Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 111137

Area Forecast Discussion
National Weather Service Quad Cities IA IL
637 AM CDT Wed Oct 11 2017


Issued at 350 AM CDT Wed Oct 11 2017

At 3 am the surface low associated with the system that brought
widespread rain Tuesday was over east central IL. Area radars
showed widespread rain continuing north and northwest of this
feature across northern IL into far eastern IA, under and ahead
of the weakening upper level low, evident on water vapor imagery
over east central IA. A seasonably cool and damp airmass was in
place in the resulting cyclonic low level flow with temperatures
ranging from the mid 40s northwest to mid 50s in the far east.


.SHORT TERM...(Today and Tonight)
ISSUED AT 350 AM CDT Wed Oct 11 2017

Main focus is on the lingering cool and cloudy conditions with possible
drizzle as an inversion keeps a deep layer of low level moisture
trapped in place throughout.

Today: The surface low continues east with its cyclonic low level
flow hanging back over the forecast area through early afternoon.
Lingering rain and drizzle early this morning will exit east as
with the passing upper level wave exiting into lower MI by midday
and in-building shortwave ridge. Will continue to watch trends,
but for now have the light precipitation ending by 9 am in the far
east. Thick cloud cover will prevent much of a diurnal rise and
have lowered highs to the mid 50s to around 60.

Tonight: A developing warm advection southwest flow aloft sets up
with forecast soundings showing a sharpening inversion and
favorable signature for drizzle and at least patchy fog. This
setup focuses especially across the west where a developing low
level southeasterly flow enhances moisture convergence and
have drizzle and patchy fog beginning there late, spreading east
toward sunrise. Cloud cover and lingering low level moisture will
hold low temperatures in the lower 50s.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 350 AM CDT Wed Oct 11 2017


Moisture and stratus deck trapped beneath a persistent low-level
inversion around 900mb may lead to some drizzle into the mid morning
hours. Time-height sections show weak omega in the stratus layer
likely from a subtle convergent flow. NAM sub-inversion layer is
more moist than the GFS, so went with patchy drizzle in the grids.

There may also be patchy fog. Latest hi-res models hold a corridor
of dense fog to our west across central and south-central Iowa down
into Missouri. In this area, clearing skies and the axis of the sfc
high pressure ridge are collocated. One model outlier is the NMM,
which works dense fog into the far SW forecast area. Due to low
confidence on the occurrence of widespread fog, mentioned as patchy
in the grids at this time.

Large-scale subsidence will win out into the afternoon with cloud
cover gradually thinning and temps reaching near 70 F SW to low
60s NE (where clouds may hang on until evening).


Warmer into the low 70s north to near 80 F across the south. A cold
front is forecast to slide through the CWA from NW to SE from late
afternoon into the night, bringing with it scattered showers and
isolated thunderstorms. At this point, any rainfall amounts would be
on the light side.

Saturday through Sunday morning

No significant changes to the weekend forecast.

There are signals for an anomalously moist atmosphere and
atmospheric river setting up from the Southern Plains into the
Midwest. Model mixing ratio plots continue to show moisture
originating over the east-central Pacific in the ~700-500mb layer
and over the Southern Plains/Western Gulf between 925-850mb, then
advecting northeastward into the Midwest. The confluence and
superposition of these moisture plumes along and ahead of a cold
front is manifested as very high integrated WV transport (IVT) and
PWATs over Missouri, Iowa, Illinois, and Wisconsin.

Across E Iowa/NW Illinois, NAEFS progged PWATs (near 1.75") and IVT
values (exceeding 750 kg/ms) are approximately 3 standard deviations
above the mean for the middle of October. Raw GFS PWATs are near
1.85 inches, slightly lower than the previous few model runs.

Besides another likely round of rain, there is potential for periods
of heavy rain and possibly localized flash flooding from heavy
rainfall rates. Of course, the details and placement of the heaviest
rain can still change this far out.

It`s too early to delve into the details regarding severe weather
potential, but deep layer shear and instability look favorable for
some severe storms in a zone from south-central Kansas to west-
central or NW Illinois Saturday into Saturday Night.

Sunday Afternoon through Tuesday

Conditions should be drying out Sunday afternoon and evening as the
system moves to the east. After a cool start in the 40s Monday
morning, expect seasonable temps in the 60s for the afternoon. Near
to slightly above normal temps in the 60s are forecast for Tuesday.


.AVIATION...(For the 12Z TAFS through 12Z Thursday Morning)
ISSUED AT 637 AM CDT Wed Oct 11 2017

Low clouds lingering behind a departing storm system will remain
in place today through tonight. IFR ceilings this morning are
expected to improve to MVFR levels of 1000 to 2000 ft over all the
terminals this afternoon. Lingering moisture and weakening wind
fields are then likely to lead a deteriorating conditions
overnight with lowering ceilings and developing fog and drizzle.
Latest forecasts have IFR conditions returning due to the low
clouds and fog after midnight. While not included due to low
forecast confidence, LIFR conditions will be possible by 12z




LONG TERM...Uttech
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