LNU Lightning Complex, Northern California

This wasn’t the first time people in Napa and Sonoma Valley had to evacuate their homes. Actually, this was becoming an annual occurrence. For the second time in 3 years, many evacuated their homes as the LNU Complex exploded.

Tensions were running high as the middle of August rolled around. The area was quite dry and the rain was no where to be found. But the weather pattern changed and it looked as though much of the area could see a round of thunderstorms, potentially with dry lightning.

The LNU Complex is made up of 3 fires within the same area, all started by lightning. The first is the Hennessey Fire located in Napa Valley that surrounded Lake Berryessa. The second is Walbridge Fire located northwest of Santa Rosa in Sonoma County. The third is the Woodward Fire located northwest of San Francisco.

Let's learn a little about the northern Bay Area. The wettest months in central California is the winter. The summers are quite dry so it wasn't abnormal for the Sonoma County Airport (chosen due to the proximity of it being between all 3 fires) to see little to no precipitation in June and July. Looking at the maps on the left, California was not in any drought with only the northern border being abnormally dry. Listed below are what normal precipitation is for each month with what actually occurred in parenthesis.

2020 Precipitation for Sonoma County Airport by Month

• January - 7.05" (2.38")
• February - 6.40" (0.00")
• March - 4.98" (1.05")
• April - 2.09" (1.19")
• May - 1.33" (1.20")
• June - 0.24" (Trace)
• July - 0.01" (Trace)

If you add all the months above, Sonoma County Airport normally receives 22.10" from January through July. But for the year 2020, they received a measly 5.82" over the same months, less than that they typically receive in the month of January or February. The year 2020 was 26% of normal.

The days leading up to the event were extremely hot. An excessive heat watch had been issued to include much of the Bay Area. Record or near record heat with many temperatures reaching 95° to 108° for August 14 - 19. This would cause fuels to dry out even further. Below is an image of the observed temperatures at 12:45 PM PDT on August 14, the first of 6 days that would see temperatures around 100°.

• August 15th at 2:19 AM PDT: Fire Weather Watch issued for Fire Weather Zone 506. Light winds and relative humidity as low as mid teens with poor overnight recoveries. High base, fast moving storms will lead to a risk of isolated dry lightning strikes through the period.
• August 15th at 10:27 AM PDT: Red Flag Warning upgrade for Fire Weather Zone 506. Light winds and relative humidity as low as mid teens with poor overnight recoveries. Increase in wildfire starts in proximity to thunderstorms due to lightning. Fires may spread rapidly due to dry fuels and, if nearshore, breezy onshore winds. Not all locations will see thunderstorms but any lightning strikes will likely lead to new fire starts given the current heat wave. A secondary pulse of moisture and instability arrives later Sunday into early Monday

The Red Flag warning that was issued on the morning of August 15 was continued through August 17 with a few minor changes with each update. This Red Flag Warning was in conjunction with an Excessive Heat Warning stating that the areas could see temperatures into the low 100s which had been in effect from August 13 to August 19.

On the afternoon of August 16, lightning began to move north through San Francisco and up to Sonoma and Napa counties. A Severe Thunderstorm Warning was issued for this are for penny size hail and winds up to 60 mph. Within the warning as well was the statement "Continuous cloud to ground lightning is occurring with these storms. Move indoors immediately."

Jeff Boyce managed to capture the light over the Bay Area between 3-4am.

After the storms passed, NWS Bay Area did a summary image of where lightning strikes occurred on the morning of August 16. Lightning data is provided by Vaisala. You'll notice both positive and negative strikes on the image. The main difference between the two types of strikes are that positive strikes are much more powerful than negative and are less common, making up less than 5% of all strikes. Both positive and negative strikes can start fires but with the extra power behind a positive strike, it is likely that the positive strikes in the image below started fires.

The storms in the morning would not outshine the record heat that occurred later on in the day. 8 locations within NWS Bay Area CWA broke record high temperatures, including Santa Rosa which would be in the middle of fires within the LNU Complex.

But the storms from the 16th would not be the last. Another round of moisture would allow for thunderstorm formation in the early hours of August 17.

Alert Fire cameras caught one of the fire starts for the Hennessey Fire, which would later surround Lake Berryessa.

Winds were a little breezy but nothing that would be alarming for fire spread. The fuels were so dry though that the fires in the LNU Complex would rapidly spread. One of the struggles with this fire was the limited recourses that were available. There were dozens of other large fires around California that took many recourses away from the Bay Area.

The day after the storms, it was apparent that NWS Bay Area would not only be dealing with the LNU Complex (Listed in the image below as LNU Complex, Hennessey, and Gamble Fires). There was also the CZU and SCU Lightning Complex that was started as well.

Winds became gusty on the 19th as the pressure gradient began to tighten. Winds near the LNU complex reached gusts up to 40 mph, allowing for the fire to continue spreading.

NASA issued a blog about the burn scars visible from satellite using MODIS. Visit the link below for more information.

Within 3 weeks, the LNU Complex became the 4th largest fire in California history. It would continue to stay at 4th when the fire was contained on October 2.

The LNU Complex was contained on October 2, 46 days after the fires began, LNU Complex was contained. Now it was time to see the extent of the damage. Below is an application from CalFire showing the extend of damage to buildings within the LNU perimeter. You can zoom out and view the Woodward Fire and Walbridge Fire.

The Hennessey Fire was the largest of the fires within LNU Complex. CalFire put together an image showing the progression of the fire within the first 13 days of the fire.