NOAA Updates The Outlook For September 2020 – Early Autumn Impacts Some Crops

Written by Sig Silber

At the end of every month, NOAA issues an update to their Early Outlook for the following month, in this case, September 2020, which was issued as usual on the Third Thursday of the month. Today, eleven days later, NOAA has issued their Updated Outlook for September. Everything is connected and these changes appear to be related to the development of La Nina but NOAA denies this. NOAA is not quite ready to announce that we are in La Nina but we expect it soon. Autumn is intruding but areas where the Monsoon failed will remain warmer than usual for this time of the year.

Some housekeeping: On August 20, 2020, we published Part I of our analysis of the NOAA and JAMSTEC Three- to Four- Season Outlook and that can be accessed here. In that report, the September 2020 Early Outlook was issued. This article presents the NOAA update of their Early temperature and precipitation Outlook for September and the most recent Drought Forecast update for the month of September.

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Now, we will compare the NOAA Early Outlook for September 2020 with the newly issued update.

Early Outlook Temperature

Updated Temperature Outlook

Early Outlook Precipitation

Updated Precipitation

Here is the discussion released with the forecast.

30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2020

The 30-day update for September 2020 is based on official temperature and precipitation outlooks for constituent time periods (Week-1, 6-10 days, Week-2, and Week-3/4), and on the latest dynamical model guidance. Other factors, such as MJO activity, recent temperature and precipitation observations, decadal trends, and soil moisture, all played a minor role in the updated outlooks. As noted in the previous discussion (issued August 20th), the expected transition from ENSO-neutral to a La Nina remains on track, with a 60% chance of La Nina development during autumn 2020. However, any forcing from a La Nina is predicted to have little overall effect on the temperature and precipitation outlooks for September.

The updated temperature outlook for September indicates elevated chances of above normal mean temperatures for practically all of Alaska, the western CONUS, and parts of the Southeast. Southern Alaska is predicted to be anomalously warm through most of September, while temperatures across Northern Alaska are forecast to be relatively cool during the first half of the month, followed by a moderating trend. Probabilities for above normal temperatures exceed 60% over southwestern portions of the state. Contributing factors include dynamical model guidance, the official suite of temperature forecasts made out to one month in advance, and the relative proximity of anomalously warm water surrounding most of Alaska. Across the western CONUS, dynamical models predict an extensive period of above normal temperatures associated with a late season heat wave. This heat will likely be exacerbated by the lack of a substantial summer monsoon over the Southwest, with significant strengthening of the monsoon considered unlikely during its final climatological month. Probabilities of above normal temperatures exceed 60% over much of the interior West. In Florida, dynamical model output has consistently predicted anomalous warmth, in addition to the proximity of anomalously warm Gulf/Atlantic waters. Below normal temperatures are favored for a large part of the central and east-central CONUS in September, associated with an expected prolonged period of anomalously cold temperatures. The forecast magnitude of this cold air anomaly will have a dominant impact on this region for at least the first 2-3 weeks of the month, and is expected to carry the month as a whole. Some areas of the Northern Plains are expected to receive frosts and freezes, likely ending their growing season. In areas where there are weak climate signals and/or conflicting tools, Equal Chances (EC) of below, near, and above normal mean temperatures are indicated.

The updated precipitation outlook for September favors elevated odds of above normal precipitation from the south-central Plains eastward across the south-central Mississippi Valley, the Tennessee Valley, parts of Alabama, and the East Coast states from about Long Island, NY, southward to Florida. This precipitation is associated with a mean baroclinic zone that stretches across much of this region. The predicted rainfall bulls-eye over southeastern Oklahoma and western Arkansas is forecast to receive 5-7 inches of rain just within the first week of September. Chances of above normal precipitation exceed 70% over the vicinity of the Ozark Mountains. Over the Southeast, models generally favor above normal rainfall during the month. In addition, the Southeast is favored to experience increased influences from tropical cyclone activity; especially those tropical systems that form within the Main Development Region (low-latitude Atlantic south of 20N). Above normal precipitation is also favored over southwestern and south-central Alaska, consistent with a ridge amplifying into this region. Odds for above normal precipitation exceed 50% for southwestern portions of the state. Below normal precipitation is favored for most of the West, and extending eastward across the northern tier to the Upper Great Lakes region. Odds for below normal precipitation exceed 50% from the Cascades eastward to the western Dakotas. The official Week-1, Week-2, and Week-3/4 precipitation outlooks continue to go with a very weak monsoon this summer across the Southwest. The prospects for above normal temperatures, coupled with a weak monsoon, favor a widespread area of below normal rainfall across that region. For far southern California and southwestern Arizona, EC is indicated and is based on several considerations. With class limits very low over this region in late summer, even a small amount of precipitation can result in above normal totals. A brief burst of monsoonal moisture, or perhaps moisture from a recurving eastern Pacific tropical cyclone, could result in above normal rainfall for the month. Elsewhere, where there are no signals , Equal Chances (EC) of below, near, and above normal precipitation amounts are favored.

Visual Consistency Testing.

It is useful to see how the full-month forecast fits with the set of partial-month forecasts that we have. These will continue to update in this article. It is important to remember that with the partial-month forecasts, we generally have about 25 days of the subsequent month to look at when we do this at the end of the month as we are doing now. It depends on what day of the week the month ends. So this month, which ended on a Monday, we are only missing a forecast that includes the last five days of the month. So we are in very good shape for the visual consistency testing this month.

First Temperature

The new monthly forecast is to the right. Does it appear to be reflective of the four parts of the month for which we have forecasts? →			↑ ← The Week 3 – 4 forecast was updated Friday, August 28 so we will probably not need to update the commentary when the Week 3 – 4 update is reissued on September 4. At this point, we have forecasts for 25 days of the 30 days in September and the fit looks pretty good.

And Precipitation

The new monthly forecast is to the right. Does it appear to be reflective of the four parts of the month for which we have forecasts? →			↑ ←The Week 3 – 4 forecast was updated Friday, August 28 so we will probably not need to update the commentary when the Week 3 – 4 update is reissued on September 4. At this point, we have forecasts for 25 days of the 30 days in September and the fit looks pretty good.

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.

And with respect to drought, this was also issued on August 31, 2020

Latest Monthly Assessment – ENSO conditions currently remain neutral (leaning negative), with the potential for La Niña to develop later in September. During August, drought coverage expanded across the CONUS, mainly in the West. The much below-normal monsoon season and much above-normal temperatures resulted in expansion and intensification in the Desert Southwest and Four Corners Region. The remainder of the western CONUS was dominated by enhanced ridging through much of August, leading to some development in the Central Rockies and Western Plains. Areas east of the Rockies saw a mixture of drought improvement and degradation. Western Iowa and New England saw the most degradation, while most other areas experienced general improvement. An amplified pattern is favored to dominate much of North America during September, with ridging building in the West and mean troughing progressing from the central CONUS into the eastern CONUS during the period. The predicted mean ridging in the West favors above-normal temperatures and below-normal precipitation, favoring drought persistence with some development in the Pacific Northwest, Northern Rockies, and Central Plains. Heavy precipitation is likely during the first week of September, enhancing the likelihood for drought improvement for much of Texas and western Oklahoma. With increasing odds for below-normal precipitation in the latter half of the period, drought persistence is favored for western areas of the Corn Belt. The Ohio Valley and the Northeast are predicted to see some removal and improvement in the first half of September, with above-normal precipitation favored through the Week-2 period. However, some drought will likely still remain for areas experiencing long-term (90 days and beyond) deficits in the Northeast. The Southeast has experienced scattered, persistent rainfall in recent weeks. With the peak of the hurricane season during September, coupled with increased odds for above-normal precipitation throughout the period, no development is likely in this region.

D1 drought (Kodiak Island) and abnormal dryness (D0) developed in portions of Alaska in August. The predicted amplified pattern favors mean southwesterly flow and above-normal precipitation through the first half of September which should be enough to remove D1 on Kodiak Island. In Hawaii, although the western islands have increased odds for above-normal precipitation during the first half of October, a drier airmass is favored for the second half of the month. As such, drought persistence is favored for Hawaii, with some development on the leeward slopes of the Big Island and Kauai. The passage of Isaias in late July and Laura in August eliminated all drought in Puerto Rico, and with the expectation for an active tropical season, no development is expected during September.

Looking back on August to relate the forecast for September to the actuals in August

First August Temperature (30 out of 31 days).

And then August Precipitation (30 out of 31 days).

We then show the new forecast and the prior month actuals (less one day) side by side.

Conclusion

The purpose of this article was to present the updated Outlook for September 2020 and compare it with the Early Outlook. It is not to critique the updated Outlook for September based on our opinions but we point out possible inconsistencies if we find them. We have assessed the extent to which the monthly outlook seems to fit with the other forecasts provided by NOAA. In reality, all forecasts are obsolete when issued as the situation changes day by day or six hours by six hours i.e. each new model run. In our other articles, we continually examine the current forecasts from NOAA including Day 1- 5, 6-10, 8-14, and Week 3 and 4. So we are continually looking at approximately 25 days into the future.

On September 10, 2020, IRI/CBC will issue their ENSO forecast (and we expect them to declare that we are in La Nina Conditions or will soon be in La Nina Conditions) and we will present that and critique it. It is an ongoing process and we attempt to provide our readers with the best current assessment of the next approximately 25 days (except when we cover the Four-Season Analysis or medium-frequency cycles such as ENSO), so our analysis is pretty much ongoing. We also publish a “LIVE” Weather Article which updates in real-time to reflect NOAA Weather Headlines and for those interested provides detailed information down to current warnings in place. All of our reports including past versions of these reports can be located via the Directory/Archive which can be accessed here.