Moderate NorCal storm in short term; potentially very cold event in long term

Filed in Uncategorized by on January 1, 2007 2 Comments

The new year is off to a beautiful start here in CA. The high temperature today here in the San Rafael hills eclisped the 60 degree mark for the first time in nearly a month. This dry and warm weather (with chilly overnight lows) will continue for one more day before a fairly dramatic change in the presiding weather pattern begins to develop. A cold and strong Gulf of Alaska trough will drop sharply southward down the West Coast beginning later tomorrow in the far north and will slowly work its way down toward SoCal by Friday. There is a fair amount of moisture with this system–no spectacular tropical tap exists, but the system is not moisture-starved either. Cold air advection will be quite strong , esp. near frontal passage, so baroclinic instability will be significant. The end result will be a fairly robust frontal band moving down the coast, bringing a period of heavy rain and poss. iso. thunder along with gusty winds up to 45 mph in favored locations. Total rainfall could be fairly heavy in far NorCal–locally as much as 4 inches–and the Bay Area should see 1-2 inches for the most part. SoCal will generally see less than 1 inch, except possibly in favored mountain areas. All in all, a very typical winter storm.

The long-term, however, is a great deal more interesting. The GFS has been indicating for several days now (12-15 run cycles) that a major blocking pattern will attempt to form over the central/eastern Pacific Ocean out beyond days 7/8. As indicated by the past few runs, however, the potential implications of such a pattern are quite impressive. The most recent model run–the 12z–indicated an almost unbelievably amplified pattern over the Central Pacific (see picture below). What is most significant, though, is the placement of the retrogressive (contra-mean flow–west-moving instead of east-moving, that is) trough and developing low pressure area. The nearly meridonal (north-south) flow on the downstream side of the block will allow for some extremely cold air aloft to stream southward, offshore of WA, OR, and CA. The jet fueling this Arctic plunge will be quite strong, allowing for much of this cold air arriving relatively intact to much lower latitudes. We usually talk about “modified Arctic airmasses” causing winter cold and snow outbreaks along the West Coast, but it is conceivably possible that much of WA and even OR could see virtually unmodified polar air make an appearance in the next 2 weeks. The timing is also rather perfect for the advection of extremely cold air into the West–temperatures over the Alaskan interior will drop to nearly -60 degrees Fahrenheit tonight, with windchills below -75 degrees! This potential event–and keep in mind that the development of this cold outbreak is by no means certain–is still at least 7/8 days out, so it is far too early to get into the specifics. Generally, however, I would expect some record cold temperatures and lowland snowfall in the first half of January and possibly beyond. At their coldest, 850 mb temperatures are currently progged by the GFS to approach -20 degrees C in the Seattle area, -15 degrees C in the Portland area, possibly as low as -12 degrees C in the Eureka area, as low as -9 degrees C in the Bay Area, and as low as -5 degrees C in SoCal. 850 mb temps could fall below -3 degrees over the Pacific south and west of Baja, CA! This would produce preliminary snow levels of, respectively, sea level, sea level, sea level to 500 feet, 500-1000 feet but locally to sea level, and 500-1500 feet. Snow could also fall in the mountains on the spine of the Baja peninsula. The other major story will be the potential for significant precipitation. Extreme cold patterns here usually occur as a result of very dry, continental airmasses moving east-west to the coast (a sort of reversed zonal flow). The pattern indicated by the GFS would be highly meridonal in nature, which would allow for a great deal of overwater trajectory. Also, the retrogressive low under the main block will feed off of significant vorticity advection, possibly creating the dynamics necessary for focused precipitation. Convection, including lots of thunderstorms with hail, would be a major problem, as well. It is possible that a trough/low of this magnitude and this low latitude will pick up tropical moisture on its southern edge. If this were to occur, the impact of the cold air would be lessened under and south of the tropical plume, but these areas would be subject to prolonged heavy rains and strong winds. So…if this pattern pans out in the way the GFS currently indicates it will, all of CA and the West Coast will see some form of active weather. If the models are still indicating this development 24 hours from now, confidence in the evolution of this event will be greatly increased. An aside: this type of blocky, cold pattern is certainly not what one might expect in a year with a moderate El Nino currently ongoing. What this really serves to show is the fact that weak and moderate ENSO warm events aren’t particularly reliable indicators as to the future climate conditions in CA and the PacNW. In any case, stay tuned. Updates will be frequent in the coming days…

Jet ampmaximumcoldpatternlarge.jpg

Above left: 216 hours GFS jet stream forecast

Above right: illustration of upcoming cold pattern