Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 290437
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1137 PM CDT Fri Apr 28 2017

...06Z AVIATION UPDATE...

.SYNOPSIS...
Issued at 313 PM CDT Fri Apr 28 2017

18Z surface data has a frontal boundary from central Kansas into
northwest Indiana. Dew points were in the 20s and 30s from the Great
Lakes into the northern Plains. South of the front dew points were
in the 50s and 60s.

&&

.SHORT TERM...(This Evening through Saturday)
ISSUED AT 313 PM CDT Fri Apr 28 2017

Current radar trends show a period of dry weather late this
afternoon across the southeast half of the area.

This evening, the rain across the northwest half of the area will
end but spotty light rain or drizzle will persist or move into the
area from the southwest.

After midnight, the question becomes how quickly will the atmosphere
re-saturate under the persistent northeast flow off the lake. The
current forecast has light rain moving back into the southern half
of the area prior to sunrise. However, if the depth of the dry air
is deeper than what the models depict, like today, then most areas
south of I-80 could remain dry until sunrise Saturday.

On Saturday, the first part of the main storm system moves into the
area. Strong northeast flow will be seen across the area with rain
overspreading the area during the morning. As lift increases in the
late morning and afternoon hours, some embedded thunderstorms are
expected to develop which will increase rainfall amounts across the
southeast half of the area.

The combination of clouds, rain, and northeast winds on Saturday
will keep nearly all of the area in the 40s. The possible exception
is south of a Galesburg, IL to Kahoka, MO line were temperatures may
get just above 50 degrees.

.LONG TERM...(Saturday Night through Friday)
ISSUED AT 313 PM CDT Fri Apr 28 2017

Model trends remain fairly consistent, lending increased confidence
to active weather through the weekend, featuring heavy rain,
resulting river flooding and isolated short term flooding, and
possibly a window of severe storms. Long term river flooding is
discussed in detail below.

Synoptic models still favor the southeast third of the forecast area
for the heaviest rain axis, on the order of 3 to 4 inches. By
Saturday night into Sunday, increasingly moist soils will make
conditions prime for localized flash flooding from rain swollen
creeks, streams and ditches. Heavy rainfall rates could also produce
some significant street flooding. Most synoptic and convective
allowing models support some potential for severe weather Sunday
afternoon and evening as a surface low and warm front lift north
through the area. There are still disagreements in the eventual path
and timing of the low, but it is becoming increasingly clear that at
least some portion of the forecast area, especially the southeast,
will be at risk. Would not be surprised to see an upgrade from
marginal to slight risk, at least in the south. Clouds may limit the
amount of destabilization and areal coverage of severe storms, but
even a modest amount of MUCAPE would support a QLCS wind/tornado
threat given the strong shear and favorably aligned 0-3km shear
vector.

For Monday, a well-developed TROWAL will sustain lingering showers,
with afternoon temps struggling to get out of the 40s. Tuesday
through Friday, expect mainly quiet weather with near to slightly
below normal temperatures.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday Night)
ISSUED AT 1123 PM CDT Fri Apr 28 2017

Patchy drizzle and ifr to lifr cigs will begin the taf cycle at
MLI and BRL. Meanwhile, mvfr conditions will be found at CID
and DBQ to start. Latest hrrr cig height plots continue to suggest
cigs will lift to vfr overnight at CID and DBQ, as northerly flow
in 925-850 mb layer advects in drier air. This has been pretty
consistent and matches well with obs upstream and therefore
continued to follow these trends and advertise vfr conditions
developing into CID and DBQ overnight. Hrrr would support vfr
reaching MLI toward daybreak and even BRL by mid morning
Saturday. I have trended to vfr at MLI but with BRL on the edge
will only improve to mvfr at BRL for now. Otherwise, next slug of
rain expected to overspread the terminals by mid morning through
midday Saturday, with mvfr to ifr conditions. This should once
again taper off toward evening as the forcing pulls away. Winds
will be from northeasterly direction through the taf cycle at
around 10-15 kts while becoming gusty on Saturday to near 25 kts.

&&

.HYDROLOGY...
Issued at 1141 AM CDT Fri Apr 28 2017

Updated morning forecast guidance from the North Central River
Forecast Center now includes 72 hrs of QPF, through the bulk of
the heavy rain event for the forecast area. Confidence is
increasing on the potential for widespread heavy rain through the
period. Parts of west central and northwest Illinois remain
favored for the heaviest amounts, potentially 2 to 3 inches and
locally higher.

These amounts place the Illinois tribs and the Mississippi River
south of the Quad Cities at the most risk of significant flooding.
Moderate flooding is possible into next week on the Rock River,
as well as the La Moine River, and the mainstem Mississippi from
New Boston to Burlington. Other faster responding points in
eastern Iowa will also be watched closely this weekend.

There is still some uncertainty on the axis of heavy rain, and those
with interests along area rivers, streams and creeks should check
back for updates through the weekend.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...08
SHORT TERM...08
LONG TERM...RP Kinney
AVIATION...McClure
HYDROLOGY...RP Kinney


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