Special Weather Statement, NWS Quad Cities, IA

Area Forecast Discussion
National Weather Service Quad Cities IA IL
315 AM CDT Tue Aug 20 2019

.SYNOPSIS...
Issued at 300 AM CDT Tue Aug 20 2019

High pressure beginning to shift into the Western Great Lakes has
led to return flow and increasing theta-e advection through
western and central Iowa. Over the last few hours, IR imagery has
shown explosive development of elevated convection across western
Iowa in a region of low-level jet induced isentropic lift and
convergence. MUCAPE is this area is around 4000 J/kg. Locally, for
eastern Iowa and northwest Illinois, it has been a muggy but dry
night with temps and dewpoints in the mid 60s to low 70s.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 300 AM CDT Tue Aug 20 2019

Today

Thunderstorm Potential: There is an Enhanced risk for severe
thunderstorms this morning roughly in the counties along and south
of I-80. An organized MCS or squall line is expected to quickly
move through about the SW half of the forecast area. The
stratiform comma-head precip shield with embedded thunder will
likely extend into parts of the northern CWA. The primary threat
is damaging straight- line winds of 50-70+ mph with the highest
gusts from bowing segments or surges in the line. Large hail is
also possible in embedded supercell structures. The HRRR seems to
be a little too far north with the track of the MCS compared to
the 3 km NAM and other HREF members.

Timing: The most likely timing for the MCS to reach the western
counties is between 5-8 AM, the Mississippi River corridor between
7-10 AM, and then areas to the SE in the 8-11 AM window.

Environmental Setup: The atmosphere will become primed for a severe
thunderstorm complex to track from central Iowa into west-central
Illinois this morning. Storm clusters will initially form in a
region of elevated instability across central Iowa on the
periphery of a 30 kt low-level jet early this morning. Continual
theta-e advection manifest as a surge in both surface- based and
elevated CAPE, will provide the fuel to sustain a mature MCS into
parts of E Iowa/NW Illinois. While the deep layer shear isn't
terribly strong (at 30 kts or less), moderate to high instability
and more than sufficient 0-1 km shear of 20-25 kts will foster
robust convection and a sustained/balanced cold pool.

For storm motion, the instability gradient, 1000-500mb thickness
contours, and forward-propagating Corfidi vectors all support an
ESE trajectory with the line. The tornado risk is low because 0-1
km shear vectors are below 30 kts, but an isolated spin up
tornado is possible. Again, this is mainly a wind threat event,
with some opportunity for large hail especially across the western
CWA where residual supercellular structures may still be intact.

Rainfall amounts shouldn't get too out of hand given storm motion of
40-50+ mph. However, PWATs around 2 inches will lead to extremely
heavy downpours with hourly rates over 2 inches a possibility. For
event totals, thinking 0.50 to 1.50 inches for most areas that
get impacted by the MCS. Lower amounts are possible across the
north and northeast.

After the MCS shifts east by late morning to midday we may have
to watch for a period of strong winds on the backside of the MCS
due to a wake low/meso high tight pressure gradient. It will be
very warm and humid during the afternoon with peak heat indices in
the 90s across the west and south.

Tonight

Majority of the night will be quiet, but another complex of
storms may develop to our west across eastern Nebraska and western
Iowa and then track to the ESE. Models varying quite a bit on the
timing of this round of storms, which could hit the SW forecast
area prior to 6-7 AM. Right now the consensus is near or after 7-8
AM. This time tracking a little more to the southwest than this
morning's round. Uttech

.LONG TERM...(Wednesday through Monday)
ISSUED AT 300 AM CDT Tue Aug 20 2019

There are timing and placement issues through the long term period.
The main forecast concerns are chances of precipitation through the
period.

The long term period is dominated by a progressive zonal flow across
the northern CONUS. The long term period opens 12 UTC on Wednesday
as a piece of shortwave energy dig southward into the base of a
trough over southeastern Canada that digs into the northeast US.
This energy will phase with another shortwave moving eastward across
Iowa and drive a cool front across the area Wednesday morning. The
NAM has come in wetter and farther to the north with the latest run
which has expanded POPs northward across the area in the forecast.
THe GFS, ECWMF, and Canadian limit convection to south of I-80
during the day on Wednesday.

Disagreement between the models on the Thursday to Friday time frame
as to how far south the Wednesday cool front sinks into Missouri.
The GFS pushes the front south of Interstate 80 with any
precipitation in the region closer to the frontal boundary while the
Canadian and ECWMF keep the front further to the north and lingers
precipitation across northern Missouri and southern Iowa through the
day on Thursday. The Canadian and ECWMF solution seems to be more
likely as the front will be parallel to the the winds aloft and that
will slow its southward progression. This will help to keep the
chance of showers and thunderstorms across southern Iowa and
northern Missouri into Thursday.

Lowered chances of showers and storms on Friday and Saturday as
models showing high pressure lingering over the Great Lakes with
high temperatures in the upper 70s and dewpoints in the upper 50s to
lower 60s. Friday and Saturday look to be pleasant days.

Another 500 HPA trough will move into the Canadian Prairie Provinces
and Upper Midwest during the day on Sunday. At this point, the GFS
develops northwesterly flow aloft as a ridge builds in the Pacific
Northwest while the ECWMF leaves the ridge off shore and develops a
zonal flow across the region. Either solution results in several
prices of shortwave energy moving across eastern Iowa, northwest
Illinois, and far northeast Missouri. This will result in daily
chances of precipitation with the best chances on Sunday. Monday
through Tuesday will usher in temperatures warming into the mid 80s
with dewpoints back into the mid 60s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday Night)
ISSUED AT 1232 AM CDT Tue Aug 20 2019

Currently VFR conditions at all TAF sites early this morning. A
line of thunderstorms is forecast to move southeastward across
eastern Iowa from approximately 11 to 15 UTC this morning. Expect
MVR ceilings and visibilities with periods of IFR to as low as
LIFR with the strongest storms. The highest winds associated with
these storms are expected at KCID and KBRL. Ahead of the
thunderstorms fog of 3 to 5 miles will be possible at KMLI, KDBQ,
and KBRL. Conditions will quickly improve to VFR in the wake of
these thunderstorms.

&&

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

SYNOPSIS...Uttech
SHORT TERM...Uttech
LONG TERM...Cousins
AVIATION...Cousins

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Special Weather Statement
National Weather Service Quad Cities IA/IL
339 AM CDT Tue Aug 20 2019

IAC011-095-200915-
Benton-Iowa-
339 AM CDT Tue Aug 20 2019

...A STRONG THUNDERSTORM WILL AFFECT SOUTHERN BENTON AND NORTHERN
IOWA COUNTIES UNTIL 415 AM CDT...

At 338 AM CDT, a strong thunderstorm was located over Belle Plaine,
or 12 miles northwest of Marengo, moving east at 30 mph.

Winds in excess of 40 mph and nickel size hail are possible with this
storm.

Locations impacted include...
Marengo, Belle Plaine, Atkins, Newhall, Blairstown, Norway, Luzerne,
Amana, Middle Amana, South Amana, Homestead, West Amana, Upper South
Amana, Hannen Lake Park, Watkins, Koszta, High Amana and Iowa County
Fairgrounds.

LAT...LON 4197 9230 4200 9186 4173 9186 4177 9230
TIME...MOT...LOC 0838Z 270DEG 27KT 4187 9229

Cousins