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 November 2009 through March 2010
- 4. Most recent experimental forecast guidance for October 2009 through March 2010
- 5. Discussion of forecast guidance
- 6. Executive Summary
Outline for latest forecast webpage (updated on October 23rd, 2009; next update by November 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 November 2009 through March 2010;
4. Experimental forecast guidance for October 2009 through March 2010 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 September 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.esrl.noaa.gov/psd/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 positiveSST anomalies across the basin, mostly above +0.5C, with a growing patch of +1C to +2C mostly east of the dateline, indicating moderate El Niño conditions. Westerly wind anomalies have been centered near the equatorial dateline over the last month, weakening the trade winds in this area, and 'nursing' the burgeoning warmth underneath. The El Niño event of 2009-10 has thus increased its strength significantly since the last update of this webpage. 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) dropped slightly last month, remaining at weak-to-moderate strength in the El Niño sense for the period August-September 2009.
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 stalled at just below +1C from July through September, a feature not anticipated by most models, including this one. The most recent forecast anticipates a modest rise of Nino3.4 SST anomalies to just over +1C before the end of the calendar year, followed by a slow decline to weak El Niño conditions by April 2010. This prediction is less optimistic than last month's, probably in response to the stalled warming of the last three months. This figure is taken from the ( ECMWF seasonal forecast website) that is updated monthly around the 21st.
Latest comparison plot of 13 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 SEPTEMBER 2009), this graph shows the average outcome. All but four dynamical forecast models produce an SST anomaly of +1C or higher later this year, while six out of eight statistical models reach that threshold. Compared to forecasts earlier this year, the difference bewteen statistical and dynamical model predictions has shrunk quite a bit (mostly around 0.2C higher for the dynamical models), showing continued convergence in outcomes. The consensus is for a peak to occur in early winter (around December/January), most likely at +1 to +1.5C (peak value for six dynamical and six statistical forecasts). La Niña remains out of the picture through early 2010 (only one model drops slightly below 0C by mid-2010). 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 October 2009 CPC ENSO discussion states that "El Niño is expected to strengthen and last through the Northern Hemisphere Winter 2009-2010."
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 October through December, a season that requires only six 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.
ditto for January through March, a season that requires nine core regions to describe the bulk of co-variability across the domain.
3. Most recent Climate Prediction Center forecasts for November 2009 through March 2010
The most recent U.S. government (CPC) temperature forecast for November 2009 shows a tilt of the odds towards warmer temperatures that covers all of the western U.S., albeit mostly below 10% (a 40% contour refers to tilts of 7%, 50% equals 17%, etc.). This reflects long-term warming trends, with additional warming in the high plains due to expected El Niño impacts. The precipitation forecast for November 2009 (below) is left as "EC" (as in equal chances of below-normal, near-normal, and above-normal precipitation) for our region of interest, since this time of the year does not have a strong El Niño-precipitation signal. Both forecasts are taken from the monthly outlook website that is updated on the third Thursday of each month (as well as on the last day of that month.
From the same website, the next set of forecast maps shows the national CPC forecasts for January through March 2010. Given the expectation of strengthening El Niño conditions, long-term warming trends are expected to be enhanced from Colorado northward, while superimposed El Niño-related cooling reduces the tilt in the odds to "EC" over southernmost Arizona and New Mexico (first map below). The precipitation forecast (second map below) reflects mostly El Niño composites for this season, giving the best chances for a wet late winter in southwestern Arizona and southeastern New Mexico, while leaving Utah and Colorado mostly undecided ("EC").
4. Most recent experimental forecast guidance: October-December 2009 and January-March 20109 (last complete month of observed data used: September 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):
A. October-December 2009 precipitation for seasonal core regions 1 - 6 (this is the same set of forecasts as last month).
B. January-March 2010 precipitation for seasonal core regions 1 - 9.
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 TEN years (1999/2000-2008/2009); 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 1999-2008
A1. OND-1=NC UT/CO -.5 -.4 0.6 +16 0# +/-0% none undefined (#)
A2. OND-2=S UT/W CO 0.8 -.0 1.8 +28 +19 +3% none 0# (9)
A3. OND-3=most of AZ -.1 -1.0 1.7 +50 +22 -17%* none +17 (9)
A4. OND-4=NM/SC CO 0.1 -.4 1.4 +22 +25 -11%* none -33# (9)
A5. OND-5=E CO 37-40N -.1 -.1 0.9 +34 +22 +1% none +25 (8)
A6. OND-6=NE CO/SE WY+ 0.3 -.1 1.5 +19 +13 -1% none +36* (7)
TIME-REGION PRED. ANOMALY PRED. HSS HSS CROSS-VALIDATED CHANGE FROM HSS PREDICTAND FULL MEDIAN RANGE FULL IND. TERCILE SHIFT PREVIOUS FORECAST 2000-2009
B1. JFM-1=NW Utah -.1 0.1 2.3# +31 +13 +/-1% none +33* (9)
B2. JFM-2=AZ + 0.3 -.2 2.0 +41 +44* -11%* -22%# +10 (10)
B3. JFM-3=SE NM -.0 0.7 1.6 +16 +19 +/-1% +/-0% +40* (5)
B4. JFM-4=NE NM 0.7 0.8 1.8 +59 +38* +33%* +/-0% +40*(10)
B5. JFM-5=NW NM+SC CO 0.1 -.2 1.6 +38 +25 +2% -1% -33# (9)
B6. JFM-6=SE CO 1.2 -.1 2.9# +34 +09 -2% -3% +36* (7)
B7. JFM-7=NE CO -.6 -.6 0.8 +31 +13 -15%* -5% -20#(10)
B8. JFM-8=NC CO -1.5 -1.5 0.5* +63 +53* -33%* -32%# -5#(10)
B9. JFM-9=SE UT+W CO -.7 -.2 1.0 +31 +13 -9% -6% +25 (8)
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 (2000-2009) verification Heidke skill scores at or above +30, as well as to predicted ranges of 0.5 or less. For OND'09, three out of six predictions achieve this distinction, due to large probability shifts (regions 3,4), or a high verification score (6). For JFM'10, seven predictions feature this attribute, four due to a high verification skill score (1,3,4, and 6), four due to high probability shifts (2,4,7, and 8), three due to high cross-validated skill (2,4, and 8), and one due to a low predicted range (8). In contrast to September, one one of the high cross-validate skill scores matches a high verification skill score (region 4).
In contrast, a pound (#) symbol indicates a very large spread (more than two 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 OND'09, three predictands are flagged in this fashion, all associated with poor verification skill scores (1,2,4), as well as due to a low cross-validated skill score (HSS IND=0). For JFM'10, six predictands end up with this stigma, three times due to poor verification scores (5,7, and 8), twice due to big shift from the previous forecast (2,8), and twice due to a range beyond two standard deviations (1,6). Sometimes, regional forecasts feature both symbols (* and #), which also reduces the confidence one should place in those forecasts. In this forecast round, OND'09 features no such 'mixed-message' forecasts, while JFM'10 is handicapped by an unusually high number of such forecasts (1,2,6,7, and 8)).
The first forecast map below shows the calibrated shifts in the tercile probabilities for OND 2009, while the second one shows the same for JFM 2009:
Forecasted shifts in tercile probabilities for October-December 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% (one), 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% (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 10%, it is considered significant, even if recent verification skill has been wanting. Most of Arizona and New Mexico are covered by significantly dry odds this season. Note that this forecast remains completely unchanged from last month.
ditto for tercile probabilities for January-March 2010. Most of Arizona and Colorado is covered by a significant tilt in the odds towards dryness, while eastern New Mexico shows a significant tilt towards a wet late winter.
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 Autumn (October-December 2009) forecasts - repeated from last month, no changes!
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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 ten 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, unchanged from last month.
OND-2=S UT/W CO features the first of two mild positive tilts (this one at +3%), along with moderate cross-validated skill (+19), even though the last ten years have shown just as much success as flipping a coin (HSS'99-08=0). In other words, climatological odds apply here as well.
OND-3=most of AZ shows the most significant shift in the odds towards a dry fall season (-17%), along with the second highest cross-validated forecast skill score (HSS=+22), and positive skill in the last ten years (+17). After what appears to be shaping up as a dry summer monsoon season, this is an unfortunate threat of continued drought conditions, despite the apparent return of El Niño.
OND-4=NM/SC CO is covered by another large shift in the odds towards dry conditions (-11%), supported by the highest cross-validated skill value (+25) of this go-around. However, verification skill has been very poor (-33), so that this forecast region has more frequently verified outside the predicted tercile than would have been expected by chance. Again, if El Niño were to exert its typical influence on this region, a wet fall would be more likely than not.
OND-5=E CO 37-40N joins in with the weakest positive tilt of this forecast round (+1%). Moderate historical (+22) as well as verification skill (+25) are 'wasted' on an 'EC' forecast, unfortunately. Incidentally, El Niño tilts the odds towards wet rather than dry in the fall season, especially in October.
OND-6=NE CO/SE WY+ features a small negative tilt in the odds towards a dry fall (-1%), in the biggest change from last month's forecast (it was +7%). Cross-validated skill is moderate (+19), while verification skill is at the highest level for this season (+36), slightly higher than last month's skill score (+25). As for the previous region, this part of Colorado tends to be wet with El Niño in the fall, so there is still hope despite a climatological forecast.
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Discussion of winter (January-March 2010) forecast guidance
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JFM-1 (NW UT) shows enhanced odds of being either in the driest or wettest tercile, with no clear tilt towards one sign or the other (+/-1%), but a very mild preference for either tail (just 1% each). This undecided forecast comes with modest historic forecast skill (+13), and high verification skill in the last decade (HSS'00-09=+33). This translates into a forecast of climatological odds, unchanged from last month.
JFM-2 (AZ +) shows a significant shift of the odds towards dry conditions (-11%), along with high cross-validated forecast skill (HSS-IND=+44) and modest verification skill (HSS'00-09=+10). This forecast tilt is an exact reversal from last month's wet forecast tilt of +11% which was backed up by much higher verification skill (+33). Arizona is a region with a strong linkage between El Niño and wet winters, so a wet forecast would have been consistent with anticipated El Niño conditions through the winter season, while a dry foreast is most decidedly not consistent. We will have to wait until next month to see whether this forecast guidance will reverse itself yet again, or whether it will continue dry to buck a substantial El Niño signal.
JFM-3 (SE NM) has much improved historic and verification skill (+19 for HSS IND. and +40 for HSS'00-09) over last month's negative values. Unfortunately, it shows yet another undecided (+/-1%) forecast. Climatological odds remain the safest bet for now, even though this region has shown a historic preference for wet El Niño winters.
JFM-4 (NE NM) features the most optimistic forecast for this season (+33%), along with outstanding cross- validated and verification skill (+38 and +40, respectively). This new forecast round has reaffirmed this optimistic forecast which is also consistent with expected El Niño conditions.
JFM-5 (NW NM & SC CO) shows a modest tilt towards a wet winter (+2%). However, cross-validated skill is much better (HSS-IND= +25) than verified skill (HSS'00-08= -33). Nevertheless, El Niño winters have shown a tendency to be wet in this region, so cautious optimism is still justified.
JFM-6 (SE CO) repeats the theme of 'wasting' good forecast skill (+36 verified skill in last decade) on small tilts in the forecast odds (-2%), while cross-validated forecast skill is the lowest of this forecast round (HSS-IND=+9). This is the part of Colorado that has the most consistent wet signal year-round with El Niño conditions, so more optimism than the odds indicate is justifiable.
JFM-7 (NE CO) joins in with a significantly dry forecast (-15%). This pessimistic outlook comes paired with modest cross-validated and poor verification skill (+13 and -20, respectively), and worse than September's numbers which were also in support of a dry forecast. While Marches tend to be wet with El Niño, January and February are often on the dry side. If El Niño sticks around into the spring season, a wet March often more than makes up for a dry winter, but we are stuck with a dry forecast for now.
JFM-8 (NC CO) has shown the biggest discrepancy between cross-validated forecast skill (+53!) and verified forecast skill since 2000 (-5). The new forecast tilt of -33% should thus be taken with a very large grain of salt, even though El Niño winters do tend to be dry in northern Colorado's higher elevations.
JFM-9 (SE UT & W CO) features a moderate shift in the odds towards dry conditions (-9%), along with modest cross-validated forecast skill (HSS-IND=+13), and decent observed skill since 2000: +25. A dry late winter is consistent with general dryness to the north and east during El Niño.
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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 tropical storm activity over the western and eastern Pacific is flagged by this color, while suppressed cloudiness (yellow-orange) flags drought conditions over India and Indonesia, consistent with El Niño.
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. 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. A fairly unsettled and cool pattern appears to be shaping up for next week, just in time for Halloween.
6. Executive Summary (updated on October 23rd, 2009; next update by November 25th, 2009)
1. The La Niña of 2007-09 ended about six months ago. Weak-to-moderate El Niño conditions became established in June, and have increased in strength just over the last month after a three-month interlude of stagnant conditions. This event should continue at least into the boreal winter season, with at least moderate strength expected for the next few months.
2. October brought moisture to the eastern plains of Colorado, as well as from New Mexico into south-central Colorado while Arizona and eastern Utah into western Colorado have remained dry so far. Next week should return cold and unsettled weather for much of the interior southwestern U.S., including the potential for significant snow along the northern Front Range of Colorado.
3. My experimental forecast guidance for the fall season (October-December) is pessimistic (dry) for New Mexico and Arizona, but more neutral for Colorado and Utah. El Niño fall seasons tend to be wet in these four states, so this forecast may reflect the weak nature of El Niño conditions through September. The new late winter forecast (January-March) is also surprisingly dry for Arizona in particular after a much more promising forecast last month. The outlook for New Mexico remains wet, consistent with El Niño expectations, while Colorado and Utah show a generally dry forecast beyond the typical mid-winter dryness for higher elevations of northern and central Colorado.
4. Bottomline: The fledgling El Niño of 2009 has gained noticable strength over the last few weeks, going into the winter season with at least moderate strength. Much of the southwestern U.S. has a decent chance of a wet fall due to El Niño, despite a less optimistic experimental forecast. The outlook for the winter season has changed towards drier conditions, but has no data input from the changed October conditions in the tropical Pacific, so there is hope for a possible rebound next month.
Questions about this webpage should be addressed to:
(Klaus.Wolter@noaa.gov),
(303) 497-6340.