2009

California has been experiencing an unusually active late-spring period in many eastern mountain areas. Thunderstorms have been nearly a daily occurrence in the Sierras and occasionally the Coastal Range and the Transverse Range. May is usually a quiescent period for most of the state, but some of these mountain thunderstorms have been quite strong and locally even severe. The distribution and intensity of these storms is actually likely to increase further in the coming days as deep southeasterly flow develops and advects extremely unseasonable monsoonal moisture into the state. This will be the main weather story over the next five days. Some storms may drift over foothill or even far eastern valley locations before falling apart through the upcoming weekend. The pattern gets even more interesting starting next Monday, however…

The models are currently in somewhat remarkable agreement that a low pressure area will cut off in the far Eastern Pacific over the weekend and slowly make its way to the southeast, setting up shop not far from the CA coast. The exact placement of the low is not certain, but this is one of those rare scenarios that almost any placement offshore would produce nearly the same result–lots of thunderstorms. The low is progged to be fairly deep, so moist southerly to southeasterly flow at the surface and mid levels combined with strong divergence aloft on the east side of the low would be more than sufficient to produce widespread convective activity not anchored to (though perhaps enhanced by) elevated terrain. The low is also expected to have some relatively cold temperatures aloft, and this will add some surface-based instability into the mix, as well. In all, if the present solutions of the GFS and ECMWF pan out (or even some similar setup occurs), much or all of CA will be looking at at least a 48 hour period of significant thunderstorm potential. Several model runs indicate that this low will stick around for a while, giving Central California a weather pattern more reminiscent of the Gulf Coast. Also worth noting is the possibility of severe thunderstorms. This type of pattern would be just about ideal for organized thunderstorm development in CA, and it would not be at all surprising, should this pattern actually occur, to have an outbreak of multiple severe storms over the mountains and perhaps even in the valleys. It is rare to see a synoptic pattern that favors severe weather in CA; the vast majority of the relatively few severe storms that do occur result from local processes. This is still a ways out, but the models are currently in remarkably good agreement and have been so now for over 24 hours. I will update as time permits, but expect at least one more full update by mid-weekend.

Side note: many of you have been discussing the possible progression to positive phase of ENSO (an El Nino event). SST anomalies are currently small, but are trending positive over all of the major Pacific basins. Numerical guidance from the CPC does indicate that continued warming and perhaps at least a weak El Nino event is likely to unfold by the mid-summer months. Some of you have pointed to a particular graphic that shows predicted anomalies up to 3 degrees F; keep in mind that this forecast only applies to the Nino 3.4 region, and that other regions are not forecast to have SST anomalies quite as high. This, however, may change in the coming months. Stay tuned…