Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 201106

Area Forecast Discussion
National Weather Service Quad Cities IA IL
606 AM CDT Mon Aug 20 2018


Issued at 339 AM CDT Mon Aug 20 2018

MSAS indicated a surface low pressure centered over northwest
Missouri. This was just east of a well defined upper level low
seen on water vapor imagery over SE corner of Neb. A surface warm
front was analyzed se and east of the low roughly along the I-70
corridor. A large area of rain was in the deformation axis north
of the low over most of central and western Iowa, while a narrower
axis of showers and isolated thunderstorms was in an area of
elevated thetae convergence from SE IA southward along the MS
River Valley. An axis of surface pressure falls over SW IA
supports the model advertised northward advancement of the surface
low, expected to lift into far southwest IA this morning, then
track eastward into west central IL by evening.


.SHORT TERM...(Today and Tonight)
ISSUED AT 339 AM CDT Mon Aug 20 2018

A strong summer storm system will spread rain and thunderstorms
across the forecast area today. Main challenges center on
placement of heavy rainfall likely with this system and
possibility of severe weather later this afternoon.

In the near term, the arcing band of showers and isolated embedded
thunderstorms, occurring with the thetae convergence in a
moisture rich airmass, will continue to advance northward through
the forecast area this morning. Limited elevated instability has
prevented any lightning from occurring up until roughly the past
half hour. This instability will increase today with the warm,
moist advection and resulting steepening thetae lapse rates. Upper
forcing ahead of the upper level circulation will enhance the
convection. A dry slot will likely then follow into the south
from mid morning onward, following the advancement of the
northward shifting surface warm front, causing the showers and
thunderstorms to be more isolated south, but remaining focused
north of I-80 late morning into afternoon.

The potential for severe weather will be primarily centered on the
enhanced convergence and enhanced low level shear near the warm
front in the afternoon hours. Otherwise, forecast bulk shear
values will be marginal for severe weather and the amount of low
level instability will be closely tied to any breaks in the cloud
cover. SPS has a marginal risk out for today for mainly east of
the MS river, where these aforementioned features may come
together for at least a small threat for supercells with a threat
of tornadoes and damaging wind. This will be monitored closely

Heavy rainfall will be possible with any thunderstorms today due
to the seasonably high PW feeding into the system. The synoptic focus
for heavy rainfall will transition north through the area by
early afternoon, then become focused further northwest and then
north, based on parameters in the 00z model run. Widespread QPF
over 1 inch looks possible for today through this evening over the
entire forecast area with heaviest amounts of 1.5 and above
likely along and north of the highway 20 corridor. With several
days since the last significant rainfall, flash flooding is not
expected, but there could be some drainage issues in urban areas
with heavy rainfall rates especially north I-80 and toward the
highway 20 corridor to watch for.

Tonight, the upper low passes eastward and opens into an open wave
by early Tuesday over the western Great Lakes. At the same time,
the surface low rapidly weakens and becomes less organized over IL.
This should focus lingering rain and isolated thunderstorms to a
weakening deformation axis across the north with scattered weak
showers ending elsewhere.

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 339 AM CDT Mon Aug 20 2018

Mainly dry until late week. Temperatures initially near normal
through Friday before going above normal for the weekend.

Assessment...medium to high confidence

The storm system exits the area on Tuesday. There may or may not be
some lingering rain east of the Mississippi during the morning
hours. If there is any rain it should be done by mid morning.
Tuesday afternoon looks dry across the entire area.

Right now the model consensus has slight chance to low chance pops
east of the Mississippi for Tuesday morning.

Tuesday night through Wednesday night
Assessment...high confidence

All models show high pressure moving through the Midwest resulting
in dry conditions for the area.

Thursday on...

Thursday and Thursday evening
Assessment...high confidence

The dry conditions will continue Thursday into Thursday evening as
high pressure moves into the Ohio Valley. Attention then turns to
the next storm system.

Late Thursday night through Friday night
Assessment...medium confidence

The global models show a seasonably strong system moving through
late Thursday night/Friday with it departing the area Friday night.
However, there are slight differences in timing and strength of the

The ECMWF/CMC global are similar on timing but differ slightly on
overall strength of the system. The GFS is slower on timing and
interestingly stronger than the ECMWF/CMC global.

Right now the model consensus has slight chance to chance pops west
of the Mississippi late Thursday night with chance to likely pops
Friday and Friday night.

Saturday through Sunday
Assessment...high confidence on warm temperatures, low on rain

Next weekend mainly dry and very warm conditions are expected across
the area. The global models do have weak upper level disturbances
moving through in the flow aloft. Given the expected above normal
temperatures the potential is there for diurnally driven convection
in the afternoon and evening. If this scenario does occur, 85-90
percent of the area would likely remain dry.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday Morning)
ISSUED AT 605 AM CDT Mon Aug 20 2018

Low pressure passing through the region will push a warm front
northward, causing conditions to deteriorate to MVFR with periods
of showers and isolated thunderstorms through the day. This
boundary could also trigger strong thunderstorms in the afternoon,
especially in the MLI vicinity. Conditions will likely improve to
VFR in BRL this afternoon. Tonight, the showers and thunderstorms
will end from west to east with low clouds and light fog expected
to bring periods of IFR conditions overnight at all sites.




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