Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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138
FXUS63 KDVN 281759
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1259 PM CDT WED SEP 28 2016

...AVIATION AND HYDROLOGY 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 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 100 PM CDT Wed Sep 28 2016

MVFR cigs becoming VFR as the afternoon progresses and then these
conds continuing tonight into Thursday. North winds 10 to 20 knots
this afternoon then around 10 knots tonight into Thursday morning.

&&

.HYDROLOGY...
Issued at 100 PM CDT Wed Sep 28 2016

Overview: Most significant change was at New Boston on the
Mississippi River where the crest forecast increased from moderate
to major flood stage. Otherwise, river forecasts are largely on
track. See site specific information in the following discussion and
in recent Flood Warning Statements.

Cedar River...

Vinton: Now below major flood stage.

Cedar Rapids: Forecast to fall below major flood stage Thursday
evening or night.

Conesville: Major flooding is occurring as river level rises
rapidly. Timing of crest forecast is Thursday afternoon into
Thursday night. Confidence is moderate to high on a crest a half
foot on either side of 18.30 feet.

Wapsipinicon River...

Anamosa Shaw Rd: Forecast to fall below major flood stage this
evening.

De Witt 4S: High confidence in reaching major flood stage. Thinking
current crest forecast is on the upper end of possible outcomes.
Reasonably high probability the final crest is lower. Please
monitor for updates.

Iowa River...

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

Moderate confidence is associated with the crest forecasts. Current
thinking is crest forecasts are toward the upper range of what is
possible; observed crests may come in lower. Please follow forecast
information closely throughout this event.

Mississippi River...

Minor to Major flooding continues to be on track. Greatest forecast
change was at New Boston with an increased crest forecast from
moderate to major. Otherwise, latest forecasts show little to no
adjustments. Majority of the crests are projected to occur over the
weekend or early next week. Uttech

&&

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

$$

SYNOPSIS...McClure
SHORT TERM...McClure
LONG TERM...Gibbs
AVIATION...Haase
HYDROLOGY...Uttech



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