Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 200816
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
316 AM CDT MON JUN 20 2016

.SYNOPSIS...
Issued at 316 AM CDT Mon Jun 20 2016

Very warm and somewhat humid conditions were found across the area
early this morning, as 3 am temperatures were in the 70s with
dewpoints from the mid 60s to around 70. This was due to the
enhanced mixing and S-SW level flow ahead of a weak cold front
approaching from the north. This front extended south from an area
of low pressure over NW Ontario across NW WI to near La Crosse,
then W-SW along the IA/MN border into far northeast Nebraska. A
weakening thunderstorm complex accompanied this boundary across NW
WI with more scattered redevelopment noted across far N and NW IA
along and ahead of the SE moving boundary.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 316 AM CDT Mon Jun 20 2016

The main challenge centers on thunderstorm chances today and tonight
as the cold front drops south through the area. In the near term,
looking at recent radar trends, the hi-res models may be a bit
too aggressive and fast with the elevated thunderstorms in
northern and NW IA reaching into eastern IA early this morning.
However, the overall trend looks reasonable, with thunderstorms
likely reaching the highway 20 corridor in eastern IA as early as
6 am, then spreading south and east to the I-80 corridor and NW IL
by mid morning. By this time, the elevated moisture advection will
be weakening as winds veer to more northwesterly and elevated
instability will also wane. This should lead to dissipating of the
convection and have trended pops along these lines.

Current consensus timing has the front advancing to a line from north
central IL, SW into SE IA by noon, then exiting to the south and
SE by mid afternoon. With thunderstorms potential largely
dependent on the frontal boundary, this will lead to a dry
afternoon over all but the far south. Lower dewpoints advecting
into the area behind the front will gradually lower humidity
values, but deep mixing into only slightly cooler air aloft should
still lead to highs at least into the lower 80s north, to the
upper 80s to around 90 central and south.

Tonight, thunderstorm chances will remain across the south early,
spreading north and northeast toward sunrise as the elevated front
begins to return back northward. Low temperatures will range from
the upper 50s north to mid 60s south.

.LONG TERM...(Tuesday THROUGH Sunday)
ISSUED AT 316 AM CDT Mon Jun 20 2016

Main forecast concern for the long term remains the threat for
organized convection and severe weather on Wednesday.  Active
pattern, with sufficient shear and instability to warrant the threat
for severe weather on Wednesday.  Once this cold front and upper
level feature advect out of the area, quieter weather returns for
the end of the week, before another front pushes into the area early
this weekend.  Temperatures once again this weekend appear to be
hot.  However, it is worth noting, that solutions past four or five
days seem to be rather inconsistent, leading to a lower confidence
in the extended periods of this forecast.

Tuesday afternoon into Wednesday morning...the cold front that is
pushing through the area today will return as a warm front across
the area starting Tuesday afternoon.  Sufficient instability with a
constant feed of high thetae.  It is important to note that NAM and
other hires guidance from 00z tonight, had an issue with derived
PWATs across much of the CONUS.  This means that the NAM instability
for Tuesday and Wednesday are likely well over done. That said, the
GFS also has CAPE values that will support convection overnight.
Multiple rounds of showers and thunderstorms will track along the
warm front as it slowly shifts north through the overnight into
Wednesday.  This nonetheless will lead to a possible flash flood
setup for Wednesday across the area.

Wednesday...SPC has the area in a day 3 enhanced with severe weather
outlook.  Diving through shear and instability parameters makes it
clear that all the parameters exist for severe weather.  Warm front
across the area, with deep layer shear increasing to 50+ knots,
along with 0-1km shear at 30 knots and 0-3km shear vector of 50
knots, along with ample instability and two speed maxes running
through the flow are the players for the event. Ongoing morning MCS
will likely travel through the area, leaving OFBs in its wake. With
increasing deep layer shear, sufficient speed and directional shear
in the low to mid levels, the threat for tornadoes seems exists,
especially with any afternoon redevelopment.  With the lack of upper
level jet, believe that these supercells may turn rapidly into HP
supercells.  This would likely mean that the supercell tornado
threat may be short lived at the beginning of the event.  However,
rapid growth upscale into MCS with a strong cold pool/shear balance
suggests that the main severe threat will be damaging winds. Think
embedded tornadoes will also be likely in the large MCS. With
earlier rain from the overnight across the area, afternoon storms
may be enough to cause flash flooding.  The only plus to this not
happening is the weak llvl moisture transport into the area until
00z. Think that a severe and hydro event look increasingly likely
across the CWA.  There are still differences in placements of the
mass fields between the guidance.  These differences are enough to
either bring the main warm front north of the CWA, or place it
directly in the middle of it.

Post Wednesday, into Thursday and the weekend...current guidance
suggests quiet weather before the next warm front makes its way into
the are for Saturday.  The GEM seems to have the heaviest QPF and
likely has swayed the Superblend towards higher pops.  With this
being the outlier, decided to go with low chc to schc pops before
the front becomes more active on Saturday across the area.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z Monday NIGHT)
ISSUED AT 1125 PM CDT Sun Jun 19 2016

VFR conditions with south winds will continue overnight, as a cold
front slowly moves toward the area. By morning, the front will
gradually move south from along highway 20 by 12Z to near highway
34 by 21Z. A very low chance for a shower or thunderstorm can be
expected in a 2 hour window of time as the front passes through.
This is not expected to produce more than a brief isolated threat
of MVFR conditions. Northwest winds around 10 kts can be expected
behind the front. Late Monday night, additional showers and
thunderstorms may brush through far southeast Iowa. This low
potential is not yet included in the TAF at BRL.

ERVIN
&&

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

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Gibbs
AVIATION...Ervin



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