Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 kdvn 201734 
afddvn

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1234 PM CDT Tue Aug 20 2019

Update...
issued at 1233 PM CDT Tue Aug 20 2019

Last night's mesoscale convective system has exited well to the east and south with a few
lingering storms along a lingering elevated warm air advection
axis over far northeast Missouri into west central Illinois early
this afternoon. This continues to trend east and southeast and
should be out of the area within the hour. Have introduced low
chance pops for a persistent, but recently weakening, linear
complex of storms moving south out of southeast Minnesota and western WI.
At the current rate, these would enter eastern Iowa around 2 to 3
PM.

&&

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, infrared 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 mesoscale convective system 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 County Warning Area. 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 mesoscale convective system compared to
the 3 km NAM and other href members.

Timing: the most likely timing for the mesoscale convective system to reach the western
counties is between 5-8 am, the Mississippi River corridor between
7-10 am, and then areas to the southeast 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 mesoscale convective system into
parts of east Iowa/northwest 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
east-southeast 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
County Warning Area 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 mesoscale convective system. Lower amounts are possible across the
north and northeast.

After the mesoscale convective system shifts east by late morning to midday we may have
to watch for a period of strong winds on the backside of the mesoscale convective system
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 east-southeast. 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 Continental U.S.. 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 18z tafs through 18z Wednesday afternoon)
issued at 1233 PM CDT Tue Aug 20 2019

VFR conditions are expected for this afternoon and evening. A weak
front moving out of Minnesota and WI may trigger scattered thunderstorms
that could affect the Cid, dbq and mli terminals by mid afternoon.
However, confidence that these will hold together and impact these
sites was too low to include mention in the forecasts. Overnight,
lingering low level moisture and clouds moving in behind the cold
front may lead to IFR conditions, which was included at all
sites. There should be an improving trend Wednesday morning, but
forecast confidence is low in the timing.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

National Weather Service Glossary of Abbreviations