Ward Forecast 8 SoCal
Current Outlook Valid at 18:10 UTC:
Let's begin by analyzing current surface observations, radar and satellite imagery to see what has been going on, and what is currently happening right now. Current water vapor imagery shows a low pressure system over Nevada which is currently producing precipitation off to the west and south, with heavy low level cloud coverage trailing to the southwest. A disturbance and jet streak is currently coming ashore near the LA area providing the forcing for the associated precipitation just to the north of LA. Current surface observations from the SPC Mesoscale Analysis Page show that the surface low pressure system over western Nevada is at 1001 mb, and temperatures throughout the LA area are in the mid to lower 50's with dewpoint temperatures ranging from the low 40's inland to near 50°F along the coast. Currently the winds are from the south at 10-15 knots and further up the coast, these winds shift over to from the west and even further up the northwest all associated with this low pressure system and current trough.
In higher elevations, this precipitation falling along the Highway 99 corridor is mostly rain, except for areas in higher elevation where it is cold enough for snow which is what Sandberg is currently reporting.
Short Range Forecast (Today through Monday 00Z):
Currently expecting to keep rain in the forecast for much of the LA area throughout the day and into the evening with higher elevations in the mountain seeing snow. Based on the 12Z run of the NAM for April 13th, a strong cyclonically curved jet stream at 250 mb is coming ashore with strong jet streaks imbedded within of 120 knots. Based on the jet stream model for cyclonically curved jets, a nice region of upper level divergence is expected just to the north allowing for the further enhancement of this precipitation, coupled with sufficient differential vorticity at the base of this trough and some 700 mb geostrophic temperature advection, sufficient forcing is present throughout the day. 12Z observed soundings from VBG shows some surface CAPE of 190 J/kg so I'm expecting a couple of embedded thunderstorms and strong downpours of rain, especially with so much low level moisture coming in from the Pacific. With the cloud coverage and light showers throughout the day, expecting high temperatures to get into just the low 60's.
Strong cyclonically curved jet lingers in the area throughout the day from 250mb heights and winds at 15Z. Source: twisterdata.com
For Saturday, a few lingering showers seems to remain behind this trough according to the NAM, so I would keep the chance for a few isolated showers in the day, especially for the mountains as orographic lifting could continue any chance for precipitation. Plenty of low level moisture remains over the area however, so Saturday should remain mostly cloudy with high temperatures reaching around the mid-60's along the coast and cooler inland, and keeping low temperatures in the low 50's to upper 40's along the coast.
NAM 850 mb Relative Humidity and Winds for 12Z Saturday
Saturday night into Sunday, a Pacific high pressure system starts to move eastward, by then completely clearing out any chance for precipitation for Southern California and sets up to be a major factor for the first part of the week.
Long Range Forecast (Monday through Thursday):
Models seem to be in fairly good agreement for the rest of the week, with only one or two noticeable exceptions. The GEFS shows sufficient agreement with only one or two runs digging in a deep trough into the region Wednesday morning into Thursday. The high pressure system according to the 12Z run of the GFS seems to be keeping the strength of this high pressure at around 1024 mb or so, and holding it far enough east to keep the forecast area under predominantly northerly flow. Comparing the GFS and the ECMWF at the same time, they seem to be in agreement as to the location of this high pressure system that's going to dominate the pattern through the rest of the week. The GFS also shows strong zonal flow aloft which is important for the forecast area as a low pressure system up in the Gulf of Alaska swings through late Tuesday night and swings a strong upper level trough across with it into the BC and Pacific Northwest area. If the model verifies, the strength of this high pressure system and zonal flow should keep the trough and associated precipitation from impacting the southern parts of California. With this strong zonal flow, expect temperatures to be around to slightly above normal temperatures with highs steadily increasing from the 70's into 80's as the week progresses.
GFS 500 mb relative vorticity for 6Z Wednesday showing the PVA across the Pacific NW and strong zonal flow.
Let's begin by verifying the precipitation forecast for LA area valid from 12Z the 13th to 12z the 14th. Most of the heaviest amounts of precipitation fell just to the north of LA due to the orographic uplifting in the Angeles National Forest region where amounts of 2.0 inches were observed. The precipitation based on radar data tampered off around 3Z on the 14th with higher elevations receiving spotty snow/rain showers. Daily climate report for LAX had the high at 59F at 10:23 am and the low of 50F at 4:15 pm with winds as high as 33 mph from the west and 0.44" of rain setting a new record. My high temperature for the 14th was way off with the belief that clouds would linger throughout the entire day, but the low temperature seemed okay as the recorded high was 60F and the low was 45F which was a record. Leaving spotty rain showers in the forecast for the remainder of Saturday verified in the higher elevations to the east of Bakersfield and to the north of LA after 12z on the 14th.
NAM40 24 hour precipitation ending at 12Z on the 14th from the 1200Z run on the 13th showing that the model actually did a fairly good job matching the observed precipitation. The bulls eye north of LA was only just slightly under forecasted in terms of QPF, but the general regions of maxima matched fairly well.
GFS from the same time showing 24 hour QPF was a lot more generous giving a much broader area significantly more precipitation than what was observed. Still, the flood warnings were warranted as the observed precipitation was massive.
One interesting analysis was that RUC never seemed to deepen the low over Nevada enough initially in comparison to what was observed as the surface pressure. NAM model runs actually did a fairly well job handling the 6 hour precip for the forecast area Friday into Saturday, even picking up on some of the late day Saturday orographic precip to the north.
For the long range forecast, the GFS is still in agreement keeping the strong high pressure off to the west and some ridging in place. This is going to keep any rain from the low pressure system from the Northwest away from the forecast area.