Alert archive / California / Mesoscale Discussions in 2011

Mesoscale Discussion 239

March 20, 2011Alpine County, Tuolumne County, California

The Storm Prediction Center issued this mesoscale discussion at Mar 20, 2011, 9:51 AM PDT for Alpine County, Tuolumne County, California. It was in force for 4 hr.

The area

The outline as issued.

The facts

Issued: Mar 20, 2011, 9:51 AM PDT (16:51 UTC)
Expired: Mar 20, 2011, 1:45 PM PDT
In force: 4 hr
Issued by: the Storm Prediction Center

Counties Alpine County, CA, Tuolumne County, CA

The radar

The San Joaquin Valley radar (KHNX), 148 km from the middle of the area, began a volume at 16:53 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.

The nearest sounding before it

Reno, NV (KREV), launched 12:00Z, 4.9 hours before the alert went out and 237 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 0 J/kg
Mixed-layer CIN: No buoyant parcel
Mixed-layer LCL: 595 m above ground
0-6 km bulk shear: 87 kt
0-1 km storm-relative helicity: 307 m²/s²
0-3 km storm-relative helicity: 434 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

427 
ACUS11 KWNS 201652
SWOMCD
SPC MCD 201651
CAZ000-NVZ000-202045-

MESOSCALE DISCUSSION 0239
NWS STORM PREDICTION CENTER NORMAN OK
1151 AM CDT SUN MAR 20 2011

AREAS AFFECTED...THE SIERRAS OF E-CNTRL CA AND EXTREME W-CNTRL NV

CONCERNING...HEAVY SNOW

VALID 201651Z - 202045Z

THE THREAT FOR SNOWFALL WITH RATES OF 2 IN/HR WILL SHIFT SWD ACROSS
THE SIERRAS OF E-CNTRL CA AND EXTREME W-CNTRL NV THIS AFTERNOON.

A ZONE OF STRONG SLY TO SWLY FLOW IN THE BASE OF DEEP-LAYER LOW
PRESSURE AND AN ATTENDANT TROUGH OVER THE ERN PACIFIC IS IMPINGING
ON THE SIERRAS /500-MB SWLY WINDS OF 70 TO 80 KT ACCORDING TO RUC
ANALYSIS/. THE COMBINATION OF AMPLE MOISTURE TRANSPORT AND UPSLOPE
FLOW CONTINUES DRIVING PRECIPITATION OVER THIS AREA. SUFFICIENTLY
COLD TEMPERATURES OVER THE HIGHER ELEVATIONS...PARTICULARLY AOA 4000
FT ABOVE MSL...WILL SUPPORT HEAVY SNOW INTO THIS AFTERNOON ACROSS
THE CNTRL/SRN SIERRAS. MORNING SFC OBSERVATIONS INDICATE HEAVY SNOW
IN THE LAKE TAHOE AREA...LIKELY IN ASSOCIATION WITH A SLOWLY MOVING
COLD FRONT. THE AFOREMENTIONED DEEP-LAYER TROUGH IS FORECAST TO
AMPLIFY TODAY...SHUNTING THE ZONE OF STRONGEST DEEP-LAYER FLOW AND
ASCENT SWD. THIS WILL GRADUALLY DRIVE THE FOCUS FOR POTENTIAL
2-IN/HR SNOWFALL RATES SWD...AS SUGGESTED BY THE 14Z RUN OF THE
EXPERIMENTAL HRRR. HOWEVER...AS MID-LEVEL LAPSE RATES STEEPEN ACROSS
NRN PORTIONS OF THE DISCUSSION AREA THIS AFTERNOON...THE
PRECIPITATION COULD ASSUME MORE OF A CONVECTIVE NATURE WITH AN
ATTENDANT THREAT FOR 2-IN/HR RATES. THESE CONDITIONS ARE EXPECTED TO
PERSIST INTO THE EARLY EVENING HOURS...BEFORE TAPERING OFF TONIGHT.

..COHEN.. 03/20/2011

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

ATTN...WFO...VEF...REV...HNX...STO...

LAT...LON   35691811 35921856 36771911 37631983 38402040 39022051
            39462038 39432009 38961980 38261937 37601879 36251796
            35691811

Nearby

Mesoscale Discussions in California, 2011 Every mesoscale discussion in 2011

Sources

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.