023
FXUS63 KDVN 241107
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
607 AM CDT Mon Jun 24 2024

.KEY MESSAGES...

- Dry conditions today overall, save for a low chance of showers
  and storms early this morning and again late tonight, some of
  which could be strong to severe tonight

- Heat and humidity increases today, with heat indices well into
  the 90s to nearly 100 this afternoon for our southwestern half
  of the area

- Very conditional forecast is on tap for Tuesday, with another
  round of severe weather, hot and humid conditions, and flash
  flooding all dependent on how things play out tonight

- Quiet conditions expected Wednesday and Thursday before
  another round of storms moves in for Friday and Saturday

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 409 AM CDT Mon Jun 24 2024

An area of high pressure currently over our region will gradually
shift eastward with time this morning. Southerly return flow will
develop in its wake today, which will lead to increasing heat and
humidity across the area later today. Early this morning, we are
watching a cluster of thunderstorms that has developed over
southwestern Iowa. This appears to have developed in a corridor of
850-700 mb layer warm air advection, ahead of a nocturnal low-level
jet. CAMs largely keep this activity out of our forecast area, save
for our far southwestern areas, so don`t be surprised if a storm or
two moves through that area later this morning. Otherwise, the heat
and humidity will be the main story during the daylight hours today.
850 mb temperatures look to increase pretty steadily to around 20 to
22 degrees C per the GEFS ensembles. Additionally, dew point
temperatures should increase back to the upper 60s to lower 70s,
resulting in heat index values into the 90s to nearly 100 degrees in
our far southwest. I wouldn`t be surprised if some isolated
locations see heat indices reach 100 degrees, but the more robust
warm air advection doesn`t move through until Tuesday, and
confidence is higher that it will only be a brief bout of ~100
degree heat indices, which precludes a Heat Advisory for today.
However, we did decide to use a Special Weather Statement to draw
attention to the sub-Heat Advisory level heat and humidity. Likely
our entire southwestern forecast area sees 95+ degree heat indices.

Attention then turns to the potential for strong to severe storms
late tonight into early Tuesday morning. This is one of the more
uncertain forecasts I`ve had to work in a while, as a few scenarios
could play out. Looking at the big picture, we are watching a
strengthening upper-level jet that will be situated along the
US/Canada border, with a mid-level impulse translating over
Minnesota by sunset this evening. A complex of thunderstorms, likely
as an MCS-type system, appears to dive southeastward through the
overnight hours along the instability gradient. The latest suite of
CAMs don`t provide much clarity on how this MCS will evolve, with
some keeping it just northeast of our forecast area or clipping us.
Complicating the convective picture is a mid-level shortwave that
will eject from the Intermountain West region, and translate towards
western Iowa before sunrise Tuesday. Some guidance has another
complex of storms developing in this area, which may move through
our southwestern areas as well. All of this to say that there
remains high uncertainty on how things will play out. A fairly
strong southwesterly low-level jet is likely to be in place during
the early morning hours Tuesday, with values between 35 to 45 knots
per the HREF ensemble mean, so there will be support for convection
in our region. We will have to keep a close eye on this activity as
the latest guidance comes in. Given the uncertainty, SPC has
introduced a fairly large Slight Risk (level 2 of 5) for areas along
and north of Highway 34, with Marginal Risk (level 1 of 5)
elsewhere. With the LLJ in place, and MUCAPE on the order of 1500 to
2500 J/kg per the HREF, damaging winds will be the main threat,
along with large hail.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 409 AM CDT Mon Jun 24 2024

Tuesday will be the day to watch for several hazards, as a surface
cold front slowly sinks southward behind the departing MCS system.
The nocturnal LLJ will gradually veer with time and dissipate, with
the flow becoming more west to east. Forcing for ascent will
maintain along the baroclinic zone/convergence area, as well as
additional shortwave energy moving through. With the enhanced
convergence, chances of thunderstorms will continue throughout the
day Tuesday. Additionally, any outflow boundaries that develop from
previous convection will play a role in convective initiation
location, which these details won`t be ironed out until after we see
how the storms behave late tonight. Heat and humidity will stay with
us, with highs likely into the lower to middle 90s, and heat indices
in the upper 90s to lower 100s for several locations. This should
result in oodles of CAPE, with both the GEFS and ECMWF ensemble
probabilities of 3000 J/kg or greater around 50 to 80% for Tuesday
afternoon ahead of the front. Additionally, effective shear is
expected to increase as the mid-level jet sinks southward, resulting
in values around 30 to 40 knots. This convective parameter space
should support the chance of storms becoming strong to severe, with
damaging winds and large hail being the main threats. PWATs will
also once again be quite high, with values progged around 1.8 to 2.1
inches. If PWAT values exceed 2 inches for our evening balloon
launch Tuesday evening, that would be a top 15 highest PWAT value
for the month of June! One major concern I have is the threat for
flash flooding, given these very high PWAT values, along with the
Corfidi forward propagation vectors oriented from west to east,
parallel to the boundary, which would support training convection.
With all of this said, how things play out will be highly dependent
on how the late-tonight storms evolve with time, since a more active
early Tuesday morning could alter the environment to affect how
things play on later on Tuesday. This would not only have impact on
severe potential, but rainfall/flash flooding and heat index values
too, so again a highly conditional forecast here, with lots of
uncertainty. In any case, SPC has our entire region highlighted in a
Slight Risk (level 2 of 5) for severe weather, while WPC has much of
our region highlighted in a Slight Risk for heavy rainfall. This
activity will lingering into the overnight hours, so things won`t
end after sunset Tuesday!

As we go into the day Wednesday, any lingering showers or storms
should diminish from increased subsidence from an area of high
pressure moving into the area. This high pressure system will remain
in control through the day Thursday, resulting in more seasonal
temperatures for both Wednesday and Thursday, with highs in the
lower to middle 80s. Friday and Saturday could be active, with more
chances of showers and storms.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 607 AM CDT Mon Jun 24 2024

VFR conditions are expected throughout the TAF period. The only
exception might be late tonight for DBQ, when a low chance for a
thunderstorm is possible, which could result in brief MVFR
conditions. Otherwise, BKN to SCT mid-level clouds will stick
around over the next several hours. Southerly winds generally
around 10 to 15 knots will increase late this morning into the
afternoon, with gusts between 20 to 25 knots expected at CID. A
period of low-level wind shear will develop around midnight
tonight as a 40 to 50 knot low-level jet develops.

&&

.HYDROLOGY...
Issued at 929 PM CDT Sun Jun 23 2024

No significant changes for the Cedar River basin this evening
with the new forecasts. The rise on the Cedar River at Cedar
Rapids has been slowed some and still may reach 14 ft or
Moderate flood stage early Friday.

For the Mississippi, flood watches have been converted to flood
warnings for Dubuque Lock and Dam, Camanche, and Rock Island
Lock and Dam sites this evening. Additional warnings can be
expected in the coming days.

.Previous Discussion.

The Mississippi River is expected to reach flood stage at
multiple locations, including the Rock Island gage, by the
middle of this week. Long term we expect the Mississippi to
reach Moderate and/or Major flood stages the week of the Fourth
of July, with the crest at the Rock Island gage possibly
reaching just over 18 feet early on the Fourth of July.
However, a larger margin of error should be taken into
consideration when forecasting precipitation amounts and
locations in the 7-10 day timescale. Watches are currently out
for most of the Mississippi River and warnings will follow in
the next few as a clearer picture is drawn on what
precipitation and routed water we have coming downstream. In the
latest forecast package, we have opted to upgrade Gladstone and
Burlington to a River Flood Warning, aided by the recent
rainfall north of the area.

The Rock River is also expected to reach flood stage in Joslin,
with an increase noted into Action Stage at Moline. A River Flood
Warning has been issued for Joslin in this latest forecast
package, which is forecast to hit flood stage tomorrow
afternoon. We will continue to see additional rainfall through
this coming week, which may lead to further fluctuations in the
river levels. If heavy rainfall is seen, then increases can be
expected.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...Schultz
LONG TERM...Schultz
AVIATION...Schultz
HYDROLOGY...Gross