Alert archive / California / Mesoscale Discussions in 2021

Mesoscale Discussion 2025

December 13, 20216 counties in California

The Storm Prediction Center issued this mesoscale discussion at Dec 13, 2021, 3:01 PM PST for 6 counties in California. It was in force for 4 hr.

The area

The outline as issued.

The facts

Issued
Dec 13, 2021, 3:01 PM PST (23:01 UTC)
Expired
Dec 13, 2021, 7:00 PM PST
In force
4 hr
Issued by
the Storm Prediction Center
Concerning
Heavy snow

Counties Alpine County, CA, El Dorado County, CA, Mono County, CA, Nevada County, CA, Sierra County, CA, Tuolumne County, CA

The radar

The Reno radar (KRGX), 143 km from the middle of the area, began a volume at 23:02 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, 11.0 hours before the alert went out and 179 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-pSkew-T log-p diagram, Reno, NV at 12:00Z on 2021-12-13Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel. CAPE 0 joules per kilogram.LCL10008507005003002001001 km2 km3 km6 km9 km12 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed.
HodographHodograph, Reno, NV at 12:00Z on 2021-12-13The wind through the lowest 10 kilometers. Zero to 1 km storm-relative helicity -8 square meters per square second.1939587897117136Sfc1 km3 km6 km9 kmRMLM
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
774 m above ground
0-6 km bulk shear
72 kt
0-1 km storm-relative helicity
-8 m²/s²
0-3 km storm-relative helicity
209 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0

Open this sounding in Storm Lab

The text

As transmitted.

566 
ACUS11 KWNS 132301
SWOMCD
SPC MCD 132301 
CAZ000-NVZ000-140300-

Mesoscale Discussion 2025
NWS Storm Prediction Center Norman OK
0501 PM CST Mon Dec 13 2021

Areas affected...the central Sierra

Concerning...Heavy snow 

Valid 132301Z - 140300Z

SUMMARY...Snowfall rates in excess of 2 inches per hour are expected
above 5000 to 5500 feet with snowfall rates possibly as high as 4
inches per hour above 6000 feet.

DISCUSSION...A strong atmospheric river has been bringing
low-elevation heavy rain and mountain snow to much of northern and
central California through the day today. Cooler air has moved in
aloft with snow levels now dropping below 5500 feet based on central
Sierra observations. Expect these to continue to fall through the
evening. Simultaneously, a renewed push of heavy precipitation can
be seen on composite radar reflectivity across coastal and central
California around 2230Z. In addition, guidance indicates the
low-level jet is expected to strengthen from around 50 knots to 55
to 60 knots and veer more southwesterly. This will lead to
increasing upslope flow which will further enhance already heavy
precipitation. Therefore, very heavy to extreme snowfall rates are
expected with the greatest rates expected between 23Z and 08Z.

..Bentley.. 12/13/2021

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

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

LAT...LON   38061899 37411838 36881836 36851895 37451961 37952006
            38622053 39092068 39612093 39872075 39812046 39472020
            39251999 38931986 38641956 38381927 38061899

Nearby

Mesoscale Discussions in California, 2021Every mesoscale discussion in 2021

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.