A Record-Breaking Atmospheric River Hits California and Strong Marine Cyclone Offshore of Washington

A lot to talk about.   It is pouring outside as I write this....really coming down

The forecasts of an extreme atmospheric river hitting California proved highly accurate.   To start, here is a water vapor satellite image (the satellite sensor observes a wavelength for which water vapor is a very good absorber and emitter) for 5 AM this morning. 

You can see the very long plume of moisture stretching from Hawaii to the central WA coast.   A very large cyclone (the swirl) is directly west of Washington State (more on that later!)

The moisture content of this atmospheric river is extraordinarily unusual, unprecedented in many ways.  To prove this to you, here is the climatology of precipitable water (PW) at Oakland, California and the value of PW  observed this morning from the twice-daily radiosonde weather balloon ascent.  Precipitable water is the sum of the water vapor in a vertical column of air.

The value (1.69 inches) blows away the daily record (the thin red line).  It is higher than any day from late September through early July.  This is huge. 

The highest value observed at this location during the wet season of the year.

And note that this is the time of the year when atmospheric moisture at Oakland is generally lowest.

With such large values of moisture heading into California, rainfall totals have been large.  As shown below (click to enlarge), a number of locations over the western sides of the coastal mountains and the Sierra Nevada have received 4-6 inches of rain over the past 48 hours--several more than 6 inches.  They would have gotten much, much more if the incoming winds had been stronger.

Some rivers in the region are already at flood stage (red circles in plot below) and CA reservoir operators are releasing substantial amounts of water to ensure to safety of their dams, especially the heavily damaged Oroville Dam.

And the rain is not over for California. ANOTHER system is expected next week, with substantial rains from northern CA into the Northwest (see 72h total ending 5 AM next Saturday).

Here in the Northwest, heavy rain is falling over Washington State as a frontal band associated with a very strong Pacific cyclone moves through (see infrared satellite image at 7 AM this--Saturday--morning).   Beautiful, large storm with the clouds swirling into the low center.

The 3-h forecast for this time suggests we are dealing with a 972 hPa low center, with an intense pressure gradient (change of pressure with distance) around the storm.  That means strong winds.

 In fact, the forecast wind gusts  for 11 AM predict 60-65 kt gusts reaching the Oregon and Washington coasts.

The maximum gusts for the 24-h ending 7 AM show strong winds already, with some favored locations in the coastal mountains getting to 70 mph, with 40-50 mph gusts at some places along the coast (click to expand map).

The low is going to slowly drift our way as it fills (decreases in strength) as shown by the forecast map at 5 AM tomorrow morning. 

By 2 PM today, winds will be really cranking along the Washington coast and over NW Washington (e.g., San Juans), as shown by the forecast winds at that time.  50 kt gusts will be common in those areas.  That means some power outages.

Winds over Puget Sound should increase today as the low approaches, perhaps to 30-40 mph, later today and Sunday morning.  Not enough for major impacts.  And showers will continue until mid-day Sunday for most folks on the western sides of the Cascades.




from Cliff Mass Weather and Climate Blog https://ift.tt/2EsQ4Y2