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838
FXUS63 KDVN 281141
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
641 AM CDT WED SEP 28 2016

...12Z AVIATION UPDATE...

.SYNOPSIS...
Issued at 315 AM CDT Wed Sep 28 2016

Deep upper low and attendant surface low was sagging south of
Lake Superior early this morning. Radar is showing a few light
showers over portions of northern Illinois attendant to a
shortwave diving south on the westward flank of the low. MSAS
surface analysis shows a cold front over the southern cwa.
The incoming cold advection was leading to a decent thermal
gradient across the region at 2 AM, with a few mid 40s along the
Hwy 20 corridor while several sites hovering around 60 degrees
across the far southern cwa. The incoming cold advection and
differential-cva is squeezing out a few patches of extremely light
rain or sprinkles here at the office and in other areas north of
I-80, which is not being picked up well by radar as it`s occurring
beneath the beam.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 315 AM CDT Wed Sep 28 2016

The deep upper low and attendant surface low are forecast to drift
southward into the Ohio Valley by late tonight.

For today, expect generally mostly cloudy to cloudy skies between
advection of stratus from Wisconsin on northerly winds, and cold
advection aiding steep low level lapse rates favorable for
stratocu development with any breaks. Anticipate some sprinkles
and isolated light showers at times, especially near and east of
the Mississippi River early this morning and again this afternoon.
Highs will be modulated by the amount of solar insolation, but
overall with 850 mb temperatures around 4C and generally a mostly
cloudy sky would anticipate cool highs in the upper 50s and lower
60s, with northerly winds 10-20 mph making it feel rather blustery
at times. What to watch for this afternoon is the potential to
see more breaks in the clouds develop with increasing subsidence
in wake of the passing shortwave. Should more solar insolation
occur then could see more in the way of lower to middle 60s for highs.

Tonight, expect continued subsidence building in on the northwest
flank of sagging low, which should lead to eventual decreasing
clouds. Overnight the low level flow turns more from the northeast
over far eastern cwa and this could shuttle in some lake enhanced
clouds east of the Mississippi River, which will need to be watched
for potential impact on lows. Overall though expecting skies
becoming mostly clear to partly cloudy. Sufficient northerly wind
5-10+ mph should keep boundary layer mixed to keep lows near guidance
blend of upper 40s to lower 50s.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 315 AM CDT Wed Sep 28 2016

Main forecast concern for the long term remains the closed low over
the IL and IN this morning.  With a ridge building west of the area,
the low will move very little to the east before it retrogrades back
to the west northwest. This closed low will have a great affect on
our sensible weather and make for a tricky forecast.

Thursday and Friday, winds will remain slightly up under the low
across the eastern CWA.  On Thursday this may lead to warmer temps
than currently forecast.  On Friday, the low begins to move slightly
west.  Waves around the low may induces showers across the eastern
CWA.  Friday will be cloudy, and cool day across the area.

This weekend, guidance attempts to push the low east later on
Saturday.  Before this, the chances for pops have increased across
the area.  Still think that temps may continue cooling.  Most of the
blends have been swayed by the BC grids which have been trying to
make up for the past few days where stronger winds allowed warmer
temps to mix down.  Operational models still so highs this Saturday
in the 60s across the area.

Next week, guidance agrees that ridging will lead to a warm up
across the area before another trof moves across the area.  The ECM
and GFS differ greatly on the actual structure of the wave.  One
thing seems clear, we will see another chance for thunder and rain
Tuesday into Wednesday.  The ECM depicts a case where some of the
storms could be strong to severe.  I have low confidence in this
occurring. However, I have more confidence that the guidance will
change for next week, mainly due to this closed low and the models
usual poor performance with closed low movements.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday Morning)
ISSUED AT 630 AM CDT Wed Sep 28 2016

MVFR CIGs have overspread KDBQ and KCID terminals and expected
to develop at KMLI and KBRL next couple of hours, with
generally MVFR CIGs prevailing the rest of this morning. This
afternoon anticipate CIGs lifting to high MVFR and VFR with
aid mixing on gusty north winds 10-20 kts. A few sprinkles or
light isolated shower cannot be ruled out today, especially
east of the Mississippi River. Limited coverage and lack of
impact with visibilities staying at or above 6SM precludes
any mention. Eventually should see clouds scatter out by evening
with increasing subsidence, while northerly winds diminish to
around 10 kts or less.

&&

.HYDROLOGY...
Issued at 315 AM CDT Wed Sep 28 2016

Main change to the rivers overnight was that the warning at
Independence was cancelled.  Otherwise no change to previous forecast
or discussion.  Latest discussion is attached below.

Previous Discussion...

River forecasts largely on track. See site specific information in
following discussion and in recent Flood Warning Statements.

Cedar River...

Vinton: Major flooding continues. Forecast to fall below major flood
stage Wednesday afternoon or evening.

Cedar Rapids: Currently cresting near 22 feet (unofficially the
second highest on record). Expected to remain above 21.5 feet into
this evening before beginning to steadily fall.

Conesville: Major flood stage is also forecast here by Wednesday
evening, persisting into the weekend. Forecast crest confidence is
moderate due to uncertainty associated with the effects of
attenuation as the high water routes downstream.

Wapsipinicon River...

Anamosa Shaw Rd and De Witt 4S: High confidence in reaching major
flood stage, moderate to high on the crest forecasts which have been
adjusted down slightly - partly based on flow measurements taken
yesterday evening at Anamosa.

Iowa River...

Excessive amounts of water continue to propagate downstream through
the Iowa and Cedar Rivers. There is high confidence on significant
flooding, especially during the middle to end of the week. Exact
timing varies by site.

Moderate confidence is associated with the crest forecasts, or by
how much a river level exceeds either moderate or major flood stage
due to attenuation of the routed flow. Uttech

Main Stem Mississippi River...

Minor to Major flooding continues to be on track, with the sites
hitting Major flood stage being Keithsburg, Gladstone LD18, and
Burlington. In the ongoing assessment of routing water from
upstream, as well as tributary input, the latest forecasts show
little to no adjustments from previous runs from last night.
Generally most sites have kept the same crest value, or have been
adjusted a tenth or two of a foot downward. Besides at Dubuque
(Thursday into Friday), crests on the Mississippi at most of the
sites are forecast to occur over the upcoming weekend. Then most of
the same gage sites are projected to start to experience river level
decreases Monday into next Tuesday. With higher confidence that the
river will not reach the flood stage at Bellevue LD12, have dropped
the river flood watch for that site.     ..12..

&&

.DVN Watches/Warnings/Advisories...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...McClure
SHORT TERM...McClure
LONG TERM...Gibbs
AVIATION...McClure
HYDROLOGY...Gibbs/Uttech/12