
southern half of the forecast area through the night. The
combination will continue to produce mostly light snow over
especially areas north of the I-80 corridor well into this evening.
The heaviest will be mid to late this afternoon, as an apparent
mesoscale feature seen on KDVN 88D and area of favorable thetae
convergence in model data will produce heavy snowfall rates as
already observed in the Cedar Rapids, where 1 to 2 inches in less
than one hour has been reported. This will track eastward roughly
between the I-80 and highway 30 corridors between 3 pm and 5 pm and
possibly result in low visibilities and quick 2 plus inch
accumulations.

Elsewhere, the late afternoon and evening period will see widespread
1 to 2 inches, which may push totals to 7 or 8 inches over the far
northwest, where snow began around midnight. The aforementioned
features will significantly weaken this evening, and little more
than a dusting is expected after midnight. The continued light
freezing drizzle in the south may result in a light glazing of ice
accumulation, before cooling thermal profiles allow the
precipitation to change to light snow. Thus, will anticipate
allowing the Winter Weather Advisory for our southern tier of
counties to expire at 6 pm. The rest of the advisory that is set to
expire at midnight, may be cancelled early, depending on how fast
the accumulating snow winds down this evening.

After holding nearly steady in the mid to upper 20s today,
temperatures tonight will fall into the teens by morning as winds
shift to the north. The north winds of 10 to 15 mph may cause some
low end drifting on east to west roads toward morning especially
along the Illinois highway 20 corridor.

.LONG TERM...(Monday through next Sunday)
ISSUED AT 307 PM CST Sun Feb 17 2019

Monday and Tuesday...A few lingering light snow pockets or flurries
acrs northwest IL along lingering LLVL trof axis through mid
morning, otherwise a nil wx period with fresh cold advection. Sky
cover will be key, as any clear out Monday night into Tuesday
morning will make the fcst lows still too mild taking into account
the fresh snow cover especially along and north of I80. With a clear
out, there will be some sub zero readings in the normal cold air
drainage locations in the northwestern third to half of the DVN CWA
with several inches of fresh snow cover. High temperature recovery a
bit of a question for Tuesday with more clouds streaming in ahead of
the next system and east to northeast LLVL flow...see a lot of 20s.

Tuesday night and Wednesday...The latest suite of 12z medium range
model runs continue to advertise the next ejecting upper trof with
associated LLVL wave and trof complex rolling up through the region
in steering southwesterlies this period. While the real heavy precip
with this system streams up the lower MS RVR Valley and southern OH
RVR/TN Valley regions, a secondary def zone of at least moderate
precip may sweep acrs the area from the southwest starting Tuesday
evening and lasting through at least late morning Wednesday. Still
some uncertainty of QPF/moisture feed-conveyor, the thermal
profiles the system will drag up wiht it for precip types, and
strength. But currently indicated strength of mid level vort and
upper level low suggest at least the potential for moderate precip
amounts even with secondary moisture feed/TROWAL wrapping in from the
southeast off heavier precip fields.

Taking into account modifying LSR`s but still rather high with the
incoming saturation and early detected dendritic growth fields, see
widespread advisory level snows(except for maybe the far southeast
and east central CWA) by mid Wed morning, with the potential for
lower end warning amounts acrs the northwestern third of the CWA.
With an elevated warm draw, the GFS even breaks out a wintry mix of
sleet and freezing rain acrs the southeastern third of the CWA late
Tue night after midnight and into Wed morning with some icing
possible. But the 12z ECMWF and NAM keep the freezing rain at bay
just off to the southeast of the DVN CWA through Wed morning. With
the uncertainty will cut back on the freezing rain extension to the
northwest, but keep it in the south of I80 or more confined to the
southeastern third of the fcst area for now. As has happened many
times this winter, as the deeper saturation gets eroded from the
southwest behind exiting system, lingering lift may produce freezing
drizzle on Wed in the wake of exiting snows, of courser where sfc
temps are below freezing. Several models have the southeastern half
of the CWA warming into the low to mid 30s for Wed afternoon, but
currently low confidence in those parameters being that mild with
precip and lingering snow cover.

Thursday through next Sunday...A post system chill down for Thu
along with nil weather, then eyes look to the weekend for yet
another potentially significant system and large trof possibly
phasing and impacting much of the mid west from Friday night through
at least early Sunday. Much will depending on the phasing of the
trof and if it comes out in one or two slugs of energy during this
time. But thee are signs of pre-system warming enough to make precip
types a challenge again, as well as post-system cold advection for
more of a wintry regime on it`s back end. Too much uncertainty to
hammer out details at this point, but something impactful may be
looming out there for next weekend. Rain, freezing rain, snow, and
winds may be all possible at times.   ..12..

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday Evening)
ISSUED AT 525 PM CST Sun Feb 17 2019

A trof of low pressure remains over the area with widespread light
fluffy snow falling north of Iowa City to Galesburg this evening.
This snow will gradually weaken to a non accumulating intensity by
the 03Z to 06Z time frame this evening. Winds will slowly increase
from the north tonight to around 10 to 15 kts by midnight.
Otherwise, look for IFR conditions improving to MVFR by morning,
and likely remaining there all today Monday.


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until midnight CST tonight for Benton-
     Buchanan-Cedar-Clinton-Delaware-Des Moines-Dubuque-Henry IA-
     Iowa-Jackson-Jefferson-Johnson-Jones-Keokuk-Linn-Louisa-
     Muscatine-Scott-Van Buren-Washington.

     Winter Weather Advisory until 6 PM CST this evening for Lee.

IL...Winter Weather Advisory until midnight CST tonight for Bureau-
     Carroll-Henderson-Henry IL-Jo Daviess-Mercer-Putnam-Rock
     Island-Stephenson-Warren-Whiteside.

     Winter Weather Advisory until 6 PM CST this evening for Hancock-
     McDonough.

MO...Winter Weather Advisory until 6 PM CST this evening for Clark-
     Scotland.

&&

$$

UPDATE...Ervin
SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...12
AVIATION...Ervin


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