SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0238 PM CDT Sun Sep 18 2022
Valid 182000Z - 191200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM
SOUTHEASTERN IOWA AND NORTHEASTERN MISSOURI INTO WEST-CENTRAL
ILLINOIS...
...SUMMARY...
A few intense supercells, capable of very large to giant hail, along
with tornadoes, one or two of which may be strong, may evolve into
an MCS producing scattered severe wind gusts. This appears most
likely across portions of south-central to southeast Iowa, northeast
Missouri, and west-central Illinois, beginning after 5 PM CDT and
continuing through tonight.
...Discussion...
The prior outlook remains reflective of current expectations, with
storm development expected to occur late this afternoon/early this
evening across the central/southeastern Iowa/northeastern Missouri
vicinity. With the airmass to continue gradually destabilizing over
the next couple of hours, and an increase in both mid-level
west-northwesterlies and low-level west-southwesterlies with time,
risk for an all-hazards -- and locally significant -- severe-weather
event remains likely.
..Goss.. 09/18/2022
.PREV DISCUSSION... /ISSUED 1130 AM CDT Sun Sep 18 2022/
...IA/MO/IL...
A low-amplitude shortwave impulse over the western Dakotas will move
east across the Upper MS Valley to the south of a deeper shortwave
trough shifting from MB to far northwest ON. This evolution will
yield an amplifying mid-level jetlet (with 500-mb winds in excess of
50 kts) across southern MN and IA into WI by this evening. A
strengthening baroclinic zone across IA, aided by robust
differential boundary-layer with low 90s into northwest MO and low
70s behind the cold front in northern IA, will support initial
thunderstorm development in the 22-00Z time frame centered on
south-central IA as MLCIN wanes. The initial environment will be
characterized by weak 0-1 km SRH, but large 0-3 km SRH with further
increase in wind speeds through the mid to upper portion of large
buoyancy amid rather steep mid-level lapse rates. This setup appears
favorable for potentially a few long-lived discrete supercells with
the primary hazard being very large to giant hail given the expected
hodograph structure.
The degree of tornado threat is more uncertain. Low-level
southwesterlies will strengthen this evening, but primarily across
northern MO and displaced largely south of expected convective
development. The greatest tornado threat may evolve with the
southern-most supercell which would have uninterrupted inflow and
greater 0-1 km SRH near the IA/MO/IL border during the early
evening. As such, have shifted the inherited 10 sig tor area south,
but this threat appears more conditional than previously forecast.
Consensus of latest CAM guidance still suggests potential upscale
growth into a small MCS as supercells congeal near/east of the MS
River, with the relatively greatest severe wind threat across
southeast IA and far northeast MO into west-central IL. How far
downstream this threat lasts is more uncertain, as there's a
reasonable signal for low-level warm theta-e advection to support
potential regenerative supercells on the upshear (northwestward)
side of broader-scale convective outflow within the feed of steeper
mid-level lapse rates. This may yield a persistent threat for large
hail into the early morning before convection weakens further into
the overnight.
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