Weathernotes Archive February 2007

Two Fronts — Not Enough Rain.
Weathernotes for Wednesday, February 28, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

If nothing else, this rain season has been consistent. Only the first of two cold fronts held together well enough to produce measurable rain throughout the Los Angeles area. Rainfall amounts from Monday's frontal passage generally ranged from few hundredths to a few tenths in the basin and valleys, with up to an inch or so in the mountains.

Downtown Los Angeles (USC) recorded 0.04 inch of rain on Monday, no rain on Tuesday, and a trace of precipitation today, bringing the water year total to 2.42 inches, which is 8.38 inches below normal. This is less rainfall for the date than in the record dry water year of 2001-2002.

At the moment, most guidance suggests precipitation in Southern California will remain below average through mid March.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Rainy Monday with More on the Way?
Weathernotes for Monday, February 19, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

As recently as Friday, some runs of the NAM were projecting little or no rain would result in the downtown Los Angeles area from the system currently affecting Southern California. Given the systems trajectory, there was a question of how much moisture it would be able to entrain.

A GOES-11 water vapor image from 6:30 a.m. shows the wrap-around circulation and moisture associated with the upper low and trough. Here is an Intellicast composite radar image from the same time as the GOES-11 water vapor image.

The system has produced widely varying rainfall intensities and amounts across the area, with CNRFC reporting as little as 0.01 inch at Malibu, and as much as 0.71 inch at Browns Canyon for the 24 hr. period ending at 8:00 a.m. So far Downtown Los Angeles (USC) as recorded 0.03 inch since midnight and 0.09 inch the last 24 hours.

Computer models show the system moving rapidly to the east, with precipitation ending in the Los Angeles area by this evening. Another system is forecast to affect the area on Thursday. This system appears to have a more typical rainy season synoptic structure than most systems we've seen this Winter. Current forecasts suggest the highest rainfall amounts will be in Northern California, but Southern California should get some additional, and much needed, rain. We'll see!

Update 02/23/07 7:00 A.M. Yesterday afternoon, the axis of the upper trough became more negatively tilted as it moved onshore and this had the effect of narrowing and accelerating the frontal zone as it moved into Southern California. This resulted in less rain in the Los Angeles area than hoped for, but still produced a good shot of much needed precipitation. Rainfall amounts in the basin and valleys generally ranged from about 0.25 inch to about 0.50 inch, with as much as an inch or so in some foothill and mountain locations. According to the NWS, Downtown Los Angeles (USC) recorded 0.38 inch for the storm, bringing the water year total to 2.38 inches — 7.64 inches below normal.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Will Los Angeles Be Short-Changed Again?
Weathernotes for Friday, February 9, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

A strong Pacific storm system with a very moist sub-tropical connection is forecast to produce more than an inch of precipitation over much of California the next 72 hours, with as much as 5 inches (liquid equivalent) forecast in some areas of the central Sierra Nevada. Given our meager amount of rainfall this rain season, it is unfortunate that the projected amount of precipitation falls off dramatically south of Santa Barbara.

Today's 18z NAM/WRF generates only 0.35 inch of rain at Los Angeles through Monday morning, and this is consistent with the HPC 3-day precipitation forecast for the area. The system looks very impressive on satellite and it wouldn't take much of shift to produce a dramatic increase in rainfall. However, the 09z SREF puts the probability of a 24 hr. period with more than 0.25 inch of rain at Los Angeles at about 50%, and for more than 0.50 inch at about 10-25%. The 15z SREF also supports the NAM/WRF and HPC forecasts for the Los Angeles basin, and areas to the south and east of the basin.

That being said, a BUFKIT analysis of 18z NAM data for Van Nuys shows maximum precipitable water values on the order of an inch, with west-southwest winds at 5000 ft. of about 20 knots. Accordingly, southwest to west facing slopes could see increased precipitation from terrain induced lift. We'll see!

Update 02/12/07 7:00 A.M. Short-changed or not, any rain is better than no rain at all. Rainfall amounts in the Los Angeles area for the storm generally ranged from about 0.25 inch to 0.50 inch, with some favored foothill and mountain locations ranging from about 0.5 inch to as much as 1.0 inch. To the probable benefit of the denuded slopes in the 2006 Day Fire area, the Santa Barbara and Ventura coastal mountains did not see the dramatically enhanced rainfall that had been anticipated -- generally recording amounts ranging from about 0.5 inch to 1.0 inch. According to the NWS, Downtown Los Angeles (USC) recorded 0.42 inch for the storm, bringing the water year total to 1.92 inches, which is now 6.63 inches below normal.

More information concerning Southern California weather can be found using our WEATHER LINKS page.

July-January 5th Driest on Record. Warm Superbowl Sunday. Pattern Change Ahead?
Weathernotes for Thursday, February 1, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

While some locations in the Los Angeles area were pounded by heavy showers on Tuesday, with rain rates on the order of a 0.25 inch/hour, Downtown Los Angeles (USC) recorded only 0.02 inch of rain. This increased the water year total (since July 1) to 1.50 inches, which is 5.62 below normal, and the 5th driest water year on record.

Clearing skies and a warming trend could push temperatures into the eighties in warmer Southland locations by Superbowl Sunday

Enjoy the good weather this weekend, ESRL/PSD's Reforecast Ensemble suggests an increased chance of California rain from about Wednesday February 7 through at least February 16, with the largest mean precipitation anomaly currently forecast for Central California February 10-11 and Northern California February 13-15. This wetter weather would result from a pattern shift that would effectively open California's door to the Pacific and afford additional opportunities for rain in our area. We'll see!

More information concerning Southern California weather can be found using our WEATHER LINKS page.