Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 110926
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
326 AM CST Wed Dec 11 2019

.SYNOPSIS...
Issued at 325 AM CST Wed Dec 11 2019

The narrow clipper snow track is non track this morning, and has not
deviated much from the expected axis. Looking at radar paired with
surface observations, the snow is lining up most prolifically along an
a line from Spencer to Fort Dodge, to Cedar Rapids, to the Quad
Cities. This seems to be the very specific axis where amounts just
over 1 inch may occur. The rate of fall should limit amounts to no
higher than 2 inches, as this is only a 5 hour long accumulating
event in any one location.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 325 AM CST Wed Dec 11 2019

By 6 AM, the snow back edge should be near Cedar Rapids, and by 8
AM, near the Quad Cites, and out of the eastern counties by 10 AM.
The combination of robust WAA within a dendritic layer combined with
light winds and very cold temperatures, will support a fluffy snow
ratio of 18 to 25/1. Thus, amounts may yield over 1 inch in the
highest band, and a half to 1 outside of the main band. We will
continue to focus energy communicating impacts of snow covered roads
and the timing for the impacts, and end of precip for the morning
commute.

The remainder of the day will see clearing by afternoon, and another
cold day north and milder day south.  This evening is actually the
most interesting time in my forecast thoughts. I do think we`ll have
a snow cover heading into tonight in the northern 1/2 to 2/3rds of
the CWA, and for a 2 to 4 hour window, we appear likely to see light
winds and clear skies, before southeast winds increase overnight and
clouds become completely opaque. Thus, the low temperature is very
likely to occur around 7-8PM. We could really drop fast, but for
now, will undercut guidance by 2-3 degrees where the snow cover is
most likely to be in place, the same axis where we discussed above.

ERVIN

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 325 AM CST Wed Dec 11 2019

Thursday...00z model runs in decent agreement of maintaining
progressive short wave clipper system from the northern plains to
the heart of the GRT LKS this period. Deepening and robust southwest
flow and warm air advection ahead of this system to drive warming
sfc temps to above normal along with blustery southwest winds of 15
to 25 MPH during the day. Mixing depth and cold ground with some
lingering light snow cover in spots may temper the warming
potential, but will advertise upper 30s in the north to mid to upper
40s in the southwest despite expected cloud cover.  Most solutions
have adjusted an associated WAA wing of precip back further south
again where the northern quarter of the CWA/north of Hwy 30 may get
skirted with some light precip Thu morning into early afternoon.
Fcst vertical thermal profiles and elevated F-gen forcing suggest
initially mainly light snow developing into the northwestern CWA
northwest of Cedar Rapids before 15z, then spreading east north of
Hwy 30 and probably closer to Hwy 20 through noon to early
afternoon before migrating northeastward of the area.  Elevated WAA
to cause the snow to mix with rain and then eventually transition
to all light rain by midday. Will not advertise it now, but a low
chance for some light freezing rain/snow mix of short duration
along Hwy 20 if sfc temps can`t recover before WAA aloft takes hold.
Some light snow accums of at least a few tenths possible along the
Hwy 20 corridor especially west of Dubuque Thu morning.

Thu night...Post-clipper boundary and elongated sfc ridge lays out
NE-to-SW acrs the CWA by Friday morning. Bulk of the models are dry
and will maintain the loaded dry blend, but the NAM has enough
shallow moisture depth that it suggests drizzle soundings as
upstream upper trof moves east toward central IA by Friday morning
and provides lift. Of course, where sfc temps drop below freezing
this may be problematic and lead to freezing drizzle later Thu night
into early Friday morning. Something to keep in mind if other model
low to mid level moisture profiles come in line with the NAM.

Friday and Saturday...One last above normal day Friday with some
spotty light precip possible if moisture depth maintains pooled over
the area and LLVL boundary retreats as a warm feature again as the
upper trof edges in from the west. There may be some fog issues
Friday morning but low confidence on that aspect right now. Any
precip Friday should be light rain/sprinkles or drizzle with a low
deck/fog as opposed to frozen crystals or freezing droplets. Highs
in the upper 30s to low-mid 40s. Still some upper trof and phasing
issues to be handled and worked out be additional model runs, but the
approaching upper trof with some sfc wave development may translate
into an increase in precip with southeast fetch moisture ingest as
Friday night progresses. There has been a southward shift in the
latest runs with this possible system, but wrap-around precip may
still affect the bulk of the area especially after midnight.
Developing LLVL cold conveyor will look to change rain over to snow
with possible snow accums into early Sat morning, but how much and
where still very much uncertain with model descrepancies.  After
whatever potential system can impact the region migrates east to
southeast, a cold drying influx should take hold of the upper
Midwest into Sunday. If we manage to get more snow cover, currently
indicated cold air advection and thickness decreases suggests lows
in the single digits acrs the northwest third by early Sunday
morning, and low to mid teens in the southeast.

Sunday through Tuesday...Cold and mainly dry Sunday expected with
lingering Canadian ridging in place, then eyes will turn to upstream
phasing process between northern and southern stream short waves and
potential resultant low pressure storm system. The various 00z run
medium range solutions and ensembles suggest anywhere from a less
phased system skirting the southern CWA with light snow from late
Sunday night and Monday, to a full on winter storm system with
several inches of snow right acrs the area. For now, will keep CHC
POPs and snow mention as a place holder for Monday.   ..12..

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
ISSUED AT 1025 PM CST Tue Dec 10 2019

Clipper system will bring a period of light accumulating snow
after 08z until 14z. CID/MLI appear to be most favored for the
accumulating snow and a period of generally MVFR to low VFR
ceilings and MVFR to IFR visibility during the snow. Mainly
flurries and VFR conditions anticipated at DBQ and BRL during this
timeframe, with a chance for a brief period of MVFR light snow.
Conditions will return to widespread VFR from mid morning through
midday Wednesday, and remain VFR throughout the rest of the TAF
period.

&&

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

$$

SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...12
AVIATION...McClure


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