Rueter forecast #8

Nik Rueter

4-13-12

EAS 434

The key to forecasting without using special programs that are not available to the general public like nmap2 and GEMPAK is finding data that is of equivalent value on the web. The forecast process will be the same, with verification of what has been occurring for the forecast area, and short-term and long-term forecasts based on forcings and variables analyzed.

For my short through long range forecast for Los Angeles, I began by looking at the radar, surface observations, and current flow of the jet. This can be found on the NWS page for Los Angeles, as well as the weather.rap.ucar.edu site. All the sites that were used were included below. The NCAR site has multiple variables that can be run on the GFS model, as well as the NCEP Model output on the Purdue weather discussion page. These two sites along give you all the model runs with most of the variables that you need to analyze to determine the flow, forcings, vorticity, etc. In the case of severe weather, which is not the case for this forecast, the SPC page also has all of the severe convection variables as well as soundings needed for that analysis. So it is clear that these sites with all of the variables and model runs give you as much information as nmap2 can.

The current conditions in Los Angeles are cool and rainy with temperatures in the high 50s. A strong jet core is currently located just off the coast, as is providing the strong positive forcing for this precipitation as can be seen on the 500mb vorticity plot. Rain will continue throughout the day and taper off by tomorrow morning as the trough pushes through and to the east. This trough will eject from the four corners region and is expected to provide the forcing and instability for the strong chance of thunderstorms in the Great Plains Saturday night.

Expect clearing skies by tomorrow, as a strong ridge and high pressure quickly build in from the East Pacific. Strong subsidence will provide clear sunny skies and allow temperatures to return to normal highs in the upper 70s by the beginning of the workweek. The entire week looks to be dry and clear due to this strong ridge that will be in place over the West. A few small shortwave disturbances will affect the Pacific Northwest, but the entire Southern California and Los Angeles area will have very stable conditions with negative forcing and highs in the upper 70s.

Sources used:

http://weather.rap.ucar.edu/model/displayMod.php?var=gfs_500_vrt&loop=1- NCAR RAP

http://wxp.eas.purdue.edu/wrfdata/wxbrief/- Purdue source of NCEP model output

http://forecast.weather.gov/MapClick.php?map.x=272&map.y=143&minlon=-121.29&maxlon=-117.61&minlat=32.65&maxlat=35.81&mapwidth=354&site=lox&zmx=1&zmy=1- NWS page for Los Angeles

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Verification

4-17-12

The forecast above for the Los Angeles area was made at about 18Z on the 13th. We will take the verification from this point on. The forecast discussion was mainly focused on the timing of the precipitation and the track of the low pressure system and the jet stream. No amounts of precipitation were forecasted, and a part of the verification will also include precipitation that occurred in the mountains surrounding Los Angeles.

As far as the timing of the pressure center and associated precipitation, at 12Z on the 13th, the precipitation was heavy over central California, especially as it pushed east towards the Sierra Nevada mountain range. This orographic lifting provided the forcing for heavy precipitation as it moved inland, and dumped over a foot of snow around the Tahoe area, as well as the San Bernadino mountain range north of Los Angeles. Moderate rainfall began to push south into Los Angeles and San Diego by around 21Z. A few isolated pockets of thunderstorms occurred to the frontogenesis and positive vorticity advection, with a few storm reports of wind in San Diego. While much less than in the mountains because of the downslope effect, Los Angeles received 0.41 inches of rain. The low pressure was over central Nevada at this time with the cold front trailing behind this precipitation. The water vapor image for this time shows the ample moisture and shows the dry slot as the cold front pushed through.

The models handled the timing of this low pressure track very well, as the forecasted timing of the precipitation being over by early morning Saturday was the case as precip was out of the area by 06Z on the 14th. The precipitation quickly deteriorated over the desert valley in southeast California, as the upper level support was gone and likely due to the downslope from the mountains. Temperatures Saturday morning were far below average, with lows in the low 40s around Los Angeles.

This deep diving trough quickly tracked through the four corners region as forecasted, and was responsible for a strom tornado outbreak that spawned over 100 tornadoes throughout Oklahoma, Kansas, and Nebraska Saturday evening. This area was forecasted with a severe weather outlook very accurately, and shows how consistent the models were in their forecasts of this system.

A few lingering areas of precipitation remained over the mountains, but the coastal areas of Los Angeles were dry and clear as the ridge quickly moved in. Temperatures were forecasted to rebound into the 70s quickly with clearing skies, and this was definitely the case as the observed temperature on the 16th was 70 degrees with clear skies. It seems that precipitation events are much quicker and generally less precipitation around the Los Angeles area because alot of the moisture is lost over the mountain ranges to the north and the Sierra Nevada range. Also, upper level support is less frequent as it has to take a significant dive south to impact Southern California. Since the prevailing weather pattern is much different for this area, more verifications would help me to understand these effects and how the models handle them. For this case however, both the short and long term forecasts were very accurate.