Written by Sig Silber
Updated at 3:30 pm EDT August 21, 2020 to reflect the updated Week 3 – 4 forecast. From the forecast…”The amplified upper-level ridge favors a large spatial area of below normal precipitation throughout the West, Great Plains, Corn Belt, and Great Lakes.” So that bears watching.
On July 9, 2020 NOAA Updated their ENSO Alert System Status to La Nina Watch which they maintained on August 13, 2020. Here is the August 20, 2020, NOAA 15-month forecast for CONUS and Alaska including the Early Outlook for September 2020 and forecasts out through November 2021. It covers a period that begins one month later than the forecast last month and extends one month longer. The prior forecast included the Early Outlook for August and this forecast has the Early Outlook for September. Also included is the three-month drought forecast. NOAA has again chosen to issue a forecast that is essentially a forecast for a marginal La Nina. We expect that next month and even with the end of month update on August 31, 2020, it will be slightly more a forecast of a typical weak La Nina. That is not a huge difference but that is the direction we see the forecast trending.
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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 for shorter time frames. 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 September 2020. It is called the Early Outlook because it will be updated at the end of August. Only the September Outlook will be updated at that time.
Temperature
Precipitation
We have nothing to compare these maps with as NOAA does not provide in their Update 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 September *. For Temperature, it is warm in Alaska, the West, along the Northern Tier and for Florida but there is a large EC area the surrounds the Mississippi River Valley. Given the warming trend, EC really means colder would be expected. For precipitation, the Southeast and part of the Mid-Atlantic is wet as the Alaska Panhandle and there is a large dry anomaly centered on Utah and Colorado but extending over a large area involving all are part of seventeen states. The is a lot of area that is EC.
* 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 only July so only the third of the three maps is useful for this purpose. The third map extends the coverage through the first 11 days of September. The Seasonal Outlook was issued fairly late this month since the third Thursday fell on August 20, 2020 close to the latest possible issue of the forecast which would be August 22. Eleven days of shorter forecasts is not an acceptable basis for doing this visual consistency testing but it is what we have to work with and there are thirty days in September. By the time most people see this report we will have eighteen days of shorter forecasts which is a reasonable amount of information. That has now been done.
First Temperature
 |  |  |
| The new monthly forecast is to the right. Does it appear to be reflective of the parts of the month for which we have forecasts? → |
| ↑ ← It is properly too soon to do this analysis. We have done an update on Friday, August 21 as this now provides seven more days of the early month forecast to work with. The large EC area is verified by the new Week 3/4 Forecast. We will also revisit the forecast at the end of August when the September forecast is updated. |
And then Precipitation
 |  |  |
| The new monthly forecast is to the right. Does it appear to be reflective of the parts of the month for which we have forecasts? → |
| ↑ It is properly too soon to do this analysis. We have now done an update on August 21 so we now have seven more days of the early month forecast to work with. With the additional information the September forecast looks reasononable. We will also revisit the forecast at the end of August when the September forecast is updated. ← |
Sometimes it is useful to look at the Week 3-4 Discussion. We have now added that.
Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks NWS Climate Prediction Center College Park MD 300PM EDT Fri Aug 21 2020
Week 3-4 Forecast Discussion Valid Sat Sep 05 2020-Fri Sep 18 2020
The Madden-Julian Oscillation (MJO) continues to exhibit a coherent wave-1 pattern with anomalous upper-level divergence (convergence) over the Western (Eastern) Hemisphere. Dynamical models remain consistent that the MJO, with varying forecasts of amplitude, propagates eastward through early September. This eastward shift of its enhanced phase across the tropical Atlantic and Africa is likely to promote a favorable large-scale environment for tropical cyclone (TC) development during the next few weeks and coincides with the climatological peak in TC activity in the main development region of the Atlantic basin. Also, the low-frequency climate state, which includes suppressed convection across the equatorial western and central Pacific, is likely to result in reduced vertical wind shear across the tropical Atlantic. The increased chances of an active Atlantic basin through at least early September played a role in the Week 3-4 precipitation outlook along the East and Gulf Coasts. In addition to the likely MJO influence on TC activity in the Atlantic basin, the temperature and precipitation outlooks are based on dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and SubX multi-model ensemble (MME) of experimental and operational ensemble prediction systems along with decadal climate trends.
The CFS, ECMWF, and JMA models along with the MME are in good agreement about the 500-hPa pattern, with an amplified trough extending from the Bering Sea south to the Gulf of Alaska and a corresponding ridge over western North America. This well-agreed upon 500-hPa pattern during week 3-4 represents an eastward shift in the longwave pattern from week-2. The amplified ridging aloft supports above normal temperatures across the West and High Plains. Although the influence of anomalous soil moisture on surface temperature is waning, extremely low soil moisture (below the 5th percentile) across the Four Corners region and western Texas is expected to be at least a minor contributor to above normal temperatures for these areas. The predicted position and amplitude of the ridging upstream over western North America favors intrusions of anomalously cool temperatures across parts of the east-central CONUS. Based on a consensus among the dynamical models including the MME, below normal temperatures are most likely across the Middle Mississippi Valley and Ohio Valley. Compared to the previous outlook released on Aug 14, a change from above to below normal temperatures being favored across these areas was necessary due to the evolving longwave pattern and expectation that anomalous warmth times off after the beginning of September. To the east of the trough axis, above normal temperatures are favored along the East Coast.
The amplified upper-level ridge favors a large spatial area of below normal precipitation throughout the West, Great Plains, Corn Belt, and Great Lakes. Probabilities of below normal precipitation taper off closer to the coastal Pacific Northwest due to the proximity of troughing upstream over the Gulf of Alaska. Only modest probabilities of below normal precipitation are forecast for the Desert Southwest due to weak and varying precipitation signals among the model guidance. Regardless of any impact from tropical cyclones during week 3-4, the amplified trough upstream favors above normal precipitation across the mid-Atlantic and Southeast. Also, the favored area of above normal precipitation across the mid-Atlantic is consistent with decadal trends. Despite the CFS and ECMWF models indicating elevated probabilities of below normal precipitation along the Texas Gulf Coast, equal chances of below or above precipitation are forecast due to the potential for a tropical cyclone to enter the Gulf of Mexico.
The predicted trough axis, extending southward from the Bering Sea, favors southerly flow and above normal temperatures across much of Alaska. Positive sea surface temperature (SST) anomalies elevate probabilities of above normal temperatures for coastal western Alaska, the Alaska Peninsula, and the Aleutians. Enhanced onshore, southerly flow favors a tight gradient between areas of above and below normal precipitation across southeast Alaska. Increased chances of above normal precipitation from the Kenai Peninsula east to the Alaska Panhandle are consistent with the predicted longwave pattern and flow at the surface, while below normal precipitation is probable north of the Alaska Range.
Model guidance strongly supports above normal temperatures throughout the Hawaiian Islands. The largest probabilities of above normal temperatures are forecast at Honolulu and Lihue where weekly SST anomalies, surrounding Kauai and Oahu, remain above +0.5 degrees C. Equal chances of below or above normal precipitation are forecast for Hawaii due to weak precipitation signals along with an expected transition in the MJO-related anomalous precipitation.
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g. September/October/November is shown as SON. You will see such abbreviations often in this report.
Prior Temperature Outlook for SON 2020
New Temperature Outlook for SON 2020
Now Precipitation.
Prior forecast for SON 2020
New Precipitation Outlook for SON 2020
Now let us focus on the long-term situation.
First Temperature
Prior 14 Month Temperature Outlook: SON 2020 – ASO 2021
New 14 Month Temperature Outlook: OND 2020 – SON 2021
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: S0N 2020 – ASO 2021
New 14 Month Precipitation Outlook: OND 2020 – SON 2021
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 months.
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 months.
*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) released by NOAA on August 20, 2020. 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 (AugustJuly), the new NOAA Summary for ASO, and finally the remainder of the 15-Month Forecast.
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
Oceanic and atmospheric observations across the equatorial Pacific indicate ENSO-neutral conditions persisted into August. Observed sea-surface temperatures (SSTs) for the previous 30 days showed positive anomalies west of 170E and negative anomalies from about the Date Line to the coast of South America. The equatorial central pacific cooled while the eastern Pacific warmed during the past 30 days, though the eastern Pacific still remained below average. The four Niño indices were negative during the latest week, with the Niño-3.4 value at -0.5C. Positive subsurface temperature anomalies continued to be confined to the western Pacific, with negative temperature anomalies to a depth of 150 meters in the eastern Pacific. Integrated upper-ocean heat anomalies declined during the second half of July and early August. Easterly low-level wind anomalies were present over the western low-latitude Pacific during the previous 30 days, with westerly upper-level wind anomalies along the equator over the western equatorial Pacific Ocean. Positive outgoing longwave radiation (OLR) anomalies, during this same period show suppressed convection across the western and central equatorial Pacific. Negative OLR anomalies were noted over portions of the Maritime Continent. A La Nina watch is in effect.
PROGNOSTIC DISCUSSION OF SST FORECASTS
The CPC SST Consolidation for the Nino 3.4 region indicates an SST anomaly declining to near -1.0C during late autumn. The CCA and Markov model inputs CPC SST Consolidation remain in ENSO-neutral territory. Among the NMME models, there is a range from far below normal (-2.0C) to cold-neutral. The C3S models all get close to, or below, -0.5C for at least one month. The CPC/IRI consensus forecast favors a La Niña development during Northern Hemisphere fall 2020 (60% chance) and continuing through winter 2020-21 (55% chance).
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2020
The expected transition from ENSO-neutral to a La Nina remains on track, with a 60% chance of La Nina development during autumn 2020. These odds are about 5% greater than they were last month. However, any forcing from a La Nina is predicted to have little overall effect on the September 2020 temperature and precipitation patterns for the United States, with a more substantial impact possible during the autumn season. The combination of MJO and Kelvin wave activity over the eastern Pacific has likely contributed toward enhanced tropical cyclone activity over that region during the past 1-2 weeks. The MJO is predicted to remain active during the subsequent two-week period, with its enhanced convective phase expected over the Western Hemisphere (RMM phases 8 and 1). However, lagged composite impacts are minimal in coverage when masked by statistical significance. MJO influence is therefore likely to be manifest mainly through modifications to tropical cyclone activity in the eastern Pacific and Atlantic basins. The September temperature and precipitation outlooks are based primarily on statistical and dynamical model guidance, with consideration of recently observed conditions, and decadal trends . Impacts from soil moisture/evapotranspiration on temperature and precipitation typically decline significantly during September, so their contribution to the outlooks is minor.
Temperature
The temperature outlook for September 2020 favors above-normal mean temperatures over the western half of the contiguous U.S., the northernmost portions of the Mississippi Valley, the Great Lakes region, upper Ohio Valley, Northeast, Mid-Atlantic, and Florida. Most of the NMME model suite, including the CFS, as well as decadal trends , and most of the official temperature outlooks for the next few weeks, favor at least a 50% chance of above-normal temperatures over the Southwest and California. It seems very unlikely that the lackluster monsoon will be able to recover in its final climatological month, September, after rather poor performance so far. A weak monsoon favors less extensive cloudiness and precipitation, and increased solar insolation, which in turn favors warmer-than-normal temperatures. Probabilities for above-normal temperatures still remain only moderate (40%-50%), since any warming effects from La Nina forcing are likely to occur later in the autumn. Above-normal temperatures are also favored for northernmost portions of the Mississippi Valley, the Great Lakes region, upper Ohio Valley, Northeast, Mid-Atlantic, and Florida, supported by the NMME temperature guidance, decadal trends , and official temperature outlooks valid for the next several weeks. Odds for above-normal temperatures exceed 50% in northern New York and northern New England. The remaining portion of the contiguous U.S. (which covers much of the eastern CONUS) is forecast to have Equal Chances (EC) of above, near, and below-normal mean temperatures. In Alaska, there are elevated odds of above-normal temperatures for most of the state (the exception being EC for much of the Panhandle). Odds in excess of 60% are forecast for northern and western portions of the state. This is based on a combination of NMME temperature guidance, anomalously warm water near the coast, and decadal trends. The climatological delay in sea ice formation near the northern coast of Alaska, and the accompanying enhanced odds of above-normal North Slope temperatures, is particularly pronounced in October and November but not as much in September.
Precipitation
The precipitation outlook for September 2020 has considerably more uncertainty than does the temperature outlook. In Alaska, most of the NMME precipitation forecasts favor above-normal precipitation for the Alaska Panhandle, which is consistent with an enhanced climatological onset of the stormy season, and the southward migration of the polar jet stream. However, the precipitation outlook for the southwestern portion of the state is not as clear cut. Some of the dynamical models favor below-normal precipitation, which aligns better with recent observations. Other models favor near-normal precipitation, in better agreement with climatology, or even above-normal precipitation though it may be early for La Nina impacts which favor above-normal precipitation for southwestern Alaska during autumn. It was decided that EC may be the best bet for this area, given the conflicting information. For the remainder of Alaska, where no clear climate signal is present, which includes most of the mainland, EC is favored. Over portions of the western and central CONUS, there are increased chances for below-normal precipitation in September. This is based on reasonable support from official CPC precipitation outlooks that extend several weeks into the future, some NMME precipitation guidance such as the NCEP CFSv2 and the IMME, and the expectation of a weak monsoon over the Southwest CONUS. Though moisture from a recurving eastern Pacific tropical cyclone cannot be ruled out for the region in September, this is more likely to occur in October. A weak extension of this area of below-normal precipitation is more likely to extend across Nebraska and Iowa during the first half of September, rather than the second half. Probabilities are 33%-40%, which reflects the weak tilt in the odds towards below-normal precipitation for this extension into the Midwest. Over the southeastern CONUS, above-normal precipitation is favored in September, particularly in Florida and southern Georgia. This is supported by the NMME, IMME, GFDL FLOR, GFDL CM2.1, and the CanCM4i models, and has general support of the official precipitation outlooks during the next few weeks. The above-normal precipitation favored for this region is also based on historical tropical cyclone climatology, especially for the long-track Main Development Region (MDR) systems which often recurve from the tropics towards the Southeast CONUS. Elsewhere, which includes a large fraction of the CONUS, EC is favored, due to weak climate signals and/or conflicting tools.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
Tools used for the seasonal outlooks included dynamical model guidance such as the North American Multi-Model Ensemble (NMME), including the Calibration, Bridging, and Merging (CBaM) post-processing applied of the NMME. The projected state of ENSO is considered generally, and specifically through a regression of temperature and precipitation as included in the seasonal consolidation tool. Additionally, various statistical tools such as CCA and constructed analogs on SST patterns were viewed but considered quantitatively as part of the consolidation. In the further leads, beyond winter 2020-21, decadal trends were the primary source of climate signals .
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (focus on SON)
Temperature
The September-October-November (SON) 2020 temperature outlook favors above normal temperatures across all of the contiguous U.S. and Alaska.
Precipitation
The SON precipitation outlook favors above normal precipitation from the Pacific Northwest to the northern plains, portions of the Southeast, Southwest Alaska, and along the north coast of Alaska. Elevated odds for below normal precipitation stretch from the Southern Sierra Nevada mountains to the central Rockies and Southern Plains. Equal chances (EC) of below, near and above normal precipitation are indicated for central Alaska, and for all remaining areas of the contiguous U.S.
PROGNOSTIC DISCUSSION OF OUTLOOKS – SON 2020 TO SON 2021
TEMPERATURE
During SON 2020, the outlook favors above normal temperatures for the CONUS, based on model outputs, trends , and relationships with SST patterns. Uncertainty is higher across the Pacific Northwest and from the Northern Great Plains to the Southeast. The outlook therefore calls for slightly lower odds in the Pacific Northwest than the SON outlook issued last month, and higher odds for above normal temperatures in the Northeast and portions of the Southwest. The changes in odds are small, and related to slightly more certainty about the projected state of La Nina. Similar reductions in odds for above normal temperatures in the Pacific Northwest were carried through DJF 2020-21, with below normal temperatures now favored from Washington to North Dakota in JFM 2021. The changes are again related to more certainty of impacts related to projected state of ENSO, and more recent dynamic and statistical guidance. The shift toward colder outcomes across the northern tier of the CONUS was carried through MAM 2021. Minor or no changes were made in the outlooks for Spring and Summer of 2021, as the core guidance for those leads is trend. Outlooks across Alaska remained largely unchanged as the impacts for La Nina were already largely accounted. Some below normal temperatures were indicated across Southeast Alaska in some of the C3S models and statistical tools, but other tools had little to no signal, so EC remained until there is more certainty.
PRECIPITATION
The status of ENSO and likelihood of La Nina developing through the winter of 2020-21 played a role in the outlook. The SON 2020 outlook is similar to the outlook from last month for the same target period. The odds for above normal precipitation in the Southeast were extended up the Carolina coast while the odds for below normal precipitation were trimmed back slightly along the western Gulf Coast. Both related to the updated model guidance and potential for late season tropical cyclones to develop in the Gulf of Mexico, aligned with the seasonal hurricane outlook. Outlooks covering the autumn through winter were modified to reflect more certainty with respect to La Nina and potentially a more intense impacts, so the odds for above normal precipitation were increased across the Pacific Northwest with a drier signal into the Great Basin. Outlooks for the core winter seasons of DJF and JFM do include a signal for below normal precipitation into portions of California and higher odds for below normal precipitation across the entire southern tier of the CONUS. Outlooks for the spring of 2021 reflect a little more uncertainty across the Northern Plains as the transition period from a La Nina is uncertain. Enhanced odds for below normal precipitation were added to the Pacific Northwest for late spring and through the summer to reflect trends , while the rest of the outlook was largely unchanged.
We will discuss this more when we compare the JAMSTEC to the NOAA forecast. But it is useful to look at the JAMSTEC discussion especially as it applies to Autumn.
Aug. 13, 2020 Prediction from 1st Aug., 2020
ENSO forecast:
Observation shows that a La Niña is developing at present. The SINTEX-F predicts that the La Niña-like condition will persist in the latter half of this year with weak anomaly in December-February.
Indian Ocean forecast:
Observation shows that the tropical Indian Ocean is slightly warmer-than-normal at present. The ensemble mean suggests that the warmer condition will persist in autumn, then will return to a neutral state from winter.
Regional forecast:
On a seasonal scale, the SINTEX-F predicts that most part of the globe will experience a warmer-than-normal condition in boreal autumn, except for southern Australia, India, and some parts of northern Africa. In boreal winter, the model also predicts that most part of the globe will experience a warmer-than-normal condition, except for western Brazil, India, and some parts of Southeast Asia.
As regards to the seasonally averaged rainfall in boreal autumn, a drier-than-normal condition is predicted for southern/central part of U.S.A., Mexico, some parts of the South American Continent, southern Africa, West Africa, southern China, some parts of the Russian Far East, and most part of Europe. In contrast, western Canada, India, some parts of East Africa, Southeast Asia, Philippines, Indonesia, northern Europe, and northern Russia will experience a wetter-than-normal condition. In boreal winter, a wetter-than-normal condition is predicted for Alaska, northwestern coastal area of Canada, northern Europe, northern part of the South American Continent, most part of Australia, Philippines, and some parts of Indonesia. In contrast, western/southern U.S.A., Mexico, Argentina, DRC, Mozambique, southern Europe, eastern China will experience a drier-than-normal condition.
The model predicts most part of Japan will experience warmer-than-normal condition in autumn and winter as a seasonal average. As regards to the seasonally averaged rainfall, most part of Japan, except for Hokkaido, will experience wetter-than-normal condition in autumn and drier-than-normal condition in winter.
New Seasonal Drought Forecast (It is not set up to auto-update but you can always find the new Seasonal Drought forecast here) But it will not update until September 17August 20, 2020.
Latest Seasonal Assessment – Currently, ENSO conditions are neutral, but leaning negative. A La Niña watch remains in effect for the Fall, with a ~60% chance for development (up from 50-55% in July). If La Niña conditions were to materialize during the September-October-November (SON) period, there is a 55% chance that they will continue through winter 2020-2021. La Niña conditions typically indicate a warmer and drier pattern across much of the southern tier of the CONUS, cooler and wetter conditions in the Pacific Northwest, and increased chances for precipitation in Hawaii. A wild card for this outlook is the increased potential for a continued active Atlantic hurricane season. This is due to the potential for the active phase of the Madden-Julian Oscillation (MJO) to shift into the Atlantic Ocean, temporally coinciding with the peak of the hurricane season in early September. Therefore, several synoptic-scale forcing mechanisms are considered in this forecast, in addition to the short-, medium-, and long-range model guidance and seasonal normals.
Drought continued to expand in the past month over much of the western CONUS. Much of the West is expected to remain in drought, with some additional development likely in the Southwest during the SON season, as warm and dry conditions are favored for much of the West. The exception is the Pacific Northwest, where above-normal precipitation and drought removal (D1) and improvement (D2 and worse) are favored. The dry and wet signals in the Southwest and Pacific Northwest, respectively, could further be amplified by the potential development of La Niña. In the Northern Plains and Midwest, drought removal (D1) and improvement (D2 and worse) are favored due to decreasing evapotranspiration rates during the Fall and climatologically wider-spread synoptic events. The Central and Southern Plains will be a wild card, due to the climatological positioning of the storm track coinciding spatially with increased odds for above-normal temperatures and below-normal precipitation during the valid period. Additionally, the potential for enhanced tropical activity further reduces forecast confidence for those areas. The favored above-normal hurricane season carries a bit more weight in the forecast for the Southeast, with removal favored for all areas currently depicting drought. In the Northeast, above-normal temperatures are favored during the SON period whereas there is an absence of a precipitation signal. Based on 20 years during which La Niña conditions were present, weak positive and negative precipitation signals are depicted in the Northeast and Ohio Valley during the SON period, but have trended wetter in more recent La Niña years. In addition, the potential exists for recurving tropical cyclones during the forecast period. Therefore, slight improvements (D2) and drought removal (D1) are favored.
Abnormal dryness (D0) is not expected to persist in Alaska due to the climatological southward shift of the storm track along the southern coast during the boreal Fall months. Drought conditions have improved in the last month in Hawaii and, despite the dry signal in the statistical model guidance during September, drought removal and improvement are favored to continue. In addition, Hawaii is transitioning into a wetter time of year, and the potential onset of La Niña could further increase odds for above-normal precipitation, particularly later in the SON season. Puerto Rico saw drought removal during the past month due to above-normal precipitation, largely aided by the close passage of Isaias in late July. Long-range model guidance suggest wetter-than-normal conditions in Puerto Rico, favoring no drought development.
ENSO Considerations
Here is what NOAA released last Thursday, August 13, 2020 which is the official time (second Thursday) for them to make that release.
No change in the status but it looks like there may be a change next month.
This survey of meteorologists has been the most influential in determining the NOAA assessment of ENSO status. Notice the forecast gives the edge to La Nina for the period ASO. We are in August now. Usually, I pay the most attention to the middle letter in the triplet ie. September. Of it things proceed as forecast we probably will see a change in ENSO Status on September 10, 2020.
This graphic is one I added as it shows the state of the Equatorial Pacific subsurface at about the time NOAA released their status advisory. I could provide a more up to date version but really it makes more sense to show what NOAA was looking at when they issued their advisory.
On a sea surface temperature basis it is La Nina now. But the atmosphere is still behaving closer to ENSO Neutral. There is not much difference between Neutral with a La Nina bias and a marginal La Nina so there is no pressing need to change the status. There will be a new Seasonal Outlook Update on August 20 so there will be more discussion on this topic at that time.
NOAA has their own proprietary model which they rarely use.
Right now it is clearly forecasting La Nina and looks a lot like the BOM forecast.
BTW here are the BOM and JAMSTEC forecasts.
Here is the Australian Bureau of Meteorology (BOM) forecast.
And here is the forecast the BOM issued on August 18, 2020
La Niña ALERT—likelihood of La Niña in spring has increased
La Niña ALERT
ENSO Outlook
The El Niño–Southern Oscillation (ENSO) is currently neutral. However, continued cooling of the surface of the tropical Pacific Ocean, changes in tropical weather patterns, and climate model outlooks shifting further towards La Niña mean the Bureau’s ENSO Outlook has moved to La Niña ALERT.
La Niña ALERT indicates the chance of La Niña forming in 2020 is around 70%—roughly three times the average likelihood.
ENSO indicators in the tropical Pacific Ocean are consistent with the early stages of La Niña development. Surface waters are cooler than average, while sub-surface temperatures have cooled further over the past fortnight.
In the atmosphere, trade winds are stronger than average, while the Southern Oscillation Index (SOI) remains neutral but positive. Equatorial cloudiness near the Date Line is also below average.
All international climate models surveyed suggest the tropical Pacific Ocean will cool further, with three of the eight models reaching La Niña thresholds in September, and two more in October.
The Indian Ocean Dipole (IOD) is currently neutral, despite the index dropping sharply in the last week. Large parts of the Indian Ocean are warmer than average, with some weak cool anomalies near the Horn of Africa. Half of the six surveyed models indicate negative IOD thresholds could be met or exceeded during spring.
Both La Niña and negative IOD typically increase the chance of above average rainfall across much of Australia during spring.
The Southern Annular Mode (SAM) is currently negative, but is expected to soon become weakly positive. At this time of year negative SAM is typically associated with above-average rainfall over southern Australia.
The Madden–Julian Oscillation (MJO) is in the eastern Pacific and is expected to continue eastwards towards Africa. At this time of year an MJO pulse in this region has little influence on rainfall over northern Australia.
Here is the JAMSTEC forecast
B. Conclusion
We seem to be headed into La Nina. We are pretty sure the September 10, 2000 NOAA ENSO Advisory will upgrade to La Nina and the September 17, NOAA Seasonal Outlook will be slightly more definitive as a La Nina Forecast.
But we think the current forecast is probably pretty much on target.
