022
FXUS63 KDVN 210531
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1231 AM CDT Tue May 21 2024

.KEY MESSAGES...

- Scattered showers and storms are possible mainly NW of the
  Quad Cities late tonight.

- There is a Moderate Risk (level 4 of 5) for severe weather on
  Tuesday. Large hail, damaging winds, and tornadoes will all
  be possible. Some significant wind gusts over 70 mph may
  occur.

- 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 WEDNESDAY/...
Issued at 215 PM CDT Mon May 20 2024

Late Afternoon through Tonight

A MCV currently over north-central Illinois and far southern
Wisconsin will continue to track to the NE into this evening,
taking the majority of the scattered storms with it. A few
isolated storms could develop late this afternoon and evening;
however, most of the area will stay dry. By late tonight,
increasing warm air advection aloft will likely lead to the
development of scattered showers and storms across mainly the NW
portion of the forecast area. The primary threats from this
round of storms are small hail, gusty winds, and heavy
downpours.

Tuesday

A potent shortwave trough is forecast to track from the Central
Plains into the Upper Mississippi Valley. The trough will become
negatively tilted by the afternoon and evening which will lead
to a deepening surface low down to ~988mb as it reaches central
Minnesota. This is a classic setup for severe weather locally
across portions of Iowa and Illinois as the warm sector pulls
northward into the region amidst sufficient deep layer shear
for supercells and organized linear convection. There are
several potential rounds of storms on Tuesday. The first two
rounds during the morning and through mid/late afternoon will
likely remain scattered in coverage and favor areas north of
I-80. Large hail and damaging winds are the primary threats,
with a few tornadoes possible during the afternoon period.

The highest confidence on severe storms will be associated with
the cold front during the evening into early nighttime period.
CAMs indicate either a line of storms or broken line of
supercells rolling across the area from west to east. Forecast
soundings and HREF environmental parameters are supportive of a
higher end severe threat with all hazards possible, including
large hail, damaging winds, and tornadoes (a few of which could
be strong -- EF2+). CAM forecast soundings display high low-
level shear and favorable thermodynamics for tornadic potential
with 0-1 km bulk shear near 30 kts, large clockwise hodograph
curvature contributing to 0-1 SRH values of 150-250 m2/s2, and
reasonably low LCLs under 1000m where moisture pools ahead of
the cold front. 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. Fortunately, the storms will
be moving along at 50 - 60 mph, thereby decreasing the overall
threat for flash flooding.

We may eventually see a transition in storm morphology from a
broken line of supercells to more of a severe QLCS capable of 60
to 80 mph winds, with a continued tornado risk due to embedded
supercells and potential for mesovortices with 0-3 km line
normal shear near 30 kts. Tomorrow is a day to 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.

Timing of Storms:

Round 1: Warm air advection driven, mostly elevated storms are
possible across the NW portion of the outlook area from late
tonight through late Tuesday AM.

Round 2: Scattered storms may form during the early to mid
afternoon well out ahead of the cold front. Confidence with this
round is low, but potential is there for scattered severe storms
developing in the open warm sector along a convectively induced
warm front.

Round 3: The highest severe weather threat is between 6 PM to
11 PM as a cold front and widespread storms track west to east
through the outlook area.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 215 PM CDT Mon May 20 2024

Active weather will continue across the area through the upcoming
holiday weekend. The long term period begins on Wednesday with
southwesterly flow aloft from the Desert Southwest into the Upper
Great Lakes. On Wednesday morning, Tuesdays storms system will be
lifting northeastward with a secondary disturbance swinging across
the area late Wednesday morning into the afternoon. This will bring
lingering showers and storms to the area through the day on
Wednesday.  High temperatures on Wednesday will be in the lower to
mid 70s.

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...None.
IL...None.
MO...None.

&&

$$

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