Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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131
FXUS63 KDVN 201732
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1232 PM CDT Fri Oct 20 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 326 AM CDT Fri Oct 20 2017

This morning high pressure was centered over the southeastern
United States. This high was sliding east of the region as return
flow begun moistening the low levels across the central US. High
clouds associated with a weak short wave in the flow were
advecting north as the next system approaches our area. This
return flow and shortwave will drive the weather in the short
term period.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 326 AM CDT Fri Oct 20 2017

Numerous different forecast challenges in the short term this
period. The first are high temperatures for today, the second are
dewpoints and subsequent fire weather danger, the last are
chances for rain late in the period.

Most guidance has lower highs today than yesterday across the
area. This seems unlikely as H85 temps are slightly warmer than
yesterday and mixing should result in higher temps this afternoon.
Used the RAP for a starting point for high temps today. Then did
some editing from there. The main story on temps are temperatures
that are near 10 degrees above average. With dewpoints in the 50s
and 40s this afternoon it will feel warmer than yesterday even
with a stronger wind. With these stronger winds, also expect drier
afternoon dewpoints. Went with a consshort blend for dewpoints
this afternoon.

The lower dewpoints and cured farm fields have led to very high
fire danger for ag fields this afternoon. The best threat for
these conditions exist across Iowa counties.

Later in the period, multiple weak short waves are forecast to
transverse the area. With returning moisture, there are chances
for precip late tonight, especially near 12z Saturday. These pops
are slower than previous forecasts. CAMs suggest that we could see
sprinkles this afternoon into the evening. At this time, decided
to leave that out.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 326 AM CDT Fri Oct 20 2017

Showers and some storms arrive mainly for Saturday night lingering
into Sunday east. This will be followed up by generally colder and
below normal temperatures much of next week, with a risk of frost and
possibly a freeze sometime during the mid and/or late week timeframe.

Saturday-Sunday... Broad warm/moist advection will bring the threat
for a few showers during the day on Saturday, mainly west of the
Mississippi River. Best rain chances though look to arrive Saturday
night lingering into Sunday (east of Mississippi River) as a cold
front and attendant mid level trough arrival coincident with good
moisture surge (PWATs increasing to near 1.5 inches). Shear and
instability supportive of the potential for discrete cells forming
well to the west of our cwa Saturday afternoon. Then would
anticipate evolution into a quasi-linear convective system Saturday
evening ahead of sweeping cold front and near parallel surface flow
while making it into the cwa west of the Mississippi River. SPC has
upgraded the western cwa into a slight risk for Saturday, with a
marginal risk covering nearly the remainder of the cwa. Appears to
be a narrow window of opportunity for a few strong to possible
severe storms Saturday early to mid evening west of the Mississippi
River with deep shear values increasing to 30-40+ kts. Main threat
is strong winds in strong predominately low level shear, with a
small risk of tornado if surface winds remain backed enough
enlarging 0-1km hodos despite low CAPE. Do anticipate convection
weakening while further progressing eastward across cwa, but
increasing lift aided by right entrance region of Midwest to Great
Lakes H25 jet should assist in maintaining some rain across rest of
the cwa into Sunday morning. Anticipate potential for 1-2 inch
swaths across the west/southwest cwa tapering of to 0.25 to 0.5 inch
generally elsewhere. Gusty south winds and moisture advection will
lead to a mild night Saturday night. Front will pass late Saturday
night and Sunday morning leading to cooler highs Sunday mainly in
the 60s.

Monday-Thursday... Still seeing some model differences with respect
to timing/strength of upper level energy and potential phasing, but
overall the theme is for a colder and below normal stretch for most
if not all this period. Pretty good agreement on a strong PV Anomaly
arriving Monday into Tuesday accompanied by a chance of showers with
cold pocket aloft and cyclonic flow, especially on Tuesday. Could be
quite a windy day on Tuesday depending on degree of phasing energy
and extent of cyclogenesis over the Great Lakes and/or Ohio Valley.
NW winds combined with highs only in the 50s Tuesday will make it
feel quite raw/blustery and certainly a dramatic change from the
above normal stretch of late and for much of the fall for that matter.
If the boundary layer can decouple and winds diminish Tuesday night
with a quick passing surface ridge then this will be the first window
of at least frost potential by Wednesday morning. Plenty of differences
begin to show up mid to late next week but there is some signal for
a brief warm-up Wednesday and especially Thursday ahead of a another
stronger NW flow shortwave. In fact the models show 850 mb temps
surging into the range of 12c to 17c on Thursday, and should some
of that be tapped into by gusty surface winds then potential to be
several degrees warmer for Thursday than currently forecasting with
potential for widespread low to mid 60s for highs (with a few upper 60s
possible south of I-80). This passing wave late week will be
accompanied by shower chances, and then followed up by a shot of even
colder air to which another frost and even freeze appears possible either
Friday or Saturday depending on frontal timing.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
ISSUED AT 1226 PM CDT Fri Oct 20 2017

VFR conditions will be seen through 18z/21 as a storm system moves
into the Plains. After 00z/21 the potential is there for LLWS that
will end around mid-morning Saturday. There may or may not be
some sprinkles or isolated light showers falling out of a 9-12kft
deck prior to sunrise Saturday.

&&

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

$$

SYNOPSIS...Gibbs
SHORT TERM...Gibbs
LONG TERM...McClure
AVIATION...08



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