Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 210837

Area Forecast Discussion
National Weather Service Quad Cities IA IL
337 AM CDT Sat Oct 21 2017

Issued at 336 AM CDT Sat Oct 21 2017

Early this morning, strong southerly flow in tandem with a series
of mid level shortwaves has led to an increase in boundary layer
moisture across the central CONUS. This was ahead of a strong wave
over the Rockies that is expected to push east through the day
today. Closer to the area, a stronger wave across western IA lead
to development of showers and storms near the Missouri river.
Over our area, drier low levels have kept most of the radar
returns from reaching the ground as anything more than sprinkles.


.SHORT TERM...(Today and Tonight)
ISSUED AT 336 AM CDT Sat Oct 21 2017

Main forecast concern in the short term are the chances for severe
weather this evening and total storm coverage later today. As
noted last night, the current runs are even slower with the
progression of the main wave as the best CVA doesn`t occur until
near 00z Sunday.

Let`s discuss storm coverage first. Discrepancies between the
global models and CAMs in overall QPF, leads to a lower than
average confidence in storm coverage this evening. The latest
NAMnest suggests that convection falls apart as it spreads east
across our CWA. The Euro has a QPF bullseye across the central and
eastern parts of our area. While this is the case, it is clear
that a H85 jet along with strong surface convergence will likely
favor more of a Euro forecast with better coverage of storms later.
It will be telling to see how the CAMs trend today with coverage
towards 06z Sunday. If the CAMs start to trend towards the Euro,
then I would think the chances for severe weather are increased
into the late evening.

As far as severe weather goes, another typical Fall setup with
strong dynamics, a H25 jet entrance region developing over the
area after 00z Sunday, and strong low level shear will lead to
a chance of severe weather. Model consensus of surface winds
staying up after sunset and backing to east of south as the cold
front approaches leads to 1km low level shear of 30 to 40 knots.
More than enough for tornadoes. CAPE is forecast to be near
800-1000 J/kg. While higher CAPE values are important for the
strength of severe weather, the distribution of CAPE in the
profile and the location of the shear in that profile are much
better indicators of tornado potential. In this case, the best
vertical motion due to CAPE is closer to the layer where the
highest shear exists. If any updraft can tilt this shear into the
vertical, the there would be a tornado threat. At the current time
it appears that the best chances for this are across the western
zones between 1z and 6z this evening. With H85 winds increasing to
around 45 to 50 knots, and a mixed boundary layer due to higher
surface winds, damaging winds will also be possible with these

.LONG TERM...(Sunday through Friday)
ISSUED AT 336 AM CDT Sat Oct 21 2017

The cold front looks to exit to the east early to mid-morning on
Sunday. Frontogenetical (FGen) banded post-frontal rain/showers
will gradually diminish from west to east into Sunday afternoon.
A couple of models (ECM, GEM) trending wetter on qpf and no too
surprising given the impressive layer Fgen entrance region of
upper jet ejecting from the Mid-Mississippi Valley to the
Great Lakes. Given that wouldn`t be surprised to see some moderate
rain amounts of 0.25 to 0.5 inch additional on Sunday along and
especially east of the Mississippi River if forcing remains strong.
Highs Sunday along/east of the Mississippi River will likely occur
in the early morning before falling into the 50s at which point
any recovery will be dependent on timing of end to rain and any
breaks in clouds. Best chances for some partial sunshine will be
found across our western counties as surface ridging begins to
build in by late day, which may allow temps to recover into the 60s.
Min temps Sunday night will be quite challenging due to continued
weakening of low-level cold advection on developing light southerly
winds due to rapid progression of ridge axis through CWA, as main
core of surface high slides to our south. Result may be limited
temp drop far west, while a quick drop central/east before leveling
off. The quick drop could see some areas down into the 30s before
generally rebounding to around 40 or in the 40s rest of Sunday night.

Monday and Monday night...
SW flow continues to increase during the day ahead of next cold front.
Should be set up for a decent mixing day and nice rebound on highs
well mainly in the mid to upper 60s with at least partly sunny skies.
Some lower PoPs are present later Monday night lingering into
Tuesday and seem prudent given concentration of moisture and lift
along mid-level frontal boundary. In wake of surface cold frontal
passage, will see brisk NW winds settle in attendant to strong CAA for
Monday night with lows dropping through the 40s.

Tuesday-Tuesday night...
General consensus is for a phasing of energy from the south with a
digging trough from the northwest, with this phasing likely to
occur to our east over portions of the Ohio Valley, Mid-Atlantic and
Great Lakes regions. Brisk NW winds of 20-30+ mph in lingering tight
pressure gradient and attendant strong cold push will knock temps
down by some 10-15+ degrees from Monday, with highs upper 40s and
lower 50s. Brisk winds will make it feel even cooler and more like the
lower to mid 40s making for quite the fall feel.

Wednesday through Friday...
This period looks to be quite challenging more-so than usual as
models struggling with timing of several amplifying shortwaves within
active and strengthening northern CONUS jet. Uncertainty also
compounded by potential impacts on Typhoon Lan on downstream pattern.
In general though the suggestion is for the potential of a couple
of additional cold frontal passages followed by quick bouts of
surface ridging during this time frame. This will lend support
to the typical fall roller coaster on temps, with potential for
decent mixing on gusty SW winds propelling highs at least in the 60s
in advance of the cold fronts, with temps then dropping back into
the 30s/40s in post-frontal CAA. Some rain chances will exist in this
regime, especially post-frontal being wrung out in CAA, but overall
not seeing signs for any significant moisture with any rain mainly light.

As mentioned above regarding Typhoon Lan watching the western Pacific
for big implications (colder) on our weather late next week and
especially next weekend. Often times a recurving typhoon in the western
Pacific, as the case with Lan, results in a buckling of the jet
orientation over North America, in turn leading to a central and
eastern US trough and trend toward colder conditions. Typically if
a system recurves over or east of Japan, as appears to be case with
Lan, this takes about 6-9 days to occur where as if a system were to
recurve further west toward China this may take about 2 weeks to develop.
In any event, the bottom line is trends support a transition to a colder
pattern next week and just beyond with increasing risk for a


.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.




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