Summer begins to show signs of a transition to fall…

Last week’s weather excitement in far SoCal and in the Sierras were caused in large part by the remnants of Hurricane Dean. A remnant circulation that was visible on satellite imagery may have been the actual center of circulation that was originally the core of the monster cat. 5 storm, but it’s hard to say for certain. Isolated parts of San Diego county recieved record rainfall and intense to severe convective activity, as did pockets of the Southern Sierras. Of course, most populated areas saw next to nothing aside from a few light showers. The GFS currently indicates the potential for a fairly significant slug of moisture to move up from the south during the next 24-48 hours. There is even a bit of precip being generated by the model over parts of the Central Coast and NorCal. This scenario is still highly uncertain and not supported by other models, but it’s worth at least a bit of consideration. It the meantime, it will be quite hot over much of CA for the forseeable future, though the heatwave will peak in the next 48 hours.

The sparsity of upper-air data over Mexico this summer has resulted in the models performing terribly during periods of southerly/easterly flow (where Mex. is the atmospheric source region), compared to the reasonably accurate forecasts that were produced for vernal monsoonal patterns in past years with enhanced funding from the North American Monsoon Experiment (NAME). This just goes to show how crucial observational data are in model initialization and the subsequent impact on forecast quality. The most substantial improvements that will be made in numerical forecast models in coming years will probably be of this nature; cloud physics and convective parameterization may be exceptions, but otherwise atmoshperic processes are fairly well represented in operational models and the greatest advances will come in the form of more widespread surface observations and vertical profiles in areas where there are currently large data voids (oceans, countries without reliable weather monitoring networks). That’s enough of a tangent on the future of weather forecasting; now back to the current synopsis…

The GFS has been indicating two things for several model runs now. First: that a tropical system will develop in the Eastern Pacific (which has been notably quiet lately) and move some distance off the Baja coast by days 7-9. Second: that an unusually strong, deep, and retrogressive low pressure area and associated digging jet will develop over the West Coast by day 10 and possibly persist for a while, possibly bringing the possibility of precipitation and convection to all of CA/NV and much of the West, if the GFS solutions are to be believed literally without any interpretation. This would be unwise, as climatology argues strongly against such an anomalous pattern. If the retrogressive low were to take up residence off the CA coast as currently depicted by the GFS, thunderstorm and shower activity would almost certainly occur. The remnants of the Baja tropical system, however, would be pushed too far to the east to affect CA substaintially. However, if the trough is nowhere near as deep as the GFS currently indicates (which I believe will likely be the case), the tropical remnants wil have a better chance to drift northwards and potential affect CA weather. It’s worth keeping an eye on, at least…