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Extraordinary winter warmth and dryness to persist as West Coast ridge dominates

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Well below average precipitation has been widespread so far this winter across most of the American West. (Nov 3-Jan 31; NOAA/Weather West)

by on February 1, 2018  read full Weather West Blog post here

Summer in January; Sierra Nevada snowpack nears record low

Well below average precipitation has been widespread so far this winter across most of the American West. (WRCC)

Despite the calendar, it sure hasn’t felt much like the middle of winter across California in recent days. While well above average temperatures and below average precipitation have been widespread throughout the state, recent warmth and dryness have been especially concentrated across Southern California. Immediately following the warmest autumn (and before that, warmest summer) on record, a legitimate mid-winter heatwave baked the southern third of the state this week, setting numerous high temperature records. In fact, anomalous offshore flow brought daytime temperatures near 90 degrees and overnight lows above 70 degrees near the Pacific coastline–temperatures that would be well above average in these areas in June, let alone January….

“Severe drought” has returned to parts of Southern California

Today’s update of the U.S. Drought Monitor suggests that “severe drought” conditions have returned to parts of Southern California–most notably, Santa Barbara and Ventura Counties, where huge tracts of land have recently been burned by the nearly 300,000 acre Thomas Fire (in December) and/or the localized but devastating debris flows and flash floods (in early January). That might not come as a surprise to local residents–many of whom will point out that California’s multi-year, statewide 2012-2016 drought never really ended locally (where Lake Casitas, the only source of water for several communities, is running at about 35% of capacity). While the Northern California reservoirs are generally in much better shape due to carry-over from last year’s deluge (and California’s big cities, which are all tied into the state water system, are therefore unlikely to experience water restrictions this year), California’s wildlands and ecosystems will most likely experience more immediate adverse impacts from yet another warm, dry winter. The recent drought was a major factor in the bark beetle-linked mortality of well over 100 million Sierra Nevada trees in recent years. Despite last year’s wet and cooler reprieve, these drought and beetle-stressed forests have yet to recover from the intense multi-year drought that preceded it–so there is serious concern that forest mortality could accelerate once again during the upcoming dry season….

A big West Coast ridge has returned…and it looks pretty resilient

GEFS and ECMWF ensembles both strongly suggest persistent ridge will keep West Coast dry for 2+ weeks. (NCEP via tropicaltidbits.com)

At risk of sounding like a broken record: a strong, persistent, broad, and anomalous ridge of atmospheric high pressure has yet again set up along the West Coast. All indications are that it will probably stick around for the foreseeable future (certainly for the next 10 days, and plausibly for the next 2-3 weeks). This may sound a bit odd to those accustomed to the typical weather prediction mantra that anything out beyond about 7-10 days is essentially unpredictable. But recent evidence suggests that under certain circumstances, large-scale atmospheric predictability can be much higher over longer periods. The present instance of prolonged, stable ridging near the West Coast appears to be one of these situations, given the remarkable multi-model ensemble agreement that the large-scale flow pattern is unlikely to change much through at least mid-February (and perhaps even longer than that)….

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