The month began with cooler than normal low temperatures in the wake of a fairly strong trough of low pressure that brought rather cool air to the region. Morning lows dropped to as low the upper 30s in parts of the San Joaquin Valley. However, daytime temperatures quickly rebounded as an upper-level ridge of high pressure began building into the state; afternoon highs in the central and southern San Joaquin Valley warm to near normal (around 80 degrees).
The ridge continued to strengthen during the first several days of May. On the 2nd and 3rd, temperatures warmed several degrees, with central and southern San Joaquin Valley highs reaching the mid to upper 80s. May 4th saw both Bakersfield and Fresno reach a high of 90 degrees, the first 90-degree day for either city in 2011. The next day saw San Joaquin Valley highs reach the mid 90s, as the ridge continued to strengthen ahead of an approaching upperlevel trough. The ridge began weakening on the 6th, bringing a modest cooling trend; much stronger cooling arrived the next day as the trough began moving into California.
Winds began increasing during the afternoon of May 7th over the Kern County mountains and deserts, and a few gusts on the 8th exceeded 60 mph as mountain waves briefly touched down on the higher elevations of the Piutes and Tehachapi Mountains. The central and southern San Joaquin Valley saw temperatures fall around 11 degrees from May 6th to the 7th, and as much as 13 degrees on the next day as temperatures fell to well below normal.
As the trough began to move inland on the 9th, thunderstorms with small hail, brief heavy rain, and gusty winds to around 50 mph developed over the southern Sierra Nevada. The airmass over the San Joaquin Valley was forecast to be unstable during the afternoon of May 9th, and this proved to be true as the thunderstorms that formed over the southern Sierra Nevada moved southwest into the central and southern San Joaquin Valley. By 5 PM, thunderstorms were moving quickly south through the Fresno/Clovis metropolitan area, and also along the west side of the Valley���including the Interstate 5 corridor���from Coalinga south. The storms on the east side of the Valley continued moving south across Tulare County before weakening. Hail up to ��-inch in diameter was reported, and wind gusts to around 50 mph toppled trees in Fresno and Tulare Counties.
The city of Fresno saw rapid temperature changes as the thunderstorms moved through. Ahead of the storms, the temperature at Fresno-Yosemite International Airport rose from 64 degrees at 2:53 PM to 75 degrees at 3:50 PM, and then dropped to 67 degrees 3 minutes later. By 4:53 PM, the temperature had fallen another 9 degrees, to 58, before bottoming out at 55 degrees an hour later.
Temperatures remained rather cool on the morning of the 10th. However, another ridge of high pressure began to develop over the area, although it was not as strong as the previous one. Nevertheless, temperatures began to rebound back to around normal values by the 11th and into the mid to upper 80s on the 12th and 13th.

Thunderstorm Wind in TULARE, CALIFORNIA
From 09-MAY-11 16:50:00 to 09-MAY-11 16:50:00 (PST-8)
Location: EXETER
Magnitude: 45.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained weather spotter reported trees downed from thunderstorm winds and blowing dust. Winds estimated up to 50 mph.