Dramatic pattern change once again

Filed in Uncategorized by on April 28, 2009 35 Comments

Meteorological spring is traditionally a period of great atmospheric flux, with wide variations in temperatures from day to day and large contrasts between wet and dry periods. Usually, much of California experiences something of a muted spring, with the more rapid oscillations in sensible weather being relegated to more inland parts of the North American continent. This year, however, California has played host to a very wide range of weather patterns, and is about to experience another (and perhaps even more unusual) dramatic shift over the next 72 hours.

Presently, a cold upper low centered over northeastern Oregon is sending cold air aloft over much of CA, producing cool temperatures, locally windy conditions, and some high-Sierra snow showers. This low was originally forecast to move further south and west into CA, producing copious convective activity, but this has obviously not been the case. Instead, as the main low begins to lift northeast and lose its influence on the state, a powerful low-latitude Pacific jet will punch through the mean west coast ridge. The models have been struggling tremendously with this upcoming pattern shift, and understandably so: this is a very unusual pattern for this time of year, especially given the antecedent heat waves earlier this spring. In any case, it now appears that some significant precipitation will occur over at least Northern California and potentially further south. The models are still apparently trying to wring out the details, but the NAM, GFS, and ECMWF have all trended much wetter and stronger with the weekend storm (and potentially with another storm  early next week) in recent runs. This storm is progged to have a deep tropical tap, and based on current water vapor imagery over the Pacific, there is currently a tremendous amount of moisture being pulled up from the subtropics. It is possible that the models are actually underestimating the magnitude of the moisture extrainment, especially given the built-in climatological bias  in precisely the opposite direction. This could proved to be a fairly interesting storm: The will be some wind and probably a fair bit of rain, but what is really remarkable is the surface temperatures in NorCal. Even with steady rainfall, temperatures may surpass 70 in the Central Valley this weekend, which would be quite remarkable. With all this moisture, warm temperatures, and energy aloft, I would suspect that there will be at least a chance of convection at some point. This storm is worth keeping an eye on, because if the subtropical tap has been underestimated, it is possible that SoCal could also see significant precipitation out of this event. Stay tuned.

Some of you have commented on the “gustnado” that occurred in Woodland last Friday.  This event was actually the product of a very interesting convective outbreak over Solano, Yolo, and Sacramento counties that occured very suddenly between 5:10 and 5:30 PM. Synoptically, a small but cold upper low had just developed over the Northern Sacramento Valley earlier in the day and had slowly been drifting to the south all afternoon, forcing a “cool” front to thesouth ahead of the low. At the same time, clear skies for most of the day in the Southern Sacramento Valley had led to surface heating. In response to the north-south temperature gradient in the Valley, a strong Delta breeze developed  by mid-afternoon, with sustained winds 20-25 mph and gusts over 35 mph in Davi. This strong southerly flow at the surface converged with the south-moving “cool” frontal boundary and produced a rather spectacular and very narrow arc of moist convection. Due to the dryness of the boundary layer, most of the initial precipitation evaporated before reaching the ground, which likely increased winds even further at the surface as the downward momentum of the cooled air parcels hit the ground. Just as the line appraoched Davis, precipitation finally began to fall from the base of the clouds, and vertical development continued. Just to give an indication of how quickly these storms formed, I was in North Davis at the time (when I snapped the pictures below). When the line moved over, I experienced some moderate precipitation (though the raindrops were extraordinarily large and they were mixed with ice chunks from the downdraft). By the time the band reached South Davis, torrential precipitation was occuring, and as the line moved south across Interstate 80, the band began to produce lightning and hail. The NWS did issue a Severe Thunderstorm Warning on this band near Vacaville for the strong wind potential. It’s not clear if this ever verified, but there was indeed some apparent wind damage in Woodland. The NWS classified the wind event as a gustnado, which makes sense, considering the cumulus clouds were only 10,000 feet tall at that time with little precipitation. It is interesting to note that the gustnado occured with such minimal convection overhead–a testament to the extreme local convergence of the day. A very rare event in these parts, that’s for sure. Pictures below!

Click for larger images!