Historic weather day today in the San Joaquin Valley....
I first thought it was going to be an interesting day around midnight early Sunday morning. Skies were clear with a 28F temperature here in Turlock at that time, but showers were already moving onshore in the San Francisco Bay Area. Altocumulus and stratocumulus rapidly spread over the area and southeast winds picked up over the next three hours - but by 0400, these elements had only risen the temperature to 30F. Radar had indicated light precipitation over the area since 0300, but nothing had reached the ground yet. That would quickly change.
At 0400, very light rain began falling here - mixed with snowflakes. The rain/snow mix fell until 0520, actually falling moderately from about 0440 to 0520. Temp crept up to 32F as the rain and snow picked up in intensity, so all of the snow melted as it hit the ground. But at 0520, the temp dropped back down to 31F and the precip changed entirely to snow. A very light, very brief slushy accumulation occurred between then and 0600 - but was not enough to measure. It continued to snow until 0640 - then the temp climbed back up to 32F and the snow mixed with rain until 0740. Precipitation was pretty much over for the day in Turlock after that, but the fun had yet to begin in other parts of the valley....
By mid-afternoon, the temp climbed to 42F and towering cumulus began popping up all over the area. As the Arctic boundary moved in around 1500 - one tower about 10-15 mi to my E became a Cb. Other cumulus towers in the area began dissipating as this Cb grew - a promising sign. The Cb still had a hard textured appearance and showed no apparent signs of weakening at 1545, so I got in my car at this time in pursuit of this storm - which by now was moving into northeastern Merced County from southeastern Stanislaus County.
My plan was to drive as far east as I could without entering the heavy precipitation, then head south and follow the storm along its western periphery. I headed east on Route J17 from Turlock into Merced County. Once I got to the intersection of Route J17 and Oakdale Road, the storm less than two miles to my east-southeast, so I decided to begin heading southeast on Oakdale Road.
About a mile up the road, near the intersection of Oakdale and Turlock Roads (about 5 mi NE of Ballico - elevation approx 200 ft), I started getting into a mix of snow and graupel, which would continue over the next eight miles of my journey. Temp was only 33F as I drove through the precip - a bit cool at the for only 1600 in the afternoon. Snow and graupel accumulated on grassy surfaces to about 1/10 to 1/4 inch, with a trace accumulation on paved surfaces. Most of the graupel pellets were pea-sized, although there were a few pellets about the size of golf balls (which made very loud thuds when they hit the car). Once I got near the intersection of Oakdale Road and Hwy 59 (11 mi NW of Merced - elevation approx 225 ft), the graupel ended and the precipitation was all snow. I continued getting snow until I was about 5 mi north of Merced - but by now was only seeing trace accumulations on grassy surfaces. By then I was out of the storm, and once I got to the intersection of Hwy 59 and Hwy 99 in Merced, I turned north on Hwy 99 and headed back to Turlock.
In all, we ended up with a trace of snow here in Turlock - our first snowfall since Feb 8, 1989 and only the fourth snowfall I've ever seen here (I've lived here since 1977). Trace amounts were generally the rule in the northern San Joaquin Valley - except for the eastern half that saw the convective snow shower. Although I saw 1/10 to 1/4 inch where I was, I didn't even get into the core of the heavy precipitation. I'd say parts of eastern Stanislaus and northeastern Merced Counties may have had 1-2 inches of snow Sunday afternoon - if not more. None of the areas that storm crossed are any higher than 250 feet above sea level.
Skies cleared quickly after sunset, but gusty NW picked up around mid-evening, keeping temps from dropping too low. It was still down to 27F at 0300 Monday morning despite a 20 mph wind, which puts wind chill values down in the single digits. It will be interesting to see how cold it gets once the wind dies down. If this cold snap hit at just about any other time during the winter, we could be talking about record lows. But we're at the same time of year that we set our all-time record lows in 1990 (18F on the 22nd, 14F on the 23rd, 18F on the 24th, 18F on the 25th), so those records will probably be safe.