FIRE WEATHER WATCH FOR NORTH BAY MOUNTAINS. CHANCE OF DRY LIGHTNING BEGINS LATE SUNDAY.

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A series of weather factors are lining up to bring potentially dangerous dry lightning storms driven by monsoonal moisture into much of Central and Southern California beginning today. This moisture may reach our area Sunday afternoon, and bring a threat of high-based thunderstorms, with potential dry lightning, through Monday morning.

With that said, it is highly UNLIKELY this push of monsoonal moisture will bring anywhere near the magnitude of storms that arrived last August and set off lightning caused fires throughout the state. Now the details.

As of Saturday, monsoonal moisture can be seen streaming into the desert Southwest. This seasonal surge of subtropical moisture brings thunderstorms and much needed rain to the Southwest each year from June to September. Moisture is rotating into the region around a high pressure system near the Four Corners. (Image 1)

The so called “Monsoonal High” will push westward toward California by Sunday, and moisture rotating around it headed northward will increasingly be directed toward California. (Image 2)

As you can see, the moisture stream will rotate around the high, and also be directed by a low pressure trough to our north. This arrangement may cutoff the deepest moisture from reaching the North Bay. That’s still uncertain.

What is fairly certain is that the amount of moisture arriving in the mid levels of the atmosphere (around 20K feet up) will be VERY high for this time of year compared to normal. This shows those moisture levels (green) on Saturday, with the monsoonal surge beginning to arrive in Southern California (Image 3) and early Monday as it arrives in Northern California. (Image 4)

As you can see in the second picture, some models want to cut off the flow of moisture over the North Bay (due to low pressure to our north), which could keep the main area of dry lightning (if it develops) south of our area. The NWS has already mentioned this in it’s discussion of the upcoming moisture surge.

Importantly, the moisture from the south will arrive in the mid levels of the atmosphere, near 20K feet, and not at the surface. This is where thunderstorms, if they happen, will form. With very dry air remaining in place below that, the likelihood of dry lightning, without associated rain, is higher.

In any case, by later in the day on Monday the moisture is forecast to move away from the area ending the threat of high based thunderstorms.

As we all know, fuels remain at critically low moisture levels and the wildland fire potential from any lightning starts is extremely high. The Fire Weather Watch (Image 5) may be upgraded to a warning as the event develops. One bit of good news is that weeks of fog and marine stratus has helped to improve fuel moisture levels along coastal areas. That’s one reason that the valleys of the North Bay and Bay Area are not included in the Fire Weather Watch for now. We will keep monitoring the developing weather this weekend.