Explosive weather hits the Golden State

  • Pressure map for California on Monday 20.02.2017 at 12.00 UTC. Low pressure to the west of the Golden State, which will give the next wave of bad weather.

    Forecasted 24 hr rainfall totals for California from Monday 20.02.2017 00UTC.

    12hr rainfall observations from weather stations around the Los Angeles area.

  • Explosive weather hits the Golden State
    18.02.2017 17:39

    Over the past few years whenever California has been mentioned about weather events it has been about heat and drought. However, since the turn of the year it has seen numerous severe storms giving record breaking rain and snowfall amounts. Whilst it is welcomed to replenish the depleted water sources, it has and continues to cause a serious amount of disruption.


    One of the biggest storms to hit California in years hit last night due to an area of low pressure which deepened rapidly.  The rapid deepening is often referred to as explosive cyclogenesis, which is where the centre of a low pressure system deepens by 24 hPa or more in a 24 hour period. While not an uncommon phenomena, it is fairly rare to see this situation occur across southern California.

    A couple of weeks ago we looked at the meteorological factors in bringing such vigorous storms to the California coastline. The combination of the jet stream, atmospheric rivers (locally known as the Pineapple express) of moisture and the sharp temperature gradient have come together to fuel these severe storms.

    The most recent storm, which hit on Friday night, was a result of a very powerful Jetstream across the Pacific feeding moist sub-tropical air towards the west coast of the United States. The relatively warm air then collided with much colder air aloft which aided the rapid intensification of the low pressure system, ultimately leading to explosive cyclogenesis.

    The storm on Friday night into Saturday produced some exceptionally heavy rainfall with some places in the Los Angeles area seeing 53mm in the 12 hour period from 00UTC to 12UTC today. There have been reports that some weather stations have recorded up to 200mm over the last 24 hours. All this rainfall has led to severe travel disruption with sinkholes, flooding and mud slides making roads impassable. With dams now full from the heavy rainfall and the ground saturated, it will continue to be a dangerous situation for some time. Flights have also been affected due to wind gusts up to and exceeding 80mph causing up to 300 flights to be cancelled.

    There is no let-up in the bad weather as another low pressure system feeding moist tropical air into the western seaboard of the United States at the beginning of next week. Even though the next storm will not be as powerful as the one that went through on Friday night into Saturday, it is likely to bring more heavy rain and snow to the mountains. As seen in the graphics on the side of the page, places in California in the 24 hour period from midnight on Monday could see 116mm of rain as a result of the deep area of low pressure sat to the west of California. With these values further records could be beaten along with continued disruption with more flooding and landslides.

    By: Tom Whittaker