2008

The last 24 hours have brought significant cold rains to the northern half of the state, and heavy snowfall in places that don’t often see it. 8 or more inches of snow has fallen in the Santa Cruz Mountains and in parts of the Diablo Range, with over a foot in parts of the Santa Lucias. The peaks of the Northern Bay Area have seen over 5 inches of snow, and even the hills down to 1000 feet have accumulated 1-3 inches of the white stuff. The Sierra foothills have also seen quite a bit of snow. Snow briefly accumulated at the north end of the Sacramento Valley (in Redding) yesterday, but didn’t make it down much below 500-800 feet. The key message here: more is on the way. A rather interesting-looking cut-off low is currently spinning off the CA coast, bringing periods of rain and snow above 1500 feet . Isolated lightning strikes have occurred near the center of the low, mainly offshore at the moment. This will likely change, however, by tomorrow morning. As the low picks up even more Pacific moisture and some breaks in the clouds develop, dynamic instability (combined with good wind shear and helicity) will almost certainly generate some thunderstorm activity from the Bay Area southward to the Los Angeles area and inland in the Sacramento Valley. Some of this activity could become quite strong, with a strong jet streak at 500 mb rounding the base of the low and lending a great deal of upper-level divergence. The primary threat would be copious amounts of hail and grapuel, which could accumulate enough in some places to cause major travel problems (even to sea level if you’re under a strong cell). Waterspouts are a very real possibility, esp. over the SoCal Bight, as relatively warmer water temps increase lapse rates and island-effect convergence patterns lead to increased local wind shear. Snow levels should rise above the very low levels of the past 24 hours to 2000 feet in the north and 3000 feet in the south, but that’s still quite low and some additional accumulations are possible above 2500 feet. Once this system finally begins to clear out on Thursday (though some scattered cold showers will remain) a new and likely more powerful storm will dive south from the Gulf of Alaska late on Thursday. This system will have much stronger dynamics and more moisture to work with, leading to a stronger cold front and potentially even more interesting weather. Heavy and intense precipitation is likely with the front itself, as dynamic and static stability will be quite low and lead to significant convective activity in addition to the stratiform precip. Thus…thundertstorms are likely to develop along and behind the front late Thursday into Friday. This front may be strong enough  to produce some local flooding. The other major consideration is the cold temperatures aloft this system will usher in.  850 mb temps never rise above 0 C before this system moves in, so it will be a reinforcing shot of modified Arctic air to an already cold airmass. Just how cold these 850 mb temps will b, however, is a matter of contention. It is possible that this system could be significantly colder than the current one, especially according to the NAM, which brings 850 mb temps of -6 C to NorCal on early Friday morning along with significant precipitation. This would indicate snow levels in NorCal of no higher than 1000-1500 feet, and very possibly to sea level. The GFS is not quite as cold, and the ECMWF is not nearly so cold. I think it is likely that the ECMWF is underestimating the amount of cold air that will be present, but the GFS might have the right idea. Even then, the slight possibility of sea level snowfall in NorCal will have to be watched. If the NAM were to verify, there would likely be briefly accumulating snows in the Central Valley and even in the urban parts of the Bay Area. Don’t get too excited yet–but certainly stay tuned. Needless to say, thundetstorms with lost of hail are likely once again on Friday. Cold showers taper off again on Saturday before another very cold system…with snow down to 1500 feet or lower…drops in from the north. Depending on how sharp this trough is and how far south it digs, there is also the slight possibility of sea-level snow in NorCal from this one, as well. And it doesn’t end there–very cold and moist storms are forecast every 24-48 hours through the end of the 16 day run of the GFS. SoCal will see it’s fair share of the action, as well–snow levels will not be quite so low (but still down to 1500-2000 feet at times), but heavy rainfall could occur at times through the weekend. Update: new 18z GFS indicates the potential for the Friday system to have a major tropical tap in far SoCal (San Diego area). THAT would make for a VERY impressive storm for that area. We’ll see…