Colorado (and Interior Southwest) forecasts

Outline for latest forecast webpage (updated on August 22nd, 2008; next update by September 20th, 2008)

This webpage consists of six parts:

1. Status and Outlook for ENSO (El Niño/Southern Oscillation), the most important global climate variability factor on year-to-year time scales;

2. Background information on regional climate variability of the interior western U.S;

3. "Official" CPC (Climate Prediction Center) forecasts for September through December 2008;

4. Experimental forecast guidance for OCT-DEC 2008 precipitation in the interior southwestern U.S.;

5. Discussion of experimental forecasts, and useful links to shorter-term forecasts;

6. Executive Summary of this webpage.

The most recent forecasts are based on observational data through July 2008. This website will remain online until further notice.

You are welcome to use any of the material from this website, but proper acknowledgment would be appreciated, especially when referring to figures, forecasts, and assessments unique to this website. PROPER ACKNOWLEDGMENT should state that the used material "was provided by Klaus Wolter at the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from his website at http://www.cdc.noaa.gov/people/klaus.wolter/SWcasts/". The forecasts presented in section 4 are new and experimental - users are cautioned that no responsibility for any losses can be assumed by CDC. For questions, contact me under (Klaus.Wolter@noaa.gov)

1. Current status of the El Niño/Southern Oscillation (ENSO) phenomenon and prospects for the next six to nine months

Recent sea surface temperature (SST) and wind conditions in the tropical Pacific from Australia to just west of South America show negative SST anomalies of more than -0.5C only north of 5N and near the dateline (reduced from last month), while positive SST anomalies above +0.5C continue to cover much of the equatorial stretch between 140 and 100W. At the same time, wind anomalies show slightly enhanced trade winds across much of the basin. This figure is taken from the ( TAO/TRITON website) that is updated daily.

Since this figure reflects conditions over the last five days, the reader may be interested in an ENSO discussion that pays more attention to the longer time scales of this phenomenon. A monthly updated ENSO discussion provides for this by the end of the first week of each month, and features a comparison figure of similar ENSO situations near the end of that web site. The Multivariate ENSO Index (MEI) has stabilized near zero, indicating near-neutral ENSO conditions. At this time of year, the boreal spring 'window of opportunity' for ENSO-phase transitions has closed. Therefore, a shift into El Niño conditions does not appear imminent despite the recent weakening of La Niña.

Latest European coupled model forecast for central Pacific tropical SST anomalies. The so-called "anomaly plume" (a.k.a. "spaghetti plot") refers to fifty different forecasts that were computed during the last month from slightly perturbed initial conditions, in order to encompass the range of possible outcomes over the next six months. As the stippled blue line indicates, Nino3.4 SST anomalies have risen dramatically from almost -2C in February to above 0C in July. While earlier forecast runs missed the mid-winter drop in SST, the recent increase was fairly well anticipated by February. However, almost all forecasts underpredicted the speed of warming that took place. Compared to last month's forecast, the range of possible outcomes is narrower than before, consistent with the higher degree of persistence through the remainder of the calendar year. Roughly 80% of the forecasts remain between 0C and +0.5C, while a handful of runs each barely reach above 0.5C (weak El Niño) and below 0C by next January. This figure is taken from the ( ECMWF seasonal forecast website) that is updated monthly around the 21st - just in time for this month's SWcasts update.

Latest comparison plot of 15 dynamical and 8 statistical model forecasts for central Pacific tropical SST anomalies. For those models that are run in ensemble mode (like the ECMWF forecast - note the one shown here is from LAST month, summarizing the information from July 2008), this graph shows the average outcome. The range of possible outcomes appears to favor near-neutral conditions, similar to last month. Compared to last month, dynamical models are not quite as different from statistical models in slightly favoring El Niño-like conditions, with six dynamical models reaching or exceeding the +0.5C threshold in this plot, while two of the statistical models do so. For comparison, there are now two dynamical models as well as two statistical models that drop their Niño 3.4 SST anomalies back to at -0.5C or lower. This figure is taken from the ( IRI ENSO model website) that is updated by the third Thursday of every month.

For an independent interpretation of the current situation, I recommend reading the latest NOAA ENSO Advisory which represents the official and most recent Climate Prediction Center opinion on this subject. The August 2008 CPC statement expects ENSO-neutral conditions to continue into our fall season.

2. Regional climate background information

Average timing of wettest three-month season during the course of the year is shown for just over 1,000 historical climate records in the interior Southwest. This figure includes both cooperative observing stations (mostly in the lower elevations) and automated SNOTEL sites (in the montane and subalpine regions of the Rocky Mountains). The number shown refers to the CENTRAL month of a three-month season (JAN=1, FEB=2,...,DEC=B). Note the spatial extent of the winter/early spring wet season (red numbers) over Colorado and points west: mostly at high elevations. By comparison, the summer monsoon season brings the most moisture to almost all of New Mexico and much of southern Colorado, southeastern Utah and eastern Arizona (blue numbers).

This companion map to the average seasonal wetness peak shows the average driest season during the course of the year. Note the lack of precipitation over eastern Colorado and New Mexico during the winter season, while Arizona experiences its driest season during late spring prior to the monsoon.

This map shows regions of similar precipitation variability for the Interior Southwestern U.S. for July through September, a season that requires ten core regions to describe the bulk of co-variability across the domain. COOP stations are indicated by circles, and SNOTEL sites by triangles. The amount of color in each station symbol represents the amount of local variance that is explained by the index time series created for each core region.

3. Most recent Climate Prediction Center forecasts for September through December 2008

The most recent U.S. government (CPC) temperature forecast for September 2008 shows a tilt of the odds towards warmer temperatures covering the interior southwestern U.S., including all of Arizona and New Mexico, most of Utah, and the southwestern half of Colorado. The rest of Colorado and Utah is left blank ("EC"), translating into equal chances of below-normal, near-normal, and above-normal temperatures. Where the forecast is "warm", it is mostly due to long-term warming trends in that region, while the lack of significant warming (or cooling) trends resulted in the "EC" label. La Niña is not a factor in this forecast cycle anymore. The precipitation forecast for September 2008 (below) shows all of the interior southwestern U.S. left blank ("EC"), due to the absence of significant trends. Both forecasts are taken from the monthly outlook website that is updated on the third Thursday of each month.

From the related seasonal outlook website, the next set of forecast maps shows the national CPC forecasts for October through December 2008. Given the expectation of continued near-neutral ENSO conditions, long-term trends dominate the maps for both temperature and precipitation. In the case of temperatures, this means a warm forecast for most of the southwestern U.S., except for most of Utah (see first map below). In the case of precipitation, this translates into "EC" (no tilt of the odds) over most of the western U.S., except "below-normal" over southern Arizona and southwestern New Mexico (second map below).

Other links one might want to browse are the IRI forecasts, as well as our in-house model-derived CDC forecasts that are based on statistical modeling of impacts of global SST on atmospheric circulation models. While the IRI forecasts normally mimic the CPC forecasts shown above, the CDC model-derived forecasts for the next three months often do not. Not unexpectedly, they show a warm forecast in September-November for the interior Southwest, but no tilt towards wet or dry for much of our region of interest.

4. Most recent experimental forecast guidance - for October-December 2008 (last complete month of observed data used: July 2008)

The forecast guidance presented here is based on a screening stepwise multiple regression procedure that requires that each new predictor explains at least an additional 10% of the predictand's variance. These calculations were performed for the full record (1951/2-98/9), as well as for five subsets with a decade of data being withheld. The predictors I use include a variety of ENSO indices (including spatial differences and recent tendencies in time), regional SST averages in the Indian Ocean, Eastern Pacific and Caribbean, regional precipitation anomalies within the southwestern U.S., and a few general sea level pressure indices such as the North Atlantic Oscillation and centers of action both east and west of Australia.

For this go-around, I include calculations for the following targets (or predictands):

October-December 2008 precipitation for seasonal core regions 1 - 6.

FORECAST TABLE:

First column is predictand (name of regional rainfall anomaly index);

2nd column is predicted standardized anomaly for the named region based on full training period (Water Years 1951-99);

3rd column is predicted standardized anomaly based on the median of the cross-validated forecasts (i.e., the middle of five predicted values;

4th column is the range of predicted values from all six forecasts;

5th column is Heidke Skill Score (HSS) for full training period (based on 3x3 contingency table using terciles - a score of 0 means that the forecasts are no better than what would be expected by change (one hit (correctly predicted tercile) out of three tries), a score of +100 would mean "perfect" forecasts (three hits out of three forecasts), and -50 would be the worst possible forecast skill (no hits at all); for a comprehensive discussion of forecast verification issues, I recommend this link , while a "light-weight" definition of this score can be found here.

6th column is HSS for all five cross-validated decades combined (independent verification);

7th column is the calibrated shift in the probability distribution towards positive or negative anomalies (upper or lower tercile);

8th column shows the change in the 7th column from last month, if applicable;

9th and last column shows a preliminary HSS for the last eight years (1999/2000-2006/2007); the number of forecasts with a tilt of at least 3% is indicated in brackets.

Note that the 7th column mimics the CPC forecast scheme in which the middle tercile category is left at 33.3% (unless otherwise indicated), and the upper and lower terciles add up to 66.7%, with the gain of the upper tercile meaning the loss of the lower tercile, and vice versa. If the data were normally distributed, and the skill in forecasting completely unbiased, a median predicted anomaly of +0.26 would translate into a shift in the terciles of +10%, with the lowest tercile being less likely than "normal" at 23.3%, while the highest tercile would be more likely at 43.3%.

See also the discussion section following this table. 

PRECIPITATION FORECAST GUIDANCE


TIME-REGION             PRED. ANOMALY   PRED.   HSS     HSS     CROSS-VALIDATED   CHANGE FROM             HSS
PREDICTAND              FULL   MEDIAN   RANGE   FULL    IND.     TERCILE SHIFT  PREVIOUS FORECAST      1999-2007


1. OND-1=NC UT/CO        -.7    -1.0     0.6    +16       0#       +/-0%                              undefined (1)


2. OND-2=S UT/W CO       -.9     -.6     1.6    +28     +16         -9%                                  -13#(8)


3. OND-3=most of AZ      -.6     -.8     1.5    +50*    +25        -14%*                                 +17 (9)


4. OND-4=NM/SC CO        0.7     -.1     2.4#   +22     +28         -4%                                  -25#(6)


5. OND-5=E CO 37-40N     0.1     0.2     1.6    +34*    +13         -3%                                  +14 (7)


6. OND-6=NE CO/SE WY+    0.0     -.2     1.7    +19     +16         -9%                                  +17 (9)

I have marked with an asterisk (*) those forecast probability shifts that equal or exceed 11% (or, roughly, a doubled risk of one tercile category vs. the opposite one). The same applies to cross-validated as well as 1999-2006 verification Heidke skill scores above +30, as well as to predicted ranges of 0.5 or less. For OND'08, one out of six predictions qualify for an asterisk, due to a significant forecast tilt (region 3).

In contrast, a pound (#) symbol indicates a very large spread (at least 2 standard deviations), weak cross-validated or 1999-06 HSS's (0 or less), or a large change from last month's forecast. Such forecasts should be taken with a 'grain of salt'. For OND'08, three out of six regions share this stigma, one due to poor cross-validated skill (region 1), one due to the wide range in ensemble members (region 4), and three due to poor or undefined verification skill (regions 1, 2, and 4). Sometimes, regional forecasts feature both symbols (* and #), which reduces the confidence one should place in those forecasts. In this forecast round, none of them feature such contradictions - a rare situation!

The map below shows the calibrated shifts in the tercile probabilities for OND 2008:

Forecasted shifts in tercile probabilities for October-December 2008. In order to be shown on this map, a forecast tilt in the odds has to reach at least 3% either towards wet, dry, or near-normal. Shifts towards the wettest (driest) tercile are indicated in green (red), with a green plus sign for shifts between +3% and +5% (none), and a red minus sign for equivalent shifts towards the negative (one over southeast Colorado and one over New Mexico). Tilts towards near-normal are indicated by the letter "N", if at least by 3% (none). Question marks denote a forecast with a greater likelihood of being either wetter OR drier than near-normal at the expense of near-normal odds (northern Utah and northwest Colorado). Positive or negative shifts of over 5% are contoured in 5% increments. If any shift reaches over 10%, it is considered significant, even if recent verification skill has been wanting. There is only one region that reaches that threshold: most of Arizona (along with positive, if not significant verification skill).

NEW: To access an archive of earlier public experimental forecasts and verifications, click on the season and lead-time of interest, once you get to the respective directory. Average Heidke Skill scores for each forecast season are listed here. I will update this archive later this summer.

5. Discussion of forecast guidance

To repeat from the beginning of this webpage: This forecast guidance is experimental, and should be used with caution. The Climate Diagnostics Center cannot assume any responsibility for losses incurred related to them.

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Discussion of first fall (October-December 2008) forecast guidance

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OND-1=NC UT/CO shows enhanced odds of being either in the driest or wettest tercile, with no clear tilt towards one sign or the other (+/-0%). Since almost all forecasts of the last nine years showed this type of forecast, and since the cross-validated Heidke Skill Score is zero as well, this is truly a region for which a climatological forecast (i.e., equal chances of below-/near-normal/or above-normal precipitation) is all I can offer.

OND-2=S UT/W CO features the first of five negative tilts (-9%), along with moderate cross-validated skill (+16), even though the last nine years have shown less success than flipping a coin (HSS'99-07=-13). In other words, climatological odds apply here as well, despite the fact that this dry forecast matches the following four regional forecasts.

OND-3=most of AZ shows the only significant shift in the odds towards a dry fall season (-14%), along with a fairly high cross-validated forecast skill score (HSS=+25), and positive skill in the last nine years (+17). After what appears to be shaping up as a 'normal' summer monsoon season, this is an unfortunate threat of renewed drought conditions, despite the apparent demise of La Niña.

OND-4=NM/SC CO is covered by a modest shift in the odds towards dry conditions (-4%), supported by the highest cross-validated skill value (+28) of this go-around. However, verfication skill has been exceptionally poor (-25), so that this forecast region has more frequently verified outside the predicted tercile than would have been expected by chance. If La Niña were to make a return this fall, a dry fall would indeed be more likely than a wet one.

OND-5=E CO 37-40N joins in with the weakest negative tilt of this forecast round (-3%). Modest historical (+13) as well as verification skill (+14) caution against too much pessimism. Incidentally, La Niña does not tilt the odds as much towards dry conditions as further south.

OND-6=NE CO/SE WY+ features a moderate shift in the odds towards a dry fall season (-9%). Cross-validated skill is moderate (+16), similar to verification skill levels (+17), so that this dry forecast needs to be taken seriously, similar to Arizona.

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Links to further pertinent websites

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Given continued drought conditions in much of the western U.S., there is great concern about streamflow and reservoir levels. To help with the near-term assessment of precipitation prospects, I am enclosing a few links of interest:

1. Outgoing Longwave Radiation ( (OLR) anomalies for the last week) show up as blue for deep convection (over the tropics). Recent thunderstorm activity over India and parts of Indonesia is indeed flagged by this color, while suppressed cloudiness near the dateline shows up as yellowish-orange colors - all of these are still consistent with La Niña.

2. Under the following link, click on "latest forecast", then on "precip" at the top (This link is based on work originated by Jeff Whitaker) and forecasts generated here at CDC, with a recalibrated version of the Medium Range Forecast (MRF) model that was originally developed at NCEP. As of yesterday, this tool was indicating near-normal precipitation in much of the southwestern U.S. for "Week 2" (straddling Labor Day). During the next week, we may see a major storm digging into Colorado around Wednesday, but it is too early to know for sure. A related user-friendly link to "zoomed-in" forecasts is maintained by ( Gary Bates) who takes the recalibrated MRF forecasts and puts them into an interface where the user can pick and choose the grid box for which a daily temperature or precipitation forecast out to fifteen days is desired.

6. Executive Summary (updated on August 22nd, 2008; next update by September 20th, 2008)

1. The 2007-08 La Niña event appears to have faded, at least for now. Most forecasts show continued near-neutral conditions into the upcoming winter, but a return of La Niña is still possible, especially since moderate-to-strong La Niña events like the last one often return for an encore performance.

2. July was dry and hot in Utah and Colorado, while Arizona and New Mexico enjoyed an active monsoon period. After a hot start to the month, August has brought much cooler and wetter weather to eastern Colorado in particular. This has ameliorated drought conditions significantly over much of that region. The next two weeks appear on track for more unsettled weather, especially during the last week of August.

3. My first experimental forecast guidance for the late fall season (October-December 2008) is somewhat pessimistic (dry) from Arizona into northeastern Colorado, hinting at the return of atmospheric circulation anomalies with a La Niña footprint. While August has bucked this dry trend, a rapid transition into El Niño and its associated fall wetness in much of the southwestern U.S. is indeed not likely.

4. Bottomline: After the expected last 'hurrah' of summer-time heat around the beginning of August, the last three weeks have finally witnessed the long-predicted surplus of late summer moisture in eastern Colorado. During this period, the summer monsoon has remained more or less 'norma' further west and south. This has led to an overall improvement of drought conditions in the southwestern U.S., especially in eastern Colorado. The next month (September) could bring more of the same in Colorado, while eastern New Mexico might finally dry out, if my earlier forecasts verify. The overall ENSO situation remains somewhat unsettled, leaving the door open for a return to La Niña this fall, or a gradual transition into El Niño. Unfortunately, my first outlook into October-December is more consistent with the former scenario than the latter.

Questions about this webpage should be addressed to:
(Klaus.Wolter@noaa.gov), (303) 497-6340.