889
FXUS63 KDVN 201033
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
533 AM CDT Mon May 20 2024

.KEY MESSAGES...

- There is a marginal risk (level 1 of 5) for severe weather
  today. Hail up to quarters and 60 mph winds will be the
  primary threats with the strongest storms if they occur.

- There is an enhanced risk (level 3 of 5) for severe weather on
  Tuesday. Large hail, damaging wind, and tornadoes will all be
  possible. Some significant wind gusts over 65 kts may occur.

- Drying out late Wednesday through Friday with more seasonable
  temperatures and humidity.

- Precipitation chances increase over the holiday weekend, but
  it will not be a wash out.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 248 AM CDT Mon May 20 2024

Through sunrise two separate storm induced upper level
disturbances will affect the area. The first will move into
Wisconsin while the second rotates through eastern Iowa and
northern Illinois. Forcing provided by both disturbances will
keep convection going across the area. The very short term
models are having difficulty handling the disturbances. However,
trends from the HRRR/RAP suggest most of the areal coverage of
the convection looks to be along and west of the Mississippi.

Storms along and west of the Mississippi will persist through mid-
day Monday but gradually decrease in areal coverage as the second
upper level disturbance moves into Wisconsin. The potential
exists, albeit only a 20 percent chance, of much of the
afternoon hours remaining dry as downward motion behind the
departing upper level disturbance caps the atmosphere.

However, boundaries left over from the earlier storms combined with
peak heating should allow isolated to scattered storms to develop
mid to late afternoon. Any storm that develops may bring a
hail/wind threat. The area favored looks to be south and east
of a line from Dubuque, IA to Ottumwa, IA. The overall recovery
of the atmosphere will dictate the severe potential for late
afternoon and evening. The steep mid-level lapse rates that
support storm development are actually over central and western
Iowa. Thus activity over eastern Iowa and east of the
Mississippi could remain isolated in nature.

Tonight will be interesting. Storms that fire in the Plains
this afternoon will likely grow upscale into one or two lines
that move east overnight. Steep mid-level lapse rates will
initially help maintain the storms as they move east but cooling
of the atmosphere with sunset should result in storms becoming
weaker with time. Thus there are three possible scenarios.

Scenario 1 is the evening remains fairly quiet with little or no
storms followed by an increase in storm coverage as the low level
jet shifts east feeding moisture into the approaching storm system.
The HRRR is beginning to suggest this scenario and to a lesser
extent the RAP.

Scenario 2 is the entire night remains mainly quiet due to
decreasing mid-level lapse rates and the low level jet remains out
in the Plains. The NAM suggests this scenario.

Scenario 3 is that isolated to low end scattered convection persists
overnight while the stronger convection gets no further east than
western Iowa. The NSSL WRF and to a lesser extent the FV3 suggest
this scenario.

Scenario 1 is currently favored given the slightly better results
the HRRR/RAP had with the Sunday night convection.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 248 AM CDT Mon May 20 2024

Tuesday...00z model solutions continue to show a very active severe
weather day for the local area. Scattered showers and thunderstorms
to be ongoing to start the period primarily in the northern half of
the CWA in the WAA regime ahead of 996 surface low in northern KS
aided by overnight LLJ. With some MLCAPE (1000 J/Kg), 30 kt deep
layer shear, and PWs around 1.5" wouldn`t be surprised to have an
elevated hailer or two with penny/nickel hail during the morning
hours along with heavy rain. This activity will lift north out of
our area as the warm front lifts north near the IA/MN border in
response of the surface low tracking into NW Iowa by 22z. This low
location is further west and slower than previous model runs and
makes sense with a negatively tilted trough. The question then
becomes how quickly the atmosphere recovers in the warm sector.
Steeping mid level lapse rates in an elevated mixed layer (EML)
combined with a moist BL will allow for destabilization to
occur ahead of cold front across the eastern 2/3rds of Iowa. RAP
soundings in the afternoon show very high SBCAPE and MLCAPE
values in excess of 2500 J/Kg, 55kt effective shear, and 25 kts
of 0-1 km shear all supportive of supercells initially in the
warm sector with large hail and tornadoes (possibly strong).
After CI occurs, large scale ascent and forcing along the front
will allow for rapid upscale growth into a QLCS moving across
Iowa. 0-3 km shear vectors over 40kts become close to line
normal with the surface front which supports hazards
transitioning into a damaging wind/QLCS tornado threat. Some
significant wind gusts over 65 kts not out of the question as
this system grows upscale. In addition, some heavy rainfall will
be possible with this event, with PWs around 1.5 to 1.7 inches,
which would be near the daily max for this time of year per SPC
climatology. Thankfully, storm motions will be quite fast (over
50mph!) leading to not much residence time of heavy rainfall at
any one point.

Regarding timing...latest CAMs and RAP/NAM model progs suggest this
would develop late afternoon Tuesday (4pm) through late evening
(11pm). An interesting note from the new 00z MPAS model run from
NCAR shows several members initiating convection slightly further
west over western IA at 21z before quickly moving east, which may
suggest a slightly later time of storms arriving here. In any case,
be weather aware Tuesday and stay up-to-date with the newest
information as it unfolds.

Wednesday On...surface low to continue to fill and weaken over
western Lake Superior. This will bring comfortable humidity
levels, seasonable temperatures, and dry conditions across the
CWA to end the work week. 850mb temps to remain below 11C
through Friday. Highs in the 70s and lows in the 50s will be
seen. Going into the holiday weekend, the 00z models diverge on
their respective solutions and forecast confidence in
precipitation chances decreases. Will maintain slight
chance/chance PoPs through Sunday night for now, but there will
be dry hours and this will be further refined in later
forecasts.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 521 AM CDT Mon May 20 2024

VFR conditions will prevail, but residual showers and storms will
bring brief MVFR conditions. Cigs may drop between 2000-3500
ft, with decreases in visibility to 3-5 SM, especially in
heavier showers. BRL should mostly be clear of these
restrictions, but the remainder of the terminals will have these
impacts through the morning. Thus, a TEMPO is out to cover this
until 16z. From there, we should start to see things clear out
and cigs increase, as clouds start to scatter out. Winds are
quite variable at the moment, as multiple rounds of
showers/storms resulted in several outflow boundaries, impacting
wind direction throughout. Overall, easterly winds will shift
southeasterly early, and then most areas will be southwesterly
by 18z. Gusts upwards to 25 KTs will be possible, especially
with any showers/storms.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...08
LONG TERM...Gross
AVIATION...Gunkel