Weathernotes Archive January 2008

Eight Days of Los Angeles Rain One of Longest Wet Periods on Record.
Weathernotes for Monday, January 28, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

According to NWS Preliminary Climatology Data (CF6), Downtown Los Angeles has recorded measurable rain on each of eight days from January 21 to January 28, 2008. This puts it in the top six of the station's wettest periods from 1921-2007.

Downtown Los Angeles (USC-KCQT) has recorded 5.78 inches of rain over the period, bringing its water year total to 11.73 inches -- nearly 5 inches above normal for the date. Even if Los Angeles were to receive no rain through the entire month of February, we would still go into the month of March ahead of normal.

And it doesn't look likely that we'll have no rain this February. Today's NWS 6-10 Day Precipitation Outlook for Southern California is Above Normal, and the 8-14 day outlook is Normal. We'll see!

Here is an archived PDF of a NWS Public Information Statement with some preliminary rainfall totals for our most recent storm -- from 4:00 p.m. Saturday to 10:00 a.m. this morning.

Update 01/29/08. Early this morning the NWS released a Public Information Statement (PDF) with the rainfall totals for the eight day period from January 21 to January 28.

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

Heavy Rain Expected Overnight Saturday.
Weathernotes for Friday, January 25, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

Computer weather simulations can be impressive, but sometimes the models just don't get it. That was the case overnight and into this morning as the Los Angeles area was hit by thunderstorms, heavy rain, and hail. Rainfall totals around the area generally ranged from about 1.5 to 3 inches, with some foothill and mountain location recording much more. OPIDS Camp, near Mt. Wilson in the San Gabriel Mountains recorded 6.27 inches for the 24 hour period ending at 8:00 this morning.

As has been the case several times this rain season, the GFS provided a better rainfall guesstimate than the NAM, forecasting 1.5 inches at Los Angeles through today, compared to the NAM's paltry 0.5 inch. Here are a CNRFC graphic of 24 hour rainfall totals in the Los Angeles area, a PDF of some preliminary Los Angeles County rainfall totals, and another PDF of a NWS Public Information Statement with some preliminary rainfall totals for the ongoing rain event. Several stations have recorded more rain since Monday than during all of last year's rain season.

Now attention turns back to the large upper level low spinning off the coast. The extensive circulation around this low is forecast to interact with the southern branch of the jet stream and an associated plume of sub-tropical moisture, and draw the jet and moisture into Southern California overnight on Saturday. According to NAM, GFS and high resolution NMM and ARW, the strongest dynamics and precipitation are forecast for Northern and Central California, however heavy precipitation is also expected in Southern California.

The 12z NAM and GFS are in better agreement than in recent days, projecting about 2.5 inches at Los Angeles from early Saturday morning to early Monday morning, with peak rain rates around 1:00 a.m. on Sunday. A BUFKIT analysis of 12z VNY data shows a strong, saturated, veering, southeasterly to southerly inflow, turning southwest over a six hour period from about 9:00 p.m. Saturday to 3:00 a.m. Sunday. This could produce significant orographic enhancement of precipitation on favored foothill and mountain slopes. Wet antecedent conditions at the lower elevations, and the possibility of rain on snow at the middle elevations, could increase runoff.

Update 01/26/08. Today's models appear to be indicating stronger dynamics in the south with more embedded vorticity, more diffluence aloft, and more favorable jet stream dynamics, but the 18z NAM and GFS are still cranking out about 2.3 inches at Los Angeles, beginning this evening, continuing through Sunday night, and tapering off Monday morning. Strong southerly inflow could produce much more than this in the foothills and mountains, particularly on south facing slopes. As the upper level low moves onshore on Sunday afternoon, thunderstorms could produce very heavy rain in areas where the soil iis already saturated.

Here is a NWS Public Information Statement with the preliminary rainfall totals for the period 4:00 p.m. Monday afternoon, to 4:00 p.m. Friday afternoon.

Please refer to your local NATIONAL WEATHER SERVICE FORECAST OFFICE for official forecasts and warnings.

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

New Upper Level Low May Tap Tropical Moisture.
Weathernotes for Thursday, January 24, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

You can see why the weather models had such a tough time with yesterday's forecast for Los Angeles. Much of the day the main rain band associated with the upper level low spinning off the coast was stalled and producing moderate to heavy rain in areas of Ventura, Santa Barbara and San Luis Obispo counties. New daily rainfall records were set at Santa Barbara, Santa Maria, and Paso Robles airports.

With just a slight change in the position of the low, that rain band might have spent the day over Los Angeles instead. As it was, the rain did finally move into the area during the afternoon, resulting in an inch or more of precipitation in many areas, and snow in the mountains -- closing I-5 over the Grapevine. Here are PDFs of some preliminary Los Angeles County rainfall totals, and a NWS Public Information Statement with some preliminary rainfall totals for the ongoing rain event.

As wet as it's been, it could get wetter. This morning's 12z NAM shows the upper low currently off the coast being replaced by another -- stronger -- upper level low. The new low is forecast to tap into an area of high precipitable water in the tropics and draw that moisture into Southern California Saturday night. The result could be copious rainfall. The wet antecedent conditions would likely increase runoff. In addition, thickness values and 850 mb temps are forecast to rise, which could result in a higher snow level, with rain on snow a possibility.

But keep in mind we're dealing with another upper level low. The models continue to have difficulties with the forecast. The 12z GFS is astronomically wet, generating 6 inches of rain at Los Angeles between early this morning and Sunday afternoon. A BUFKIT analysis of 12z GFS data for Van Nuys shows a peak rain rate of 1.5 inch/hour Saturday night! The GFS is also wetter in the short term, producing about 2 inches over the next 36 hours. In comparison the 12z NAM generates only about 2.75 inches at Los Angeles through Sunday, and less than an inch through Friday. We'll see!

Update 01/24/08 4:00 p.m. The 18z NAM and GFS are not as wet as the 12z runs. The 18z GFS generates about 4 inches through Sunday evening, and 1.5 inches through Friday evening. The 18z NAM produces about 1.75 inches through Sunday evening, and 0.4 inches through Friday evening.

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

Upper Level Low Spinning Showers into Southern California -- Heavier Rain May Be On Tap.
Weathernotes for Tuesday, January 22, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

In a rain season characterized by upper level lows, yet another one has set up shop off the coast and has been sending some showers our way. As of 8:00 a.m. this morning rainfall totals around the Los Angeles area have generally ranged from a few hundredths of an inch to as much as about 0.25 inch in a few locations. Here is an archived copy of a NWS Public Information Statement with some preliminary rainfall totals.

As is often the case with this kind of scenario, the models have been having a difficult time handling the situation, and there has been a lot of variation from run to run. The 12z NAM and GFS remain at odds. The GFS is much wetter at Los Angeles through early morning Thursday, generating about 1.3 inches vs. the NAM's 0.5 inch. From early Thursday through Friday afternoon the NAM is a little wetter, generating 1.6 inches of rain at Los Angeles vs. the GFS's 1.2 inches.

In any case it looks like the water year rainfall total at Downtown Los Angeles has a good chance of remaining above normal as we head into February. We'll see!

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

Series of Storms Soaks West.
Weathernotes for Monday, January 7, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

The last of a series of storms that have been producing rain and snow in the western U.S. the past several days moved eastward and (mostly) out of the region today. According to preliminary NWS data, Downtown Los Angeles (CQT) recorded 2.00 inches of rain in the course of the event, bringing its water year total to 4.41 inches -- 1.54 inches above normal.

Precipitation totals in the Los Angeles area from Friday morning to this morning generally ranged from 2-5 inches in the basin and valleys, and from 5 to 10 inches or more in the local foothills and mountains. San Marcos Pass, near Santa Barbara recorded nearly 11 inches of rain, and OPIDS Camp, near Mt. Wilson, recorded nearly 13 inches of precipitation. Here is an archived copy of a NWS Public Information Statement with preliminary rainfall totals for the rain event, and an AHPS Precipitation Analysis of California precipitation for the 7 day period ending 4:00 a.m. this morning.

According to today's Summary of Snow Water Equivalents from California Cooperative Snow Surveys, the series of storms increased the Sierra Snowpack to 111% of normal for the date -- up from 60% at the end of 2007.

Another fast moving shortwave is forecast to primarily affect areas north of Pt. Conception Tuesday into Wednesday, with only a small chance of showers in the Los Angeles area.

So far this rain season, Southern California has dodged a La Nina bullet. This AHPS Precipitation Analysis for the water year to date indicates much of the area having received near normal to above normal precipitation. Even though the Climate Prediction Center's revised precipitation outlook for January, issued December 31, indicates above normal precipitation in the western U.S., the JFM Precipitation Outlook, issued December 20, remains pessimistic. We'll see!

Updated January 10, 2008. The ESRL-PSD Composite ENSO plots page was updated today to correct an issue that resulted in the wrong set of years being used for its Winter La Nina composites. As a result the La Nina composite precipitation map in the October 9, 2007 Weathernotes has been updated. Note that the average La Niña rainfall indicated for coastal Southern California is in the 7.0-10.5 inch range, rather than the 10-15 inch range previously indicated in the ESRL-PSD graphic.

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

Heavy Rain Expected Friday Night into Saturday.
Weathernotes for Thursday, January 3, 2008.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

Forecast details have changed, but the main message is the same regarding a deep trough and 504 mb H5 low developing off the west coast. Strong, deep-layered, moist southwesterly flow is forecast to produce copious precipitation Friday night into Saturday, with lesser amounts preceding and following this period.

HPC's quantitative precipitation forecast issued this morning for the 5 day period ending Tuesday morning indicates totals as high as 11 inches (water equivalent) in the Sierra Nevada, and up to about 8 inches in the mountains of Ventura and Los Angeles. The 12z runs of the NAM and GFS generate about 3 inches of rain at Los Angeles between now and Sunday afternoon -- a little more forecast by the NAM and a little less by the GFS.

BUFKIT analysis of 12z NAM data for Van Nuys continues to indicate a low level jet of approximately 50 kts developing overnight Friday, with precipitable water values of 1.2 inches and cloud velocities exceeding 60 kts. Such a scenario -- if the forecast verifies -- could produce extraordinarily heavy, orographically enhanced rainfall in the foothills and mountains, with flash flooding and debris flows possible, particularly in areas that have been recently burned.

A new wrinkle, just introduced by the 12z GFS, is a more southerly track of a shortwave progged for mid-week next week, with a chance of rain developing Wednesday into Thursday. We'll see!

Please refer to your local NATIONAL WEATHER SERVICE FORECAST OFFICE for official forecasts and warnings.

Update 01/06/08 1:00 P.M. Archived copy of a NWS Public Information Statement with preliminary rainfall totals for the ongoing rain event.

Update 01/05/08 9:00 A.M. Rain event progressing more or less as expected with storm totals so far generally ranging from about 1 to 5 inches in the basin and valleys, to 3 to 7 inches or more in the foothills and mountains. San Marcos Pass, near Santa Barbara has recorded nearly 9 inches of rain, and OPIDS Camp, near Mt. Wilson, has recorded about 8 inches of rain. Here is an archived copy of a NWS Public Information Statement with preliminary rainfall totals for the storm, and an AHPS Precipitation Analysis of California precipitation for the 24 hr. period ending 4:00 a.m. this morning.

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