(see attached for full discussion with images)


Los Angeles area is currently experiencing below normal temperatures with very high RH.  Winds are coming in from the SW at a mild 5-10 kts. There are overcast skies with some reports of rain in the valley with snow reports coming in for the upland mountainous regions as well. LAX has 0.09 in accumulated over the last 6hrs. The 500 mb low is situated just off the CA coast, directly to the west (figure 1).

Figure 1 - Four panel view, NAM12Z for 1800Z


At 850 mb, the MM5 shows CAA brushing over a relatively warm landsurface (figure 2). Air temp and SST are about the same however, past weather has been on the warming trend. The IR loop over the last 12 hrs shows increased vertical extent of cld tops as the diurnal heating progresses. WV loop over the last 12 hrs shows relatively dry air associated with the CAA coming in over the area about now (last image was at 1715). Current conditions suggest a changeover of system, very much conducive to the rain event expected for the short term.

Figure 2 - from MM5 12Z for 1800


Current radar (figure 3) shows moderate rain occurring over the LA region, with winter type reflectivity values occurring over the mountainous areas.

Figure 3 - Unisys Weather, current radar 1745Z.



A deep trough will dig in over the SW, bringing cooler temperatures behind it. The 500 mb map shows a jet core approaching the area. Though intensified, the same pattern is seen at 250 mb. The forecast area is located on the right exit region and is receiving upper level support from this setup. Slight PVA is associated with the trough moving through the area, the degree is questionable as the propagation of the vort max is rather parallel to the geostrophic wind. Looking at other possible drivers of lift, the 12Z sounding from KVBG shows a conditionally unstable environment. Given our degree of saturation, we have some instability present. CAPE values are not in the severe range but there is no cap at fhr 6 and out, so storms could deliver large QPFs. The rain occurring right now will continue through the evening, breaking up overnight into Saturday early morning - with the exit of the low. Patchy rain events are expected as evidence does not suggest synoptic scale lift but more terrain induced effects. Indices are not suggestive of severe weather with this event. Most of Saturday and all Sunday are expected to be clear as flow becomes less amplified and more zonal across the area.


Friday: high in the mid 50's with showers through the evening, low in the mid 40's

Saturday: high in the upper 50's, low in the mid 40's

Sunday: high in the mid 60's, low in the mid 50's



Flow become much more zonal, offering a string of mostly clear to clear days. A drying out will occur, indicated by the 700 mb RH on both the GFS and CMC models. Not much confidence is given to the forecast starting Tuesday as the CMC and GFS have no agreement with the 500 mb flow.


Monday: high in the mid 70's, low in the mid 50's

Tuesday: high in the mid 70's, low in the mid 50's

Wednesday: high in the mid 70's, low in the mid 50's

Thursday: high in the low 70's, low in the low 50's


high/low/precip verification at Los Angeles Airport....

FRI 13th: 59/50/0.44(rain)
Temperatures were off with an underestimation in both the high and low temperatures. Need a better way to look at temperature advection and valley interactions. Reviewing sounding data may offer improvements as well. Rain was in the forecast, which checks out. Also, the forecast mentioned that the event would not be severe. This was a decent assessment as the storm reports for that day consisted of two wind damage reports from the San Diego area, almost 120 miles south of the forecast area.

SAT 14th: 60/45/0.0
Temperatures were in range and precip was also forecasted well.

SUN 15th: 62/48/0.0
The high temperature was slightly overestimated, while the low temperature was slightly underestimated. Again, need a better way to look at temperature advection and need to give more consideration to valley interactions. Rain forecast verified.

MON 16th: 67/52/0.0
Temperatures were off base. At this time, confidence was low anyway as models were not agreeing well. But, after another look at the 500 mb maps from the GFS, UKMET, and CMC (13APR2012 12Z runs) there was actually very good agreement and a better forecast should have been possible. Difficulties were most likely due to me scrolling improperly across the Weather Portal fhr bar. More practice should abate that problem.

TUES 17th: 67/52/0.0
Temperatures generally verified and precip checked out too.

Michael Baldwin,
Apr 13, 2012, 12:24 PM