SPC MD 1629

MD 1629 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF FAR SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA

Mesoscale Discussion 1629
NWS Storm Prediction Center Norman OK
0415 PM CDT Fri Sep 08 2017

Areas affected...Portions of far southeast California and southwest
Arizona

Concerning...Severe potential...Watch unlikely 

Valid 082115Z - 082345Z

Probability of Watch Issuance...5 percent

SUMMARY...Isolated strong-to-severe storms are expected through the
afternoon across portions of southeast CA and southwest AZ. A WW is
not expected due to the isolated nature of the threat.

DISCUSSION...Thunderstorms are beginning to increase in
coverage/intensity across portions of far southeastern CA, with
Multi-Radar/Multi-Sensor data indicating periodic short-lived strong
updraft pulses. These storms are forming amidst strong diurnal
heating (e.g., surface temperatures in the mid/upper 90s) and rich
low-level moisture (e.g., surface dewpoints in the upper 60s), which
is contributing to moderate instability (e.g., MLCAPE 1000 to
locally 3000 J/kg). Poor deep-layer shear (e.g., less than 20 kts
effective bulk shear) will lead to outflow-dominant storms with
pulse-type and transient multicell modes. Steep low-level lapse
rates coupled with inverted-v profiles (evident in 20Z 1Y7 sounding)
will allow for strong gusty outflows, as evaporative cooling
enhances downward momentum transfer in collapsing updraft cores.
Thus, isolated instances of strong-to-severe wind gusts are possible
with the most robust/persistent updrafts. While hail should
generally remain sub-severe, a couple of near-severe hailstones are
possible owing to sufficient CAPE through the hail growth zone. 

Activity is expected to weaken as it moves northward into a less
favorable environment where instability is much weaker. A WW is not
expected given the isolated nature of the threat.

..Elliott/Grams.. 09/08/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...PSR...VEF...

LAT...LON   32581565 33631580 34001588 34341576 34621553 34691512
            34751485 34771449 34691424 34491397 34021394 33551399
            33211404 32911411 32731419 32471421 32411439 32581565 

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