237
FXUS63 KDVN 210842
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
342 AM CDT Tue May 21 2024

.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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1217 AM CDT Tue May 21 2024

A warm front will lift north through eastern IA and
northwest IL before 12z, with scattered showers and isolated
thunderstorms developing primarily north of I-80 early this AM.
This may bring some MVFR vsbys to CID/DBQ. After 15z, south
winds will begin to increase with gusts over 30 kts at times
this afternoon. Latest guidance is mixed on another round of
showers/storms occurring in the afternoon hours, but confidence
remains low on placement and timing at this time and left out
any mention. Strong storms, possibly severe will track along a
cold front moving east across Iowa late this afternoon and
evening. Current guidance has these arriving in the 22-02z time
frame at the terminals and have included PROB30 groups with this
TAF cycle. Winds will switch out of the northwest behind the
fropa late this evening.

&&

.HYDROLOGY...
Issued at 905 PM CDT Mon May 20 2024

River flood watches have been issued for portions of the
Wapsipinicon, Skunk and Iowa rivers. These river forecasts
incorporate forecasted rainfall for the next 24 hours.
Significant rainfall of 2+ inches is being forecasted in
the headwaters of these basins. This combined with generally
0.5 to 1 inch of forecasted rainfall across the channels of
these rivers into E Iowa is leading to significant rises, and
the potential for minor to moderate flooding on portions of
these rivers over the next 2-7 days. Additional rises are
possible on other rivers, likely mostly within bank. However,
the response on area rivers will all depend on how much rain
occurs in the coming 24-48 hours and the location of the heaviest
rains.

&&

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

&&

$$

SHORT TERM...Gross
LONG TERM...Gross/Cousins
AVIATION...Gross
HYDROLOGY...McClure