Mesoscale Discussion 42

Issued

Jan 27, 2021, 10:00 PM PST (06:00 UTC)

Expired

Jan 28, 2021, 4:00 AM PST

In force

6 hr

Issued by

the Storm Prediction Center

Concerning

Heavy snow

Counties Alpine County, CA, El Dorado County, CA, Mariposa County, CA, Nevada County, CA, Placer County, CA, Sierra County, CA, Tuolumne County, CA, Storey County, NV

The Reno radar (KRGX), 130 km from the middle of the area, began a volume at 06:00 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.

896 
ACUS11 KWNS 280601
SWOMCD
SPC MCD 280600 
CAZ000-NVZ000-281200-

Mesoscale Discussion 0042
NWS Storm Prediction Center Norman OK
1200 AM CST Thu Jan 28 2021

Areas affected...The California High Sierra

Concerning...Heavy snow 

Valid 280600Z - 281200Z

SUMMARY...Very heavy snow with rates of 3-5 in/hr are expected to
continue overnight across portions of the Sierra Nevada above 4,000
feet. Blizzard and whiteout conditions may also occur with the very
heavy snow and strong winds.

DISCUSSION...Evening water vapor imagery showed a mid-level trough
across the eastern Pacific and northern California coast. A 60-80
knot mid-level jet streak was located along the eastern periphery of
the trough extending onshore into western Nevada. 

Widespread strong ascent is occurring along much of the West Coast
with the trough and jet streak manifesting as a strong poleward
moisture flux in the low and mid-levels. Regional radar and
satellite observations show a large band of enhanced precipitation
associated with the strong moisture flux orthogonal to the Sierra
Nevada Range. Very heavy precipitation is falling with this band as
confirmed by spotter reports and several ground stations around 7000
ft MSL (KBAN & KMMH) which have reported continuous heavy snow for
the last several hours. Heavy snow is expected to continue for at
least the next 6 hours as; forcing for ascent from the
aforementioned trough and jet streak continue to move onshore,
persistent strong orographic ascent from mid-level flow interacting
with the Sierras, and decreasing static stability/moistening from
relentless moisture transport into central/southern California. 

The previously mentioned factors will support very heavy snowfall
rates of 3-5 in/hr (locally higher amounts possible) from 4000-8000
ft through tonight. The heaviest snow is forecast to remain across
the central and southern Sierra, but will likely expand northward
slightly as the mid-level trough and associated forcing for ascent
pivot onshore later tonight. Heavy snow may also spill over the
crest into portions of western Nevada tonight where snowfall rates
close to 1 in/hr have been observed periodically.

..Lyons.. 01/28/2021

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

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

LAT...LON   39691926 39421952 39201973 38931987 38691971 38461950
            38151916 37891885 37421861 37071863 36911871 37001917
            37281963 37612001 38292038 38852071 39162080 39592072
            39882035 39981988 39921946 39691926

Mesoscale Discussions in California, 2021Mesoscale Discussions in Nevada, 2021Every mesoscale discussion in 2021

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.