Tools of the Trade (well one of them...)

This entry is part of our series called "Tools of the Trade".  The idea behind the series is to share with you some of the cutting edge tools that go into our decision making process.

Today we're going to look at relatively new tool, West Coast Atmospheric River Landfall Tool, that is becoming a go-to way to help us examine the potential for atmospheric rivers reaching the west coast. We may abbreviate atmospheric rivers as AR, because sometimes we don't want to type out all the letters of a word. The tool has been developed out of several years of research from the CalWater project at NOAA's Earth System Research Laboratory with assistance from Plymoth State University and the University of California-San Diego Scripps Center for Western Weather and Water Extremes. A significant portion of our winter precipitation comes from just a handful of these atmospheric rivers making landfall in northern and central California. During drought winters we may only see 1 or even zero, in normal winters it can be 3-5.

The image below is the graph of the AR tool from a model run on 00Z Sunday March 8th. In local time, that is the late afternoon of Saturday March 7th. The horizontal scale is days from the current Model Run, while the vertical scale is latitude along the west coast. The color bar indicates the probability of an atmospheric river reaching the west coast. Red and purple are a high probability, while colors in blue and green are a very low probability. In this example, this highest probability is from the afternoon of Wednesday March 11, (+3 days from the model run time) to the afternoon of Thursday March 12th (+4 days) between 48.0 and 42.0 north latitude. A second small AR may reach the coast on March 14th and 15th.  From a forecast standpoint for the Sierra and western Nevada, and a personal one too with this (insert your own adjective) drought, we want a lot of purple moving into the west coast between 42.0 and 36.0 north latitude. Unfortunately this tool doesn't show the magnitude of the atmospheric river, but we have other tools for that.


The probabilities in the graph are calculated from several model runs, we call ensembles.  Ensembles are atmospheric models run with slightly different initial conditions. The big take away here is if several ensemble runs are showing the same outcome, our forecast confidence increases.  Now if the ensembles show the same solution over several days (we call this model consistency in our discussions), well that's more gold in our bucket of forecast confidence.

The next loop below is from the atmospheric river event that occurred in early February, which resulted in much needed rainfall across central and northern California. AR events are usually accompanied by very high snow levels and this event was no exception. There was a moderate increase in the snowpack but only for elevations above 8000 feet. Notice the large area of purple reaching the west coast, which gave NWS offices along the west coast a huge amount of confidence in this event. 
The atmospheric river tool loop from the February 5-8, 2015 storm.

Now that we are somewhat familiar with what this tool displays, let's check out the loop for next week.  Notice how there isn't a lot of red and purple, booo!!!!  For now it appears that we are going to see a pattern change, which means the ridge will breakdown by Tuesday, with two systems possible for Wednesday and sometime over the weekend. Unfortunately it doesn't appear that we will be getting very much precipitation. For now our interpretation is we're going to a get a little precipitation moving into the west coast, but the confidence in a big event for the Sierra and western Nevada is pretty low.
AR tool for the potential "storms" the week of March 8th, 2015.
As always thanks for your interest and always check out the latest Forecast Discussion for more details on the official forecast.

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