Colorado (and Interior Southwest) forecasts

Outline for latest forecast webpage (updated on July 29th, 2008; next update by August 23rd, 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 August through December 2008;

4. Experimental forecast guidance for JUL-SEP 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 data through June 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 just between the dateline and 170W along the Equator (reduced from last month), while positive SST anomalies above +0.5C have expanded again 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 increased considerably since March, now reflecting 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 mayin February to less than -0.5C in June. While earlier forecast runs missed the mid-winter drop in SST, the recent increase was well predicted by February. Compared to last month's forecast, there is now less aggressive warming advertized right into northern winter. The majority of ensemble members rise above 0C by August, with a wide range of possible outcomes by January: from about -0.5C for the "coldest" member to about +0.9C for the "warmest" member. In fact, less than 25% of all ensemble members exceed the conventional El Niño threshold of +0.5C at that point, after a brief peak in October 2008 when about half them do indeed reach that threshold. This figure is taken from the ( ECMWF seasonal forecast website) that is updated monthly around the 21st - 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 June 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 continue to show a more positive (El Niño-like) outcome than the statistical models. In fact, six dynamical models reach or exceed the +0.5C threshold in this plot, while none of the statistical models do, while four statistical models drop at least briefly back to at least -0.5C, as compared to only one dynamical model. 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 July 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 August through December 2008

The most recent U.S. government (CPC) temperature forecast for August through October 2008 shows a tilt of the odds towards warmer temperatures covering the interior southwestern U.S., including all of Arizona and New Mexico, with most of Colorado and Utah 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 August through October 2008 (below) shows above-normal covering much of Colorado, while the rest of the interior southwestern U.S. is left "EC". This surprising forecast is based on a blend of statistical and dynamical forecast tools, with the "Coupled Forecast System" (CFS) model of CPC contributing the lion's share of the wet forecast over Colorado. Both forecasts are taken from the seasonal outlook website that is updated on the third Thursday of each month.

From the same website, the next set of forecast maps shows the national CPC forecasts for October through December 2007. Given the expectation of continued near-neutral conditions, long-term trends dominate the maps for both temperature and precipitation. In the case of temperatures, this means a warm forecast for about half of the southwestern U.S., except for most of Utah, western Colorado, and northwestern New Mexico (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 August-October for the interior Southwest, but no tilt towards wet or dry for much of our region of interest, except for southeasternmost New Mexico.

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

July-September 2008 precipitation for seasonal core regions 1 - 10.

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 (1951-98);

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 FOR JUL-SEP 2008


TIME-REGION              PRED. ANOMALY   PRED.   HSS     HSS     CROSS-VALIDATED   CHANGE FROM             HSS
PREDICTAND               FULL   MEDIAN   RANGE   FULL    IND.     TERCILE SHIFT  PREVIOUS FORECAST      2000-2007


1. JAS-1=NW UT            -.5     -.5    1.9      +38    +13         -1%       	      +2%                  0#(6)


2. JAS-2=SE UT/NW CO     -1.4     -.4    2.5#     +44    +13 	Neutral: 6%           -2%                +70*(5)


3. JAS-3=most of AZ      -1.0     0.2    1.8      +22     +3         -1%            +/-0%                -29#(7)


4. JAS-4=SW NM            1.1     1.1    2.0#     +19     +9         +8%             none                +50*(6)


5. JAS-5=NE NM/Sangres    -.6     -.3    1.8      +22    +13         -5%            +/-0%                +36*(7)


6. JAS-6=SE NM/NW TX     -1.2     -.3    1.3      +44    +16        -12%*            none                -31#(8)


7. JAS-7=NC NM/SC CO      -.0     0.0    1.6      +56    +19         +3%             none                +14 (7)


8. JAS-8=SW/C CO (Mtn)    0.5     0.3    1.5      +34     +6         +4%             none                -13#(4)


9. JAS-9=E CO (plains)    0.2     0.4    1.8      +41    +19         +8%            +/-0%                +14 (7)


10.JAS-10=N Front Range   -.5     -.3    1.1      +41    +19         -1%              +7%                  0#(6)

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 JAS'08, four out of ten predictions qualify for an asterisk, three of them for good verification skill (2, 4, and 5), and one for its significant forecast tilt (6), in a repeat from last month.

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 JAS'08, seven out of ten regions share this stigma (unfortunately, one more than last month), five of them due to poor verification skill at this lead-time (1, 3, 6, 8, and 10), and two due to the wide range (> 2 sigma) in ensemble members (2, 4). Sometimes, regional forecasts feature both symbols (* and #), which reduces the confidence one should place in those forecasts. In this forecast round, JAS'08 features three such contradictions - only the forecast for region 5 remains untainted.

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

Forecasted shifts in tercile probabilities for July-September 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% (one in southwest Colorado), and a red minus sign for equivalent shifts towards the negative (none). Tilts towards near-normal are indicated by the letter "N", if at least by 3% (one in northeast Utah). 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 (none). 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 region that reaches that threshold: southeastern New Mexico, and, yes, verification skill in that part of the domain has been poor.

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 summer (July-September 2008) forecasts:

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JAS-1=NW UT shows a negligible tilt of the odds towards the driest tercile (-1%), supported by moderate cross-validated skill (HSS-I=+13), but poor verification skill (HSS'00-07=0). Interestingly, last month's forecast featured a bigger negative tilt as well as much better verification skill, hence I would still give this region a better chance of being dry rather than wet. However, this is also the driest time of the year for this region, so that the difference between "wet" and "dry" can be quite small.

JAS-2=SE UT/NW CO features a moderate tilt of the odds towards the middle tercile (+6%), cross-validated skill is moderate (+13), while verification skill has been very high (+70). Given the biggest ensemble spread of this go-around (2.5 standard deviations), climatological odds are the safer bet, with the hope that near-normal rainfall will accumulate this summer.

JAS-3=most of AZ has a minor tilt towards a dry monsoon (-1%), similar to last month, but compared to -8% two months ago. This "EC" forecast comes with the lowest cross-validated (+3) skill of this forecast round, and poor verification forecast skill (-29). This is the only forecast region with a known, albeit weak tendency for wet monsoons during La Niña conditions, and its dry winters also tend to be followed by a wet summer. Since last winter was not dry, and La Niña has essentially collapsed, one should not expect a wet forecast. All in all, this remains a case for climatological odds.

JAS-4=SW NM features a moderate above-normal forecast tilt (+8%) for this season. Skill levels have been modest in the cross-validated sense (+9), but very good in the verification sense (+50). Unfortunately, the spread in the ensemble members is disturbingly high (2.0 sigma), and La Niña summers are not known for wetness in this region. On the other hand, this forecast confirms earlier optimistic outlooks, and La Niña has been weakening. This foreast remains unchanged from last month.

JAS-5=NE NM/Sangres shows a moderate tilt in its foreast (-5%), recomputed, but unchanged from last month's forecast tilt. The new forecast is supported by moderate cross-validated skill (+13), and high verification skill (+36). This is the single forecast region for which good verification skill is not counteracted by an extra large spread in ensemble members. A dry outcome would be consistent with lingering La Niña effects in this region.

JAS-6=SE NM/NW TX continues to feature the largest tilt in the precipitation odds for this season (-12%), unchanged from last month's forecast. This is backed up by decent cross-validated skill (+16), but contradicted by poor verification skill (-31). Cautious pessimism seems to be the safest bet, unfortunately in the wake of a dry fall and winter.

JAS-7=NC NM/SC CO shows a modest positive tilt in the forecast odds (+3%), along with moderate cross-validated skill (+19) as well as moderate skill since 2000 (+14). The preference for a wet monsoon appears to extend a slightly eastward shifted monsoon axis from southwestern New Mexico into Colorado (see also next two regions). This forecast remains unchanged from last month.

JAS-8=SW/C CO (Mtn) has a modest tilt of +4%, along with modest cross-validated skill (+6) and poor verification skill (-13). This forecast remains unchanged from earlier months, and fits in with the forecasts for regions 7 and 9.

JAS-9=E CO (plains) features a moderate positive tilt in the forecast odds (+8%), supported by decent cross-validated (+19) and verification (+14) skill levels. Since the spread of the ensemble members is slightly reduced compared to last month, this wet forecast is considered slightly more reliable than last month's outlook, even though the tilt in the odds remains the same.

JAS-10=N Front Range features the biggest change compared to last month: the new forecast tilt is -1% insted of -8%. While this is good news for this region, the verification skill for this region has been poor (0), despite good cross-validated skill (+19). On the other hand, last month's forecast was supported by decent verification skill (+14). Therefore, I still believe that a dry outlook is a better reflection of this forecast situation than neutral odds. Incidentally, La Niña would support a dry forecast, but it would have to strengthen again to be consistent with that influence. It will be interesting to see how this region will fare compared to other parts of Colorado that have better odds for a wet monsoon season.

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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 Indonesia is indeed flagged by this color, while suppressed cloudiness from the dateline eastward show 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" (early August), while the near-term looks dry. 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 July 29th, 2008; next update by August 23rd, 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's like the last one are often with us for more than one year.

2. The late spring season delivered below-normal moisture to much of the southwestern U.S., consistent with lingering La Niña impacts. At least, temperatures stayed cool or near-normal into June, allowing the abundant snowpack to linger into the summer, and to alleviate drought impacts for those that had access to the runoff. However, July has brought a return to above-normal temperatures in Utah and Colorado, along with mostly dry conditions, while Arizona and New Mexico have enjoyed an active monsoon season so far. Over the weekend, Hurricane 'Dolly' did pull in additional moisture to New Mexico in particular, but gave Colorado only spotty relief, leaving the hardest hit drought regions of southeastern Colorado mostly dry. Normally the peak of the summer monsoon season, the next two weeks do not promise more than typical moisture for Colorado in most forecast models, with extreme heat possible during the first week instead.

3. My experimental forecast guidance for the late summer season (July-September 2008) is somewhat optimistic from southwestern New Mexico into eastern Colorado, corresponding to an enhanced monsoon over that region. Given the lingering La Niña footprint in the southwestern U.S. this spring, a dry monsoon season for eastern Colorado remains a distinct risk. In this context, the dry spring of 2008 was preceded by a "wet" experimental forecast as well, serving as a cautionary warning. Interestingly, the latest CPC forecast for August-October is also "wet" over eastern Colorado, apparently due to wet early fall expectations. It is conceivable that August could remain dry, while September could turn wet to make this outcome possible, but it would be too late for most agricultural interests.

4. Bottomline: The last four weeks have seen a rapid worsening of drought conditions in eastern Colorado, while the summer monsoon has been more active further west and south in particular. During the next week, hot and dry weather is expected to cover much of the interior southwestern U.S., worsening the drought situation. The monsoon should make a return by early next week, but may not deliver more than average moisture north of New Mexico. While there is still some hope that August and September could recover some lost ground, the damage to dryland crops may already be irreversible by then. 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. For most of the southwestern U.S., the former would increase the risk of a dry fall, while the latter would decrease that risk.

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