Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 180552

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1252 AM CDT Wed Apr 18 2018


Issued at 1000 PM CDT Tue Apr 17 2018

Based on recent hi-res model runs, delayed the onset of
precipitation for tomorrow morning by a few hours. Also, with
confidence increasing on convective heavy snow showers associated
with the initial band of warm air advection and strong 700mb
frontogenesis, increased snow ratios across the northwest forecast
area toward 12:1. Shouldn`t be too difficult to realize ratios
above 10:1 given the convective nature of the precipitation and
strong omega in the lower portion of the dendritic growth zone.
Using these updated snow ratios, have storm total snowfall amounts
a few inches higher northwest of a line from Cedar Rapids to
Dubuque - generally 3 to 6 inches with isolated 7-8 inch amounts
possible in parts of Buchanan and Delaware Counties.


Issued at 306 PM CDT Tue Apr 17 2018

18Z surface data has the low of the next storm system in northeast
Colorado. A frontal boundary ran from the low east southeast into
the bootheel of Missouri. Dew points were in the 20s from the Great
Lakes and Ohio Valley through to the central and northern Plains.
Dew points in the 40s and higher ran from the Tennessee Valley west
into the southern Plains.


.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 306 PM CDT Tue Apr 17 2018

Bottom line up front...

The winter storm watch is going to a warning and is being expanded
with staggered start times of 6 AM and 8 AM Wednesday. Winter
weather advisories have been expanded southward to I-80 in Iowa
including the QC metro area. Winter weather advisories have been
expanded southward in Illinois down to Whiteside county.

The risks are snow and ice in the warning areas with more ice than
snow in the advisory areas.

Through sunset and this evening quiet and dry conditions will be
seen across the area. Attention then turns to late tonight.

As expected there has been a southward shift with the storm system.
Atmospheric profiles show considerable dry air being pulled into the
area which is slowing down the arrival of the precipiation a little.
The possibility does exist that the storm system may shift a bit
further south.

However, the forcing developing late tonight into Wednesday morning
is very impressive. Thus the dry air will quickly get overcome in
the far west in the pre-dawn hours.

The intense forcing combined with an unstable layer of air aloft
means that thunderstorms will be possible late tonight and Wednesday

The dry air will also allow the atmosphere to quickly cool Wednesday
morning forcing the precipitation over to all snow for some areas.

The sensible weather Wednesday will be quite interesting. The
potential is there for thundersleet, thundersnow, and potentially
thunder freezing rain.

Wednesday afternoon the strongest forcing moves east of the area
allowing the overall intensity of the precipitation to decrease.

The cooling of the atmosphere means that temperatures will likely
stay right around or just under freezing for much of the day. High
temperatures on Wednesday may not occur until mid to late Wednesday

.LONG TERM...(Wednesday Night through Tuesday)
ISSUED AT 306 PM CDT Tue Apr 17 2018

Main focus is on the exiting winter storm Wed evening. Remainder of
the forecast is more benign with continued below normal temperatures
this week, then moderating and possibly returning closer to
climatological normal by early next week. The next significant
strong storm over the weekend is still trending well to the south.

Wed night, precipiation will be winding down as the surface low
moves across OH into PA. General model consensus has a low level
deformation axis in the cyclonic flow extending westward into
northwest IL into eastern IA into early evening, which may be
generous looking at the timing of the departing upper level forcing.
Will keep low to medium pops for what should be just light snow or
flurries based on thermal profiles going until midnight with a
possible additional half inch in the Freeport area and a dusting
elsewhere. Cloudy skies and low level cold air advection continues
through the rest of the night with temperatures falling to the
mid 20s north to around 30 south.

Thursday should then see wrap around stratus clearing early under
an in-building mid level ridge and a surface high that migrates
through the area through Friday. The northerly low level flow and
Canadian airmass will hold highs in the mid 40s to lower 50s
Thursday, then Friday should moderate to the 50s area-wide. Under
the center of the surface high, Friday morning`s forecast lows
from the lower 20s north to the upper 20s south may be too mild
and somewhat dependent on the coverage and magnitude of remaining
snow cover.

The next upper level low rolling out of the southwest U.S. is still
progged to track well to the south, passing through the lower MS
River Valley Sunday. This would keep the local area influenced by
low level subsidence and at least modest warming aloft to allow
daytime highs to reach the 50s Saturday and then at least lower 60s
Sunday through Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
ISSUED AT 1241 AM CDT Wed Apr 18 2018

A fast moving area of a wintry mix of moderate to locally heavy
rain, freezing rain, sleet and snow with embedded thunder will
sweep through the region after 12z through 16z, with conditions
deteriorating rapidly to IFR/LIFR. Greatest impacts expected at
CID and DBQ terminals where freezing rain/sleet should quickly
switch to snow within 1-2 hours after precipitation onset, with
snow then persisting through the afternoon before tapering off
into the early evening. Sleet and snow accumulations of at least
1 to 2 inches are expected on pavement. At MLI and BRL expecting
rain and sleet for a few hours by mid morning before transitioning
to bouts of snow by afternoon with minor accumulation possible on
pavement at MLI during any bouts of heavier snow.


IA...Winter Weather Advisory from 8 AM this morning to 7 PM CDT this
     evening for Cedar-Clinton-Muscatine-Scott.

     Winter Storm Warning from 8 AM this morning to 7 PM CDT this
     evening for Dubuque-Jackson-Jones.

     Winter Weather Advisory from 6 AM this morning to 7 PM CDT this
     evening for Iowa-Johnson.

     Winter Storm Warning from 6 AM this morning to 7 PM CDT this
     evening for Benton-Buchanan-Delaware-Linn.

IL...Winter Weather Advisory from 8 AM this morning to 7 PM CDT this
     evening for Carroll-Rock Island-Stephenson-Whiteside.

     Winter Storm Warning from 8 AM this morning to 7 PM CDT this
     evening for Jo Daviess.



LONG TERM...Sheets
AVIATION...McClure is the U.S. government's official web portal to all federal, state and local government web resources and services.