120
FXUS63 KDVN 020902
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
402 AM CDT Tue Jul 2 2024

.KEY MESSAGES...

- Flash flood watch is in effect for the area this afternoon
  into evening. Very heavy rainfall is possible from these
  storms.

- Severe thunderstorms are possible today and tonight, with all
  hazards possible.

- Unfortunately, Independence Day is looking to be a wet day, as
  a warm front brings the potential for widespread showers and
  storms once again. Too soon to determine the severe risk.

- Moderate to Major River Flooding continues, with further
  changes to river forecasts expected as we transition from
  predicted rainfall to observed rainfall over the next 36 hrs.


&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 402 AM CDT Tue Jul 2 2024

We are sitting on the precipice of what could be a very
impactful flash flooding/severe event for the area. An inverted
trough located west of the area paired with a strong 50-60kt
LLJ was leading to showers and storms across the area. Moisture
transport with this jet was bringing large quantities of
moisture to the area. The wave that induced the LLJ will move
east of the area this morning as showers and storms either
fester or die out. Another stronger wave is expected to move
into the area this afternoon and evening. This inverted trough
will become a cold front as this wave approaches and will slowly
move SE across Iowa. Moisture will pool ahead of this front
leading to an even more saturated effective inflow layer. As the
LLJ increase with the wave, even more moisture will surge into
any ongoing storms. As far as moisture amounts go, the
deterministic models are all predicting over 2.5 inches this
evening. This would be above record PWAT values for our area!
The HREF, suggests 2 to 2.4 inch PWATs, especially across our
southern CWA. We have a lot of moisture around and all we need
are thunderstorms to turn this into heavy rainfall. Models have
this moisture and instability stopping near the river. I`m not
convinced this will happen and later shifts will need to
determine if the flash flood watch will need to be extended to
the east.

There remains questions on how today`s event will evolve. 1st,
will we see mainly re-development or festering of ongoing
convection today. If it`s redevelopment, the OFB from today`s
storms could wash out or move and thus the area for
redevelopment would be more difficult to ascertain. If it`s
festering, and this seems more likely due to no cap and weak LLJ
during the day, there should be a well-defined E/W thermal
boundary for storms to fire on. To make identifying that
location even more difficult are the differences in guidance.
The 06z HRRR says HWY 30 corridor is where this should occur.
The 06z NAMnest suggest redevelopment out west and turning
quickly into a line. The spectral models, except for the ECMWF
which agrees mostly with the NAMnest, has the heavier rain
across the HWY 20 corridor. I`m not confident where this will
occur at this time. However, we should have a much clearer
picture this AM when we can whats happening with the convection.
We should be able to nail this down better and should message
that.

Now on to impacts. Severe weather is possible and flash flooding
is looking better and better each time I look at it. Whatever
happens as far as evolution, supercells should start out first.
If they form out west, then its a line when it gets here. If
they form on the E/W thermal boundary two things are possible.
1. They will stay supercells longer and 2. if they latch on to
that boundary we may see an even greater tornado and flash flood risk
where that occurs. Now, I want to talk about the 06z HRRR. That
depicts a worst case scenario for both tornadoes and flash
flooding. The 06z HRRR has supercells that train and become
embedded in a line. This would favor a better tornado threat, in
fact the updraft helicity tracks support this. Also, if we have
rotating updrafts in any part of these storms, we can expect
even higher rain rates. The dynamic pressure forces caused by
the mesos induce even greater low pressure and upward motion.
This, if the 06z HRRR occurs, would be a high impact event with
tornadoes and flash flooding and would require a step up in
messaging. It`s one model run, but we need to understand the
environment is primed for that potential. Luckily the 07z HRRR
is more in line with the NAMnest, however, as I said before I`m
not confident on storm evolution at this juncture and until we
nail that down, we must respect this environment.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 402 AM CDT Tue Jul 2 2024

Previous discussion still looks good and is attached below.

A brief break in the wet weather is expected Wednesday, but the
emphasis is on brief, as a surface warm front passes north into
the area as early as late Wednesday night/early Thursday AM
with convection potential increasing (40-70%) south of I-80.
Then, the shower and storm potential increases over the rest of
the area on the 4th of July into Thursday evening, as a vigorous
shortwave trough evolves into a closed low while traversing
near the international border to the north. Ensembles point to
PWAT values of 2.00+ inches again pooling near and south of the
boundary supporting a heavy rain threat. Where this warm front
sets up will be critical to where this heavy rain threat
and potential severe weather threat eventually sets up, and
will bear watching in the coming days.

Next weekend the pattern looks to transition to a mean trough
over the north central U.S. Thus, the active pattern looks
to persist with additional shower and storm chances with
lower predictability on timing given the lower amplitude
shortwaves. Many periods have PoPs Saturday-Monday, but there
will likely be plenty of dry hours. Temperatures look to be
near normal.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1255 AM CDT Tue Jul 2 2024

Active period of showers and storms is expected for the next 30
hours across the area. Strong to severe storms are possible this
afternoon across the area. Wind gusts with the strong storms
tonight could bu in excess of 50kts. VFR conditions are expected
outside of storms. As rain/storms start MVFR vsbys and CIGs are
expected. In the strongest storms, IFR could be possible.

&&

.HYDROLOGY...
Issued at 425 PM CDT Mon Jul 1 2024

The river forecasts this morning included 48 hours of QPF in
them instead of the normal 24. This was done to minimize any
significant jumps in the crest forecasts later this week.
Tonight`s and the Tuesday morning river forecasts will have 36
and 24 hours of QPF in them respectively.

The flood crest on the Cedar River is near Conesville, IA and
will enter the Iowa basin Tuesday morning.

Discussion...

We are looking at a potential heavy rain event across the area
Tuesday into Wednesday with a bulk of the rainfall occurring
Tuesday night. There is some uncertainty where the heaviest
rainfall will occur. The moisture is being supplied from the
remnants of tropical storm Chris. However, at the same time,
there will be a dry layer of Saharan Dust closely following the
tropical moisture. Given the data available it appears that a
general 1 to 1.5 inch rainfall will occur with pockets of 2-4
inches across eastern Iowa. If the rainfall is lighter/heavier
than forecast, this will result in the eventual crest forecasts
being lower/higher later this week.

As a result of the 48 hour QPF used in the morning river
forecasts, the Wapsipinicon River near Dewitt is now forecast to
reach major flood stage late this weekend. Much of the
Mississippi River will reach major flood stage late this week
into next week. Additional rises are expected on the Iowa,
Cedar, English, Wapsipinicon and Maquoketa basins.

Manchester on the Maquoketa River is under a flood watch based
on the 48 hour QPF. Additional flood watches are possible over
the next 24 hours for parts of the Cedar, English and
Wapsipinicon Rivers if the overall forecasted QPF remains
fairly consistent.

Outlook...

As expected the MJO has turned more active over the Indian Ocean
and is currently in a phase 3 and is forecast to go into a phase
4 as it moves into the Maritime Continent. Phase 3 has a high
correlation with above normal precipitation across the upper
Midwest and mid-Mississippi Valley. While the correlation is
not as high, phase 4 does support a signal of above normal
precipitation across the mid- Mississippi Valley.

The current Climate Prediction Center outlook for the second
week of July has a 33 percent chance favoring above normal
precipitation.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Flood Watch from 3 PM CDT this afternoon through late tonight
     for IAZ040>042-051>054-063>068-076>078-087>089-098-099.
IL...Flood Watch from 3 PM CDT this afternoon through late tonight
     for ILZ009-015-016-024>026-034-035.
MO...Flood Watch from 3 PM CDT this afternoon through late tonight
     for MOZ009-010.

&&

$$

SHORT TERM...Gibbs
LONG TERM...McClure/Gibbs
AVIATION...Gibbs
HYDROLOGY...08