478
FXUS63 KDVN 271023
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
523 AM CDT Sat Apr 27 2024

...Updated for 12z Aviation...

.KEY MESSAGES...

- Active period ahead by this evening through Sunday with several
  rounds of showers/storms.

- Flash Flooding remains a concern for areas that receive repeated
  rounds of thunderstorms tonight/Sunday with the potential
  for areas of 2-4 inches of rain. Confidence however, remains low
  on the placement of the heaviest rainfall.

- Severe weather is also possible, particularly by this evening,
  but also into Sunday although prior convection may have an impact on
  the magnitude/timing/location of severe weather Sunday PM.

- Unsettled weather continues through next week, with periodic rain
  chances.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 305 AM CDT Sat Apr 27 2024

Latest surface analysis places a 994 mb low into SW Minnesota with
a cold front trailing through the Missouri River valley while a
warm front extends eastward from the low near the Wisconsin/Illinois
state line. Low level water vapor imagery shows the shortwave
aiding the outbreak of tornadoes yesterday across portions of Nebraska
through central Iowa lifting northward into Minnesota. Last bit
of glancing vorticity advection was pulling away leading to a marked
decrease in precipitation into portions of NE Iowa. Meanwhile, a
low amplitude perturbation on it`s southern flank and a nocturnal
LLJ and attendant isentropic ascent was aiding a few storms into
portions of north central Illinois.

Overall, anticipate a general lull in precipitation through the first
half of today with a waning LLJ, and as weak mid level height rises
develop over the region in the wake of the departing shortwave and
ahead of a secondary upper level low emerging from the Four Corners.
Sun peaks/breaks and a SSW wind gusty at times will promote deeper
mixing into 850 hPa temps of 13c to 15c, which will lead to highs
from the mid 70s to lower 80s and a summer-like feel aided by surface
dew points in the lower to mid 60s. Winds could be near advisory
criteria for a period of time with gust 35 to 45 mph at times this
morning through midday mixing into the near 35-40 kts of wind around
900 hPa.

This afternoon, the cold front will stall in/near our northwest
cwa as it becomes nearly parallel to the mid level flow. Proxy
model soundings would support a weakening of a capping inversion
in place as temperatures breach the mid/upper 70s. As a result,
we could see widely scattered storms develop early to mid PM
(1-3 PM) particularly across our north/northwest service area in
closer proximity to the boundary and right entrance region of a 70+ kt
700 hPa jet streak. Deep layer shear 0-6km of 40+ kt and MLCAPE
1000-1500+ j/kg would support potential for Supercells initially
but proxy soundings show evolving to more linear hodographs to where
we could see splitting and possibly evolution to multicells.
Main threats would appear to be large hail and damaging wind gusts.

Anticipate an burgeoning of storms this evening, with an increasing
nocturnal LLJ attendant to a lead shortwave ejecting out ahead of the
upper low emerging along the front range. These will have mainly a
large hail and damaging wind threat. The tornado threat would appear
low this evening with largely veered low level flow, but it can`t be
totally ruled out if low level winds were to back more than forecast or
given any storm scale interaction with boundaries, etc.

The severe threat may largely wane by late evening/overnight and
transition more to heavy rain and a local flash flood threat, as
the LLJ continues to foster bouts of showers and storms. PWATs
surge again to around 1.5 inches and flow parallel to the boundary
may lead to repetitive convection and enhanced areas of 2-4+
inches of rain. Confidence on just where this axis of heavier rain
sets up is low, but it could be more of a concern if it occurs
across our south which some areas saw over 1 to 1.5+ inches of rain
in the past 18 hours.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 305 AM CDT Sat Apr 27 2024

Sunday has the potential to be active again with a severe weather
threat, as the Rockies shortwave ejects in negative tilt fashion across
the Upper Midwest through 12z Monday. The lingering precipitation
may play a pivotal role in the extent of destabilization and
subsequent magnitude/location/timing of the severe weather risk.
That said, in general later Sunday afternoon and evening would appear
the most favored window attendant for any severe storms tied
to the arrival of the synoptic scale ascent. Effective shear of 35-
40 kts would support organized convection capable of producing
severe weather. Large hail and damaging winds would be the main
concerns, but a tornado threat cannot be ruled out with increasing
low level shear. The 2-6 km flow becomes nearly parallel to the cold
front, which may lead to a transition from semi discrete to more of
a linear mode.

In the wake of the cold front, the start of next week looks to be quieter
and drier. However, there`s signs that an active pattern will return
mid to late next week with the flow becoming semi-zonal shuttling impulses
across the Midwest from the Pacific along a meandering mid level baroclinic
zone.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 520 AM CDT Sat Apr 27 2024

MVFR to VFR ceilings and gusty SSW winds to 35+ kts will be found this morning.
Showers and storms are anticipated this afternoon, but initial coverage is
somewhat uncertain given a lack of substantial trigger. However, by
this evening showers and storms are expected to increase in coverage warranting
TEMPO to prevailing mention. Storms will be capable of producing hail, gusty
winds and heavy rainfall, which may result in periods of IFR conditions. SSW
winds will remain gusty to 35 kts through the afternoon, but then diminish some
tonight although they will be variable and gusty in/near storms.

&&

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

&&

$$

SHORT TERM...McClure
LONG TERM...McClure
AVIATION...McClure