Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 102331

Area Forecast Discussion
National Weather Service Quad Cities IA IL
531 PM CST Fri Nov 10 2017


Issued at 327 PM CST Fri Nov 10 2017

It was a cold start this morning with near record lows in the teens
for many areas. Dubuque actually set a daily record low of 12 F.

The very dry, modified arctic air mass has been slow to warm today,
despite full sun. As of the late afternoon, sfc temps ranged from
the upper 20s around Dubuque and Independence to the lower to middle
30s elsewhere.


.SHORT TERM...(This Evening through Saturday)
ISSUED AT 327 PM CST Fri Nov 10 2017


Chilly with lows in the 20s and wind chills in the teens. We`ll have
increasing mid-level clouds ahead of a trough of low pressure that
will bring light precip to E Iowa/NW Illinois this weekend.


Slightly warmer into the mid/upper 30s far north to mid 40s far
south as SSE sfc winds advect milder air northward. Model time-
height sections reveal mid-level WAA/isentropic driven ascent
through the day, but a dry layer from the sfc-925mb persisting
through much of the afternoon/evening. Therefore, it will take some
time for the low levels to saturate and precip to reach the ground.

Best chances for light mixed precip (rain/sleet/snow) prior to
Saturday night are late in the afternoon and during the evening,
along and NW of a Marengo to Dubuque line.  In this area, max wet-
bulb temps aloft are 0.5 - 2.0 C and sfc wet-bulb temps may be close
to 32 F. However, even here chances for measurable precip are
relatively low at 30-50%. If precip occurs, it will be light and sfc
temps should be warm enough to prevent impacts to travel. Uttech

.LONG TERM...(Saturday Night through Friday)
ISSUED AT 247 PM CST Fri Nov 10 2017

Initial focus is with a mainly light precipitation event
Saturday night into Sunday. The shortwave responsible
is just coming ashore in the Pacific NW early this afternoon.
This low amplitude wave and attendant surface trough will
push across the region overnight Saturday night through
the first half of the day on Sunday. Moisture transport/
advection, while persistent during this time, remains fairly
weak while forcing in general broadly weak thus mostly light
amounts around 0.1 inch or less. However, there is also some
suggestion and potential for fairly narrow or concentrated
corridor of stronger forcing attendant to mid level
frontogenesis and deformation along/north of vort max that
could potentially result in a swath of higher amounts of near
0.25 inch. GFS shows this band across portions of northern IA
waning into northeast IA Sunday AM, while ECMWF hints at this
band south of I-80 thus low confidence exists with exact
location of this band and hoping for more agreement next
12+ hours as system getting better sampled by RAOB network.
Despite the uncertainty with the banded potential for much
of the area this looks to be a light precipitation event.
Precipitation type will be a challenge, as initially
Saturday evening moisture ramps up in low levels while weak
lift and drying is noted above, which would support drizzle
and perhaps some freezing drizzle over northeast IA with
surface temps possibly near freezing. Forcing and moisture
depth then become sufficient to allow for measurable precip
chances to ramp up overnight Saturday night into Sunday AM.
Precip type is still somewhat in question and will be dependent
on low level thermal parameters and amount of dry air. NAM
continues to be colder outlier in critical BL to few thousand
feet, and believe may be too cold holding 2m temps at or just
below freezing Saturday night along/n of Hwy 30 given 925 mb
flow 20+ kts from SSW. However, surface flow remains backed
from SSE and fairly light thus can`t totally discount it either.
Favoring though a compromise of warmer GFS/ECMWF with continued
moisture advection (albeit rather weak) and clouds along with
sfc-925 mb thermal ridge traversing region ahead of surface
trough. Therefore, favoring rain as the main precip type but
even with warmer solutions cooling to wetbulb would still support
some wet snow mixed in across the north 1/3 to possibly 1/2
overnight Saturday night into early Sunday AM, with little to no
snow accum. If colder NAM were to verify precip type would tend to
be predominately snow during period of best lift overnight into
early Sunday AM across the north, while rain south but marginal
surface and ground temps would still lend support to little if any
accum in areas that would see snow. Some slowing trend to the
system was noted in dprog/dt of models and if this persists would
need to maintain higher pops into Sunday afternoon for south/east
portions of the cwa. Bottom line, a light wintry mix of rain/snow
appears portions of eastern IA into northern IL Saturday night-
Sunday AM, and do not anticipate much if any impact to travel at
this time.

As for the rest of the extended active, fast zonal flow is set
to dominate for much of next week. This will lead to moderating
temperatures and bring periodic bouts of precipitation with
general timing of every 2-3 days, or around midweek and again
late week into next weekend for main precipitation windows.


.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
ISSUED AT 525 PM CST Fri Nov 10 2017

The clear skies early this evening will be replaced by a mid cloud
deck around 8000 feet for the overnight in through Saturday. The
weak low pressure system bringing the clouds will move over the
region through Sunday morning, and will eventually bring low
clouds, drizzle, and some rain to the region, but through Saturday
afternoon, it should only slowly saturate the atmosphere from top
down, keeping the lower cigs well to the west. MVFR cigs will
likely develop area wide after 00z/11.




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