Early Season Fire Danger & Strong Valley Inversion This Week

Think it's warm in the valley? Some locations in the foothills and western slope of the sierra are upwards of 10 degrees warmer due to the strong inversion that is trapped over the Sacramento and San Joaquin Valley.

With no precipitation in the forecast throughout northern California for at least the next week or so, there is no drought drought relief in sight. This historically dry streak over the last year has lead up to this: an early season fire weather event expected to linger through the week this week.

Never would have thought I’d be making maps having to do with fire danger in the middle of January.

Offshore winds across California are, and really already have dried out the low-levels significantly, and in addition to the very dry low-levels, the offshore wind gradient is strong enough to support some locally gusty winds throughout northern, central, and southern California.

In this graphic I’ve plotted dew points in the fill and dew point values from METAR sites across the region. The lower the dew point, the dryer the air.

What’s causing this offshore flow you ask? The core of our exceptionally strong ridge is sitting off the northern California/Oregon coast, directing some upper level winds to spiral clockwise around it, which sends northeast to easterly winds right over California. The ridge center will slowly work its way overhead Thursday and Friday, allowing winds to weaken… though, could remain locally breezy. Dew points and humidities will have a hard time recovering through the rest of the week into the weekend due to the offshore flow continuing, though weakly.

300mb heights and winds plotted through the 4km NAM indicating the clockwise flow of the super-ridge off the CA/OR coast.

In addition to dry and breezy conditions, a strong to very strong inversion has set up over the Sacramento and San Joaquin valley, where winds aren’t strong enough to mix out this warm belt a couple thousand feet above the surface. This is keeping the valley about 5 – 10 degrees cooler than most foothill and western slope locations. Generally valley temperatures have remained in the mid to locally upper 60s (except in the north valley where the inversion isn’t present, allowing them to reach the low, mid, and even locally upper 70s), while foothills and mountain communities between 1000 and 4000 – 5000ft in elevation are warming into the low to mid 70s. Overnight lows in the valley have been quite cool due to the trapped cooler air, dipping into the 30s, while in the foothills and mountains where the inversion layer is present, most lows have been in the upper 30s to mid and even upper 40s locally.

In this graphic I plotted surface (well, 2-meter) temperatures in the fill and surface temps via METAR sites. Compare temperatures from the Sacramento/San Joaquin valley to those in outlying mountain areas.

This pattern with elevated fire danger and strong inversion temperature gradients is expected to continue into the weekend, then an odd split-flow and possibly a cutoff low are expected to develop by next week, perhaps breaking up the inversion and bringing cooler temperatures and perhaps some clouds. Models are all over the place with the cutoff low portion of the forecast, with some runs dropping it and others continuing to indicate it. For instance, the ECMWF has indicated some sort of closed low for the last few days, while the GFS has been flipping and flopping… but seems to have trended way from that idea for now. The GEM also dropped the idea of a cutoff low and instead keeps our exceptionally strong ridge overhead, and in fact strengthens and amplifies it further.

I’ll keep an eye on next weeks pattern for anything interesting and have an update when confidence is high in whatever evolves out of next weeks pattern, whether it winds up being dry or perhaps a bit wet (which right now seems unlikely).