LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Fri Sep 01 2023
Valid 011300Z - 021200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
ARIZONA AND SOUTHEASTERN CALIFORNIA TO SOUTHERN IDAHO...
...SUMMARY...
Isolated severe gusts and hail will be possible today into this
evening, from southern Arizona and southeastern California to
southern Idaho.
...Synopsis...
A blocky synoptic pattern will persist across the CONUS in mid/upper
levels, anchored by a high over the south-central Great Plains, and
split flow around a cyclone over the West Coast States. Through the
end of the period, the associated 500-mb low should retrograde
south-southwestward from its initial position near SLE to near Cape
Mendocino, CA. This will help to maintain a belt of enhanced,
seasonally strong mid/upper-level winds -- with substantially
southerly component -- across much of the Great Basin and western
parts of both the Sonoran Desert and Colorado Plateau, northward
past the Wasatch Range and upper Snake River Valley.
At the surface, 11Z analysis showed a cold front offshore from SC,
becoming quasistationary to warm over central GA to a low over
west-central AL, then cold again across southwestern AL and the
northwestern Gulf to the TX coastal bend. The boundary should move
little through the remainder of the period, and will keep optimal
Gulf moisture shunted well south of the midlatitude westerlies, east
of the northern Rockies.
...AZ/southeastern CA to eastern Great Basin...
A broad plume of persistent low-level convergence and high-PW air,
of low-latitude marine origins, will combine with more-local
evapotranspiration from moist ground to support both:
1. Potential continuation and eventual diurnal re-intensification
over parts of UT, of ongoing convection moving northward out of AZ,
and...
2. Scattered afternoon and evening thunderstorms developing in
already weak, diabatically and orographically removed MLCINH across
the rest of the outlook area.
Quite often, occurrence of extensive thunderstorms over AZ one day,
with cloud cover lingering into morning the next, significantly
suppresses potential through convective stabilization and
insufficient airmass modification the second day. However, in this
somewhat atypical regime, the persistent plume of strong deep-layer
southerlies will support enough warm advection and moisture
transport to move the convectively processed boundary-layer air
northward and, to some extent, modify it, such that enough
destabilization to support more development is probable by mid/late
afternoon. That activity, in turn, should move roughly northward.
In many areas, surface heating will not be as long nor strong as
yesterday, but still enough to build a well-mixed subcloud layer,
supporting strong to locally severe gusts. With enough mid/upper
flow and deep shear to support both organized multicells and at
least transient supercells (effective-shear magnitudes of 25-40 kt),
isolated, marginally severe hail is possible again today as well.
..Edwards/Leitman.. 09/01/2023
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