IntermtnClimateSummaryJan2008

• Title: Intermountain West Climate Summary
• Date: 1/22/2008
• Description: Water Availability Task Force Meeting Report
• Basin: Statewide
• Drought Mitigation - Doc Type: Reports

INTERMOUNTAIN WEST CLIMATE SUMMARY, JANUARY 2008

Precipitation Outlook February - June 2008

The precipitation outlooks issued by NOAA/CPC are derived almost
entirely from composites of moderate La Ninas, i.e., the observed records
of past events, with probabilities adjusted to reflect the expected moderate
La Nina conditions. While there are significant temperature trends in the
Intermountain West region, precipitation trends are not significant.
Based on these La Nina composites, the NOAA/CPC precipitation
forecast for February (Fig. 11 a) calls for below average conditions across
the southern U.S. including southern Utah and Colorado and above average
conditions in the Pacific Northwest including most of Wyoming. La Nina
conditions are expected to have a greater impact on precipitation in February
than previous months.
Similarly, the outlook for February-April 2008, shows Wyoming to have
a slightly increased chance of above average conditions (Figure 11 b), while
southern Colorado and Utah remain in the region with a slightly increased
risk for below average conditions through the March-May season (Figure
11 b-c). Between these areas of opposite signals, for other areas of the
Intermountain West, the outlooks indicate "EC" or "equal chances" of above-
average, near-normal or below-average precipitation (Figures lla-c). In the
April-June season, the interior West is forecasted to have an increased risk of
below average conditions centered over Utah (Fig Ild)
An updated precipitation forecast for February 2008 will be released on
January 31st on the CPC web page. Because of the shorter lead-time, the
"zero-lead" forecast (i.e. on the last day of the previous month) often has
increased skill over the half-month lead forecasts. The next issue date for the
seasonal Outlooks is February 21 st.

Notes
The seasonal precipitation outlooks predict the likelihood (chance) of precipita-
tion occurring in the above-average, near-average, and below-average categories.
The numbers on the maps do not refer to actual precipitation values, but to the
probability in percent that precipitation will be in one of these three categories.
The CPC outlooks are 3-category forecasts based on climate models in which
the skill largely comes from the status of ENSO and recent trends. The categories
are defined based on the 1971-2000 climate record; each 1- or 3-month period is
divided into 3 categories (terciles), indicating the probabilities that the precipitation
in the period will fall into the upper third of the years (upper tercile, the middle third
of the years (middle tercile, or around average), or the lowest third of the years
(lower tercile). each with a 33.30Ib chance of occurring. The middle tercile is con-
sidered the near-average (or normal) precipitation range. The forecast indicates
the likelihood of the precipitation occurring in the below-average (8, brown shad-
ing) or above-average (A, green shading) --with a corresponding decrease in the
opposite category, The near-average category is preserved at 33.30/0 likelihood,
unless the anomaly forecast probability is very high.
Thus, areas with dark brown shading indicate a 40.0-50.00/0 chance of
below-average, a 33.30/0 chance of near-average, and a 16.7-26.60/0 chance of
above-average precipitation. Light brown shading displays a 33.3-39.90/0 chance
of below-average, a 33.30/0 chance of near-average, and a 26.7-33.30/0 chance
of above-average precipitation and so on. Green shading indicate areas with a
greater chance of above average precipitation. Equal Chances (EC) indicates
areas for which the models cannot predict the precipitation with any confidence,
representing equal chances or a 33.30/0 probability for each tercile, indicating areas
where the reliability (i.e., 'skill') of the forecast is poor. "N" indicates an increased
chance of near-average conditions, but is not forecasted very often.

On the Web

Figure 11 a. Long-lead national precipitation forecast
for February 2008 (released January 17, 2008).

Figure 11 b. Long-lead national precipitation forecast
for Feb. - Apr. 2008 (released January 17, 2008).

Figure 11 d. Long-lead national precipitation forecast for
Apr. - Jun. 2008 (released January 17, 2008).

EC = Equal A = Above B = Below
Chances .40.0-49.90/0. 50.0-59.90/0

D 33.3-39.90/0 .40.0-49.90/0

D 33.3-39.90/0

For more information and the most recent CPC forecast images, visit: http://www.cpc.ncep.noaa.gov/products/predictions/90day/.
Please note that this website has many graphics and may load slowly on your computer.
The CPC "discussion for non-technical users" is at: http://www.cpc.noaa.gov/products/predictions/90day/fxus05.html.
For IRI forecasts, visit: http://irLcolumbia.edu/c1imate/forecast/net_asmt/.
More information about precipitation distributions at specific stations in Colorado, Utah, Wyoming, and across the West can be found
at the Western Regional Climate Center, http://www.wrcc.drLedu/CLlMATEDATA.html.
The PSD experimental guidance product, including a discussion and executive summary, will be updated by January 19th, and is
available on the web at: htt ://www.cdc.noaa.gov/eole/klaus.wolter/SWcasts/index.html

FORECASTS 11 4