To begin my forecast, I looked at the visible satellite and the IR satellite images from. This showed moisture being drawn over the Southern California region by west-southwesterly winds. Next I looked at the observations over the region. Current temperatures range from 50-60 degrees with dew points from 30-55, with very weak winds.. The radar on the same site shows rain/snow falling over the higher elevations as well as more showers streaming in from the Pacific.
Next I looked at the observed sounding through the Storm Prediction Center forecast tools.. Cape over the area is around 200, and significant shear in the low levels as well as perceptible water around .75 in. There is concern for flooding over the region over the next few hours as the precipitation begins to move upslope.
Next, I look at the model data. I begin with the jet. I like to look at 250 winds and heights. I use both the GFS and the NAM and compare how the short term fits together and look for inconsistencies. Today, they seemed to be in good agreement for the short term. Next, I begin to look at each model separately. Normally, I start with the short range, CMC, UK, NAM, and HRRR. These models also showed very good consistency, which gave me more confidence in the short-term forecast. Short term also handles QPF over the region, which showed an area of more than an inch in the mountains to the east of Los Angeles. The precipitation ensembles showed this is fairly consistent throughout the models.
Next, the long term models are considered. The GFS and EMWF come into play for the long-term forecast. The GFS is less reliable than the EMWF, but the GFS is still something to consider for the forecast. The long term is where the models seemed to show a large amount of inconsistency. For Thursday and Friday, the GFS SPAG showed a few runs having an amplified synoptic scale trough moving over the region, while other runs seemed to have more of a small shortwave moving through. This shows that this area needs to be watched closely as the time gets closer.
P.M. Friday through Saturday-
Heavy rain is possible as a tough and its associated low move over the area. The vorticity maxima will begin to move over through the day of Saturday. As the vorticity max and the low begin to push east, the rain should begin to wind down. However, before this happens, some areas especially in the mountainous regions may see more than an inch of rain. A Flood Advisory has already been issued for southern Los Angeles County, and Southeastern Ventura County. Wind will also be an issue as gusts could exceed 35 mph. A wind advisory has been issued for the region. Temperatures will remain around normal with a high in the low-60’s. The temperatures will drop to 50 for Friday evening.
Sunday should be a much better day to be outside as the trough finally pushes out, and winds become calmer and shift to the north. Some clouds are still possible early in the day as the system pushes over the Rockies; however, no rain is expected. Temperatures should remain seasonable with a high in the mid-60’s and a low in the mid-50’s.
A Ridge will build in throughout the day setting the stage for a warm up. Weak low level winds will help to heat up the air. It will be a sunny day due to the stable air mass overhead. Temperatures at the airport will be in the upper 60s, but downtown will be into the 70’s. Lows are expected to be in the mid-50’s.
A zonal pattern begins to take hold as high pressure over the Pacific keeps the pattern quiet. It should be the warmest day of the week with temperatures around 70 at the airport and into the mid-to-upper 70’s downtown. Lows will be in the upper-50’s.
The zonal pattern holds on as the high to the west begins to break down. Still remaining cloudless for the time being, with temperatures in the upper -60’s at the airport and into the 70’s downtown.
Uncertainty begins to creep into the forecast for Thursday and Friday. General consensus is that a shortwave will push in off the Pacific bringing a little moisture along with it, no rain is expected however, a few clouds are possible. This is something that needs to be watched carefully for changes in future model runs. Temperatures should be in the mid-60’s and lows in the mid-50’s.
Uncertainty becomes high at this point in the forecast period. However current expectations are for warmer air to return once again, and a few clouds to hang around through the day on Friday. Temperatures will be around 70 and lows are expected to be around 55.
SPC- Mesoanalysis -http://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=12#
LA Fcst Discussion- http://www.wrh.noaa.gov/total_forecast/getprod.php?prod=XXXAFDLOX&wfo=LOX
Total Precipitation Fcst- http://www.hpc.ncep.noaa.gov/qpf/qpf2.shtml
Precipitation Ensemble Short Range-
Friday, the LA airport saw .44 inches of rain, and local cocorahs totals go from 1.60 inches to the lowest at the airport. River flooding and flash flooding was reported in the region.
Saturday, The high recorded was 60 degrees, but the low was 45. The maximum wind gust was 40 mph from the west and the maximum sustained wind speed was 33 mph. The cloud cover varied greatly throughout the day, from clear to overcast.
Sunday, The high was 62 and the low was 48. The high was correct, but the forecast low was much higher than what actually occurred. The winds calmed considerable for Sunday as the low moved to the east.
Monday, The high was 67 and the low was 52. The peak winds were only 12 mph on Monday.
All the models seemed to have done a good job handling the weather of southern California. The one problem seemed to be the underforcasted of the upslope precipitation over the region. UKMET, CMC, NAM, HRRR, and EWMF, all seemed to not do a good job at the length of time precipitation would be falling. Otherwise, all the models did well on totals. The cloud cover was another area in which all the models collectively struggled. The temperatures forecasted by NAM and GFS did fairly well, highs were close, but the lows were around 5 degrees too high consistently. This suggests a model bias from both the NAM and GFS to not accurately portray the diurnal cycle. The timing of the low movement was slightly slower than was predicted by ANY of the models but not a large enough difference to really influence the forecast. It was a difference of around 2-4 hours.