2009

Well, if you had asked me yesterday, I would have guessed that this potential event would already be in the early stages of fizzling out before it even started. Such is the tendency of early-season storms in this part of the world. At least for the time being, however, the event slated for early next week shows no signs of falling apart. Very much on the contrary–all the major computer models have (in the past several runs) significantly ramped up both precipitation totals and the potential for strong winds at the surface. The ECMWF has had much more consistency than the GFS in forecasting the potential for heavy precip and strong winds in NorCal during this event, but the most recent 00z run of the GFS is a real show-stopper. This run, in particular, brings truly phenomenal rainfall accumulations to all of NorCal over a 36-48 hour period–on the order of a foot or more. Keep in mind that the GFS does not really have sufficient resolution to incorporate the crucial orographic effects of precipitation in CA and as a result almost always underestimates basin-averaged precip during strong storms. Also very much of note is the significant potential for strong to very strong winds ahead of the cold front. A very large and deep low–perhaps sub 970 mb–is expected to deepen off the WA coast by Monday. 850 mb winds are forecast by the 12z ECMWF to exceed 75 kts over coastal Oregon and 65 kts over Norcal. The 00z GFS is now in agreement with the ECMWF forecast, perhaps even a little stronger. Were the storm to pan out exactly as indicated by the 00z GFS, it would probably the the strongest October storm ever in CA. Certainly, the rainfall output of the GFS would suggest that daily (and probably monthly) October records would be obliterated all across NorCal. As always, keep in mind that this system is still a good 5-6 days from reaching the coast, and the forecast will likely change quite a bit between now and then. Even so..the potential for a storm of this magnitude in CA in October is extremely unusual, and it bears close watching. SoCal may be spared much significant weather from this system aside from some light precipitation, but there has been a distinct shift southward with model forecasts thus far. Again: stay tuned.

What is causing this highly anomalous storm potential so early in the season? Former super typhoon Melor, now a tropical storm off the north coast of Japan after striking near Tokyo as a category 1 storm, is currently being absorbed into the Westerlies. A large volume of moisture has already been sheared off of the decaying tropical cyclone and is traversing the Pacific at this time. The remnant circulation of the typhoon is expected to become the surface low of the new extratropical cyclone that may cause all of the action next week. As mentioned previously, this is not dissimilar to the setup of the Columbus Day Storm of 1962. There is no indication that this storm will even approach the magnitude of that storm in terms of maximum wind speed, but it’s also important to keep in mind that that benchmark event affected WA and OR most severely, leaving NorCal with only moderate damage (which has since been repeated in subsequent storms in the intervening decades). In terms of its effect on CA, the storm next week very well could be similar.

Another slightly more technical note: all the models are showing a truly incredible degree of isentropic lift over NorCal during the warm frontal passage and even right up to near the cold front itself. Not only will this tend to dramatically enhance orographic effects, but there may also be an enhanced chance of convection associated with it. In addition, the new GFS has a 150 kt jet streak over CA with the left exit region over–you guessed it–NorCal. Pretty cool stuff for October, I will say…