Very active fall pattern shaping up for the West

La Nina may already be having a significant impact on the prevailing jet stream this year. An amplified northerly branch will bring storms ashore much of the Pacific Northwest over the next few weeks, at times including northern CA and eventually S. CA as well. Over the next three days, an almost incredibly deep low pressure system will develop off of the Oregon coast. Conditions are right for the low to undergo explosive cyclogenesis, or “bombogenesis” over the next 48 hours. To give a sense of scale here, the minimum central pressure forecast by the 00z GFS for this system  is about 959 mb. This pressure would equate to a major (catergory 3) hurricane, and although this will be an extratopical storm event wind speeds over the ocean will still be incredibly strong for this area and especially for this time of year. The magnitude of this storm would be comparable to the great Columbus Day Storm of 1962 (with a minimum central pressure of 958-960 mb), which is used as a benchmark on the West Coast against which powerful storms are measured. That storm produced widespread winds of 85 mph or more throughout much of OR/WA/N. CA, and recorded gusts exceeded 130 mph in at least 2 locations. That storm, which struck on October 12th, originated from the remnants of a powerful tropical cyclone in the Central Pacific. The situation this time is similar, but not identical–tropcial forcing, including the ingestion of a former powerful typhoon into the mean flow, is enhancing amplification of the trough and will likely induce rapid spin-up of the low. It is certainly interesting that this storm will be developing at exactly the same time of year (possibly to the day). The major difference here is that the strongest part of the storm will never hit land–100 mph wind gusts are likely over the ocean, but gusts on land probably won’t exceed 70 mph (in the usually windy areas) because the storm will begin to become vertically stacked and weaken significantly before impacting land later this week. That said–it will certainly be worth keeping an eye of this one, since it wouldn’t take too much of a track or timing deviation to bring some very strong winds and heavy rains to the West Coast this week. The cold front will still be very strong as it impacts the immediate coast, with gusty winds and heavy rainfall, but this will only extend inland a short distance since the front will already be in the process of falling apart. The Bay Area will probably be the southern extent of any gusty winds or briefly heavy rainfall, with some moderate rain in the Valley as far south as Sacramento and possibly some light showers further south than that. Southern California will see nothing from this system, unfortunately, although rainfall in the north is certainly going to be beneficial. The pattern actually continues to be active well into the extended period, with the potential for frequent and possibly significant rainfall for the entire state of CA during that period. Lots of uncertainty remains at this point, with tropical systems in the West Pac. and such as powerful system developing closer to home. It will certainly be an interesting week to watch the satellite loops…

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