Shaun Tanner has been a meteorologist at Weather Underground since 2004.
By: Shaun Tanner , 6:46 AM GMT on November 28, 2012
I have been watching models for the past week or so in disbelief as both the GFS and ECMWF have both been consistently predicting a very wet period for the West Coast. Perhaps this disbelief is the reason I am writing this blog so close to the beginning of the wet period.
It has been quite some time since the West Coast has seen rain like what is expected for the next several days, so let's break down what is to be expected.
The first storm will slam into the coast Wednesday, lasting into Thursday. This storm will likely be the weakest in the series, which is a testament to how intense these storms will be. This is because nearly 2 inches of rain is possible in the wettest areas in Northern California, as noted in the precipitation forecast for Wednesday through Thursday morning.
The good people at the NWS have also posted various watches and warnings mostly in California in anticipation of this precipitation. Wind Advisories are posted for parts of the Northern California as southeast winds will gust to 40 mph. Winter Weather Advisories are also posted for the western slopes of the Sierra Nevadas, but snow levels will be a relatively high 6,000 feet. In fact, this will be a theme for this wet period as these storms will be rather warm in nature. So while the rain is certainly a blessing for dry areas of California that are in the brink a more serious drought (see below), it would have been great is some more of this precipitation would fall as snow. Not only would this help the water supply, it would also help the skiing industry in the Sierra Nevadas that has suffered greatly over the past 12 months.
The next storm to slam into the coast will arrive Thursday and last through much of Friday. This storm will be considerably wetter, as the first storm will do a great job in priming the atmosphere. The wettest areas (see below) could see over 7 inches of rain. This will be on top of the rain that has already arrived on Wednesday.
This is just the first two storms. Needless to say, the West Coast will be rather wet and wild through the first week of December. There are some interesting notes I want to bring to your attention.
First, if you look at the 5 day precipitation forecast from Wednesday through Sunday, you will see that there is a bulls eye of rain expected for Northern California. The storm track has consistently pointed directly at the California coast north of the San Francisco Bay Area. It is safe to say that if nearly 11 inches of rain fall on this area over the next 5 days, there will be some flooding issues for the mountainous areas of Northern California. The good news is that the streams in the area are fairly low at this time of year.
Second, one of the reasons I have been in disbelief at the amount of rain expected from this parade of storms is that a significant amount of subtropical moisture is needed. For these types of storms, the subtropical moisture can usually be seen on satellite as a thick line of clouds that extends from near Hawaii and is pointed directly at the West Coast. When this type of pattern sets up, California can receive days upon days of heavy rain. This is often called the "Pineapple Express", or more scientifically, the "Atmospheric River". If you are interested in watching this atmospheric river set up, watch the satellite on WunderMap.
Third, while the heavy rain from these storms is expected through the weekend, I still question slightly the actual location of the heavy rain. Certainly, the heaviest rain is likely in the hills of Northern California, but any southward movement of this deep moisture could bring the bulls eye closer to the San Francisco Bay Area. However, a more northward track would bring significantly less rainfall to San Francisco. This is only worth a mention at this point.
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