Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 181718

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1218 PM CDT Fri May 18 2018


Issued at 325 AM CDT Fri May 18 2018

Large expanse of high pressure noted from Quebec into Ontario
early this morning, with the southwest flank of this high
pressure extending across the western Great Lakes. Resulting
easterly low level flow is ushering in a lake enhanced frontal
boundary via the backdoor, denoted by pronounced low level
drying (surface dew points lowering into the 30s/40s over WI
into N IL). GOES-16 satellite imagery shows thicker high
cloudiness working across southern and eastern portions of the
CWA early this morning, with regional radar mosaic depicting a
few spotty light showers from southern MO into IN. The clouds and
showers were rotating N/NW on backside of a slow moving upper
level low over the Lower Mississippi Valley that is sandwiched in-
between ridging over the Great Lakes and Central Plains. This
devolving sandwiched upper low will be in close enough proximity
owing to spotty shower chances next 24 hours.



.SHORT TERM...(Today and Tonight)
ISSUED AT 325 AM CDT Fri May 18 2018

The upper level low over southern MO will weaken - devolving to
an open wave while lazily lifting into the eastern Cornbelt Region
by Saturday morning. We will reside NW of this system, but prone
to bouts of cloudiness and a few spotty showers developing or
pinwheeling back N/NW into portions of the area through tonight,
with locations from around the Quad Cities south and east appearing
most favored for such activity. That said, CAMs and deterministic
models remain in poor agreement on coverage of showers and just
how far north/west they spread. Overall, I have kept with previous
forecast of low end chance PoPs due to uncertainty and also given
likelihood of battling drier east-northeast low level flow, which
the American models may be underplaying at least in the short term.
Tonight however, there is a signal for better coverage of showers
over eastern IL into WI with vort max and attendant surge of mid
level theta-e advection. Will have to monitor for raising PoPs in
our eastern counties. Thunder potential looks rather low with any
activity today or tonight, with generally weak lift and minimal
elevated instability as 700-500 mb lapse rates less than 6.5 C/KM
considered fairly weak for a moist parcel.

Temperatures today are somewhat challenging with cloud cover and
easterly low level flow perhaps gusty at times with more solar
insolation and deeper mixing. Progged low level thickness and
verification of highs yesterday would yield generally mid 70s to
lower 80s, and then tweaked from there to account for thicker bouts
of cloudiness at times and possible spotty precip mainly south/east.
Low tonight look to be in the 50s to around 60.


.LONG TERM...(Saturday through next Thursday)
ISSUED AT 325 AM CDT Fri May 18 2018

Saturday...Latest suite of 00z run models generally suggest a bout
of upper ridging between waves will occupy the DVN CWA on Sat. Not
much signs of any lift or forcing for any organized precip through
00z Sunday anyway, and will maintain a dry fcst for now even though
a few solutions such as the 00z ECMWF, suggest some isolated to wdly
sctrd showers popping up in daytime heating. Mixture of clouds and
some sun, with widespread highs in the upper 70s to low 80s.

Several convective parameters come together Sat afternoon and early
evening just to the lee of next ejecting upper wave acrs
northeastern KS into NE, far northwest MO and into southwestern IA.
Southwesterly 25 to 35 KT LLJ will develop diurnally and possibly
help feed an initial portion of this MO RVR Valley activity into the
southwestern CWA as a strom cluster or even small MCS toward
midnight, but more optimum timing seems to be shaping up to be
mainly after midnight into early Sunday morning mainly acrs the
south half of the CWA. The main sfc front is advertised by both the
00z GFS and ECMWF to stay down acrs northern MO, with east to
northeasterly sfc flow being maintained acrs most of the local fcst
area into Sunday morning. Thus the incoming activity will look to be
elevated in nature. The models keep upper ridging acrs IL into the
southwestern GRT LKS, and the incoming convection may weaken as it
propagates eastward. But saying that, can`t rule out at least an
elevated hail threat in the western/southwestern CWA. A low chance
for some of this activity to get sfc rooted for a damaging wind
threat in the far southern CWA by early Sunday morning...better
chances for this well off to the southwest of the area deeper into

Sunday...A low confidence day with a muddled outlook and several
possibilities suggested by the 00z model runs. The upstream east
central plains wave will look to try and eject out toward the GRT
LKS, but lee side upper ridge axis is advertised to maintain along
and just east of the DVN CWA hampering the process. A weak sfc wave
will form acrs the lower MO RVR Valley and try an roll toward north
central IA through Sunday evening, but may have trouble doing so
with western GRT LKS LLVL high pressure squeezing east flow back
acrs eastern IA. Much will depend on extent of convective debris
lingering out of Sat night, and the pull of the llvl wave on the sfc
boundary for a retreat northward some for new storm development
potential later Sunday into Sunday evening.

The 00z GFS is weaker with several of these scenarios and develops
stronger convection Sunday afternoon from the lower MO RVR
valley/northeast KS into NW MO again, and then down along the front
that is quasi-stationary acrs northern into central MO(thus mainly
southwest and south of the local area). The DVN CWA would just have
secondary elevated showers and some storms, while the stronger
convection south propagates toward southern MO and the western OH
RVR Valley as an MCS. The 00z ECMWF is a bit more bullish with the
sfc low and northeastward progression acrs north central IA. It also
suggest a potential triple point lifting up acrs the DVN CWA for a
focus for stronger convective development Sunday afternoon/evening
if we can get adequate heating. Besides at least a marginal severe
storm threat with this further northeast scenario, signs of a low
level jet fed PWAT plume of 1.4 to 1.7 inches would fuel locally
heavy rainfall under any storm cluster. All kinds of temperature
possibilities for highs on Sunday depending on boundaries, convective
debris and sfc flow, and thus a large bust potential. Will go with
an all model blend for now.

Monday through next Thursday... longer range model trends are
showing western CONUS blocking upper trof maintaining for much of the
upcoming week, with lee side ridging and split flow occupying the
mid into eastern CONUS. Under these conditions, the medium range
models suggest almost a daily chance of precip developing in or near
the area, but also plenty of dry hours. Temps to range from near to
above normal into late next week, with the GFS warmer than the ECMWF
MOS temp guidance.    ..12..


.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
ISSUED AT 1215 PM CDT Fri May 18 2018

Partly to mostly cloudy skies with VFR conditions will become mostly
MVFR and possibly high end IFR conditions as ceilings drop by late
evening and overnight to 1-3K AGL. Some 500 to 1000 ft ceilings are
possible with visibilities AOB 3 miles possible toward morning. East
and northeast winds will become northerly by morning at 5 to 10 mph.





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