# Monday, January 26, 2009

GEFS Ensemble Forecasts - For February 4, 2009 4:00 a.m. PST Click
GEFS Ensemble Forecasts
For February 4, 2009 4:00 a.m. PST

Weather is by nature chaotic, and not perfectly predictable. From a given starting point, small variations in initial temperature, pressure, moisture, and other factors in the weather system can lead to dramatically different outcomes. In addition, interactions of earth's oceans and terrain with the atmosphere further complicate predictability.

Two periods of rain were recently forecast in Southern California -- one from Wednesday into Saturday, and another from Sunday evening into Tuesday. Model projections varied from day to day and run to run. At one time it looked like the Los Angeles area might get about an inch or rain out of both systems. So what happened?

Considering the first period, Wednesday into Saturday, here are the preliminary rainfall totals from the NWS. Note how much the rainfall varies. In the Los Angeles basin, Downtown Los Angeles (USC) recorded 0.34 inch, LAX recorded 0.51 inch and Santa Monica recorded 0.76 inch. Locations that benefited from orographic enhancement recorded higher totals. For example. Pacioma Dam recorded 1.11 inch, Warm Springs 1.31 inch, and OPIDS Camp 1.54 inch. Some stations in Santa Barbara and San Luis Opisbo counties received as much as 2-3 inches of rain. A slight change in the position of low to the south on Wednesday and Thursday, or the low to the west on Friday and Saturday could have easily resulted in much more rain.

We're in the middle of the second period now. Instead of an over the ocean path forecast a couple of days ago, the cold trough and upper low that is over the southwest took a drier overland course down the West Coast. Nonetheless, there has been scattered showers in Southern California, and even isolated thunderstorms, lightning and hail. It's not a huge step to speculate the rainfall would have been more widespread if the system had taken a more westward path, and picked up additional moisture.

So what's next? Globally, some interesting things are occurring that could impact our weather over the next few weeks. The MJO and GWO are expected to continue their circuit into phase 3-4. This might open the door to more West Coast troughs, or MJO enhanced rainfall. A Sudden Stratospheric Warming (SSW) event appears to be underway and may result in polar air outbreaks that would affect the mid-latitude circulation.

However, in the short term Los Angeles rainfall remains below normal. As of January 25, Downtown Los Angeles (USC) has recorded 5.00 inches of rain this water year, which is 1.38 inches below normal. Last year, the water year total on January 25 was 9.35 inches. The ECMWF and GEFS ensembles suggest the possibility of a strong trough affecting California around February 4th or 5th, but that is a long way out, and we'll have to see!

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

Monday, January 26, 2009 8:55:48 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   | 
# Tuesday, January 20, 2009

HPC QPF Forecast - 48 hrs. Ending 4:00 pm 01/22/09 Click
HPC QPF Forecast
48 hrs. Ending 4:00 pm 01/22/09

Including today, Los Angeles has enjoyed an unprecedented ten straight January days with highs in the eighties. In the yin yang of weather, the western half of the U.S. has been enjoying unusually warm temps, while the eastern half of the country has shivered.

Since the start of the water year on July 1, Downtown Los Angeles (USC) has recorded 4.66 inches of rain. Just 10 days ago, 4.66 inches was almost exactly normal rainfall for the date. However, as is so often the case in Southern California, this apparent normality was the sum of offsetting wet and dry periods. December's precipitation was generally well above normal, but January has been dry, dry, dry. Today, the Los Angeles rainfall total is about 1.10 inch below normal, and every day it doesn't rain, our deficit increases by about 0.10 inch.

Our warm temperatures and dry weather have been the result of a high amplitude ridge, pushed up over the West Coast by a very strong and extended Pacific jet stream. Big upper level ridges such as this are consistent with La Nina, and have been a recurring theme this Fall and Winter. Much of our rainfall and cold weather this season has occurred when an extended Pacific jet collapses or contracts -- as is occurring now -- and the blocking ridge shifts westward, opening the door to cold storms plunging down the backside of the ridge from the north.

This time there is a wildcard in the mix. One of the reasons the Pacific jet has been extended is the active phase of the Madden-Julian Oscillation (MJO) has recently propagated from west to east, across the Pacific. The system that is forecast to affect Southern California Wednesday night into Thursday looks like it might be enhanced by an inflow of moisture from an area of tropical convection that may be associated with the MJO.

How much might it rain? A BUFKIT analysis of 12z NAM data generates 1.3 inches of rain at Van Nuys for the period Wednesday evening through Thursday afternoon. The HPC 2 Day forecast is not so bullish, indicating about 0.50 inch to 0.75 inch over the area. We'll see!

Medium range models suggest the weather in the West could remain unsettled into next week and beyond. Of particular interest is this morning's 12z ECMWF forecast for mid-week next week, which projects the retrogression of the West Coast ridge to 150-160°W, with a cold, wet pattern similar to what we saw in December. That would be quite a change!

Update 01/22/09. It seems the NAM and GFS models can't get a handle on the system currently affecting our area. The 12z NAM generates 1.6 inches (!) of rain at Van Nuys and the 12z GFS 1.2 inches at LAX from this morning into Saturday afternoon. On the other hand the 09z SREF Ensembles puts the probability of more than 0.25 inch of rain for the 24 hr. period ending mid-morning Friday at about 50%, and then only about 10-30% for the following 24 hr. period. Upslope enhancement may produce higher totals on south facing foothill and mountain slopes. The GFS continues to advertise the possibility of a significant rain event Monday afternoon into Tuesday or Wednesday, but given recent model performance, we'll wait and see!

Update 01/21/09. The 12z models now extend the rainy period for the first system into Saturday. The 12z NAM generates about 1.0 inch of rain at Van Nuys, beginning Thursday morning and ending Saturday midday. The 12z GFS also produces about 1.0 inch, beginning Wednesday evening and ending Saturday afternoon. The GFS adds another 0.9 inch from Sunday morning to Monday night. Somewhat higher amounts would be expected in some foothill and mountain locations. As always, we'll see!

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

Tuesday, January 20, 2009 8:57:46 AM (Pacific Standard Time, UTC-08:00)  #    Disclaimer  |   |