Weathernotes Archive March 2006

March Madness to Continue into April?
Weathernotes for Friday, March 31, 2006.
(Any updates will be found at the end of this discussion.)

Intellicast.com composite radar shows band of showers working its way through Southern California this morning, but so far only a small amount of precipitation is making it to the ground. CNRFC's Observed Precipitation shows rainfall amounts of around 0.04 inch associated with the band.

In some ways, this system is a weaker version of Tuesday's record-setting storm. GOES Sounder data shows high precipitable water values associated with the system, but the dynamics that actually trigger precipitation are much weaker. The !2z NAM and GFS shows two periods of showers; one this morning, and another as the trough moves onshore early on Saturday. The NAM forecasts about 0.2 inch total from this system at Van Nuys, and only about 0.1 inch at Los Angeles.

March's wet pattern does look like it will continue into April. A strong Pacific low pressure system is forecast to affect the area early Monday into Tuesday. The 12z GFS is saying this system should have have good dynamics and moisture, and generates over 1.75 inches of rain at Los Angeles.

A BUFKIT forecast sounding for LAX based on 18z GFS data, shows the freezing level up above 9000 ft. Sunday evening, and it is forecast to stay in that neighborhood through most of Monday. Overnight Monday it is forecast to drop to around 6000 ft., and then lower to around 5000 ft. during the day on Tuesday. We'll see!

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

Jet Energized Front Breaks Rainfall Record Set in 1895!
Weathernotes for Wednesday, March 29, 2006.
(Any updates will be found at the end of this discussion.)

Yesterday's 1.62 inches of rain at Los Angeles (USC) set a new record for the date, breaking a record set in 1895. Sub-tropical moisture and favorable placement of a 100 kt. jet max enhanced the dynamics and precipitation within the frontal zone associated with a Pacific low pressure system as the front moved through Southern California.

Rainfall amounts in the Los Angeles area generally ranged from about 1.5 inches to 2.0 inches, with higher totals at a few locations. The snow level remained higher than initially forecast, but several inches were reported above about 7000 ft. As of 5:15 A.M. Los Angeles (USC) has recorded a preliminary total of 1.68 inches of rain, bringing the (unofficial) rain season total since July 1 to 10.17 inches, which is about 3.6 inches below normal. This March has been exceptionally cool. As of yesterday, 25 of 28 days have recorded an average daily temperature that is below the norm.

Yesterday's system may be the last of our March series of storms, but then again it may not be. While the 12z NAM keeps Friday dry, the 12z GFS forecasts about 0.1 inch of rain. Today's 09z SREF (NCEP/NOAA Short Range Ensemble Forecast) says... Over the 24 hr. period ending 4:00 A.M. Saturday morning there is an approximately 30% chance of more than 0.01 inch of rain in the Los Angeles area, and a less than 10% chance of more than 0.1 inch We'll see!

Then there is April... Not normally a very rainy month, but the 12z GFS forecasts our unusual Spring weather to continue, indicating the possibility of more rain in the Sunday-Monday timeframe, and then again the week before Easter. There has been a lot of variation in the model solutions, so maybe warmer and drier weather will prevail.

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

Active March Weather Pattern Forecast to End with a Bang!
Weathernotes for Monday, March 27, 2006.
(Any updates will be found at the end of this discussion.)

Of one thing we can be sure... Beyond this week, there will be no more storms this March! Cool temperatures and periodic showers have been the hallmarks of this month, and it looks like the approaching system could be one of the strongest in the series.

In contrast to most of the other systems that have affected the area this month, this system has a longer over-water track, and is forecast to tap into sub-tropical moisture with precipitable water values of an inch or more. The 12z NAM shows a combination of mid and upper level dynamics, with diffluence aloft, and multiple vorticity centers, and a jet max rotating around the base of the trough and into Southern California. Depending on the timing of these various elements, and how they work together, there is the potential for a lot of rain. Strong low level south-southwest inflow should enhance precipitation totals in orographically favored foothill and mountain locations.

Recent runs of the the NAM and GFS have projected varied precipitation amounts. Last night the NAM forecast about an inch at Los Angeles, and the GFS about 0.6 inch. This morning the 12z NAM still forecasts about an inch at Los Angeles, but the 12z GFS has flip-flopped and is now projecting more precipitation than the NAM -- about 1.75 inch. Both models show the rain beginning overnight tonight, rain tomorrow (Tuesday), with showers extending into Wednesday.

SREF (09z NCEP/NOAA Short Range Ensemble Forecast) says... Over the 24 hr. period ending Tuesday evening at 10:00, that there is an approximately 70%-80% chance of more than 0.5 inch of rain in the Los Angeles area, and a 30%-50% chance of more than 1.0 inch. We'll see!

This morning's sounding from Vandenberg shows the freezing level at about 10,500 ft. Forecast soundings from 12z NAM data shows the freezing level dropping to near 6500 ft. this evening, and perhaps briefly lower as evaporation of precipitation cools the lower levels. The forecast sounding indicates the freezing level to remain in the neighborhood of 6500 ft. much of Tuesday, and then drop down to near 3500 ft. overnight Tuesday. This scenario could result in a foot of snow, or more, in our local mountains above about 7000 ft., and perhaps as much as 3-4 ft. in some orographically favored locations.

Update 03/28/06 9:00 A.M. Freezing level hasn't dropped as much as forecast by yesterday's 12z NAM, and this will result in less snow than expected. This morning's sounding from Vandenberg shows the freezing level still up around 8400 ft. Today's 12z NAM has the freezing level at about 8100-8300 ft. this morning, at about 7500-8000 ft. this afternoon, and about 5000-6000 ft. overnight. At 8:35 A.M., the recorded temperature at Chilao at 5450 ft. was 42°F. Today's 09z SREF (NCEP/NOAA Short Range Ensemble Forecast) says... Over the 24 hr. period ending 1:00 A.M. tomorrow (Wednesday) morning there is an approximately 90% chance of more than 0.5 inch of rain in the Los Angeles area, and a 50% chance of more than 1.0 inch. (Note that the rain is not forecast to end at 1:00 A.M. tomorrow morning. This is just the end of the 24 hour period for which the probability is calculated.)

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

Spring Begins with Below Normal Temperatures and Chance of Showers.
Weathernotes for Monday, March 20, 2006
(Any updates will be found at the end of this discussion.)

Friday and Saturday's showery system wasn't quite as cold, and wasn't quite as wet, as the storm a week ago Saturday, but did produce several inches of snow in the local mountains, and from 0.01 to about 0.5 inch of rain at the lower elevations. Yet another in our continuing series of March storms is forecast to produce some more showers this evening into tomorrow morning.

The culprit is a negatively tilted trough and associated frontal band. As of 9:30 this morning the system was producing some rain in Northern California, and south down into the San Francisco Bay area.

SREF (NCEP/NOAA Short Range Ensemble Forecast) says... Over the 24 hr. period ending tomorrow morning at 10:00, that there is an approximately 90% chance of more than 0.01 inch of rain in the Los Angeles area, a 50%-60% chance of more than 0.1 inch, and less than 10% chance of more than 0.25 inch of rain. We'll see!

According to the U.S. Naval Observatory, Astronomical Applications Department, Spring begins today, March 20, at 18:26 UTC, or 10:26 A.M. PST. The first 19 days of this March have been remarkably cool. As of the 19th, every day this month, the daily average temperature recorded at Los Angeles (USC) has been below normal. The average amount the temperature has been below normal is about 6.5 degrees. Our rainfall also continues below normal. The NWS Climate Report for March 19th reports Downtown Los Angeles (USC) has recorded 8.10 inches of rain since July 1, which is about 4.9 inches below normal.

Update 03/21/06 12:00 P.M. Front (IR image & WV image) was a little stronger than forecast. Malibu RAWS recorded south winds of about 8 kt., gusting to over 20 kt., ahead of the front. Chilao, at 5450 ft. in the San Gabriel Mountains recorded southeast winds of about 25-30 kt., gusting to over 40 kt. According to the NOAA/NWS CNRFC Observed Precipitation Map and other sources, 24 hr. rainfall totals around the area generally ranged from about 0.25 inch to 0.50 inch, with a few locations recording 0.75 inch or more. Another system is expected to produce more precipitation in Northern and Central California this weekend, but at the moment it looks like the front associated with the system should dissipate before reaching our area. The GFS has been speculating that more rain is a possibility sometime around the end of the month, but it is way too early to tell.

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

Déjà Vu All Over Again?
Weathernotes for Thursday, March 16, 2006
(Any updates will be found at the end of this discussion.)

Tuesday's frontal passage stirred up some clouds, but that was about it for the Los Angeles area. South of Pt. Conception, most of the precipitation was confined to Santa Barbara County, with only very widely scattered showers in Ventura County, and almost nothing in Los Angeles County.

In a virtual rerun of last week's cold, wet, and snowy weather, another cold upper level trough is forecast to move through Southern California Friday and Saturday, and bring with it cool temperatures, a pretty good chance of rain, and perhaps snow back down around the 2500 ft. level. At the moment, it looks like 500 mb thickness and 850 mb temperatures won't drop quite as low as last week's storm, and that the coldest temperatures won't occur until later in the storm.

The 12z NAM and the 06z GFS, both projected about 0.5 inch of rain at Van Nuys, beginning mid-morning Friday and ending Saturday afternoon. The 12z GFS just came in, and it backs off on the forecast precipitation amount to only about 0.1 inch. As with last week's system, higher precipitation amounts are forecast for areas south of the Los Angeles basin, with 0.8 inch forecast by the 12z NAM for Ontario Airport. We'll see!

Update 03/17/06 1:00 P.M. This morning's 12z NAM forecast sounding for Van Nuys shows freezing levels starting out near 5000 ft. today, dropping to around 3500 ft. overnight. The 18z NAM has further reduced forecast precipitation amounts to about 0.13 inch for Van Nuys. About two-thirds of that amount is forecast for today, with the rest in showers on Saturday, mainly in the afternoon and evening.

Update 03/17/06 8:00 A.M. This morning's 12z NAM forecasts about 0.2 inch at Van Nuys, 0.3 inch at Los Angeles, and 0.4 inch at Ontario. The 09z SREF puts the probability of more than 0.1 inch of rain in the area of the Los Angeles basin, over the 24 hour period ending 10:00 A.M. Saturday, at about 50%. The SREF probability of more than 0.01 inch of rain over the same period looks to be about 80% to 90%, and for more than 0.25 inch only about 10% to 30%.

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

Low Elevation Snow in Southern California!
Weathernotes for Saturday, March 11, 2006
(Any updates will be found at the end of this discussion.)

This morning, at about 2000 ft. in the Simi Hills, periodic snow showers added a white accent to the black, brown, and greens of a landscape in the process of recovering from the Topanga Fire. The snow was wet, and not much was accumulating, but it was SNOW in March within a mile or two of the Simi and San Fernando Valleys.

Over on the Chumash Trail at about 2200 ft., the coverage was a little better, and a couple hundred feet in elevation higher, near the trail's junction with Rocky Peak road, the scene was virtually alpine. The snow highlighted the blackened fingers of chaparral burned in the 2003 Simi Fire, and chilled the smile of this Canyon Sunflower.

Rainfall totals in the Los Angeles area have been more in line with what the GFS projected, than the NAM, generally ranging from about 0.20 to 0.50 inches. In the San Gabriel Mountains liquid equivalents have ranged up to about 1.0 inch. As the models forecasted, rainfall totals have been higher to the south of the Los Angeles basin, with totals (as of 4:00 P.M.) ranging from about 0.5 to 1.0 inch. According to data reported by the California-Nevada River Forecast Center, San Juan Capistrano has recorded 0.80 inch, Fallbrook 0.75 inch, Escondido 1.08 inch, and Ramona 1.14 inch. To our east, Palm Springs has recorded 0.54 inches.

The cold upper level trough over the area is forecast to move slowly to the east over the next 24 hours, and as it does there will be a diminishing chance of showers. Another disturbance is forecast to bring more precipitation to the northern two-thirds of California Monday evening into Tuesday, but at the moment it looks like in the Los Angeles area there's a only a slight chance of a shower. We'll see!

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

Chance of Low Elevation Snow in Southern California.
Weathernotes for Thursday, March 9, 2006
(Any updates will be found at the end of this discussion.)

The third in a week-long series of northern stream Pacific low pressure troughs is digging into the West today and is expected to produce rain and snow throughout much of California over the next 48 hours or so. A combination of low freezing levels and convection could result in snow flurries down into the 1500-2000 ft. range in the Los Angeles area overnight Friday.

Both the 12z NAM and GFS generate higher precipitation amounts to the south of the Los Angeles basin, with the NAM significantly wetter than the GFS. For the period beginning mid-morning tomorrow (Friday) and ending Saturday evening, the 12z NAM projects about an 1.0 inch of rain at Van Nuys (VNY), but nearly 2.0 inches at Ontario (ONT). For comparison, the 12z GFS generates only about 0.3 inch at VNY, and 0.6 inch at ONT. The 09z Short Range Ensemble Forecast System (SREF) puts the probability of more than 0.25 inch of rain over the 24 hour period ending mid-morning Saturday at about 60-70% in the Los Angeles area, and about 90% along the south coast.

Of particular interest with this system is the snow level. Projected 500 mb thickness values of 530 dm, 500 mb temps of around -30°C, and 850 mb temps below 0°C all suggest the possibility of low elevation snow. A BUFKIT sounding for 5:00 A.M. Saturday morning, based on 12z NAM data, shows a freezing level of about 2100 ft. Snow, or a mix of rain and snow, could occur lower than this in some areas. We'll see!

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

Series of Systems Forecast for California. Chance of Showers Monday. Weak La Niña Rebounds.
Weathernotes for Friday, March 3, 2006
(Any updates will be found at the end of this discussion.)

Frontal zone associated with Pacific low pressure system is working its way through Southern California this morning. Intellicast.com composite radar from 6:15 shows a mix of light, moderate, and some heavier showers around the area.

In part because of the convective nature of the showers, precipitation amounts have been variable, with totals for the storm as of 8:00 A.M. generally ranging from about 0.10 to 0.75 inch. More shower activity is expected today, tapering off this afternoon.

This system is the first of a sequence of three northern stream upper level lows that are forecast to affect California over the next week or so. This morning's 12z ETA and GFS move the next front into the Los Angeles basin sometime Monday morning, with the ETA earlier in the morning than the GFS. At this time rainfall amounts in the coastal and valley areas look to be on the order of 0.25 inch, but as with the current system, will likely vary.

TAO-TRITON data shows 5-day depth averaged temperatures continuing below normal in the eastern equatorial Pacific. Upwelling of this reserve of cooler sub-surface water has produced cooling at the surface over the last 2 weeks, suggesting that the weak La Niña will continue for now.

Update 03/07/06 3:00 P.M. Rainfall amounts from Monday's system generally ranged from about 0.1 inch to about 0.5 inch, with as much as an inch or so in a few stations in the mountains of Santa Barbara, Ventura and Los Angeles counties. This was system two of three forecast by the GFS back on March 3rd. At the moment system #3, slated for Friday into Saturday, may be more of a temperature event rather than a rain event. If there is precipitation, snow levels could be quite low. The 12z NAM shows 500 mb thickness dropping to below 540 dm, and 850 mb temperatures plummeting to below 0°C. That translates to cold -- especially with the wind chill of brisk west-northwesterly winds added to the mix. We'll have to see how the forecast of the trajectory of the upper level flow evolves.

Update 03/05/06 3:00 P.M. 18z NAM a little slower and wetter with approaching system than 18z GFS. Looks like the NAM has a better sub-tropical moisture tap than the GFS, and water vapor images appear to support this idea. NAM placement of jet max is also more favorable than the GFS. The GFS produces less than 0.25 inch of precipitation at Van Nuys, beginning sometime overnight. The NAM generates about 0.5 inch, beginning around mid-morning. More precipitation would be expected in orographically favored mountain and foothill locations.

If the 18z GFS forecast validates we could have several days of on-again, off-again showers or rain beginning around the end of the work week, March 10. We'll see!

Update 03/03/06 3:00 P.M. Showers have continued on and off through much of today. As of 3:00 P.M,, rainfall totals in the Los Angeles area have generally ranged from about 0.25 to 1.00 inch. Downtown Los Angeles (USC) has recorded 0.24 inches from the storm, bringing the (unofficial) water year total since July 1 to 7.56 inches -- about 3.6 inches below normal. This afternoon, at 3:35 P.M., the Whitaker Peak RAWS ( 4120 ft.), near the Grapevine, indicated a temperature of 37°F and dropping. Check with CalTrans for the latest road conditions.

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