SPC May 23, 2020 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook
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Day 1 Convective Outlook NWS Storm Prediction Center Norman OK
1254 AM CDT Sat May 23 2020 Valid 231200Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
ILLINOIS AND OVER PORTIONS OF THE HIGH PLAINS... ...SUMMARY...
Scattered severe thunderstorms, including a tornado threat, will
develop across northern Illinois later this afternoon. Isolated
severe thunderstorms, some with very large hail, will be noted
across the Plains. ...Northern IL... Upper low over southwestern IA is forecast to eject into eastern IA
by 18z, then weaken some as it ejects into southern WI by early
evening. 50kt 500mb speed max should translate across northern MO
into northern IL during the afternoon, which will enhance shear and
the likelihood for supercells immediately ahead of the upper low.
Latest short-range model guidance suggests mid 60s surface dew
points will advance north across IL into a region of favorable
large-scale forcing for ascent. Forecast soundings suggest
thunderstorms should develop by late morning across eastern IA
beneath the upper low, while downstream, portions of northern IL
will experience a bit more destabilization as partial sunshine
should allow the boundary layer to warm with surface temperatures
rising into the mid-upper 70s. As a result, MLCAPE values should
exceed 1500 J/kg within a sheared regime favorable for supercells.
Latest thinking is scattered supercells will evolve by early
afternoon near the IA/northwestern IL border then spread east toward
the Chicago metro by early evening. Hail, wind, and a few tornadoes
can be expected with this activity. ...Northern/Central Plains... Weak large-scale height falls will spread across the northern High
Plains during the day1 period as upper troughing shifts across the
northern Rockies. Downstream, high-level diffluent flow should
encourage cloud-top venting and scattered thunderstorms are expected
to develop along a surface boundary draped across the western
Dakotas into eastern WY. Strongest boundary-layer heating will
likely focus near the wind shift, thus greater buoyancy, and likely
storm intensity, should focus along this corridor. Even so,
progressive short-wave trough should nudge deep convection off the
High Plains into portions of southeastern SD/northeastern NE late as
LLJ is dislodged into this region during the overnight hours. Very
large hail may accompany late-afternoon supercells. ...Southern High Plains... Broad troughing with weak southwesterly high-level flow will persist
across the southwestern US into the southern High Plains Saturday as
the mean trough remains located near 110W longitude. Relatively cool
mid-level temperatures (-11C at 500mb) and very steep mid-level
lapse rates will lead to extreme instability just east of the dry
line over west TX. Strong boundary-layer heating across far west TX
into the western TX Panhandle will allow surface parcels to reach
their convective temperatures around 20-21z. As a result, scattered
supercells should develop by mid afternoon from the TX South Plains,
south into northern Mexico. Very large hail (possibly > 3 inches)
could be noted with these slow-moving storms, especially early in
the convective cycle. ..Darrow/Lyons.. 05/23/2020 

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