Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 151845
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
145 PM CDT Tue Aug 15 2017

.UPDATE...
Issued at 145 PM CDT Tue Aug 15 2017

Have raised pops in our southern cwa as the MCS in sc IA continues
to hold together while pushing slowly eastward. Radar indicated a
warm air advection wing of thunderstorms that extended east from
the main MCS into the dvn southern cwa.

SPC meso analysis showed PWAT`s have increased to 1.6 inches, the
low level moisture transport has increased, and SBCAPES were 3000 j/kg.
However, shear was quite weak and the storms will be moving rather
slowly eastward. The bottom line is storms should be non-severe,
but provide beneficial rainfall to the area that currently has a
moderate to severe drought. Some of the storms will produce over
an inch of rain in less than an hour. The main area of storms will
move across our southern cwa near and south of a stalled east-west
frontal boundary. Farther north the MCS may produce some very
light rain in our far west-central/nw counties this afternoon. The
HRRR has had a consistent handle on this situation but was a few
hours too slow in bringing convection into the dvn cwa.

Meanwhile, due to the persistent low stratus in our far northern
counties have lowered temperatures a few degrees.

Have raised pops to 30 percent in our south, but will continue to
monitor trends and may need to raise them further.

UPDATE Issued at 1145 AM CDT Tue Aug 15 2017

A nearly stationary east-west frontal boundary extended from along
the NE/KS border to between I-80 and Highway 34 to extreme
northern IN. To the south of the front, temperatures were in the
lower 80s and dewpoints in the lower 70s. To the north of the
front, low stratus and northeast winds continued in our northern
cwa, roughly north of a Freeport, IL to Belle Plaine, IA line.

Meanwhile, low level moisture transport west of the cwa was
supporting a linear MCS in sw IA, and this was moving slowly east.

The HRRR models suggests isolated storms to develop near this
stalled front in the dvn cwa late this afternoon/early this
evening. Other models are completely dry, so will monitor trends
and keep the slight pops going for now. Clouds in our north should
slowly diminish, but this may affect temperatures, so will monitor.

UPDATE Issued at 600 AM CDT Tue Aug 15 2017

Dealing with the fog is quite a challenge early today. In short,
this is really truly patchy dense fog. When you are in it, it`s
shallow and dense, as noted by the clearly visible sky above.
Looking only at observation sites, and a headline seems warranted
at times, and not at others. The fog reports are found in all
parts of the CWA at 11Z. However, a look at webcams, shows many
areas free of fog, and many others with very shallow fog that
seems more nuisance. Thus, we are messaging fog in SPS products,
and in social media/web graphics and not issuing a 1 hour long
advisory.

&&

.SYNOPSIS...
Issued at 312 AM CDT Tue Aug 15 2017

A weak front is positioned near highway 34 in our southern CWA early
today, and with moist air surrounding the boundary, there has been
areas of dense fog formation since about 1 AM.  The for remains
somewhat transient, as sites are reporting visibilities under 1/4
mile to over 3 and back again within each hour, which is a
reflection of the shallow nature of the fog thus far. So long as
this is the case, I will hold off on any advisory for it.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 312 AM CDT Tue Aug 15 2017

Fog formation is occurring in an absence of cloud cover which is
due to a lack of forcing along the front. That should continue to
be the case through this evening, and this is now looking like a
dry forecast through then. Overnight, broad warm advection may
allow for some showers and storms later tonight, but even that is
possibly remaining well west of the CWA through early Wednesday.
With less cloud cover and precipitation chances, it should be
moderately humid summer day of thin high cloud cover and light
winds. Highs today should be a few degrees milder than yesterday
with around 80 north to mid 80s near highway 34 and upper 80s in
northeast Missouri.

Tonight will be more complicated with respect to temperatures. The
warm front will begin to strengthen and lift north, but with the
main rainfall delayed (due to delayed forcing) the low level flow
will likely become northeast to east and draw in cooler lake air
overnight. That process will tighten the baroclinic zone along the
front for Wednesday, but may actually provide a pleasant "windows
open" for the northeast 1/3 of the area. Over all, the increasing
winds are also expected to offset the fog potential along the
front, and will keep this out of the forecast for now. I will
retain a slight chance for showers and storms from a possible
elevated waa band after midnight, but this potential is looking
much lower than previous forecasts. ERVIN

.LONG TERM...(Wednesday through Monday)
ISSUED AT 312 AM CDT Tue Aug 15 2017

Active weather pattern to continue through most of the long term.
Overall largescale pattern remains southwest to zonal, ushering
warmer air and more humidity.  With it, the chances for rain and
thunderstorms as effective surface boundary remains near the I80
corridor.

Wednesday...a lot of what happens Wednesday afternoon and evening is
completely dependent on what happens overnight Tuesday into
Wednesday.  Guidance has slowed the main wave which could keep
convection out of the area through most of the morning and early
afternoon.  If this occurs, strong storms and severe weather will be
possible.  If we see precip through the AM and or cloudy debris then
overall threat for severe weather and strong storms would be lower.
What is clear with both scenarios are that we stand to see decent
and possibly heavy rain across the area.  After 00z Thursday, LLJ
kicks in along with PWs above 2 inches.  This along with sfc
boundary and upper wave will combine to produce conditions for heavy
rain across the area.

If we do see clear skies and instability can grow Wednesday, the
threat for severe weather will increase.  Current guidance has
increased deep layer shear to 35 to 45 knots across the area.  While
I still think some of these values are due to convective feedback,
there is sufficient shear for organized storms.  Low level shear and
backed surface winds are conducive to low level mesocyclonegenesis.
Think there is a threat for tornadoes if these conditions can come
together.

Thursday...slower progression of the wave will keep rain slowly
exiting the area through the late morning and early afternoon.
Overall flow becomes NW briefly and quiet weather ensues before the
pattern returns to zonal if not SW.  The chances for rain and storms
will continue through the period.  Many of these chances will be
dependent on the boundary and mesoscale conditions.  Overall
confidence in the long term forecast is low.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
ISSUED AT 145 PM CDT Tue Aug 15 2017

Cigs will gradually improve to MVFR this afternoon at KCID/KDBQ
and mainly VFR conds at KBRL/KMLI. A few thunderstorms are
possible in southeast IA this afternoon into early this evening.
Have kept at VCTS at KBRL. Late tonight, Cigs/Vis should lower to
MVFR/lcl IFR conds at all taf sites. Northeast to east winds this
afternoon becoming southeast by Wednesday, with wind speeds around
10 knots or less.

&&

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

$$

UPDATE...Haase
SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...Gibbs
AVIATION...Haase



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