Some light precipitation this weekend, then dry once again

Filed in Uncategorized by on January 26, 2007 7 Comments

The very unimpressive-looking cut-off low spinning off the CA coast did manage to bring some light showers to coastal parts of the Bay Area today. Amounts were catergorically very light: I believe the 0.11 in. we received here in San Rafael was just about as much as anywhere else. That’s enough to wet the ground…but not quite enough for puddles. Some more occasional showers could occur throughout the weekend near the coast (that in the north and south) but rainfall accumulations will contine to be very light. Totals from this system will almost certainly remain below 0.50 inches, and well below 0.25 in. in most spots. As rainfall of this magnitude is not hydrologically significant, the main effect of this locally damp weather will be to spur the perennial grasses into a period of growth. When skies clear early next week and 8-9 hours of sunshine are available, the natural growing season should be in full swing. This is not good news at all for the upcoming fire season. Long-term precip defecits will allow the older and less responsive fuels to remain dry, but these brief and light precip events will allow the seasonal and flashy brush-type ladder fuels to grow very quickly, esp. in NorCal and CentCal. And, to answer the burning question on everyone’s mind–no, this meager rainfall does not signal a reversion to a wetter and more seasonable pattern. Here’s why:

The pattern since the start of the new year (and even before) has been unusually persistent. Even the most anamalous of winter patterns in the Eastern Pacific are, at least for the most part, somewhat progressive (transitive). Our current dryness is being caused by the presence of a powerful Omega-type blocking pattern centered over the far northeastern Pacific. The so-called “Omega Block” is named after the Greek letter ( Ω ). Here’s a crude illustration:

            Jet Stream>>>             /High\                       

                           >>>   __/Low          Low\__

This type of block effectively prevents any storm systems riding in on the westerlies from maintaining zonal trajectory. The jet is forced up and over the dominating high pressure area (which often extends much further south than indicated in this drawing) and directs energy far to the north, usually causing srtong warm air advection as warmer southern air ascends in latitude. As the jet rounds the top of the block, generally dry and cold air plunges back southward on the east side, creating a clearly-defined cold air advection pattern. We were on the right side of such a block earlier this month, and this is how the extremely cold polar air aloft reached our rather southern latitude intact. We are currently underneath the high pressure core of this rather spectacular Omega block, which is bringing moderate temperatures and stagnant flow aloft. The current weakness of the steering currents  is illustrated vividly by the sluggish movement of the cut-off currently impacting our weather. In order for this type of pattern to break down, an extremely powerful jet stream would have to punch through this extensive high pressure area covering much of the E. Pac. Alternatively, the entire block could shift 1000-1500 miles to the east, leaving CA on the recieving end (deep southery flow aloft and a near-200 kt jet) on the western side. This undercutting of the ridge by the southern (subtropical) stream of the jet always has the potential to bring heavy rains to the West Coast when it occurs. Neither situation looks likely at this point. In response to a reader question, the models (namely the GFS, as that is the only publicly-available long-range model) are indicating a continuation of the current pattern because, beyond all else, there is no convincing reason for it to change in the near future. The models have tried, at times, to start to break this tenacious pattern down, but none of these solutions have actually panned out. Given the weight given to climatological means in the GFS algorithim, I’m surprised that the model isn’t more aggressive in trying to develop a change towards a more normal, wetter pattern across the West. The GFS has actually performed surprisingly well during this highly anomalous pattern, and so there is greater-than-normal confidence in the current model solutions–that the longwave pattern will remain essentially unchanged over the next 1-2 weeks, at least. And so we remain locked in this seemingly endless dry pattern…