Tuesday, December 8, 2015

Big storm to dump 'heavy snow' through Friday

Winter is here. From the National Weather Service:
Wet Storm to Bring Heavy Snow Thursday Through Early Friday


Travel hazards from winter road conditions Thursday into early Friday, especially over Sierra passes
Snow will be heavy and wet in nature "Sierra Cement" (added)
Debris flows possible over recently burned areas
Localized flooding from wind-blown leaves clogging drains/gutters
Localized power outages & downed branches

Forecast Confidence
High for wet pattern Wednesday-Friday
High for heavy snow Thursday-early Friday
Snow level (7300 ft) Thursday morning, dropping below 5000 ft in the early evening(Slowed timing)
Medium for gusty winds Thursday afternoon and evening
Medium for exact precipitation amounts due to model variability
Low for Thursday afternoon thunderstorms
Low for debris flows over recently burned areas(added)

Timing and Strength
Starts Wednesday, heaviest precipitation Thursday, tapering off Friday
Heaviest snow impacting travel over Sierra passes expected between 10 am Thursday morning through 10 pm Thursday evening
Winds: Gusts up to 35 mph in the Valley with gusts 50+ mph near Sierra crest
Precipitation: 0.5-1.5" in the Valley, 2-4" Foothills/Mountains
Snow: 12-18" Sierra pass summit, 4-8" inches 5000 feet, up to 2 feet over high peaks(Lowered amounts)
Debris Flows: Best potential Thursday afternoon and Thursday night from thunderstorms

Weather Summary

A series of storms will bring a wet pattern from mid to late week, with periods of Valley rain and heavy high mountain snow. Warm air ahead of this system will delay the lowering of snow levels to during the day Thursday. Snow will begin to impact pass level traffic Thursday mid morning and continue to drop by Friday to around 4000 feet. Heaviest precipitation will be on Thursday, with over a foot of new snow possible above 6000 feet and the potential for 2 feet over high peaks. Rain storm total amounts around 0.5-1.5" projected for the Valley, with liquid equivalent for the mountains up to 2-4".
There is a slight chance for thunderstorms with locally heavy rain Thursday afternoon and evening. However, confidence is low as to where the storms will develop in relation to specific burn areas if they develop at all. Leaves clogging drains may cause local ponding and/or minor flooding of roads. Breezy southerly winds expected in the Valley with gusts up to 35 mph, with winds gusting to 50+ mph along the Sierra crest. Snow will be heavy and wet "Sierra Cement". This type of snow combined with the wind could cause local power outages, and whiteout conditions over mountains. Travel over Trans Sierra passes will be impacted Thursday, so plan accordingly.

Confidence continues to improved for a wet/snowy system to impact the area on the weekend, mainly on Sunday.

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