Severe Thunderstorm Warning 118, NWS Quad Cities, IA

Area Forecast Discussion
National Weather Service Quad Cities IA IL
644 PM CDT Sat Sep 1 2018

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 241 PM CDT Sat Sep 1 2018

MCS fueled by strong LLJ gradually weakened during the morning
hours, giving way to a mix of clouds and sunshine across much of the
region early this afternoon. Analyzed surfaces maps and observations
at 18Z showed a large area of high pressure over the southeast U.S,
which was keeping the Midwest under a warm, moist flow. At the
surface, a wavering frontal boundary stretched from western IA to
north central IL. Temperatures as of 2PM ranged from 77 in
Dubuque, to 86 in Fort Madison and Mount Pleasant.

&&

.SHORT TERM...(This Evening through Sunday)
ISSUED AT 241 PM CDT Sat Sep 1 2018

Challenging short term with the main concerns set on precipitation
and severe storm chances.

This evening, expecting additional showers and storms to fire up
across the region along the frontal boundary and remnant
boundaries from the morning MCS. The front is expected to set
itself up between Highway 34 and the I-80 corridor based on
current mesoanalysis of moisture transport, and current NAMnest
agrees by firing activity along a line from Williamsburg, IA to
Freeport, IL. Believe the HRRR and NAM are too far north with
development and QPF along and north Highway 20. After this moves
through, expect a LLJ to fire additional precipitation back to the
west over central IA around midnight, which will move east
through the night. With PWATs approaching 2.00" on forecast
soundings and precip moving over the same areas that saw heavy
rainfall last night, expecting the primary threats from showers
and storms to be additional heavy rainfall and isolated flash
flooding. Currently have total QPFs of 1-2 inches along and north
of I-80 to the Highway 20 corridor, but higher amounts are
possible with heavier showers/storms.

In addition, a window for severe storms exists this evening,
especially since skies have opened up early this afternoon. With the
clouds clearing, ample moisture and steepening lapse rates will
allow CAPE values to climb to or above 3000 J/kg. Combined with
forecast 0-6 km wind shear of 30-40 kts, storms that develop this
evening will have the potential to produce large hail and strong
winds. SPC maintains a marginal risk of severe storms through 12z
Sunday.

Expect a similar scenario Sunday as the frontal boundary moves
little and interacts with an upper level disturbance moving across
the region. SPC has a marginal risk of severe storms along and
north of the I-80 corridor, with threats of isolated large hail
and strong winds.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 241 PM CDT Sat Sep 1 2018

An active and wet pattern is expected to continue through the period
as abundant tropical moisture surges north into the area. While
overall pattern with H5 ridge axis may change in later forecasts,
the current set up is for heat, humidity and thunderstorms with
heavy rain.  This set up will likely exasperate any flooding
potential through the period.

Synoptic setup is for the subtropical high to be in place across the
eastern US.  To our west, a broad southwest flow will continue the
feed of moisture into the area as a weak frontal boundary will
remain across the area.  Weak waves with move through the flow from
the southwest as well as tropical waves from the the gulf of Mexico.
Each of these waves will bring chances for storms across the area.
With high PWATs, 90 percentile or better according to the NAEFS,
heavy rain looks likely.  In all likelihood, the daily occurrences of
storms will be driven by mesoscale features that will change on a
daily basis.  Overall confidence on placement of storms is low,
overall confidence on occurrence of storms is high.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday Evening)
ISSUED AT 641 PM CDT Sat Sep 1 2018

VFR to mostly VFR conditions with episodes of MVFR conditions the next
24 hours, mainly due to showers and storms. The highest risk is at
DBQ and CID terminals. Winds will be southerly at 5 to 10 mph.
Skies will tend to mostly cloudy north and mostly partly cloudy in
the south.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Flash Flood Watch through Sunday morning for Benton-Buchanan-
     Cedar-Clinton-Delaware-Dubuque-Iowa-Jackson-Johnson-Jones-
     Keokuk-Linn-Louisa-Muscatine-Scott-Washington.

IL...Flash Flood Watch through Sunday morning for Carroll-Jo Daviess-
     Rock Island-Stephenson-Whiteside.

MO...NONE.

&&

SYNOPSIS...Speck
SHORT TERM...Speck
LONG TERM...Gibbs
AVIATION...16

Hazardous Weather Outlook
National Weather Service Quad Cities IA IL
337 PM CDT Sat Sep 1 2018

IAZ040>042-051>054-063>068-076>078-087>089-098-099-ILZ001-002-007-
009-015>018-024>026-034-035-MOZ009-010-022045-
Buchanan-Delaware-Dubuque-Benton-Linn-Jones-Jackson-Iowa-Johnson-
Cedar-Clinton-Muscatine-Scott-Keokuk-Washington-Louisa-Jefferson-
Henry IA-Des Moines-Van Buren-Lee-Jo Daviess-Stephenson-Carroll-
Whiteside-Rock Island-Henry IL-Bureau-Putnam-Mercer-Henderson-
Warren-Hancock-McDonough-Scotland-Clark-
337 PM CDT Sat Sep 1 2018

This Hazardous Weather Outlook is for portions of north central
Illinois...northwest Illinois...west central Illinois...east
central Iowa...northeast Iowa...southeast Iowa and northeast
Missouri.

.DAY ONE...THIS AFTERNOON AND TONIGHT

Showers and thunderstorms are expected late this afternoon and
tonight across portions of the Outlook Area, especially along any
remnant outflow boundaries from this morning's storm activity.
Some of these storms may be strong to to possibly severe with
damaging winds the main threat, and large hail a secondary threat.
Of a more widespread concern will again be the potential for
heavy rainfall and associated flash flooding, especially across
areas that received heavy rainfall this morning. Some area
tributary rivers will also start to experience water level rises
from the run-off.

A Flash Flood Watch remains in effect for tonight for areas
generally along and north of a line from Sigourney Iowa, to the
Quad Cities, and to Sterling Illinois.

The Storm Prediction Center has placed a Marginal Risk for severe
weather over the entire area today and tonight.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY

Sunday through Friday, there is a chance of thunderstorms each
day. There is a Marginal Risk for severe weather Sunday generally
along and north of a line from Washington Iowa, to the Quad Cities,
and to Sterling Illinois. It is too soon to determine the risk of
severe weather after Sunday.

Heat and humidity will increase this weekend and into early next
week, sending heat index readings as high as the 90s during the
afternoon and early evening hours from Sunday through at least
Wednesday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this evening and tonight. Any
reports of flooding or flash flooding would be appreciated.

Speck/Gibbs

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Special Weather Statement
National Weather Service Quad Cities IA/IL
732 PM CDT SAT SEP 1 2018

IAC107-020100-
Keokuk-
732 PM CDT SAT SEP 1 2018

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT CENTRAL KEOKUK COUNTY
UNTIL 800 PM CDT...

At 732 PM CDT, radar indicated strong thunderstorms were located
along a line extending from near What Cheer to Hedrick. Movement was
east at 30 mph.

Winds in excess of 40 mph and half inch hail are possible with these
storms.

Locations impacted include...
Sigourney, Hedrick, What Cheer, Delta, South English, Martinsburg,
Hayesville, Ollie, Webster, Sig-Nor Field Airport, Lancaster, Keokuk
County Expo, Lake Belva Deer Park, Yenruogis County Park, Sigourney
Iowa Airport and Keokuk County Fairgrounds.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 4140 9241 4145 9209 4122 9201 4116 9225
      4116 9241
TIME...MOT...LOC 0032Z 252DEG 27KT 4136 9241 4117 9234

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