Mesoscale Discussion 813

Issued

May 14, 2012, 4:14 PM PDT (23:14 UTC)

Expired

May 14, 2012, 5:45 PM PDT

In force

91 min

Issued by

the Storm Prediction Center

Chance of a watch

5%

Counties Lassen County, CA, Plumas County, CA, Shasta County, CA, Siskiyou County, CA, Trinity County, CA

The Beale AFB radar (KBBX), 125 km from the middle of the area, began a volume at 23:15 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

886 
ACUS11 KWNS 142315
SWOMCD
SPC MCD 142314
CAZ000-150045-

MESOSCALE DISCUSSION 0813
NWS STORM PREDICTION CENTER NORMAN OK
0614 PM CDT MON MAY 14 2012

AREAS AFFECTED...PORTIONS OF NRN CA

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 142314Z - 150045Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...A THREAT FOR ISOLATED STRONG STORMS MAY PERSIST INTO THE
EVENING. ANY INSTANCES OF SVR STORMS ARE EXPECTED TO BE
SPORADIC/MARGINAL...THUS PRECLUDING THE ISSUANCE OF A WW.

DISCUSSION...A COMPACT CLOSED LOW IN THE MID/UPPER LEVELS HAD
EARLIER MOVED ONSHORE NORTH OF THE SAN FRANCISCO AREA PER WATER
VAPOR LOOP. THE SUPERPOSITION OF CYCLONIC VORTICITY AROUND THIS
FEATURED ON A BACKGROUND WLY CURRENT IN THE MID/UPPER LEVELS /ATOP
WEAKER LOW-LEVEL FLOW/ IS SUPPORTING DCVA OVER PORTIONS OF NRN
CA...WHERE ASCENT IS ALSO BEING AUGMENTED BY OROGRAPHIC EFFECTS. AN
INTENSE STORM THAT DEVELOPED OVER LASSEN COUNTY HAS MOVED INTO
PORTIONS OF SHASTA/TEHAMA COUNTIES AND WEAKENED. ADDITIONAL
CONVECTIVE DEVELOPMENT WILL BE POSSIBLE OVER THE NRN SIERRAS AND
LOCATIONS FARTHER NORTH/NORTHWEST INCLUDING THE SISKYOU AND KLAMATH
MOUNTAINS WITHIN A ZONE OF VERY MODESTLY ENHANCED DEEP MOISTURE OVER
THE NRN SEMICIRCLE OF THE CYCLONE -- PW VALUES OF 0.50-0.85 INCH PER
GPS DATA.

ACROSS THE AREA...STEEP MID-LEVEL LAPSE RATES WILL BE SUPPORTED BY
/1/ RELATIVELY COOL MID-LEVEL TEMPERATURES ASSOCIATED WITH THE
CYCLONE -- AROUND -17C AT 500 MB PER RAP OUTPUT...AND /2/ THE WWD
TRANSPORT OF THE DEEP MIXED LAYER ORIGINATING OVER THE CNTRL GREAT
BASIN. RAP SOUNDINGS SUGGEST MLCAPE VALUES OF 500-1000 J/KG
AFFECTING THE AREA...WHICH WILL CONTINUE TO SUPPORT THE POTENTIAL
FOR WWD/NWWD-MOVING CONVECTION INTO EARLY EVENING AMIDST MODERATE
MID-LEVEL ELYS TO THE NORTH OF THE CYCLONE CENTER CONTRIBUTING TO
EFFECTIVE BULK SHEAR MAGNITUDES GENERALLY FROM 25 TO 40 KT. THIS MAY
YIELD OCCASIONALLY ORGANIZED STORM STRUCTURES...WITH ISOLATED
INSTANCES OF SMALL HAIL /PERHAPS REACHING MARGINALLY SVR THRESHOLDS
ON A VERY ISOLATED BASIS/. DEEPER LAYERS OF STEEP LAPSE RATES AND
LARGER TEMPERATURE-DEWPOINT SPREADS OVER THE LOWER
ELEVATIONS/VALLEYS -- INCLUDING NRN PARTS OF THE SACRAMENTO
VALLEY -- MAY ALSO SUPPORT THE THREAT FOR STRONG WIND GUSTS.

STORMS COULD REACH PORTIONS OF ERN TRINITY AND WRN SISKYOU COUNTY BY
EARLY EVENING...WITH CONVECTION LIKELY WEAKENING AS THE NOCTURNAL
STABLE LAYER GROWS AFTER PEAK HEATING. THE OVERALL COVERAGE OF
STRONG STORMS WILL BE GREATLY LIMITED BY THE LACK RICHER
TROPOSPHERIC MOISTURE/GREATER BUOYANCY...WHICH WILL PRECLUDE THE
NEED FOR A WW.

..COHEN/WEISS.. 05/14/2012

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...REV...MFR...STO...EKA...

LAT...LON   39702116 40252227 40502329 40932357 41362328 41662270
            41642184 41262103 40882064 39862014 39622051 39702116

Mesoscale Discussions in California, 2012Every mesoscale discussion in 2012

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.