Brandt SoCal Fcst

Past 12 hours, current analysis:

The IR satellite loop from 6Z to 18Z showed several bands of clouds moving into Southern California throughout the period. Deeper clouds (lower cloud-top temperature) were more concentrated a few hundred km up the coast, especially in the Bay area and to the south. As the morning has progressed, however, the thickest bands of clouds have gradually been drifting down the coast (with areas to the north experiencing gradual clearing), with the thickest bands of clouds currently (as of 18Z) located NW of Los Angeles on a line from Oxnard to Bakersfield.
 
 
 

 

Rain showers were spotty overnight, with the first accumulating rain being recorded in downtown Los Angeles at 14:47Z. Since then, however, rain showers have been present in the area, with the downtown area reporting 0.15 in of rain as of 18Z.

Radar composite images show that several bands of rain streamed across the California coast overnight, the heaviest of which is currently oriented N-S stretching down the central valley and is currently located just west of the city, moving slowly towards the east.

Temperatures have remained very constant over the past 12 hours; at LAX, temperatures have not shifted more than three degrees F over this time period (between 55 and 58 degrees F).
 
 
short term- Fri-Sun
 
 

12Z NAM- 500mb height, abs. vorticity @ 21Z Fri
 
The rainy weather which affected much of California overnight and throughout the early part of the morning is associated with a low pressure system, the center of which was located around Reno, NV as of 18Z. The 12Z run of the NAM shows a fairly strong 500mb vortmax located in the base of a trough just off of the coast as of 21Z. Strong differential PVA in combination with copious Pacific moisture will continue to force rising motion and generate widespread rain showers through late afternoon... additionally, low level winds out of S-SW should bring an upslope flow which may enhance precipitation in areas just north of the city, though winds will progressively shift to become out of the WNW by early afternoon. By late afternoon, forcing looks to turn off as the 500mb trough axis comes directly overhead, and low-level CAA begins to take hold, though remnant low-level moisture will keep a chance of very scattered showers present throughout tonight and into early Saturday.
 
SPC mesoanalysis shows SBCAPE in the Los Angeles region to be around 300 J/kg as of 18Z...
isolated, embedded thunderstorms are possible with this storm system. 

12Z NAM- 850mb height, RH @ 18Z Sat
 
Aforementioned remnant low level moisture looks to keep Saturday at least partially cloudy, but the departing low will take with it chances of precipitation... Saturday should stay dry, with westerly winds and high temperatures reaching the lower-mid 60s region-wide. Look for clouds to clear out by late Saturday night/early Sunday morning.
 
On Sunday, the trough moves farther out to the east, and high pressure located off the coast will begin to dominate the weather pattern. Expect clearing skies, calming winds, and temperatures rising into the upper 60s.
 
 
 
Long Range- Mon-Thurs
 

12Z GFS- 500mb height, vorticity @ 6Z Tue
 
After the system currently affecting our area departs, weather looks fairly peaceful for the entirety of the workweek... high pressure will dominate throughout the week for Southern California. Only (trivial) exception from this dominant zonal pattern looks to be a small shortwave system entering Pacific Northwest during the day on Wednesday and bringing precipitation chances to the northern part of the state... however... Southern California looks to be unaffected by this system and no precipitation chances are forecast. Temperatures look to be in the mid-70s and skies should be clear throughout the forecast period.
 
sources--
History weather data for downtown LA: www.wunderground.com
GFS, NAM model output: http://mag.ncep.noaa.gov/
Current weather analysis: http://weather.rap.ucar.edu/
SPC Mesoanalysis: http://www.spc.noaa.gov/



Verification:

(verification made 17 Apr 2012)

The precipitation event forecast on Friday, April 13th brought significant amounts of rain to the Southern California region. Downtown Los Angeles recorded 0.49" of rain on the 13th, however, many nearby areas recorded 24-hour precipitation totals in excess of 1", especially in those areas to the NW of the Los Angeles area, as well as the windward sides of local mountain ranges. 
Radar data from the 13th show the main area of precipitation moving into Greater Los Angeles after 19Z on Friday, and departing between 22Z and 23Z. This precipitation event was dominated by broad areas of light-to-moderate rain, with embedded lines of heavier rain.

1-3 inches of snow fell in high-elevation areas in Southern California on the 13th. Mountainous areas of the San Bernardino National Forest (just NE of San Bernardino), as well as mountainous areas of the Los Padres National Forest north of Oxnard.


On the 14th, there was some precipitation to the south of Los Angeles-- close to 0.5" in the San Diego area-- but no significant measurable precipitation in the LA area.

As the system moved out, the next few days were characterized by clearing skies and a warming trend, as my forecast predicted.

Observed High/Low Temperatures, Downtown Los Angeles-

Friday, April 13th-
High: 57
Low: 46


Saturday, April 14th-
High: 62
Low: 44


Sunday, April 15th-
High: 67
Low: 51


Monday, April 16th-
High: 77
Low: 50


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