445
FXUS63 KDVN 220240
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
940 PM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Probability of storms producing heavy rain and flash flooding
  late tonight into Saturday morning has increased especially if
  storms repeat over the same areas.

- Shower and storm chances over the next 24-30+ hours, peaking
  for many later Saturday afternoon/evening.

- Potential for strong to severe storms increases Saturday PM/
  evening. Damaging wind is the primary hazard, with large hail
  a secondary threat. Heavy rain and localized flash flooding
  also remains a threat.

- Hot and humid once again on Saturday with PM heat index
  readings in the 90s to around 100 from around Hwy 30 on south.

- Dry conditions return for Sunday and generally remain through
  much of next week, outside of another round of storms
  Tuesday and Tuesday night.

&&

.UPDATE...
Issued at 844 PM CDT Fri Jun 21 2024

A flood watch has been issued for the Highway 20 corridor and
from Benton County east to Jackson County through 1 PM Saturday.
Confidence is at 50 percent that heavy rainfall will be seen in
the northern parts of the Highway 20 corridor counties.

Real data shows a weak outflow boundary from earlier storms is
now in the Highway 20 corridor west of Dubuque. Surface wind
speeds indicate this boundary will likely remain in place
through the night.

00z DVN sounding has a PWAT of 1.76 which is roughly the 95th
percentile and about 0.10 from the max. Additionally, there is a
very healthy tropical connection of deep moisture from Texas
that runs around the west side of the high in the lower
Mississippi Valley. Other short term model guidance is
suggesting the development of a boundary in the Highway 20
corridor during the night and into Saturday morning.

MCS tool has been slowly trending south the past 4-5 hours and
now suggests storms with the potential for very heavy rain in at
least the Highway 20 corridor. More disturbing are the Corfidi
vectors. They have slowly become perpendicular to each other
with a period of opposition prior to sunrise. If correct, storms
would have a tendency to repeat over the same areas with
potentially a very slow (10-15 mph) eastward speed.

Some other heavy rain tools suggest the bulk of the heavier
rainfall would remain well north of Highway 20. However, the
conceptual picture is beginning to resemble a Maddox synoptic
flash flood event with an east/west frontal boundary where the
heaviest rainfall can fall along and on the warm side of the
front.

Given the very high PWATs, rainfall rates may approach 2 inches
per hour. Flash flood guidance for the area is generally around
2 inches or just under it for both the 1 and 3 hour time
frames.

Rainfall totals by Saturday morning are expected to vary
considerably. Some locations in the watch area may see not much
rainfall. Other areas potentially could see 3-4 inches or more.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 340 PM CDT Fri Jun 21 2024

Semi-zonal to WSW flow aloft dominates the region early this
afternoon, with a trough over the Intermountain West and a a
large ridge /heat dome/ over the south-central/southeast U.S.
A frontal zone extends from a Lee low across Nebraska and
northern Iowa into southern Wisconsin. This boundary continues
to be the focus for showers and storms. A weak wave in the
flow and diurnal heating was aiding widely scattered convection
from far northern Missouri into central/southern Iowa this
afternoon. As this translates east-northeast we`ll continue to
see the potential for widely scattered activity develop into
our service area through this evening. Weak effective shear and
mid level lapse rates should preclude any severe weather potential
with this activity. Then turning toward later tonight, the
focus will be watching convective trends to our north in a
synoptic setup/pattern that resembles Maddox conceptually with
heavy rain/flash flood threat with a stalled front to our north,
and an anomalously deep moisture axis with PWATs near 2 inches
pooling near the boundary, and additional lift from a shortwave
emerging from the Intermountain west and a ramping nocturnal LLJ
30-35+ kts. In the Maddox synoptic pattern conceptual model the
heaviest rains usually occur on the warm side of the front.
Despite the deterministic and CAM guidance largely keeping the
heaviest rains just to our north, given this conceptually
brings some concern that this heavier rain axis could develop/
shift further south and possibly into northern portions of the
service area late tonight and Saturday morning with aid of outflow
propagation. Confidence is not high enough to issue a Flood
Watch at this time for our northern counties, but the evening
shift will monitor convective trends and re-evaluate this potential.

Saturday`s forecast could potentially be rather challenging with
respect to high temperatures and subsequent severe weather
threat/location pending convective and cloud cover trends through
the first half of the day. One scenario possible is if there is
more coverage of storms and cloud cover throughout Saturday AM
across our north, then the severe weather threat would likely
shift into our southern service area and further south. Highs
would be held down in the upper 70s and 80s north, with 90s
south of I-80. The other scenario would be one in which there`s
limited coverage of showers and storms with the majority of the
activity staying to our north through early Saturday afternoon.
This would allow for more heating and instability to occur ahead
of a cold front, which in turn would lead to a more widespread
severe weather threat for the mid afternoon through evening hours
ahead of the cold front. In terms of severe weather, modest
deep layer shear of around 30 kts combined with at least
moderate instability with MUCAPE over 1500-2000+ j/kg would be
sufficient for some organized severe weather potential. Warm
cloud depths and anomalously high moisture content with PWATs
over 1.8 to near 2 inches owes to lower confidence in large hail,
and higher confidence in strong winds being the primary risk via
precip loading and cool pools. LCLs will be very low with the
increase in moisture, and so can`t rule out a tornado particularly
with any storm-boundary interaction. The anomalously high
moisture will also bring the threat for heavy rain, and some
concern for localized flash flooding given the veering deep layer
and backward propagating shear vectors. However, storm motion
may be just enough to limit this threat and perhaps focus more
into urban areas.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 340 PM CDT Fri Jun 21 2024

After the front moves through late Saturday night, a much more
pleasant day is expected Sunday, with lots of sunshine and more
seasonal temperatures and humidity. Sunday is definitely the
pick day of the weekend! In fact, the large-scale pattern
appears to support a period of mostly dry conditions through
Thursday, thanks to upper-level ridging developing over the
Intermountain West states and surface high pressure settling
into the upper Midwest. The only other opportunity for showers
and storms appears to be for Tuesday and Tuesday night, as yet
another compact upper-level trough and attendant surface cold
front crosses the area. The timing of the front per the latest
global models does appear to be advantageous for more strong to
severe storms, especially with the CSU ML severe probs
suggesting strong to severe convection with a hot and humid air
mass again in place, so we will need to watch Tuesday as well.
After this front moves through the midweek period looks to quiet
down. However, by the end of next week the models show high
pressure moving off and return flow / warm, moist advection
ramping up across the Plains and Midwest for another chance of
showers and storms for the area.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 634 PM CDT Fri Jun 21 2024

Satellite shows a pronounced east/west boundary across northern
Iowa where the current SHRA/TSRA are located. A TSRA complex
will develop roughly along the MN/IA border tonight and move
into southern Wisconsin. Thus VFR conditions are expected
through 06z/22 with isolated SHRA/TSRA. After 06z/22 potential
for TSRA to impact parts of northeast IA and far northern
Illinois with MVFR/IFR conditions. Areas generally south of I-80
should remain VFR with isolated at best SHRA/TSRA.

&&

.HYDROLOGY...
Issued at 926 PM CDT Fri Jun 21 2024

Widespread 2 to 4 inches of rain is forecast near the headwaters of
the Wapsipinicon, Cedar, and Iowa river basins in the next 24 hours.
Some localized higher amounts over 5 inches is not out of the
question as well. Much of this rain is expected in a short amount of
time overnight tonight allowing for substantial runoff. As a
result, new forecasts this evening have several sites along the
Cedar going into flood in the next 3 to 5 days. Confidence in
the placement of heaviest QPF remains very low and since it is
still 3+ days out have decided to hold issuing any flood watches
for all sites except for Vinton on the Cedar River.

On the Mainstem Mississippi, heavy rainfall amounts between 4 and 9
inches in the past 2 weeks in southern MN and western WI, will
result in rises through the rest of June and into July. Went ahead
and issued flood watches for portions of the river this evening with
more likely in the coming days.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Flood Watch through Saturday afternoon for IAZ040>042-051>054.
IL...Flood Watch through Saturday afternoon for ILZ001-002.
MO...None.

&&

$$

UPDATE...08
SHORT TERM...McClure
LONG TERM...McClure
AVIATION...08
HYDROLOGY...Gross