Weathernotes Archive December 2007

Strong Pacific Storm Forecast to Pound State.
Weathernotes for Monday, December 31, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

Los Angeles will end 2007 with precipitation for the water year close to normal, and with the Sierra snowpack at about 60% of normal. However the first week of 2008 may usher in one of the strongest Pacific storms to affect California in some time, and by Monday of next week, precipitation and Sierra snowpack totals may have increased dramatically.

If recent runs of the GFS and ECMWF verify, California -- including much of Southern California -- will be deluged with heavy precipitation in the Thursday to Sunday timeframe. A BUFKIT analysis of 12z GFS data at Van Nuys indicates rain rates of over 0.7 inch an hour Friday night, and a rainfall total for the period of more than 4 inches. A second impulse Sunday is forecast to add about another 1.0 inch to the total.

CIRA Blended TPW loops show energy and moisture associated with tropical forcing in Indonesia being transferred into the the mid-latitudes, priming the system. Additional moisture is being drawn into the system from the tropics near Hawaii, and the GFS indicates a tropical connection as the system moves into California.

Details are likely to change as the week progresses, but at this point the storm's forecast parameters are impressive. At its peak, the Van Nuys BUFKIT analysis indicates a low level jet of approximately 50 kts developing, 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.

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.

Unusual La Niña Precipitation Pattern in the West.
Weathernotes for Thursday, December 20, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

According to preliminary NWS data, Downtown Los Angeles (CQT) recorded 1.24 inches of rain as a result of Tuesday night's storm, bringing the water year total to 3.69 inches. This amount is about 3/4 inch above normal for the date, and exceeds the 3.21 inch total recorded at Los Angeles during the record setting dry water year of 2006-2007.

Rainfall totals around the Los Angeles area generally ranged from about 0.75 to 1.25 inch in the basin and surrounding valleys, with as much as 1.5 to 3.0 inches (or more) recorded in some foothill and mountain locations. Here's a copy of a NWS Information Statement listing some rainfall totals for the storm.

In terms of percent of normal rainfall over the past 90 days, Southern California and the Desert Southwest have fared better than might be expected during a La Niña. The snowpack distribution in the Sierra is also somewhat unusual for a La Niña. Today's Summary of Snow Water Equivalents reports that the Sierra Southern Section snowpack is at about 77% of normal for the date, while the Central and Northern sections are at 53% and 61% of normal.

Today's NWS 6-10 Day Outlook projects above normal precipitation in Southern California, while the the 8-14 Day Outlook projects normal precipitation. We'll see!

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

More Rainfall Forecast for Los Angeles.
Weathernotes for Monday, December 17, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

SREF Ensembles have been trending wetter south of Pt. Conception over the next 36 hours with each run. The 15z run indicates >90% chance of the rainfall total exceeding 0.1 inch in the Los Angeles area for the 24 hr. period ending 7:00 a.m. Wednesday.

The last couple of runs of the NAM have also been wetter. An ARL analysis of 18z NAM data generates about 3/4 inch of rain at Los Angeles over the next 48 hours, and a BUFKIT analysis generates about 2/3 inch at Van Nuys. According to these analyses light rain is forecast to begin about 10 p.m. this evening, increase in intensity during the day tomorrow, and taper off early Wednesday morning. We'll see!

Today's NWS 6-10 Day Outlook projects above normal precipitation in Southern California, while the the 8-14 Day Outlook projects normal precipitation.

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

Los Angeles Rainfall Continues Above Normal.
Weathernotes for Saturday, December 8, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

Model performance hasn't been the best this Autumn. Sometimes this has worked in our favor, and sometimes not. The elements of Thursday night's system came together a little differently than projected and it wasn't as strong as expected.

Given the flash flood and debris flow potential in recently burned areas, the amount of rain was probably about as much as could be handled without causing too many problems. Downtown Los Angeles (CQT) recorded 0.41 inch of rain, bringing the water year total (since July 1) up to 2.44 inches -- still 0.16 inch above normal for the date. Last year at this time only 0.50 inch had been recorded.

Rainfall totals around the Los Angeles area generally ranged from about 0.25 to 1.00 inch, with as much as 1.0 to 2.0 inches (or more) recorded in some foothill and mountain locations. Here's a copy of a NWS Information Statement listing some rainfall totals for the storm.

An NRL GOES composite visible/IR satellite image from 1:30 p.m. PST this afternoon shows a strong vorticity max and upper low center near Pt. Conception, embedded in very strong north-northwesterly flow associated with a digging trough. The upper low is somewhat moisture starved, but very energetic. GOES soundings around the base of the trough indicates precipitable water values on the order of 0.50 to 0.75 inch. Radar shows some convective shower activity associated with the disturbance.

The 18z models project that the low will continue to dive south down the coast before moving onshore in northern Baja Mexico. Both the NAM and GFS generate about 0.1 inch at Los Angeles over the next 24 hours, with somewhat more to the south -- about 0.4 inch at San Diego according to the NAM. Because of the convective nature of the system, precipitation may be widely scattered and variable. Some areas may record no rain and others significantly more (or less) than the projected values. Here's the Day 1-2 QPF forecast from the HPC.

Today's (un-interpreted) NWS 6-10 and 8-14 day outlooks call for above normal precipitation in Southern California. In support of this notion, the 12z run of the ECMWF develops a full latitude trough around December 17, and the 18z GFS brings a strong system into California around December 19. A lot could change between now and then, but we'll see!

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

Los Angeles Rainfall Above Normal?
Weathernotes for Saturday, December 1, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

In a November where the other 29 days were bone dry, the 30th was surprisingly wet, with downtown Los Angeles (CQT) recording 0.56 inch of much needed rain. The storm brought the water year total (since July 1) for Los Angeles to 2.03 inches, which is a deceptive 0.15 inch above normal for the date.

Rainfall totals generally ranged from about 0.25 to 1.00 inch, with well over an inch recorded in some foothill and mountain locations. Here's a copy of a NWS Information Statement listing some rainfall totals for the storm.

Although today's (uninterpreted) NWS 6-10 and 8-14 day outlooks call for below normal precipitation in Southern California, the 12z runs of the GFS and ECMWF suggest there might be an opportunity for some wet weather towards the end of the work week. We'll see!

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