814
FXUS63 KDVN 191939
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
239 PM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Storm chances this evening/tonight have trended slower and may
  lower overall severe weather risk if trends continue. There
  remains a Slight risk (level 2 out of 5) for severe weather
  west of a line from Fairfield, IA to Anamosa, IA.

- Active weather pattern this weekend to bring several chances
  of much needed rain to the area.

- Seasonable temperatures and mostly dry conditions next week.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 228 PM CDT Thu Sep 19 2024

Radar mosaics have depicted scattered showers and thunderstorms
just to our west across north central IA for much of the day.
These storms developed along an instability gradient and have
remained below severe limits, with just a 29 mph wind gust
reported at the Waterloo airport. Latest water vapor and
satellite imagery shows CI occurring near the IA/MN border,
where there is stronger upper level support. 12z DVN sounding
showed a cap around 770 mb that has kept any clouds that
developed locally fairly shallow today. Forecast focus in the
near term is on storm trends and severe weather risk.

Assessing surface analysis and model trends today, I noticed
models were initially overdoing Tds this afternoon. This caused
some of the 06z model runs particularly the NAMNest, to convect storms
too early. In any case, recent CAM runs have matched reality
and show storms developing across southern MN and northern IA
this afternoon in the warm sector, with the greatest upper
level support. Discrete supercells are expected initially, with
some indication of 0-3 km shear values becoming perpendicular to
the storm motion and allowing for some bowing segments. A
narrow 1-3 hr corridor of a tornado threat remains between
23-01z over northeast IA tonight. Otherwise, a fat CAPE sounding
and DCAPE values over 1200 J/Kg may allow for a few severe
storms tonight with marginally severe wind/hail before the sun
goes down and instability quickly wanes. Regarding timing, the
HREF local probability matched mean (LPMM) composite
reflectivity shows storms to move east southeast through the CWA
between 7 pm and Midnight tonight.

With high PWs increasing between 1.7 and 2", any storms will be
proficient rain producers with rain rates 0.5"/hr expected.
Rainfall totals between a quarter and three quarters of an inch
are forecast. This would be much welcomed rain with most of the
area not seeing much rain since Aug 30th. The 12z HREF LPMM
Accumulated QPF even shows some potential for locally higher
amounts approaching 2" in Benton and Iowa counties overnight!
There is some potential for fog along and north of Hwy 30
overnight, but left out of the forecast for now due to expected
cloud cover.

Friday...high pressure will build overhead with clouds
diminishing in the morning leading to dry conditions. Afternoon
highs will top out in the mid to upper 80s.


&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 228 PM CDT Thu Sep 19 2024

Friday Night-Sunday Night...broad ridging aloft will begin the
period, with the latest deterministic and ensemble models in
agreement ejecting the closed low over the southwest CONUS east
of the Colorado Rockies by Sunday afternoon. A lead shortwave is
progged to move along a frontal boundary Saturday night
bringing increasing clouds and precipitation chances. High
moisture levels indicated by PWs in the 1.6 to 1.8" range (200%
of normal) and rich 850mb theta-e air in place to support heavy
rain amounts. ENS/GEFS/GEPS 24-hr probabilities of at least 0.5"
of rain are now in the 50-60% range across the CWA ending 00z
Monday. This is indeed good news for rain prospects! Regarding
the severe potential, there remains too low of probabilities and
uncertainties in instability/shear overlap at this time. That
said, Day 3 CSU machine learning probs suggest there could be
some risk added as we get closer to the weekend.

Early Next Week...the upper low mentioned above to move over the
Midwest, with continued clouds and precip chances through at least
Tuesday. Cooler, more seasonable temps, dry conditions, and
comfortable humidity levels are forecast behind this system,
with highs in the 70s and lows in the 50s Tuesday-Thursday.
Perfect for the 1st full week of astronomical Fall!

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1240 PM CDT Thu Sep 19 2024

Scattered showers and thunderstorms have remained and persisted
west across central IA today. These have formed along an
instability gradient and are expected to  before reaching
eastern IA, as they slowly lift northeast through the afternoon.
Attention then turns to storm potential ahead of a cold front
progged to move east across IA this evening. Have delayed onset
of thunder and lower VFR/MVFR conditions a few hours and
continued PROB30 wording after 03z. Behind these storms, low
clouds are expected to develop and have trended the TAFs with
MVFR cigs. The 12z HREF and RAP guidance is even suggesting some
brief 1-2 hr IFR conditions are possible, but did not go that
low on cigs with this issuance due to lower confidence. Winds
will switch to the west late this evening and eventually
northwest just beyond the TAF cycle.

&&

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

&&

$$

SHORT TERM...Gross
LONG TERM...Gross
AVIATION...Gross