The first thing I look at is a current surface map to get a big picture of the US. What is the pattern that is affecting the area that I am forecasting? I mostly look at the HPC page for this current view. I also look at observations to look at temperatures and winds along with area that are reporting precipitation. Other things that are helpful are the radar and satellite views to diagnose areas of precipitation. The NWS page also has some nice radar images that can help.
For LA today, the current HPC surface map shows a low pressure center off the coast of Oregon with a cold front of the southern portion of the low bend off away from the coast. The temperature at the airport is 57 with winds out of the south at 15 knots (17 mph). The flow in this area is generally SSW today which will keep the temperatures cool especially near the coast. Showers are forming out ahead of the front. Based on what I see in the current radar loops, chance of precipitation for LA today is 100%. There is a strong precipitation band just northwest of the city.
The place I like to start for a forecast in NCEP’s model output. This will highlight areas to watch over the next week. You can also overlay precipitation with 1000-500mb thicknesses on there to see areas of potential snow or mixed precipitation. For any short term forecast it is important to start with the current observations and forecast from there. http://home.comcast.net/~dwpusey/portal.html. This link is very helpful because you can compare several models side by side and look at the RH at 700, pressure with 1000-500mb thickness, along with 500mb vorticity and precipitation. If precipitation is found, its important to look at sounds for the times you are looking at to diagnose cape, shear and helicity to see if there is there is a severe threat.
The gfs 12Z run has the rain from about now till 6Z Saturday with the heaviest precipitation within the next 6 hours. This is very consistent with what the other models are showing. Once the front passes, the trough really digs into New Mexico which is causing the severe storm threat over the Great Plains. The pattern for LA looks to have moderate temperatures and be very dry after this low pressure system. Expect similar highs for the next two days in the low 60’s due to the WNW flow off the coast.
Long term I rely highly on the GFS model output for both precipitation and model confidence along with the previous weather portal link. I found that the MOS is a good way to verify a number to a temperature forecast. I do not trust the wind forecasts on the MOS. The site I found to be very accurate with wind predictions is weather.com. They also give you hour by hour forecasts up to 2 days or so with wind speed, direction, and POPs.
After the ridge on the back side of the low flattens out around 6Z on the 16th, LA enters an extremely zonal flow. There might be a chance for precipitation early on the 17th but models have us safe for now. The 18th shows a slight warm up with a strong southerly flow. This flow should help keep the low in the Northwest out of our area. Conditions continue to look dry and warmer with this SSW flow till about the 23rd. At this day the models are in heavy disagreement and anything at this point would be a complete guess. Expect high temperatures in the low to mid 70’s throughout next week.
If I need to analyze severe weather, I would definitely look at sounds and the SPC’s mesoanalysis page to look and different variables such as cape, helicity, and shear at different levels.
There were massive amounts of rain associate with this low that went through the Los Angeles area on the 13th. By on the national weather service, there was a pocket of 2 inches of rain just northeast of the city and .44 inches at LAX and .49 inches downtown. The central valley receive the most rain, upwards up 2.5 inches. This could drown some of the grape vines in the wine vineyards did not have their drainage systems working properly. the overall timing of the low was very close to what was predicted by the models. The models should have of had a good handle of the timing because it was only a day out.
LAX reported a low temperature of 45F which set a low temperature record. This was cooler than most of the models had project for this day. Temperature were supposed to drop after the front went through but not this low.
This image was taken off the SPC website.
SPC had the LA area in a slight risk of severe but the storms were a little stronger than predicted. There were a few hail wind damage reports associated with this system passed through.
62 was the high for Saturday and Sunday's high for downtown was 67. The models had a good handle of these temperatures. The predictions were clear and warming up and that is exactly what we saw. The high for yesterday was 77. This was on the warm side of what was predicted for this time. Overall the models underestimated the winds of the low that went through and over estimated the low temperature slightly. The conditions for Sat-Mon were very similar to what the models were showing. Temperature were slight higher due to the lighter wind which allowed the land to heat more and have less interaction with the ocean.