Written by Sig Silber
Here is the July 18, 2019 NOAA 15-month forecast for CONUS and Alaska. The assumptions on the ENSO Phase have changed so the forecast has also changed. We provide the graphics to see the changes, our description of the changes, and the NOAA discussion which is very complete. The forecast has changed substantially since last month and the Early Outlook for August is likely to change again when it is updated at the end of July.
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A. Focus on the NOAA Update
A note about terminology; the deviations from climatology/normal are color coded but also labeled “A” for more than (above) normal and “B” for less than (below) normal. The area designated EC means Equal Chances of being more or less than normal. In my comments For the shorter-term maps, NOAA uses “N” instead of “EC” as they believe they can be more definitive with respect to shorter timeframes. So the words “warm”, “cool”, “wet”, “dry” should be generally interpreted as being relative to climatology/normal for that location and time of year.
First, we will take a look at the NOAA Early Outlook for August, 2019. It is called the Early Outlook because it will be updated at the end of July. Only the August Outlook will be updated at that time.
Temperature
Precipitation
We have nothing to compare these maps with as NOAA does not in their Update provide a forecast for each of the subsequent two months separate from their three-month forecast. Thus I do not have forecast maps for August from the previous NOAA Report to compare against. And the current month is not over so we can not really compare the forecast for next month against the actual for this month. It is probably best to just try to understand what NOAA is trying to convey about August*. For Temperature it is a warm Alaska and both Coasts of CONUS and the Southern Tier but with a cool North Central. For Precipitation, it is a wet Western Alaska and a mostly EC CONUS with a wet anomaly where the cool anomaly is forecast. But the wet anomaly is larger and extends quite a bit further east.
* compared to normal/climatology for the indicated time of the year.
Visual Consistency Testing.
It is useful to see how the month forecast fits with the set of shorter forecasts that we have. The first two maps cover July not August and the third map only covers the first sixteen days of August. The Seasonal Outlook was issued fairly early this month since the third Thursday fell on July 18, 2019.
First Temperature
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| The new monthly forecast is to the right. Does it appear to be reflective of the three parts of the month for which we have forecasts? → |
| ↑ ← The full month forecast seems to fit fairly well with the Week 3 – 4 forecast. We do not have the remainder of the month in the sequence of maps above. We will revisit this question at the end of July when the August forecast is updated. |
And then Precipitation
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| The new monthly forecast is to the right. Does it appear to be reflective of the three parts of the month for which we have forecasts? → |
| ↑ ← The full month forecast does not seem to fit well with the Week 3-4 Map. We do not have the remainder of the month in the sequence of maps above. We will revisit this question at the end of July when the August forecast is updated. |
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g. July/August/September is shown as JAS. You will see such abbreviations often in this report.
Prior Temperature Outlook for ASO 2019
New Temperature Outlook for ASO 2019
Prior Precipitation Outlook for ASO 2019
New Precipitation Outlook for ASO 2019
It is a delayed onset Monsoon. From Thursday’s Drought Monitor Report
the official monsoon start date in Tucson, Arizona, based on average dewpoint temperature, was July 13, the latest onset in that location since 2005.
Now let us focus on the long-term situation.
Prior 14 Month Temperature Outlook: ASO 2019 – JAS 2020
New 14 Month Temperature Outlook: SON 2019 – ASO2020
To compare maps from one release to another, one needs to remember that the new release drops one three-month period and adds a later one. So to make the comparisons one has to shift the new maps to the right one position and that makes the map on the right drop down to become the left-most map in the next level. I do not have a computer software tool for doing that for you so you have to do it mentally. When I do the comparison, I print the two sets of maps and put them side by side and number the same three-month maps 1, 2, 3,…..,11 in both sets of maps to make it easier for me to easily compare the same three-month period in the new with the previous forecast. One uses the same procedure to compare the precipitation maps. Based on this procedure, I conclude that:
Now Precipitation
Prior 14 Month Precipitation Outlook: ASO 2019 – JAS 2020
New 14 Month Precipitation Outlook: SON 2019 – ASO 2020
If you want larger versions of each map (temperature and precipitation) you can find them here. And then each of those maps can be clicked on to further enlarge them.
Sometimes it is useful to compare the three-month outlook to the forecast for the first of the three months. It shows how much the pattern changes over the three-month period.
One can mentally subtract the First-Month Outlook from the Three-month Outlook and create the Outlook for the last two months in the three-month period.
*The concept is that the probabilities of a deviation from climatology in the First Month and the combined Month Two and Three forecast that one derives must average out to the probabilities shown in the three-month maps.
Below is the NOAA Discussion (slightly reorganized and some discussion of methodology removed) released by NOAA on July 18, 2019. Headings that are “Initial Cap” only rather than all caps were added by the Author of this Update Report for clarity. Also, we have organized the sequence of the sections of NOAA Discussion to first present the Atmospheric and Oceanic Conditions and then the Initial Month (August), the new NOAA Summary for ASO, and finally the remainder of the 15 Month Forecast.
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
Currently weak El Nino conditions persist but are expected to transition to ENSO neutral conditions within the next two months. Near average sea surface temperatures (SSTs) with some smaller scale anomalies are observed from 140W to the coast of South America, and large-scale warmer than average SSTs are observed in the central and west Pacific. The latest weekly Nino 3.4 SST anomaly is +0.4 degree C, while the latest Nino 4 SST anomaly is +0.9 degree C, indicating the current gradient in equatorial SSTs from east to west. A large area of cooler than average subsurface water at depths of 100-200 meters in the west Pacific and from just below the surface to 100 meters depth in the east Pacific has persisted in the last two months, while warmer than average water persists from the surface to depths of 100 meters from 160E to 130W longitude. Oceanic heat content, determined from near equatorial Pacific Ocean temperature anomalies from the surface to a depth of 300 meters from the Date Line to 100W, has decreased to near zero anomaly in July. Low-level wind anomalies were near average over the tropical Pacific Ocean, and upper-level wind anomalies were westerly over the far eastern Pacific Ocean. Collectively, these oceanic and atmospheric conditions represent a continuation of weak El Nino conditions in July.
Other boundary conditions relevant to the seasonal outlook are soil moisture anomalies and, in some areas, near-coastal SSTs. Widespread and persistent above normal precipitation for most of the CONUS during spring resulted in positive soil moisture anomalies for many areas of the central and eastern CONUS. Sea ice coverage for the Arctic is at record low amounts and SSTs are well above normal near the west coast of Alaska.
PROGNOSTIC DISCUSSION OF SST FORECASTS
There is a significant split on the ENSO outlook between dynamical and statistical models. Statistical models, including the Canonical Correlation Analysis, Constructed Analog and Markov models, persist in predicting warming central and eastern Pacific SSTs though Boreal autumn into winter and likely El Nino conditions, while a consensus of dynamical models from the North American Multi-Model Ensemble (NMME) predicts a return to near average SSTs over the central and eastern Pacific in the next couple months and on average some warming of SSTs by winter. Most of the dynamical models indicate either a weak El Nino or ENSO neutral. Both statistical and dynamical model forecasts predict a low probability that a La Nina will develop by next winter. The probability of average Nino 3.4 region SST anomalies above +0.5 Celsius and El Nino conditions increase through autumn and are elevated from climatology to near 40 percent by winter (DJF), but fall short of 50 percent in both the CPC SST Consolidation of statistical and dynamical models and the official CPC/IRI consensus ENSO outlook, indicating that neutral ENSO conditions are now more likely for autumn and winter with probabilities of near 50 percent.
30-DAY OUTLOOK DISCUSSION FOR AUGUST 2019
Temperature
The August 2019 temperature and precipitation outlooks are based on dynamical model guidance including the North American Multi-Model Ensemble (NMME), statistical tools, current soil moisture conditions, and potential influences from modes of tropical variability. The Madden-Julian Oscillation (MJO) weakened during early to mid-July due to interference with a strong equatorial Rossby wave crossing the West Pacific and a couple of Kelvin waves. Although a remnant MJO may eventually reemerge over the Indian Ocean by the beginning of August, the future evolution of the MJO is highly uncertain at this time. The MJO is unlikely to influence the longwave pattern across the mid-latitudes, but it can modulate tropical cyclone activity across the East Pacific and Atlantic basins during August.
Precipitation
A longwave pattern change is likely to occur during late July. Model solutions remain consistent with a retrogression of the pattern over North America. During the final week of July, the GFS and ECMWF ensemble means indicate an upper-level trough becoming centered over the east-central U.S. with anomalous ridging extending north from the Pacific Northwest to Alaska. Dynamical model output and long-term trends support increased chances for above normal temperatures across much of the western U.S. and Alaska. The highest probabilities for above-normal temperatures are forecast in parts of Alaska, related to the anomalous upper-level ridging leading into the beginning of August along with above-normal sea surface temperatures surrounding the state. Uncertainty in the temperature outlook is highest across the central U.S. due to weak or conflicting signals among the tools. However, below-normal temperatures are slightly favored across the northern and central Great Plains where above-normal precipitation is most likely to occur during August. Another contributing factor for below normal temperatures is the moist topsoil since the northern and central Great Plains have received 6 to 10 inches of rainfall, locally more, since mid-June. Good model agreement supports increased chances of above normal temperatures along the East Coast. Higher probabilities for above-normal temperatures (above 50 percent) forecast for New England are also related to long-term trends , while the NMME features probabilities of more than 60 percent across south Florida.
Enhanced odds for above-normal precipitation, albeit with limited probabilities, across the northern and central Great Plains, northern and central Rockies, Missouri River Valley, and much of mainland Alaska are based largely on dynamical model guidance. Although a favorable 500-hPa height pattern for enhanced monsoon rainfall is forecast for the Desert Southwest during late July, its duration into August is unclear. Therefore, equal chances (EC) for below, near, or above precipitation are forecast for this region. There is a slight tilt in the odds for above normal precipitation forecast across Colorado due to model guidance. Elsewhere, throughout the CONUS, EC is necessary for the precipitation outlook due to limited skill in a half-month lead during the summer. The precipitation outlook will be reassessed at the end of July when a larger coverage for either above or below-normal precipitation may be forecast.
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (Focusing on ASO 2019)
Temperature
The August-September-October (ASO) 2019 temperature outlook indicates that above normal seasonal mean temperatures are most likely for the western third of the CONUS, the Southern Plains and Southeast, the Northeast, and all of Alaska. The highest probabilities for above normal temperatures are over western areas of Alaska and the Pacific Northwest, as well as much of the Southwest, New England, and the Florida Peninsula.
Precipitation
The August-September-October (ASO) 2019 precipitation outlook indicates that above normal seasonal total precipitation is most likely for areas of western Alaska and the High Plains and northern Great Plains.
Equal Chances (EC; white areas) of below, near, and above normal seasonal mean temperatures or seasonal total precipitation amounts are where the likelihoods for these three categories are similar to their climatological probabilities. Review of subsequent seasonal outlooks and the scientific forecast basis for all outlooks are given below.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
The temperature and precipitation outlooks were based primarily on dynamical model guidance from the NMME and statistical model guidance which includes a statistical forecast of the linear impacts of ENSO combined with decadal climate trends based on the CPC SST Consolidation forecast for Nino 3.4. Potential impacts from El Nino conditions were considered for the seasonal outlook for the DJF 2019-2020 through FMA 2020 seasons, due to the slightly elevated probability of El Nino and low probability of La Nina. Soil moisture conditions were considered for the earliest leads as were coastal SST anomalies. Several statistical and dynamical models , as well as hybrid statistical-dynamical models, along with an objective consolidation of several forecast tools, were also examined in preparation of the seasonal outlooks. Decadal climate trends were considered for all leads but encompass a greater fraction of the forced climate signal in longer leads, as the uncertainty increases and signal decreases in predictable interannual climate variability.
PROGNOSTIC DISCUSSION OF OUTLOOKS – ASO 2019 TO ASO 2020
TEMPERATURE
The ASO 2019 through DJF 2019-2020 seasonal mean temperature outlooks utilize the predictability of several climate phenomena, including potential ENSO conditions and impacts, anomalous land surface conditions and sea surface temperatures, and decadal timescale climate variability or trends . Calibrated NMME forecasts based on probability anomaly correlation (or PAC) and a statistical-dynamical hybrid model combining calibrated NMME temperature forecasts with statistically bridged impacts from NMME Nino 3.4 forecasts (known as calibration, bridging and merging or CBaM) indicate that probabilities of below and above normal temperatures are nearly equal during ASO over much of the northern central CONUS from the Northern Plains into the upper and central Mississippi Valley. Some individual model forecasts for ASO from the NMME and monthly NMME forecasts indicate the potential for below normal temperatures for much of this region, especially in August 2019. Above average soil moisture anomalies and an above normal precipitation outlook for ASO 2019 also favor lower probabilities of above normal temperatures for much of the central CONUS. Supported by both the PAC calibrated NMME and CBAM forecasts, the outlooks for SON 2019 through DJF 2019-2020 indicate greater than climatological probabilities of above normal temperatures for all of the CONUS and Alaska. Probabilities of above normal are greater for the western CONUS and Alaska, where decadal climate trends are a strong influence on seasonal mean temperatures. Dynamical model forecasts from the NMME and in CBAM also indicate greater probabilities for the west than the eastern CONUS, influenced by both decadal temperature trends and the impacts of above normal SSTs in the central and western tropical Pacific. The probabilities of above normal temperatures exceed 70 percent for western and northern Alaska in the outlook for ASO and SON 2019, where well above normal SSTs are predicted to persist and observed sea ice has been substantially below normal in autumn in recent decades.
The forecasts for JFM 2020 and beyond are largely unchanged from the seasonal outlook last month, relying primarily on signals due to multi-decadal climate trends. Above normal temperatures are likely for all of Alaska and most of the CONUS, except for some areas of the northern central CONUS, where equal chances of below, near and above normal temperatures are indicated, due to weak climate trends. This area expands from JFM 2020 through JAS 2020 and is not included in the outlook for ASO 2020, when decadal trends are most widespread.
PRECIPITATION
For the ASO 2019 precipitation outlook, above normal precipitation is forecast for western Alaska, as well as northern and southern coastal regions of Alaska, following NMME and CBAM model guidance. This forecast of above normal precipitation for much of Alaska persists through all leads with some variations in regions, as it is related to signals from decadal climate trends and above normal sea surface temperatures. Following NMME and CBAM model guidance, above normal precipitation is also likely for most of the Northern and Central Plains in ASO 2019. Some of the signal for above normal precipitation persists for parts of the Great Plains into SON 2019, while a signal for above normal precipitation increases across much of the Southwest in SON 2019 and persists for parts of the region through JFM 2020. Above normal precipitation is also likely for western regions of the Gulf Coast for NDJ and DJF 2019-2020. Some of the signal for above normal precipitation arises from forecasts for a warmer than normal tropical Pacific Ocean, despite the greater chance of ENSO neutral conditions than El Nino development. Minimal changes were made to the outlooks for FMA 2020 forward as no climate forecasts for these leads have changed since last month. The outlooks for AMJ through ASO 2020 indicate likely above normal precipitation over parts of the eastern CONUS, while likely below normal precipitation is indicated for parts of the Northwest in JJA through ASO 2020, due to multi-decadal precipitation trends . [Editor’s Note: NOAA almost seems to be incorporating a Positive PDO in their forecast]
ENSO Considerations
Some of this material is from Monday’s Article plus there is some additional information. I was tempted not to include this section because we covered most of it Monday Night and NOAA covered it in their detailed discussion. But because there is not full agreement on the forecast for ENSO, I decided to include this section of the report.
First of all, it is a Modoki, not a Canonical El Nino.
Now let us look at the Nino 3.4 readings which assess the surface in what is called the Nino 3.4 Measurement Area which stretches from 170W to 120W and five degrees north and south of the Equator.
Here is a longer view of the history.
Now let us look at the forecasts.
First we look at the forecast from the NOAA proprietary model.
Comparing Last Month to This Month
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Here is the recent (prior to today) CDC-IRI Probability Forecast.
And here is the comparison of the Early July and Mid-June forecasts (the methodology is a little different.
But what is this?
This also probably was not available to NOAA when they prepared their forecast. It is hard to tell what they had and what they did not have.
This is a sneak peek of what we will be reviewing Sunday Night. Here is the JAMSTEC forecast.
Here is another forecast from outside the U.S. in this case Australia.
Looking at the subsurface. ENSO is measured on the surface, but the subsurface is a predictor of the future surface conditions.
Meeting the sea surface anomaly criteria is one part of declaring El Nino conditions. The other part is determining if the atmosphere is responding. Afterall we mostly care about the impact on land not on the ocean. There are many ways to assess the extent to which the atmosphere is responding to the pattern of the sea surface temperatures and one way is what is called the Southern Oscillation. It is an old way used prior to satellite technology. Queensland, Australia is the Keeper of the Southern Oscillation Index.
SOI = 10 X [ Pdiff – Pdiffav ]/ SD(Pdiff) where Pdiff = (average Tahiti MSLP for the month) – (average Darwin MSLP for the month), Pdiffav = long term average of Pdiff for the month in question, and SD(Pdiff) = long term standard deviation of Pdiff for the month in question. So really it is comparing the extent to which Tahiti is more cloudy than Darwin, Australia. During El Nino we expect Darwin Australia to have lower air pressure and more convection than Tahiti (Negative SOI especially lower than -7 correlates with El Nino Conditions). During La Nina we expect the Warm Pool to be further east resulting in Positive SOI values greater than +7).
B. Conclusion
There has been some substantial change in the forecast this last month mostly in terms of the precipitation forecast. Basically the date for the demise of the weak El Nino has been moved up by about six months. The changes mostly affect the Southern Tier. Some parts of the Southern Tier which were forecast to be wet are not forecast to be EC and some parts where were forecast to be EC are now forecast to be wet. This changes takes place over the six month period it is not immediate. The change proceeds from east to west but not in a smooth way probably due to seasonality.
Soon, probably on the evening of Sunday, July 21, we will compare the NOAA and JAMSTEC forecasts.
