Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 241153
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
653 AM CDT Thu Jun 24 2021

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 242 AM CDT Thu Jun 24 2021

A broad strong warm advection zone is found over the Midwest, and
with that the evening storms have evolved into an intense
destructive wind producing MCS, that at least on radar signature,
suggests this could be a long lived wind damage event.  I`m holding
off jumping on any great wind threat this morning until we actually
see it evolve, but this is quite impressive near and north of Omaha
as of 2 AM.

MU CAPE in the vicinity of the Omaha cluster is near 4000J/KG, while
farther east into our CWA, we`re running 1000-2000.  There should be
a tendency for that complex to drive southeast with time, but
limiting that so far is a mid level cap, with 700mb temperatures of
14-16C found near the south flank of the bowing segment. That cap
is probably the most important feature to monitor through the
evolution of the severe MCS, and whether any wind can impact our
CWA, which has no surface instability as of 2 AM.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 242 AM CDT Thu Jun 24 2021

1. Thunderstorms with torrential rain are going to take place in and
near our CWA through the next 24+ hours.

2. There are as many model solutions on convective handling of the
current cluster as there are models, and that is not helping any
confidence on where these storms go.

3. If the storm cluster does not turn southeast, our low levels are
not currently supportive of a damaging wind, but that could change
through the day as we warm up, so this is not an automatic switch to
a high end event, it has a likelihood to become more elevated with
time as it heads into drier air.

As mentioned above, there is a pronounced cap, of 14-16C around the
700mb level, found near the south flank of the western convection.
The wind in that layer is westerly, and the cap could advect east
with time. That will be something we can monitor through 5 AM and
can  adjust the forecast upward with respect to impacts should the
storms keep an easterly movement( at 65 mph currently).  At the very
least, should the storms drop southeast, we will have to contend
with the strong MCV moving through Iowa through this afternoon, and
that could serve to trigger additional storms over the CWA. That is
something the ECMWF continues to show in, as well as hinted at by
the UKmet.

I will continue to keep pops likely to categorical this morning, as
even if the event drops just south of the CWA, there should be a
warm advection wing of storms threat spreads through the CWA, and
possibly a large stratiform region too. There` a lot of reasons for
rain today!

Tonight, models are hinting at redevelopment in an west-east band,
near or south of the Interstate 80, then that would become an MCS
overnight. The placement of that feature is greatly dependent on
what happens with storms today, and capping placement in the south.
I`ll continue to forecast high pops south and solid chance pops
north for that event as well, but at least for now, the heavy rain
component is not confident on placement for a flash flood watch in
the near term.

We will be messaging the Enhanced Risk from SPC today, which for
now is the far southern CWA, with Slight risk elsewhere. WPC has
moderate risk for flash flooding mainly south of our CWA with
slight risk over the entire area.

ERVIN

.LONG TERM...(Friday through Wednesday)
ISSUED AT 242 AM CDT Thu Jun 24 2021

Key Messages:

1) SPC issued a Slight Risk for severe weather on Friday in our
south and a Marginal Risk elsewhere, with isolated large hail and
severe winds gusts being the main threats.

2) Very wet period to end the week and start the weekend, with WPC
issuing a Slight Risk for Excessive Rainfall.

3) Daily chances for showers and storms through next week.

Friday/Saturday...

Early on Friday, a warm front will be situated over the southern
half of our forecast area, which will act as a focus for heavy rain
and thunderstorms. Based on past model runs, it would seem that the
front does not push as far north, generally keeping it along and
south of I80. This would be due to a slightly different track of the
surface low, with latest guidance having it traverse northeast
through our southern half of our forecast area. With this in mind,
the focus for the heaviest rainfall looks to coincide within the
same area. The reason for the heavy rainfall leads us to the amount
of moisture found within the vertical profile, which is well above
the 90th percentile of values seen during this time of the year
(PWATs ranging from 2-2.5"). With these conditions in mind, guidance
wants to hint at repeating thunderstorm development. The Weather
Prediction Center has most of the forecast area under a Slight Risk
for Excessive Rainfall on Friday, therefore heavy rainfall can be
expected throughout. Thursday will be the start of the heavy rain
potential, but will continue through Friday, as the approaching
surface low leads to increasing convective coverage.  Many of the
rivers within our area are well below normal, which allows us for
some room to fill them up. Current outlooks should support within
bank rises, leading to more of a flash flood threat, especially with
local streams, creeks, or small rivers. Along with flooding
concerns, there is also a severe weather threat. The Storm
Prediction Center has issued a Slight Risk for Severe Weather south
of Highway 34, with a Marginal Risk throughout the remainder of the
area. Some uncertainty still exists with the severe potential, but
there will be and isolated risk for large hail and severe wind gusts.

Late Friday night and into Saturday morning, the initial wave will
be out of our area, with another warm front pushing north into our
area again through Saturday morning. The front is associated with
another surface low that develops with the next upper wave moving
through. Behind this warm front lies another moist and unstable
atmosphere in the warm sector. Therefore, further thunderstorm
development is expected ahead of the cold front that slides through
as the surface low passes through our north. At the moment,
extremely heavy rainfall is not represented in the guidance, but any
thunderstorm that develops has the potential to accumulate rain
quickly. With the area already worked over by several rounds of rain
and thunderstorms, the additional rainfall will lead to further
concerns for flash flooding. Please continue to monitor our future
forecasts as more details come out and confidence increases on
coverage and total rainfall.

As of right now, we are looking at the potential for between 3-6" of
rainfall through Saturday, with the highest accumulations located
along and south of I80. While these are our current forecast
amounts, fluctuations are likely, as some guidance hints at the
potential for more than that. With the amount of cloud cover that we
will likely experience in the coming days, the forecast high
temperatures may be lower than currently advertised, bringing us
below normal for this time of the year. Continue to monitor for
future updates on possible rainfall totals.

Sunday...

With there being a deep upper ridge over the west coast, we will be
left in southwesterly flow behind the waves that have passed by at
this point. While long term guidance wants to hint at most of the
forecast area staying dry on Sunday, they indicate the slight chance
for some showers and storms trailing the cold front that passed
through on Saturday, especially in our far southeast. The Canadian
model is the only one to hold precipitation for much of the area on
Sunday, with a cold front that is slower to move out of the area.

Next Week...

While much uncertainty exists beyond Sunday on the exact upper level
pattern, one thing can be relied on. The west coast ridge continues
to build, with a trough digging in from the north. With this setting
up, it would look like we fall into a brief omega blocking pattern,
which would leave us under the trough through most of the week. With
that in mind, we will fall into a pattern of daily chances for
showers and thunderstorms throughout the area. It would seem that
the chances will be more isolated/scattered, not as widespread as
this weekend is forecast. Temperatures are looking to continue
trending near or slightly below normal for the week. I suppose the
good news for next week will be that we will be dealing with some
more dry periods than wet, which will be much needed after the
upcoming heavy rain event.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday Morning)
ISSUED AT 617 AM CDT Thu Jun 24 2021

A weakening line of thunderstorms is moving across Iowa as of 11Z,
with an area of rain trailing the storms. This will move through
eastern Iowa this morning, and could cause a few rumbles of
thunder and a brief downpour as it moves through. Cigs may lower
to 3000 to 4000 ft in this rain. Early afternoon and evening will
see VFR weather with a return to move storms late, but the
placement of these strong storm is uncertain. The most likely area
is south of interstate 80, but it`s possible in all locations this
evening. A prob30 group has been included for thunder at CID with
MLI and BRL seeing a definite condition. Gusty winds and hail are
possible with storms tonight, along with torrential rainfall, and
LIFR conditions.

&&

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

$$

SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...Gunkel
AVIATION...Ervin


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