164
FXUS63 KDVN 211733
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1233 PM CDT Tue May 21 2024

...18z Aviation Update...

.KEY MESSAGES...

- Moderate Risk (level 4 of 5) for severe weather has been
  expanded today to include all except the far eastern portions
  of Bureau and Putnam counties. Large hail, damaging winds, and
  tornadoes will all be possible. Some significant wind gusts
  over 70 mph may occur this evening.

- Flood watch issued for a portion of the CWA for flash flood
  potential today.

- Drying out Wednesday and Thursday with more seasonable
  temperatures and humidity.

- Shower and thunderstorm chances return Friday through Sunday
  as a more humid air mass builds back into the region.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 302 AM CDT Tue May 21 2024

Early This Morning...elevated convection has for the most part
remained to our west and northwest thanks to the 850mb LLJ and
moisture transport vectors oriented SSW to NNE into north
central IA. This activity may clip our far northwestern
counties over the next 1-3 hours. Frequent lightning, gusty
winds up to 50 mph, and heavy rain will be the hazards. MRMS
1-hr rainfall amounts to our west have been in the 1-2.5" range
and if this continues into our CWA, an isolated flash flood
threat may exist especially if storms train over the same
location. For this reason and with collaboration from neighbors,
have issued a flood watch for the two western tier of counties
north of I-80. This area received 1-2 inches yesterday and with
FFG values in the 1.5-2" range it wouldn`t take much to cause
issues. Rises on creeks and for some rivers can be expected this
morning on the upper portions of the Iowa, Cedar, and Wapsi
Rivers. See hydrology section for more details.

Mid Morning Through Early Afternoon...a strong shortwave remains
on track to move from the Central Plains into the Upper
Mississippi Valley by 00z tonight. This wave will become
negatively tilted by the late afternoon, which will lead to a
deepening surface low down to ~988mb as it reaches central
Minnesota. A lull in the precip chances is forecast for this
period as the main forcing moves north and the region remains
fully in the warm sector. The question then becomes will
convection develop before the main show along the cold front
later in the evening. Forecast soundings from the latest
HRRR/RAP suggest a rather strong CAP will be in place that may
keep a lid on things through 21z. However, outflow boundaries
from this morning`s convection may provide a focus for storm
initiation in this period and the day shift will need to watch
for this possibility. In addition, it will become quite breezy
with wind gusts over 35 mph at times.

Early Afternoon-Evening...a very unstable atmosphere evident by
SBCAPEs over 2500 J/Kg, deep layer shear over 45 kts, and PWs
around 1.4" will be in place over our area and this all points
to a classic setup for severe weather. Latest CAMs and 00z HREF
progs suggest convective initiation to occur just west of I-35
in Iowa around 21z with discrete supercells. Very strong
dynamics (70kt 500mb speed max and 60kt 850mb winds) will
support activity quickly growing upscale into a QLCS with
embedded supercells. 0-3 km shear vectors will be oriented line
normal to the front supporting bowing segments with strong
mesos capable of producing tornadoes. This will be a high end
severe threat with all hazards possible, including large hail,
damaging winds, and tornadoes (a few of which could be strong --
EF2+). SPC has expanded the Moderate (level 4 of 5) risk across
most of the CWA with the greatest risk probabilities coming
from wind (45% or greater). There is also a risk of significant
wind gusts greater than 74 mph this evening. Pay close attention
to the weather, have multiple ways to receive warnings, and to
take appropriate action if a warning is issued for your
location.

In addition to the severe threat, a very moist atmosphere with
PWATs near 1.75" will support hourly rain rates of 1-2" with the
strongest storms. While storm motions will be very fast and
residence time for heavy rain will be low along the front, have
kept the flood watch going in our west until 9pm this evening
for ponding and an isolated flash flood threat.

To reiterate, the greatest severe risk today will be from early
evening (5pm) to late evening (11pm) as widespread storms along
a cold front track west to east through the outlook area. Once
the front moves through, the severe threat will quickly
diminish.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 302 AM CDT Tue May 21 2024

Wednesday...latest forecast models have been trending drier for
this period and have adjusted PoPs accordingly in collaboration
with neighbors. A secondary wave is still expected to track
east across IA but now only bring an increase in clouds during
the day. Afternoon highs will top out in the low to mid 70s,
with much more comfortable humidity levels.

After the wave passes on Wednesday, flow aloft becomes split
again with the northern US under the influence of the northern
branch of the jet stream. High pressure is forecast to lift across
the area at the surface and aloft and bring quiet weather to the
area Wednesday night through Thursday. High temperatures on
Thursday will be warner with temperatures in the mid to upper 70s.

Friday through Monday, a series of disturbances will move across the
area as shortwave energy from a trough in the Pacific Northwest
moves across the area. This will bring daily chances of showers
and storms through the holiday weekend, The weekend will not
be a total washout, but there will be periods of showers and
storms.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1232 PM CDT Tue May 21 2024

Isolated thunderstorms will be in the vicinity of KCID to start the
period, with potential for storms to produce heavy rain, hail up to
quarter size and wind gusts around 40-50 kts.

The main impact in the next 24 hours is still expected with strong
storms developing across central IA this afternoon and tracking
east. Some of these storms may become severe, with potential for
significant wind gusts over 65 kts in the strongest bowing segments.
Current guidance maintains these arriving in the 22-02z time frame
at the terminals, which are addressed as TEMPO grounds at all
terminals. Winds will switch out of the northwest behind the fropa
late this evening, with some guidance continuing to hint at MVFR
cigs developing at CID/DBQ late in the period.

&&

.HYDROLOGY...
Issued at 1142 AM CDT Tue May 21 2024

Very heavy rains on the order of 2-3+ inches fell across
portions of eastern Iowa mainly northwest of the Cedar Rapids
and Iowa City metros in the past 24 hours. Much of this rain
fell in the headwaters of the Wapsipinicon, Skunk, Cedar and
Iowa Rivers, which is leading to or will lead to rises over the
coming days. Flood watches and warnings are currently in effect
along portions of these rivers, with potential for minor to
moderate flooding over the next 7 days.

Additional QPF of around 0.50-1.00" is expected across these
rivers today as showers and storms move across the area. This
rain could lead to additional rises, though confidence on where
the heaviest rain will fall is low as storms will be fast
moving.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Flood Watch until 9 PM CDT this evening for IAZ040-041-051-052-
     063-064.
IL...None.
MO...None.

&&

$$

SHORT TERM...Gross
LONG TERM...Gross
AVIATION...Speck
HYDROLOGY...Speck