Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 101722
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1222 PM CDT Tue Oct 10 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 402 AM CDT Tue Oct 10 2017

Increasing north to northeast winds were pulling in a cool, dry
airmass into the forecast area early this morning. 3 am
temperatures ranged from the upper 40s along the IA highway 20
corridor to the upper 50s to near 60 over northeast MO into west
central IL. Dewpoints ranged from the upper 30s north to mid 50s
in the far southeast. This increased northeast flow was due to
the tightening pressure gradient ahead of a low pressure system
moving into SW MO and northwest AR and attendant upper low moving
through the Central Plains. The main upper forcing tonight was
concentrated well to the west, triggering showers and
thunderstorms over eastern NE into western IA, while clusters of
showers and thunderstorms were over MO into central IL along the
surface boundary and broad difluent upper level flow. Over the
local area, radars showed scattered showers along the elevated
baroclinic axis across north central into northeast IA, while weak
showers were spreading into northeast MO and southeast IA. A
cluster of thunderstorms was a close as central IL, along the low
level boundary that pushed through the forecast area Monday.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 402 AM CDT Tue Oct 10 2017

Focus is on the widespread rain, possibly heavy, that will
overspread the area today from south to north as the upper low in
the plains tracks across the region. This feature tracks across
northern MO into northern IL late this afternoon and evening, in
an initial, weakly negatively tilted fashion, which rapidly
weakens into an open wave tonight as it approaches the Great
Lakes. At the surface, models generally have the low lifting
northeast across MO into central IL by late afternoon, then moving
east into Indiana overnight.

Looking at the rain: Warm, moist advection/isentropic lift
overspreads the forecast area today with widespread coverage
mainly south of I-80 this morning, then over most of the forecast
area by afternoon. Upper level forcing is maximized from mid
afternoon into early evening over roughly the southeast half of
the forecast area. With high precipitable water and the
likelihood of convective enhancement, this would support rainfall
totals well over 1 inch to possibly 2 inches, with the heaviest
amounts likely over west central into northwest Illinois. With
the system weakening and shifting east, most of this rainfall will
be before 9 pm, with less than a quarter inch likely overnight
with pops lowering from west to east toward morning. WPC`s day one
excessive rainfall outlook paints the entire forecast area in a
marginal risk. Cannot rule out urban or small stream flooding
issues with high rainfall rates in stronger convection, but
overall, the flash flood threat is low due to the dry antecedent
conditions.

Due to high low shear values, there is a very low potential for
severe storms in the far southeast, which is greatly dependent on
how far northwest the low level low tracks this afternoon. Most
models have this low level circulation staying well southeast of
the forecast area. However, the HRRR and NAM have consistently
showed the low and critical backing of surface wind fields and
enhanced storm cells touching into the area southwest of a line
from Galesburg to Keokuk. If this were to occur, there may be low,
but sufficient MUCAPEs and strong low level shear to produce an
HSLC (High Shear Low CAPE) type severe storm with damaging wind
and a possibly spin up tornado. However, this is very low
confidence and conditional. While worth watching today, will keep
our messaging focused mainly on rainfall and non-severe storms, as
supported by the consensus low track and SPC`s marginal day 1
severe risk, which is well to the south over eastern MO into
southern IL.

Will continue to have at least a mention of thunderstorms over
the southeast half of the forecast area for the afternoon and
evening with help of the strong upper forcing and proximity of
greater instability just to the southeast. High temperatures will
be held to the 50s north to mid 60s in the southeast with blustery
northeast winds.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 402 AM CDT Tue Oct 10 2017

Wednesday and Thursday

Cloudy and cool in the 60s on Wednesday with showers diminishing
during the morning. Late at night, low-level saturation in the from
of a stratus deck trapped under an inversion around 850mb may lead
to areas of drizzle into Thursday morning.

Time-height sections show weak omega in the stratus layer likely
from a subtle convergent flow between the sfc to 875mb. NAM sfc-
850mb layer is more moist than the GFS, so only mentioned slight
chance for drizzle in the grids for now. As the clouds slowly
thin and slide east, temps will warm well into the 60s/near 70 F
for Thursday afternoon.

Friday

Warmer yet, mid 70s north to near 80 F across the south. A cold
front is forecast to slide through the CWA from NW to SE at night,
bringing with it a chance for scattered showers and thunderstorms.
At this point, any rainfall amounts would be on the light side.

Saturday through Sunday morning

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

Across E Iowa/NW Illinois, NAEFS progged PWATs (above 1.50") and
IVT values are a solid 2-3 standard deviations above the mean for
the middle of October. Raw GFS PWATs are near 2 inches.

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 determine the risk for severe weather; however,
models are showing high deep layer shear so this will be a period to
watch through this week.

Sunday Afternoon through Monday

Conditions should be drying out Sunday afternoon 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. Uttech

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
ISSUED AT 1215 PM CDT Tue Oct 10 2017

Showers and embedded thunderstorms will continue to spread
northeast across all TAF sites through the afternoon and early
evening. Expect mainly MVFR ceilings with periods of MVFR to
briefly IFR visibilities in heavier showers and storms. KBRL has
the highest risk of stronger storms during the late afternoon.
Later tonight, expect showers endings with sub-1000ft AGL ceilings
and MVFR fog. The northerly wind should stay up enough to preclude
any dense fog.

&&

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

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Uttech
AVIATION...RP Kinney



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