Model-forecast integrated water vapor initialized on Tuesday 27 November 2012, and running through Wednesday 5 December 2012. AR conditions are shown stalling over the Central and Northern California Coast for up to 2-3 days
Fig. 1. NWS precipitation forecast (inches) showing 5-day total precipitation amounts exceeding 10 inches in northern California from Tuesday 27 November to Sun 2 December 2012.
Fig. 2. NWS GFS weather model forecast of a strong atmospheric river hitting the San Francisco area late on Thursday 29 November. a) Vertically integrated water vapor (IWV) showing the characteristic spatial pattern of an AR (see Ralph et al. 2004, Mon. Wea. Rev. for the method). b) Vertically integrated water vapor transport (IVT) showing magnitude (color fill) and direction (vectors). Images are from a real-time automated AR Detection Tool developed by Wick et al. (Trans. Geosci. Remote Sens., 2012 in press).
Heavy Precipitation Event Developing in Northern California
Heavy precipitation (Fig. 1) is predicted for Northern California over the next 5 days
[for official National Weather Service (NWS) weather forecasts, visit: http://www.wrh.noaa.gov/,
and for river forecasts, visit: http://www.cnrfc.noaa.gov/]. As with most extreme precipitation events in the region, this one is
associated with a strong atmospheric river (AR).
HMT has identified the key role of ARs and has developed, tested, and implemented major new capabilities to monitor and predict them. Figure 2
shows the forecast for a strong AR at a specific time, while a loop of the forecasts over time is available
on the Atmospheric River Information page. The National Weather Service has begun applying this
concept in the forecast process, based heavily on lessons from HMT (e.g., through an
Atmospheric River Retrospective Forecast Experiment). Highlights of the
approaching atmospheric rivers are provided by new tools and research:
A 21st Century AR Monitoring Network:
Inspired by HMT and currently being installed in California for "Enhanced Flood Response and Emergency Preparedness" sponsored by the California
Department of Water Resources. For example, these real-time data show that
the soil moisture has increased in the Russian River basin in the last few weeks due to early
season storms and could reach saturation early in the event.
Determination that the duration of continuous AR conditions at a given location is key to
distinguishing flood-producing events: Ralph et al. (2012) documented that AR events lasting
more than 40 hours were associated with 6 times as much streamflow on the Russian River
than average AR events that last half as long (20 hours). The paper is in press and available online.
The results will be presented at an invited talk at the American Geophysical Union Fall meeting in San Francisco, CA on 3 December 2012.
Prototype Ensemble AR Prediction Tool: As a step toward visualizing and quantifying
the duration of AR conditions at sites along the US West Coast, this tool was developed through HMT and
led by Jason Cordeira. It uses the GFS Ensemble
Forecast System to calculate the probability of AR conditions of differing magnitudes striking the
west coast. It also determines how many 12-hour-long periods experience AR conditions as a
function of position along the coast.
The accompanying animation of model-forecast IWV initialized on Tuesday 27 November 2012, and running
through Wednesday 5 December 2012 shows AR conditions stalling over the Central and Northern
California Coast for up to 2-3 days. However, the details of this forecast are likely to change as the event progresses, and important
questions remain about exactly where it will strike and for how long. Stay tuned to the National Weather Service for up-to-date forecasts.