Colorado (and Interior Southwest) forecasts
- 1. Current status of the El Niño/Southern Oscillation (ENSO) phenomenon and prospects for the next six to nine months
- 2. Regional climate background information
- 3. Most recent Climate Prediction Center forecasts for July through September 2009
- 4. Most recent experimental forecast guidance for July through September 2009
- 5. Discussion of forecast guidance
- 6. Executive Summary
Outline for latest forecast webpage (updated on June 26th, 2009; next update by July 25th, 2009)
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 July through September 2009;
4. Experimental forecast guidance for July through September 2009 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 May 2009. 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 NOAA-ESRL PSD, Boulder, Colorado, from his website at http://www.cdc.noaa.gov/people/klaus.wolter/SWcasts/". The forecasts presented in section 4 are experimental - users are cautioned that no responsibility for any losses can be assumed by NOAA-ESRL PSD. 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 that negative SST anomalies have disappeared from the map, a continued warmup compared to last month, and that positive anomalies above +0.5C are covering most of the equatorial Pacific, with positive anomalies above +1C from about 155W eastward. Wind anomalies are mixed across the basin, and have not (yet) been as persistent nor as pervasive as often found during El Niño events. 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 risen by more than one standard deviations in the last two months, leaving La Niña conditions clearly behind, but not reaching the opposite conditions just yet.
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 persisted below -0.5C from December through March, only to be replaced by positive values since May. All ensemble members of this model (50 out of 50) are now showing a continued increase into El Niño territory (between +0.5C and +2C) for the second half of 2009. This is consistent with earlier forecasts, but with much higher certainty of this overall outcome than discussed last month. This figure is taken from the ( ECMWF seasonal forecast website) that is updated monthly around the 21st.
Latest comparison plot of 14 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 May 2009), this graph shows the average outcome. ALL dynamical forecast models produce an outcome of +0.5C or higher later this year, while only half of the statistical models (4 out of 8) reach that threshold. Compared to earlier forecast rounds, the difference bewteen statistical and dynamical has actually shrunk (now up 0.7C rather than above 1C), showing some convergence in outcomes. La Niña appears to be out of the picture for the remainder of 2009 (only one forecast models even drops slightly below 0C). This figure is taken from the ( IRI ENSO model website) that is updated around 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 June 2009 CPC statement states that "conditions are favorable for a transition from ENSO-neutral to El Niño conditions during June-August 2009".
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 June through September 2009
The most recent U.S. government (CPC) temperature forecast for July through September 2009 shows a tilt of the odds towards warmer temperatures that covers the western third of the U.S., excluding eastern Colorado and the southeastern two thirds of New Mexico. With La Niña out of the picture and El Niño not established yet, the warm forecast is mostly based on recent warming trends, while the cooler forecast to the east reflects recent wetness that has left soils moister than normal, leading to reduced surface heating. The precipitation forecast for July through September 2009 (below) is wetter than average for most of the interior southwestern U.S., with the highest odds stretching from southwestern New Mexico into central Colorado, while the western thirds of Utah and Arizona are left "EC" (equal chances of above-normal/near-normal/ below-normal conditions). This forecast is the outcome of both statistical tools and the Coupled Forecast System (CFS) model being bullish for this region, apparently unrelated to the transitioning ENSO conditions.. Both forecasts are taken from the seasonal outlook website that is updated on the third Thursday of each month.
4. Most recent experimental forecast guidance - for July-September 2009 (last complete month of observed data used: May 2009)
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 precipitation 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 nine years (1999/2000-2007/2008); 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 GUIDANCETIME-REGION PRED. ANOMALY PRED. HSS HSS CROSS-VALIDATED CHANGE FROM HSS PREDICTAND FULL MEDIAN RANGE FULL IND. TERCILE SHIFT PREVIOUS FORECAST 2000-2008
1. JAS-1=NW UT -1.0 -.4 1.0 +34 +19 -7% none +40*(5)
2. JAS-2=SE UT/NW CO -1.5 -.9 2.6# +44 +13 Neutral:+14%* Neutral:-1% +50*(6)
3. JAS-3=most of AZ -1.6 -.4 2.0# +22 +3 -6% +1% 0#(6)
4. JAS-4=SW NM 0.5 0.3 0.5* +19 +9 +1% none +57*(7)
5. JAS-5=NE NM/Sangres -.3 -.5 1.4 +22 +13 -5% -1% +63*(8)
6. JAS-6=SE NM/NW TX 0.8 1.5 1.9 +44 +16 +16%* +3% -31#(8)
7. JAS-7=NC NM/SC CO 0.1 0.0 1.4 +56 +22 -2% +/-0% +6 (8)
8. JAS-8=SW/C CO (Mtn) 0.3 0.0 1.5 +34 +6 -3% none -20#(5)
9. JAS-9=E CO (plains) 0.1 0.7 1.3 +41 +19 +9% +3% +25 (8)
10.JAS-10=N Front Range -1.0 -.2 1.0 +41 +28 Neutral:+11%* +4% 0#(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 1999-2008 verification Heidke skill scores above +30, as well as to predicted ranges of 0.5 or less. For JAS'09, six out of ten predictions achieve this distinction, due to good verification skill (1,2,4,5), large probability shifts (2,6,10), and one predicted range of 0.5 (4).
In contrast, a pound (#) symbol indicates a very large spread (at least 2 standard deviations), weak cross-validated or 2000-08 HSS's (0 or less), or a large change from last month's forecast (more than 10%). Such forecasts should be taken with a 'grain of salt'. For JAS'09, five predictands are flagged in this fashion, mostly due to poor verification skill scores (3,6,8,10), as well as due to excessive ranges (2,3), but no large changes from last month. Sometimes, regional forecasts feature both symbols (* and #), which also reduces the confidence one should place in those forecasts. In this forecast round, JAS'09 features three such 'mixed-message' forecasts (2,6,10).
The forecast map below shows the calibrated shifts in the tercile probabilities for JAS 2009:
Forecasted shifts in tercile probabilities for July-September 2009. 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). Tilts towards near-normal are indicated by the letter "N", if at least by 3% (twice). 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 are three portions of the domain with significant tilts in the odds, one towards the wet tail of the distribution (southeast New Mexico), and two towards the middle (eastern Utah and the northern Front Range of Colorado).
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. Most of this archive is updated through the end of Water Year 2008.
5. Discussion of forecast guidance
To repeat from the beginning of this webpage: This forecast guidance is experimental, and should be used with caution. NOAA-ESRL PSD cannot assume any responsibility for losses incurred related to them.
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Discussion of Summer (July-September 2009) forecasts:
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JAS-1=NW UT shows a moderate tilt of the odds towards the dry side of the distribution (-7%), supported by decent cross- validated skill (+19), and even better verification skill in nine previous summers (+40). A dry summer appears to be more likely than not in this region. This forecast remains unchanged from last month.
JAS-2=SE UT/NW CO features a significant tilt of the odds towards the middle tercile (Neutral: +14%), supported by modest cross-validated skill (HSS-IND=+13), and high verification skill (HSS'00-08=+50). However, the spread of ensemble members remains excessive (2.6 standard deviations). While near-normal precipitation appears more likely than not, the odds for this mid-range outcome are thus not as high as the computed odds indicate (47% compared to 33% for climatological odds).
JAS-3=most of AZ has a moderate tilt of the odds towards a dry monsoon season (-6%), along with poor cross-validated (+3) and verification (0) forecast skill levels. This is the only forecast region with a known tendency for wet monsoons during La Niña conditions (which are now officially over), buti its dry winters tend to be followed by a wet summer. Since this winter was indeed mostly dry, the latter tendency comes into play in contrast with last year. On the other hand, El Niño conditions may be forming fast enough to tilt the odds towards a dry summer. All in all, climatological odds are still the safest bet for this region.
JAS-4=SW NM features only a minor tilt in its forecast (+1%) for this season. This is unfortunate, since both verified forecast skill (+57) and predicted range of ensemble members (0.5 standard deviations) would support a more daring forecast. On the other hand, cross-validated skill levels are modest at best (HSS-IND=+9). For now, I recommed sticking to climatological odds for this region. This forecast remains unchanged from last month.
JAS-5=NE NM/Sangres shows a negative tilt for this foreast region (-5%), supported by moderate cross-validated (+13) and the highest verification skill of this forecast round (HSS'00-'08=+63). This is not encouraging - I would prepare for a drier-than-average monsoon season in this region, even though the odds are not overwhelming.
JAS-6=SE NM/NW TX features the largest positive tilt in the precipitation odds for this season (+16%). This is backed up by moderate cross-validated skill (+16), but contradicted by poor verification skill (-31). Climatological odds remain the best bet despite the large forecast tilt.
JAS-7=NC NM/SC CO features a weak tilt of the odds towards the driest tercile (-2%), supported by decent cross-validated skill (+22), and weak verification skill (HSS'00=08=+6). Bottomline: tilt is too small to conclude anything but climatological odds.
JAS-8=SW/C CO (Mtn) has a modest tilt of -3%, along with weak cross-validated skill (+6) and poor verification skill (-20). Yet another case where climatological odds remain most appropriate for now (this is the final unchanged forecast from last month).
JAS-9=E CO (plains) features a moderately strong positive tilt in the forecast odds (+9%), supported by decent cross-validated skill (+19), and even higher verification skill levels (+25). After a wet spring, a wetter-than-average summer appears more likely than not for much of eastern Colorado.
JAS-10=N Front Range shows a rebound in its forecast tilt, from -4% last month to a near-neutral forecast in this round. At the same time, cross-validated skill levels are the highest of this forecast round (+28), while verification skill has been equivalent to flipping a coin ("0"). All in all, climatological odds remain the best bet.
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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 the western tropical Pacific is flagged by this color, while suppressed cloudiness (yellow-orange) lingers over the central equatorial Pacific, as well as over the southeastern U.S.
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 NOAA-ESRL PSD, with a recalibrated version of the Medium Range Forecast (MRF) model that was originally developed at NCEP. This forecast page has been irregular in its updates due to computing changes at NCEP where it is now being run. Today's update, however, shows above-average probabilities for rainfal in much of the southwestern U.S. for the next 4-14 days. 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. This webpage has had similar issues with irregular recent updates. This mirrors last month's 'optimism' which ended up being justified.
6. Executive Summary (updated on June 26th, 2009; next update by July 25th, 2009)
1. During the fall of 2008, weak-to-moderate La Niña conditions returned and influenced our weather right through last winter. As of June 2009, this second season of La Niña is clearly over. In fact, at least weak El Niño conditions can be diagnosed for the last couple of weeks, but it is not clear how strong they will become later this summer, nor whether they will last into next winter.
2. The last four weeks have brought a return of cool and wet weather for much of the southwestern U.S. after a dry and warm spell that led to rapid snowmelt in the higher elevations of Colorado in particular, possibly accelerated by a heavy coating of dust in at least the San Juans and Elk Range of our state. Our recent weather has exhibited most of the features of an early monsoon pattern, foreshadowed by a really early monsoon-like moisture surge in late May. It appears that this will continue into early July.
3. My experimental forecast guidance for the monsoon season (July-September 2009) is supportive of slightly enhanced precipitation chances from eastern New Mexico to eastern Colorado, while Arizona and northwestern Utah face similar but opposite precipitation odds. Compared to last month's forecast, precipitation odds are about the same or slightly more favorable (eastern Colorado!). The overall pattern is somewhat reminiscent of last year's summer monsoon outcome, during a brief ENSO-neutral stage in the tropical Pacific.
4. Bottomline: La Niña has gone on its second summer vacation this year, leaving the door more open for a transition to El Niño than last year. In fact, this switch in ENSO conditions is apparently underway, at a more rapid pace than anticipated last month. While current indications are that the eastern plains of Colorado will (continue to) enjoy a moist summer, the odds for a wet summer would increase over much of the southwestern U.S. (except for Arizona), if El Niño were to become better established. Otherwise, the precipitation odds for July through September are presently not far off climatological odds for much of the Interior Southwest.
Questions about this webpage should be addressed to:
(Klaus.Wolter@noaa.gov),
(303) 497-6340.