301
FXUS63 KDVN 180515
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1215 AM CDT Sat May 18 2024

.KEY MESSAGES...

- Isolated to scattered showers and storms are possible
  Saturday afternoon west of the Mississippi River.

- An active pattern returns next week, particularly for the
  Sunday night through Tuesday night time frame.

- There is a Slight Risk of severe weather for Tuesday across
  the entire forecast area

- Very warm this weekend, with more seasonal temperatures coming
  for the middle of next week

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 158 PM CDT Fri May 17 2024

Mainly clear skies across the area this afternoon as high
pressure sits across the area. Scattered showers and storms
have remained to the southeast of the area. Temperatures at 2 PM
ranged from 77 degrees at Freeport and Independence to 81
degrees at Macomb.

Expect quiet weather across the area through Saturday morning as
high pressure slowly shifts to the east. Low temperatures on
Saturday morning will be in the upper 50s to around 60 degrees
with light and variable winds.

A weak shortwave is forecast to pass to our north Saturday
afternoon and push a weak cold front into the area. Moisture is
limited with some warmer air in place at 700 MB. The 12 UTC NAMNest
continues to shows an isolated to scattered line of showers and
storm developing in central Iowa and slowly moving into the
area during the late afternoon. There is 1000 to 2000 J/KG of
CAPE but shear is limited to around 20 knots. There is
disagreement among the different models on the degree of
instability tomorrow afternoon. However the Storm Prediction
Center has a Marginal or level 1 out of 5 risk of severe storms
for parts of the Highway 20 corridor but confidence is low in
this occurring. If showers and storms do form do not think that
they will last very long given the dry atmosphere and weak
shear. High temperatures on Saturday will be in the mid 80s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 158 PM CDT Fri May 17 2024

A more active pattern then takes shape for Sunday night through
Tuesday as the northern jet stream begins to amplify over the
Intermountain West, while the southern stream jet lifts northward
and strengthens as it meets the northern jet. A series of mid-level
shortwave impulses will sweep through our region Sunday night
through Tuesday night. With temperatures warming to the lower to
middle 80s Monday and Tuesday, and dew point temperatures in the
lower to middle 60s, instability values will be elevated. However,
there are some differences on just how much, with the GEFS
probabilities of CAPE 1000 J/kg or greater around 80 to 100 percent
Tuesday, but the ECMWF were more around 40 to 70 percent for a
similar threshold. The kinematics appear to be rather strong, with
both the GEFS and ECMWF ensemble mean 500 mb jet magnitude of 50+
kts on Tuesday, nosing into central Iowa. Not only will the deep-
layer shear be enhanced, but so will the low-level shear, given
clockwise-curved hodographs from the various coarser-resolution
ensembles (GEFS, ENS, GEPS). The most recent CSU machine-learning
severe weather probs are painting some decently high values for
Monday and Tuesday (especially Tuesday, which is when the better
kinematics are expected). Additionally, the ECMWF EFI for the
CAPE/Shear combination are showing values around 0.6 to 0.8 along
and south of I-80 - additional evidence for a possible severe
weather event Tuesday. Just before we sent this discussion, SPC
introduced a Slight Risk (level 2 of 5) for severe weather across
our area for Tuesday. Confidence on the exact timing of severe
weather remains uncertain, but with this new outlook, the situation
will need to be watched as we approach Tuesday as all hazards of
severe weather appear possible. In addition, some heavy rainfall
will be possible with this event, given Pwat values around 1.3 to
1.7 inches, which would be above the 90th percentile for DVN from
the SPC sounding climatology.

For Wednesday and Thursday, confidence in the forecast decreases as
models aren`t in as much agreement. The NBM PoPs reflect this
disagreement in the GEFS/ENS ensembles, with PoP values around 20 to
30 percent, despite a strong signal for rainfall Wednesday evening
from the GFS deterministic model. It does appear that temperatures
will return to more seasonal values by mid-week, however, with
values in the lower to middle 70s.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1206 AM CDT Sat May 18 2024

VFR conditions are expected through the TAF cycle. A weak cold
front will track across Iowa this afternoon and evening, with
scattered showers and isolated thunderstorms developing ahead
of it. These storms are forecast to weaken before reaching
eastern Iowa (around 00z at CID) and have left out mention for
now due to low coverage. NBM probability of precip and thunder
potential is currently less than 10%. Behind the cold front,
winds will switch out of the northwest.

&&

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

&&

$$

SHORT TERM...Cousins
LONG TERM...Schultz
AVIATION...Gross