Weathernotes Archive February 2001

Weathernotes for Wednesday, February 28, 2001

Upper level low continues to produce scattered rain and showers in Southern California. The low is being reinforced by a digging wave and vorticity sliding down from Canada. As a result, showers may persist into tonight, and perhaps tomorrow morning. Cold air aloft could result in isolated thunderstorms later today. Friday a shortwave moves through to the north. HPC forecasts most of the precipitation from this system will be to the north of us, but it would probably be prudent to consider a chance of a shower, particularly in the mountains to the north of the L.A. basin.

Earlier in the week the MRF had forecast another big storm for Southern California in the Sunday-Monday timeframe, but in recent runs has held this system offshore. There are considerable differences between the MRF and models such as the UKMET, ECMWF and the NCEP ensembles.

When in doubt, go with the ensembles, right? The NCEP ensembles currently suggest a chance of precipitation in Southern California throughout the Sunday to Thursday period. We'll see!

Yesterday's CPC/NCEP Outlooks continued to depict "Above Average" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period for Southern California, and today's ensembles suggest the wet pattern will continue for much of California.

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

Weathernotes for Monday, February 26, 2001

Yesterday, Downtown Los Angeles (USC) recorded 2.05 inches of rain, a new record for the date. the old record was 1.30 inches, set in 1958. This brings the month to date total to nearly 5 inches over normal, and our seasonal total (since July 1) to 3.7 inches above normal. Quite a February!

Pushed by a strong jet and supplied with moisture from this impressive fetch, it started to rain in Southern California Saturday morning, well ahead of schedule, and continued unabated through most of last night.

The complex upper level low responsible for this rain is still off the coast and is forecast to influence our weather through Wednesday. Multiple centers of vorticity are pin-wheeling around the low center, and the circulation around the low is resulting in east to west flow over the Sierra.

There will be a continuing chance of showers and rain over much of Southern California through Wednesday, when the low is finally kicked out by the approach of another potentially very wet system. Recent runs of the MRF bring a shortwave through the area on Friday, with a major system following close on its heels, moving into Southern California late Saturday and persisting until mid-week.

Yesterday's CPC/NCEP Outlooks continued to depict "Above Average" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period for Southern California, and today's ensembles suggest the wet pattern will continue for all of California.

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

Weathernotes for Friday, February 23, 2001

It's been wet in Southern California this month. At downtown Los Angeles (USC) we've had 5.09 inches of rain, which is 2.67 inches above normal. The month isn't over. and more rain is on the way.

This morning vorticity embedded in a trough is resulting in showers around the area. Cold air aloft with an LI near 0 could result in some heavier showers or thunderstorms.

There is an impressive fetch of moisture with very strong 150 kt winds aloft in the Eastern Pacific. Models say this system will eventually shear apart, but some of this moisture looks like it could be entrained in the system that is currently forecast to affect Southern California Saturday night into Monday.

This is another one of those cases where the details of how the storm evolves could dramatically affect the amount of precipitation in the Southern California area. At the moment guidance suggests a significant event is possible with perhaps two inches of rain in the lowlands and two or three times that in favored mountain areas.

Yesterday's CPC/NCEP Outlooks continued to depict "Above Average" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period for Southern California, but today's ensembles suggest a shift in focus to Northern California for a few days following our Sunday-Monday storm.

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

Weathernotes for Friday, February 16, 2001

Wet pattern continues for California with emphasis on northern two-thirds of state.

The first of a series of systems forecast to affect the state over the next 10 days is expected to move into Southern California Saturday afternoon and evening, with a second possibly stronger storm to follow Monday.

Ignoring the computer models and just looking at the this morning's water vapor satellite photo you would think a pretty good rain event is headed our way, but the models say that Saturday evening's storm will weaken significantly as it pushes ashore. HPC currently forecasts 0.25"- 0.50" in the Los Angeles area for the 24 hour period ending early Sunday morning, with somewhat higher amounts to the north and in orographically favored areas. Satellite derived data suggests precipitable water values of about 0.75" associated with the system, so if it does not collapse as quickly as currently forecast, precipitation amounts could be higher.

Yesterday's CPC/NCEP Outlooks depicted "Above Average" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period. We'll see!

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

Weathernotes for Tuesday, February 13, 2001

What a storm! In the 24 hour period ending 10:00 this morning, many stations across the area recorded more than 2 inches of rain, and some orographically favored stations recorded much more. For example the OPIDS Camp near Mt. Wilson recorded 4.6 inches. The wet weather has pushed rainfall totals for Los Angeles (USC) to nearly 2.0 inches above normal since July 1, and more than 5.5 inches above normal since January 1.

This GOES-10 IR satellite photo shows the developing system about 3:15 yesterday afternoon. The dynamics can be more clearly seen in this 640x480 MPEG video (550K)or 250x188 animated GIF (390K) covering the period from 4:00 Sunday afternoon to 4:00 early this (Tuesday) morning. The area of strong vorticity moves down the backside of the trough, and then, rounding the base of the trough, adds to the divergence aloft on the front side of the trough, enhancing lift. This occurred in an area with precipitable water values on the order of 1 inch. The area of enhancement rapidly broadened and eventually cyclogenesis occurred, just off the coast. Here's a visible light satellite photo taken shortly after sunrise.

While the main even is over, the current forecast is for a continuing chance of showers tonight and tomorrow. There's a possibility of more precipitation Saturday or Sunday or Monday, but at the moment there is a lot of variation in the computer models and the ensemble members.

Today's CPC/NCEP Outlooks depicts "Above Average" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period. We'll see!

Some links regarding vorticity, dynamics and difluence:

• Is Difluence the Same as Divergence?
• "Dynamics" Means More Than "Vorticity Advection"
• Vorticity Advection and Vertical Motion

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

Weathernotes for Sunday, February 11, 2001

Intellicast radar at 3:15 pm PST shows frontal band moving into Southern California this afternoon (Compare to radar summary from two days ago.) This front is associated with a dynamic upper low near the Oregon border.

This GOES-10 Water Vapor satellite photo shows the position of the low at 3:00 pm. An MPEG clip from the same period shows the moist WSW flow impinging on the Southern Sierra. This nearly perpendicular flow should be ideal for orographic enhancement. Snow sensors in the area are recording moderate to heavy snowfall.

Precipitation in the Los Angeles area is forecast to result from not only the frontal passage, but also from a region of enhanced dynamics that is expected to rotate around the base of the trough and into Southern California. Rain from the front is moving into the area this afternoon and evening. Significant additional precipitation should also occur tomorrow. HPC forecasts about 2 inches in favored areas for the 24 hour period ending 4:00 pm tomorrow afternoon. Based on QPF forecasts from NWS Oxnard, 2 inch rainfall amounts may be more widespread.

Today's CPC/NCEP Outlooks depicts "Above Average" precipitation for the 6-10 day period and "Normal" for the 8-14 day period. We'll see!

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

Weathernotes for Friday, February 9, 2001

Frontal band moving down the Central Coast this afternoon expected to bring rain to Southern California tonight. Visible satellite photo taken at 3:15 PST shows waves in southwesterly flow ahead of the front. Intellicast radar summary from same time shows extent of frontal zone and precipitation.

Computer models are not especially bullish on this initial front; suggesting less than 0.25 inch of rain in the lowlands of Southern California for the 24 hour period from 4:00 pm today to 4:00 pm tomorrow. The HPC QPF forecast for this same period appears to concur.

A somewhat stronger system is expected to affect the area Sunday night into Tuesday with ensembles and other guidance suggesting that unsettled weather may continue through much of the month.

Today's CPC/NCEP Outlooks depicts "Normal" precipitation for the 6-10 day period and "Above Average" for the 8-14 day period. We'll see!

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

Weathernotes for Tuesday, February 6, 2001

What a difference a couple of days can make. Instead of 90+ temperatures, there's talk of the mercury dropping to near freezing in the basin and valleys overnight on Wednesday. There's even a slight chance of a shower tonight, especially in the mountains, and it looks like the weekend could see some showers, with rain and snow to the north.

Today's CPC/NCEP 6-10 and 8-14 day Outlooks continue to depict "Normal" precipitation for Southern California, and the 6-10 day and Week 2 ensemble precipitation probability forecasts are looking somewhat wetter. We'll see!

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

Weathernotes for Sunday, February 4, 2001

Let me check that again... Yep, February 4th. You wouldn't know it by walking outside. It's HOT today. At 12:30 the Malibu Hills RAWS is at 87 degrees. Welby Way Elementary School in West Hills is at 88 degrees, Simi Valley ES at 93 degrees, and the Beverly Hills RAWS is at 89 degrees. The record high temperature for this date in Downtown Los Angeles (USC) is 91 degrees; set in 1995.

The culprit is a relatively warm atmosphere to start and the adiabatic heating that results from air being compressed as it is moved from higher elevation to lower. At higher elevations it is much cooler. For example, at the Acton RAWS (2600 ft.) at 12:30 it was only 70 degrees and in the San Gabriel Mountainsat Mill Creek (4970 ft.) it was only 63 degrees.

Temps are forecast to cool as the week progresses, but at the moment, no rain is expected in Southern California. Today's CPC/NCEP 6-10 and 8-14 day Outlook is for "Normal" precipitation for Southern California, but at the present time, 6-10 day and Week 2 ensemble forecasts are not especially promising. That can change. We'll see!

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