Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 190440
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1140 PM CDT THU AUG 18 2016

.UPDATE...
Issued at 1131 PM CDT Thu Aug 18 2016

The heavy rain producing MCS continues to shrink in size, but
remains very productive at producing a swath of 1 to 2 inches of
rain, mainly over Whiteside and Henry counties in Illinois...east
towards Princeton Il as well. Additional elevated showers are
developing over northeast Iowa along the outflow, and this has me
concerned that the activity may linger through the night. All of
this could greatly impact tomorrows high temperatures and cloud
cover, as well as boundary placement for PM storms tomorrow.
ERVIN

UPDATE Issued at 442 PM CDT Thu Aug 18 2016

I have made short term revisions to rain chances this evening and
overnight, first bumping up coverage for the small but intense
cores moving east through Whiteside and Bureau counties. This
activity may wane eventually, but it`s not isolated, it`s on a
boundary. Second, the severe cell in Minnesota continues to move
east southeast, and it no doubt laying out a boundary into far
northern Iowa. This boundary has potential to light up with new
cells and form an MCS tonight after dark. I have increased to
moderate chance pops as a starting point beginning north around
7 PM.
ERVIN

&&

.SYNOPSIS...
Issued at 325 PM CDT Thu Aug 18 2016

Isolated showers and thunderstorms have been developing across
portions of Bureau and Putnam counties for much of the day. These
have been along a boundary left over from heavy rain producing
thunderstorms in Ogle and Lee counties last night. Elsewhere
across the cwa isolated showers and thunderstorms have developed
along the CAPE gradient in our ne cwa with peak heating, but
shear was very weak. 3 pm temperatures were in the mid to upper
80s at most locations, along with dewpoints in the lower to mid
70s. This has pushed the heat index into the mid to upper 90s.

In southern MN a lone severe thunderstorm was southeast of Redwood
Falls and was tracking eastward. This storm was sliding along the
CAPE gradient and where deep layer shear was strong.

&&

.SHORT TERM...(This Evening through Friday)
ISSUED AT 325 PM CDT Thu Aug 18 2016

Forecast focus on thunderstorm chances.

Late this afternoon and tonight: Isolated showers and
thunderstorms should diminish quickly with loss of heating.
Thunderstorms in MN should remain north of the dvn cwa but
potentially could graze Highway 20 later this evening or
overnight if the HRRR is correct. However, most CAM`s keep the dvn
cwa dry, so confidence is low on convective trends. For now will
go with the dry look. Minimum temperatures will be around 70 at
most locations.

Friday: Other than isolated showers and storms again with the
convective temperature reached, attention will turn to our nw cwa.
We will maintain chance pops during the afternoon as impressive upper
level trough (for mid August) digs into the northern Plains. Low
level moisture transport will increase in our nw cwa during the
afternoon as large scale ascent begins to arrive. However, the
better forcing will still be west and north of the dvn cwa. Ahead
of this system and approaching strong cold front, very warm and
humid conditions will over the cwa. However, clouds should be on
the increase so expect highs in the mid to upper 80s. This will
push heat index readings into the mid 90s, especially near and
south of Interstate 80.

.LONG TERM...(Friday Night through Thursday)
ISSUED AT 325 PM CDT Thu Aug 18 2016

Main forecast concern in the long term is still the chance for
showers and thunderstorms Friday night into Saturday.  Model
solutions still show differences in timing, however they agree on
the mass fields and subsequent wave pattern.  Strong to severe
thunderstorms will be possible late Friday and into early Saturday
morning across the area, with possibly another round of storms later
in the day.

Overall a unique jet pattern where entrance regions across the
northern CWA early Saturday, transitions to a coupled jet with the
divergent zone across our east on Saturday afternoon.  This jet will
lead to different areas throughout Saturday, where convection will
be favored.  This, coupled with strong convergence along the surface
front, will lead to widespread coverage of thunderstorms and
possibly heavy rain.

Overall timing of the wave is different between the guidance with
the ECMWF now being the faster of the two.  Regardless of this
difference between guidance, there is strong signal for morning
convection across the CWA, with a prefrontal wave that is in all of
the guidance.  Deep layer shear of 30 to 40 knots, with a very moist
BL and strong frontal convergence will lead to strong to severe
thunderstorms with the main threats being damaging winds and heavy
rain.

Of question is a low end tornado threat.  The ECMWF, GFS and NAM all
suggest different scenarios ranging from isolated tornado threat
across the northern CWA Saturday AM, to no threat what so ever.  At
this time, will keep the mention of tornadoes in the AFD.

The main question for Saturday is what happens in the afternoon. The
ECMWF, says we see little to no rain and NAM regenerates convection
across our east that may be strong to severe.  Regardless,
convective debris will likely limit overall instability and lead to
a difficult forecast.  The timing of the front and any clearing will
directly affect Saturday afternoon`s chance for storms.

After this period, there are few changes to the forecast as cool dry
air is still forecast for the rest of the weekend.  NW flow will
keep the area cool and dry before H5 ridging and then SW return to
the area and lead to warmer temps and a return to the moisture.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday Night)
ISSUED AT 1131 PM CDT Thu Aug 18 2016

Scattered thunderstorms remain possible over eastern Iowa and
northwest Illinois overnight, and again Thursday evening. Many
hours will be dry in between showers, and VFR conditions should
dominate the over all weather at all sites. Winds will be light
east to south overnight, followed by southerly tomorrow around 8
to 10 kts. This pattern will remain in place through late Friday
night, when a cold front will approach eastern Iowa.
ERVIN

&&

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

$$

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



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