Record-breaking May heat wave fades away; dramatic change to come

The heat wave that occurred over the past week was truly impressive across most of CA. On the first day of the warm event, when a strong offshore gradient kept winds very breezy overnight and contributed to unprecedented compressional heating in the western Sac. Valley and parts of the Bay Area, leading to overnight “minimum” temperatures ranging from around 70 at the coast to near 90 degrees near Winters and Woodland.  San Francisco eclipsed 90 degrees, the Peninsula hit 100, and inland valley areas went over 105 in NorCal. Even Eureka hit 80 on Saturday. Needless to say, dozens of records were shattered across the region. SoCal was quite hot as well–over 100 in some areas–but fewer records were broken because intense May heat waves are not so rare down there. Regardless, this was a May heatwave to remember statewide. Today, temperatures are much cooler along the coast and cooler but still hot (90-100) in more inland locations. A very dramatic change is quickly approaching, however…

An unseasonably deep trough will force the high pressure system aloft that has been responsible for all the heat lately to the east, ushering in much colder air (both aloft and at the surface) and increasing pressure-gradient derived winds explosively. The gradient between SFO and Las Vegas will be on the order of 22 millibars, which is just about as high as I have ever seen it. A 150+ kt jet will set up in a N-S fashion over the coastal range, creating the potential for some very strong and potentially damaging N/NW winds over the ocean, higher terrain from Eureka south to San Diego, lower N-facing slopes everywhere, and possibly along the western side of the Sacramento Valley as a barrier jet sets up. Winds over the ocean will likely be storm force or higher, with sustained winds of 40-50 kts possible. Fire danger will be quite high, given the drying of fuels that has occurred over the past week. High temperatures will be 30 to locally 40 degrees cooler than the weekend in some places, with the wind making it feel even colder (highs from mid 90s in some coastal areas will drop to the mid 50s). Inland, highs in the Sac. Valley will drop from the 105 range this weekend to around 75 by Thursday. Along with the strong and gusty winds and cooler temperatures will be a good chance of Sierra thunderstorms associated with the front on Tuesday/Wednesday. The models have really been struggling with the upcoming pattern, but it does now appear that there is some potential for convective activity over parts of the state other than just the Sierras. The Coastal Range and far Southern CA coastal areas currently stand the best chance at some isolated thunderstorm activity, but even the SF Bay Area and Sac. Valley could potentially see some shower activity if the retrogressive cutoff low sends a second impulse of energy into the region next weekend as indicated by both the GFS and the ECMWF. We’ll see–a pretty dramatic weather week after a rather boring spring…

 By the way, Chaiten Volcano continues to erupt in Chile. Jeff Masters has posted an article on the possible affect of Chaitens’s eruption on world climate (which is to say: small to none).  Here’s a link…

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=942&tstamp=200805