I begin by getting a large scale picture of the current atmospheric conditions. For example is a trough/ridge in place. Then I progress to look at radar imagery, satellite and current observations. This gives me an idea of the current state. I then like to look at both the NAM and GFS to see how the models are handling the current state of the atmosphere. If one has a better handle on the current situation, I will pay greater attention to that particular model for my near term and short term forecasts. On a side note, for operational forecasting, I look to see if any watches or warnings are currently in place.
Current analysis for southern California shows a low pressure system entering off the Pacific coast in connection with a deep trough. Surface observations show good south/southwest flow bringing in moisture from the Pacific. Temperatures are in the lower 50s near Los Angeles and upper 30s with snow in the higher terrain. Winter storm warnings are in place.
Current radar shows heavy to moderate rain and mountain snow in southern California as a deep trough swings through the area. Expect locally heavy rain/mountain snow through this evening. In addition to locally heavy rain/snowfall, strong winds are also an issue. This has prompted high wind warnings and advisory for much of southern California.
Near Term (Tonight)
For tonight, the low is expected to move off to the east leaving behind scattered showers. Showers are expected to taper off west to east with the exception being along the mountains where the NAM radar reflectivity sim. holds on to the precipitation a little longer.
Short Term (Saturday-Sunday)
For forecasting both short and long term, I begin by looking at the 500mb heights/vort/winds. This gives me an idea of where troughs/ridges are expected to move as well as areas of potential PVA. For the short term I mostly look at the model which has a better handle on the current weather. With that said however, both the NAM and GFS are analyzed. Jetstream, 850 hghts/temp/wind, are also examined.
For Saturday, the upper low and deep trough begin to push off to the east placing much of southern California in NVA as is depicted by the 12Z NAM
500mb heights/vort/winds shown below. This NVA will allow skies to become clear for most areas. NAM and GFS are in rather good agreement with high
temperatures in the lower 60s and lows in the lower 50s near LAX. Colder temperatures can be expected in the higher terrain.
Sunday the upper level trough will be east into New Mexico/Texas with a ridge building in from the Pacific. 12Z GFS shows this feature as depicted in the image below. With the ridge building clear skies and warmer temperatures can be expected.
Thickness heights over southern California for Sunday will rise throughout much of the day from 552 to 556. With thicknesses rising and mostly sunny skies temperatures should also rise throughout much of the day and top out in the mid to upper 60s.
Overall a pleasant weekend looks to be on tap for much of southern California as a result of a building ridge out in the Pacific.
Long Term (Monday-Thursday)
Overall, models are in fairly good agreement through this period. Looking at the GEFS spag a persistent weak ridge looks to remain in place through at least Tuesday. ECMWF is also in agreement with this idea. Wednesday a trough appears to rotate into northern California but a zonal flow remains to dominate southern California according to both the GFS and ECMWF. Temperatures through this period look to remain steady in the upper 60’s to lower 70’s according to the extended GFS MOS.
AHPS precipitation analysis show heavy precipitation affected much of California on April 13th 2012 with rain and mountain snow..
Precipitation estimates were in the order of .01 to 1.5 inches of liquid water through much of southern California. Observations show that
that precipitation lingered along the mountains through much of Friday night as was predicted by the NAM reflectivity sim..
QPF verifications show that winter storm warnings which were posted were verified. One reporting station located in Wrightwood, CA reported 10 inches of snowfall.
Severe wind reports were reported in Orange and San Diego county in which the SPC had a see text area out.
For Saturday, models had a good handle on moving the trough eastward and bringing mostly clear skies to southern California.
Max temperatures were accurate with a high of 60 in LAX but the models failed with the record low temperature of 45 for LAX.
For Sunday, model data also had a good handle on the position of the trough which was well east of the area bringing clear skies to all of southern California..
Models handled the weak ridge very well through Tuesday as was forecasted last week. A weak trough still appears to move through
northern California on Wednesday bringing cooler temperatures as was forecasted last week by both the GFS and ECMFW..
Forecast high temperatures through this period are higher than previously forecasted by the extended GFS MOS.
Warmer temperatures are expected through the remainder of the period.