A cold winter storm that arrived in the San Diego area over the weekend brought persistent heavy rain along with widespread flooding that inundated roads and neighborhoods across the region.
It was the fourth wettest day on record in the history of weather measurements in San Diego, according to the National Weather Service. It has rained more in San Diego in the past three days than in the past three months combined. In fact, the three-day total has nearly doubled in the past three months.
But why was there so much flooding in Coronado, National City and surrounding areas? NBC 7 Meteorologist Sheena Parvin has the answer: training.
The training occurs when heavy rain moves over the same area for a long period of time, causing flash floods. Training can easily occur in many of the county's large storm systems, as long as that storm has sufficient moisture and the direction of movement allows the heavy rain to be directed over the same area. No two storms are alike, so training in a particular location usually cannot be predicted before a storm moves in.
In this case, there was a line of heavy rain and thunderstorms that was moving east over Coronado and National City. The average rainfall within that area was 1-2 inches per hour. The training results in flash floods almost 100% of the time, Parveen said.
In addition to the city's “outdated stormwater system with limited capacity,” we saw many homes flooded, along with cars, and even our transportation services were affected.
The city of San Diego said many of the city's stormwater pumping stations reached maximum capacity during the downpour and flooding caused a power outage at the pumping station in the Sports Arena/Midway area.
Here is a more detailed explanation of Parveen's explanation:
On January 22, a large and complex weather system impacted the West Coast. An area of low pressure was also near San Diego County. A band of subtropical moisture (shown by the yellow arrows in the image below) increased the moisture content of this storm system as it approached land.
The radar image below shows when rain began moving into San Diego County, around 6 a.m. Monday. Green indicates light rain, yellow is moderate, orange is heavy, while red is very heavy.
At nine in the morning, moderate to heavy rain fell in many areas of the state.
By 10 a.m., flooding was already occurring across the province with a weather phenomenon called “an exercise” In Coronado, National City and surrounding areas. The training occurs when heavy rain moves over the same area for a long period of time, causing flash floods. Training can easily occur in many of the county's large storm systems, as long as that storm has sufficient moisture and the direction of movement allows the heavy rain to be directed over the same area. No two storms are alike, so training in a particular location usually cannot be predicted before a storm moves in.
By 12pm, dry air began to move in as the storm system pulled most of the moisture away.
A closer look at Coronado and surrounding areas:
Heavy rain began moving into Coronado and areas surrounding downtown San Diego shortly after 8 a.m., shown in yellow.
With ample moisture content in the atmosphere, very heavy rain fell by 10 AM in the same areas at rainfall rates of 1-2 inches per hour.
The weather phenomenon called training occurred shortly after 10 a.m. as the storm began to form a long line of very heavy rain, all moving in the same direction, mainly from west to east.
The image below shows approximately two hours after the heavy rains began in this area, and now with heavy rain falling over the same parts of the ground. Average rainfall was still between 1-2 inches per hour.
By 10:30 a.m., heavy rain was still falling but the storm system was on its way out.
This system had sufficient moisture and zonal flow (from west to east). Which helped establish the training process for parts of San Diego County. If heavy rain causes flash flooding in an area with poor drainage or an area prone to flooding during heavy rainfall, this type of event can make matters significantly worse.
Watch more storm coverage below:
