Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 210445
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1145 PM CDT Fri Oct 20 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 256 PM CDT Fri Oct 20 2017

18Z surface data has high pressure over the Ohio Valley with a
developing storm system over the Rockies. Dew points were in the
upper 40s and 50s across the Midwest with 60s in the southern Plains.

&&

.SHORT TERM...(This Evening through Saturday)
ISSUED AT 256 PM CDT Fri Oct 20 2017

Through sunset, quiet and dry conditions will continue with winds
diminishing.

After sunset, the quiet and dry conditions will continue through the
night. Unstable air aloft will allow mid level clouds to develop as
moisture and forcing increase. Atmospheric profiles indicate the
atmosphere remains fairly dry. At worst, some isolated sprinkles may
develop during the night.

The overall persistent trend regarding the approaching front is that
the best forcing is along and behind the front. Ahead of the front
the forcing remains weak.

On Saturday, dry conditions should be seen across the area during
the morning as winds increase. At the very worst some isolated
sprinkles may be seen.

Saturday afternoon, there are still questions regarding when showers
and storms will develop. With the front not arriving until after
sunset and the forcing ahead of the front weak, the influx of
moisture may go into cloud development. If temperatures get warm
enough isolated showers and storms may begin developing by mid to
late afternoon west of a Dubuque, IA to Kahoka, MO line with a
gradual increase in coverage around sunset.

.LONG TERM...(Saturday Night through Friday)
ISSUED AT 256 PM CDT Fri Oct 20 2017

Significant rain event of showers and storms late Saturday into
Sunday Morning the main forecast challenge.

Long Term Forecast Confidence Assessment...fair or average.  Main
issue is primarily any severe risk Saturday evening with metrics not
favorable for a severe risk at this time.  Main threat suggested is
gusty winds of up to 35+ mph and localized .75 to 1.5 inches of
rain. Secondary risk is low temperatures with light winds and fair
skies days 3 through 7 are possibly 3 to 5 plus degree too high.

Saturday Night...categorical pops of a maturing and stratifying band
of showers and embedded storms to pass between 9 PM and 5 AM.
Scattered, mostly embedded thunder in the west 1/3 becoming isolated
embedded thunder in the east overnight.  Front to arrive far NW
sections around 10 PM with current worse case SBCAPE of 900 J/KG.
Local techniques based on T/Td or 68/61 suggests gusty winds of 35+
mph per BLEP algorithm possible as most significant impact with
rainfall rates over 1 inch an hour based on PW`s of 1.5".  Straight
line winds near severe to severe criteria suggests T/Td of 70+ and
65+ with some dry air intrusions needed, not expected with a
maturing line. Tornado risk currently is very low to NIL risk based
on little/no SCL buoyancy and directional shear. Cloud bases AOA 3K
AGL is generally unfavorable for low instability Tornadoes (too
high), also need winds to back to 160 or less to allow for
directional shear with winds gusts AOA 18 kts (likely higher gust
speeds) also needed for a brief, and likely weak tornado to occur.
South to southwest winds of 10 to 20 before front expected becoming
northwest at 10 to 15+ mph after frontal passage. Most locations
west should see .5 to 1+ inch rain amounts with .25 inches or less
far east and generally in-between rain totals along Mississippi
River. Area low temperatures near 50F far NW to near 60F far SE.

The latest Storm Prediction Center (SPC) day 2 outlook does have the
area in a slight risk in the western 1/3, with a marginal risk
roughly for most of the remaining forecast area.

Sunday...lingering AM light rain and showers and brisk NW winds with
clearing late with good cold air advection to keep highs upper 50s
to around 60F northwest to mid 60s far southeast.

Sunday Night and Monday...clearing cooler with forecast lows of
lower to middle 40s possibly up to 5 degrees too mild if BL wind
decouples with patchy frost not out of the question sunrise Monday.
Monday to be mostly fair with west to northwest winds of 5 to 15 mph
with highs mostly in the middle 60s. Some light showers may occur in
the far north late Monday.

Monday night through Tuesday night...upper low to pass with lots of
cold air aloft for light showers risk and lows mostly lower 40s
Monday night and highs mostly lower to middle 50s on Tuesday. This
would be coldest day so far this fall.  Clearing Tuesday night
supports good chances of frost with lower to middle 30s that may
still be at least 3 degrees too high with frost and freeze risks
an issue for later shifts.

Wednesday through Friday...mostly dry and near to below normal temps.
Highs 50s to lower 60s and lows 40s and 30s with once again more
frost risk chances with clear skies and light winds. Risk of light
showers with passing disturbances in northwest flow.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday Night)
ISSUED AT 1144 PM CDT Fri Oct 20 2017

VFR conds the remainder of tonight through Saturday afternoon,
then becoming MVFR cigs Saturday evening, spreading from west to
east. LLWS can be expected 03-15z/21. South winds 10 to 15 knots
tonight then south 15 to 25 knots on Saturday into Saturday
evening. A cold front will be approaching from the west Saturday
evening, which will spread showers and a few thunderstorms across
the area. How intense or widespread this area of rain will be is
in question will introduce only a VCSH for now.

&&

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

$$

SYNOPSIS...08
SHORT TERM...08
LONG TERM...Nichols
AVIATION...Haase


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