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 December 2009 through March 2010
- 4. Most recent experimental forecast guidance for January through March 2010
- 5. Discussion of forecast guidance
- 6. Executive Summary
Outline for latest forecast webpage (updated on November 23rd, 2009; next update by December 23rd, 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 December 2009 through March 2010;
4. Experimental forecast guidance for January 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 October 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 positive SST anomalies across the basin, mostly above +0.5C, with a stable patch of +1.5C to +2C from about 170E to 130W, indicating moderate to borderline strong El Niño conditions. Westerly wind anomalies have almost disappeared from this map during the last few weeks, inhibiting further growth of SST anomalies for now. 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) rebounded to +1.0 standard deviations last month, returning to moderate strength in the El Niño sense for the period September-October 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, Niño 3.4 SST anomalies began to increase again in October after a long period just below +1C, a behavior not anticipated by most models, including this one. Recent weekly anomalies have actually averaged above +1.5C for this region. The most recent forecast persists the recent rise of Niño 3.4 SST anomalies through the early winter months, peaking around +1.8C in December, higher than in earlier forecast rounds. Given the more pronounced (early) peak, this model shows a more dramatic drop-off to weak El Niño conditions after March 2010. This figure is taken from the ( ECMWF seasonal forecast website) that is updated monthly around the 21st.
Latest comparison plot of 15 dynamical and 7 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 OCTOBER 2009), this graph shows the average outcome. All but four dynamical forecast models produce an SST anomaly of +1C or higher in the next few months, while six out of seven statistical models reach that threshold. Only one statistical model reaches a peak of +1.5C or higher for a three-month season, while six such forecasts reside with dynamical models, continuing a theme of 'more bullish' forecasts from such models, even though the average difference between statistical and dynamical forecasts remains relatively small (+0.3C at most) compared to earlier this year. The consensus is for a peak to occur in early winter (around December/January), with seasonal values not very far from the latest weekly anomalies (say, around +1.5C). La Niña remains out of the picture through at least 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 November 2009 CPC ENSO discussion states that "El Niño is expected to strengthen and last through at least 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 January through March, a season that requires nine 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 December 2009 through March 2010
The most recent U.S. government (CPC) temperature forecast for December 2009 shows a tilt of the odds towards warmer temperatures that covers the northwestern two thirds of the U.S., including most of Utah and all of Colorado, as well as northernmost New Mexico and northeasternmost Arizona. The rest of our region of interest (most of Arizona and New Mexico) is left as "EC" (equal chances of below-normal, near-normal, and above-normal temperatures). This reflects the expected extent of El Niño impacts, enhanced by long-term warming trends. The precipitation forecast for December 2009 (below) is left as "EC" for much of the Four Corners region, except for a slight increase in the odds of above-normal precipitation in southwestern Arizona. This forecast is influenced by the expected impact of an intraseasonal ('Madden-Julian') oscillation in Californa which may spill over into adjacent Arizona, while the rest of our forecast region is not expected to benefit as much from the arrival of subtropical moisture as further west. 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 related seasonal outlook 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 our region of interest to southern Arizona and southeastern New Mexico, while leaving Utah and Colorado mostly undecided ("EC").
4. Most recent experimental forecast guidance: January-March 20109 (last complete month of observed data used: October 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):
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 2000-2009
1. JFM-1=NW Utah -.1 0.1 2.3# +31 +13 +/-1% none +33* (9)
2. JFM-2=AZ + -.7 -.2 1.7 +41 +38* -7% +4% -20#(10)
3. JFM-3=SE NM -.0 0.7 1.6 +16 +19 +4% +4% +40* (5)
4. JFM-4=NE NM 0.7 0.8 1.9 +59 +38* +33%* +/-0% +33* (9)
5. JFM-5=NW NM+SC CO 0.1 -.3 1.0 +38 +31* -17%* -19%# -25# (6)
6. JFM-6=SE CO 1.2 -.2 2.9# +34 +09 -2% +/-0% +36* (7)
7. JFM-7=NE CO -.6 -.6 0.8 +31 +13 -15%* none -20#(10)
8. JFM-8=NC CO -1.5 -1.5 0.5* +56 +53* -33%* +/-0% -5#(10)
9. JFM-9=SE UT+W CO -.7 -.6 0.7 +31 +16 -7% +2% 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 2000-2009 verification Heidke skill scores at or above +30, as well as to predicted ranges of 0.5 or less. For JFM'10, eight predictions feature this attribute, four due to a high verification skill score (1,3,4, and 6), four due to high probability shifts (4,5,7, and 8), four due to high cross-validated skill (2,4,5, and 8), and one due to a low predicted range (8).
In contrast, a pound (#) symbol indicates a very large spread (more than two standard deviations), weak cross-validated or 2000-09 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 JFM'10, seven predictands end up with this stigma, five times due to poor verification scores (2,5,7,8 and 9), once due to big shift from the previous forecast (5), 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, JFM'10 is handicapped by an unusually high number of such forecasts (1,2,5,6,7, and 8).
The forecast map below shows the calibrated shifts in the tercile probabilities for JFM 2010:
Forecasted shifts in tercile probabilities for January-March 2010. 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 (none). 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 Colorado is covered by significantly dry odds this season, 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 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 moderate shift of the odds towards dry conditions (-7%), along with high cross-validated forecast skill (HSS-IND=+38), but only poor verification skill (HSS'00-09=-20). This forecast tilt continues a surprisingly dry forecast trend established last month. 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, or whether it will continue dry to buck a well established El Niño signal.
JFM-3 (SE NM) has come around to show modest odds towards wet conditions (+4%), consistent with the expected El Niño signature in this region. This is supported by decent historic (+19) and high verification (+40) skill levels.
JFM-4 (NE NM) features the most optimistic forecast for this season (+33%), along with high cross- validated and verification skill (+38 and +33, respectively). The new forecast round has reaffirmed this optimistic forecast which is also consistent with expected El Niño impacts.
JFM-5 (NW NM & SC CO) shows a strong tilt towards a dry winter (-17%), in the biggest reversal (change) from last month's forecast. However, cross-validated skill is much better (+31) than verified skill (-25). Since El Niño winters have shown a tendency to be wet in this region, this pessimistic outlook has a pretty good chance of not verifying, even though it is consistent with the forecast for neighboring Arizona.
JFM-6 (SE CO) 'wastes' 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. In this region, Marches tend to be wet with El Niño, while January and February often end up on the dry side. If El Niño sticks around into the spring season, a wet March can easily make up for a dry winter, but we are stuck with a dry forecast for now, unchanged from last month.
JFM-8 (NC CO) has shown the biggest discrepancy between cross-validated forecast skill (+53) and verified forecast skill since 2000 (-5). The repeated forecast tilt of -33% from last month should thus be taken with a very large grain of salt, even though El Niño winters 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 (-7%), along with modest cross-validated forecast skill (HSS-IND=+16), and poor observed skill since 2000: 0, akin to flipping a coin. 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 around the Maritime Continent is flagged by this color, while suppressed cloudiness (yellow-orange) flags drought conditions over much of North and Central America, much of this not being 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 cool pattern is shaping up for early December, but moisture appears to stay mostly to the north and east for now.
6. Executive Summary (updated on November 23rd, 2009; next update by December 23rd, 2009)
1. The La Niña event of 2007-09 was replaced by weak-to-moderate El Niño conditions by last June, only to stagnate at this level through September. A secondary growth spurt in October elevated this event to borderline strong conditions. Given its present size, El Niño conditions will continue right through the boreal winter season, and potentially into next spring.
2. The last month has brought above-average moisture to the eastern plains of Colorado, including the anticipated significant snow storm to the northern Front Range in late October. The rest of the interior southwestern U.S. has remained mostly dry this fall season, in particular over Arizona. After a mild and dry Thanksgiving, this weekend should see a return of cold and somewhat unsettled weather for much of the interior southwestern U.S., especially over Colorado and New Mexico.
3. My experimental forecast guidance for the late winter season (January-March) continues dry for Arizona, in stark contrast to typical El Niño outcomes. Colorado and Utah are also covered by a generally dry forecast well beyond the El Niño- typical winter dryness for higher elevations of northern and central Colorado. On the other hand, the forecast for eastern New Mexico remains wet, consistent with El Niño expectations.
4. Bottomline: The fledgling El Niño of 2009 has gained noticable strength since early October, entering the upcoming winter season with at least moderate strength. My outlook for the winter season has remained surprisingly pessimistic for Arizona and southern Colorado in particular, where many previous El Niño seasons have brought wetter-than-average winters. Since forecasts for both states have shown marginal skill in the last decade, the more typical El Niño-related wetness is still in the cards.
Questions about this webpage should be addressed to:
(Klaus.Wolter@noaa.gov),
(303) 497-6340.