Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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784
FXUS63 KDVN 182002
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
251 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Scattered showers and storms will wane after sunset tonight.
  There is a Marginal risk (level 1 of 5) of severe weather
  today west of the Mississippi River.

- Boundary stalls south of the area as temperatures again reach
  well into the 80s as chances for thunderstorms return late.

- Active pattern through the long term with chances of strong to
  severe storms through Tuesday. Tuesday is the best day for
  severe weather in the next 5 days.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 109 PM CDT Sat May 18 2024

Temperatures well into the 80s today were found area-wide.
Dewpoints in the upper 50s to low 60s made it feel somewhat
nicer than the middle of July. To the west of the area, a cold
front was located across the center of Iowa. A shortwave passing
north of the area today will drag this front through the area
this afternoon and into the evening. The boundary will then
stall out across southern portions of the area. This boundary
on Sunday will separate the higher moisture air from the drier
air. Nonetheless, we will see highs again in the 80s, but with
drier air than today. With that boundary in place across the
south, we could see some thunderstorms redevelop along the front
later in the day and move east towards us late.

Most of the impactful weather will likely be ongoing at 00z
tonight as the cold front moves through the area. Guidance has
CAPE building ahead of the front to around 2000 J/kg this
afternoon. Shear looks very weak, so updraft organization is not
expected. Pulse like storms are expected if they develop. The
main question today is forcing. The main upper level forcing is
way north, so the front and daytime heating will be the main
drivers for thunderstorm development. CAMs have either a broken
line of storms or not much with the exception of the highway 20
corridor. Dprog/Dt of the HRRR shows run to run variability with
some having convection down to highway 30 the rest further
north. The NAMnest has convection all the way through the CWA.
Think the best chance for storms is highway 20 north. In a nod
to the NAMnest, did keep schc south through I80. Am not too big
on severe today. Could see near severe wind gusts and hail,
especially due to the steeper lapse rates, but don`t expect
cells to have much longevity to really get going.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 251 PM CDT Sat May 18 2024

Extremely active pattern with a mid-summer like pattern is
expected to last through most of the period as a stalled out
boundary resides just south of the CWA. This boundary will
separate the area between Tds in the 50s, to the more robust
deep moist Tds in the 60s and 70s to the south. A series of weak
waves and possibly MCVs/convective induced waves/ move over the
boundary later Sunday into Monday evening. A strong wave, with
better shear is expected to approach the area into Tuesday. A
deepening surface low with this wave will help to bring this
boundary north as a warm front Tuesday. This will bring higher
Tds into the area ahead of the wave and increased shear. Then
the boundary is sent south again as a cold front later Tuesday.
After this point, there are questions about where the boundary
will be and additional chances for thunderstorms.

Closer look at the guidance for Sunday evening suggests a MCV
rolling into the area after 00z. The 12Z HRRR has convection
firing with this wave like feature across the area. In fact, the
HRRR suggests some of these storms could be supercellular in
nature. As mentioned before, shear looks to be weak, however, if
this truly is a MCV, then the shear may actually be stronger
than forecast. With Tds lower tomorrow, think that these
supercells would be hailers and possibly HP in nature and a
wind threat. That said, other guidance keeps us clear. We will
need to keep an eye on convection upstream to see if this forms.
So there is a conditional risk for severe weather tomorrow
afternoon and evening.

Monday, another possible day with severe weather as the wave
induces height falls and the sfc boundary moving north again.
The NAMnest depicts yet another MCV moving through our area
Monday with redevelopment of showers and storms in the
afternoon. This is yet another potential case where we could see
higher shear than forecast. With clouds possibly keeping Ts
lower, our T/Td spread may be lower as well. Llvl winds looked
back so if we develop afternoon showers and storms monday they
could be severe as well. We currently have a mrgl for this day
as well, but could see this day updated to a slight if the MCV
comes true.

So after the next two days with conditional severe weather
outlooks we get to Tuesday when a better wave makes its way into
the area. This one will bring better shear into the area
Tuesday. Storm Mode: 0-6km shear vectors are perpendicular to
the forcing and around 40kts. This would suggest supercells and
they will likely form first west of the CWA. However, upper
level flow is weak so expect convection to quickly become cold
pool dominated. Expect a mature/ing bow to enter the area in
late afternoon. Severe Hazards: 0-3km shear vectors of 40kts
will help the production of mesovorts with localized higher wind
gusts and possible tornadoes. The LLJ is impressive for the
afternoon, and even more so after sunset, so this thing could
really ramp up later in the evening. With weak upper-level flow,
the overall system should not be moving all that fast. Some hail
is possible early on, but think this is rain/strong wind/tornado
event. Heavy Rain: In addition, some heavy rainfall will be
possible with this event, with PWs around 1.5 to 1.7 inches,
which would be above the 90th percentile for this time of year.

Wednesday On...additional showers and storms will be possible
late week. However, model timing differences in shortwaves
moving through the flow precludes changing the PoPs, which are
in the 15 to 30 percent range for each 6 hr period through
Friday. There will be some dry hours, but when this will occur
is still in question. Temperatures will return to more
seasonable values by late week, with highs in the 70s and lows
in the 40s.



&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 109 PM CDT Sat May 18 2024

VFR conditions are expected through the period. Winds will
slacken tonight and then could become gusty again later in the
period. There is a very low (<20%) chance for thunderstorms at
CID and DBQ this afternoon. There are disagreements between
guidance about this occurring. As such, have left out of the
TAFs at this time.

&&

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

&&

$$

SHORT TERM...Gibbs
LONG TERM...Gibbs/Gross
AVIATION...Gibbs