Updated Outlook For September, 2019

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, which was issued on the Third Thursday of the Month. For September, NOAA has a higher level of confidence in their temperature forecast than the precipitation forecast which can be impacted by tropical events which are more difficult to forecast.

Some housekeeping: On August 17, 2019 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 Early Outlook was issued. This article presents the NOAA update of their Early Outlook for September.

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Now let us address the NOAA Update of the September, 2019 Forecast.

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 the Short Term forecasts, NOAA has switched from using EC to N for Normal. For most purposes, EC and N can be considered to mean about the same thing. “N” implies a bit more certainty that the variation from Normal/Climatology will be small compared to “EC”.

First, we will compare the NOAA Early Outlook for September, 2019 with the newly issued update.

Early Outlook Temperature

Updated Temperature Outlook

Early Outlook Precipitation

Updated Precipitation

Here is the discussion released today:

30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2019

The updated monthly temperature and precipitation outlooks for September 2019 are based on the latest dynamical model guidance, WPC temperature and precipitation outlooks for the first week of September, CPC 6-10 day, week 2, and week 3-4 temperature and precipitation outlooks, and GEFS and CFS monthly temperature and precipitation forecasts. There is no Madden-Julian Oscillation (MJO) currently present, and no MJO is forecast to emerge during the next several weeks. Therefore, the MJO is unlikely to impact the mid-latitude circulation pattern during September.

Dynamical model forecasts for the month of September, including 700-hPa height anomalies from the CFS monthly forecasts, generally predict troughing over the Bering Sea, a ridge over western North America, and troughing over northeastern North America. Forecasts from the ECMWF and NCEP GEFS ensembles, as well as the multi-model consensus of the Subseasonal Experiment or SubX, indicate persistence of this circulation pattern overall into weeks 3 and 4, with the exception that a trough over the Northeast CONUS early in the month, as indicated by the CPC 6-10 day and week 2 outlooks, is predicted to lift northeastward, as in the CPC week 3-4 outlook.

The updated September temperature outlook continues to indicate high probabilities of above normal temperatures over Alaska and the western CONUS, with a persistent ridge and above-normal 500-hPa heights predicted throughout the month. Probabilities exceed 70 percent for coastal areas of northern and western Alaska, where sea surface temperatures remain anomalously much above average. Probabilities exceed 60 percent for areas of the Pacific Northwest and Central Rockies, under persistent above normal 500-hPa heights throughout the month of September in dynamical model forecasts. Above normal monthly average temperatures are likely for the month of September across much of Texas, excluding the Texas Panhandle, and the Southeast region, following the CPC 6-10 day, week 2, and week 3-4 outlooks. Probabilities of above normal temperatures are lower over Texas and Louisiana, following guidance for the month of September from a tool that merges GEFS and CFS forecasts for weeks 1 and 2 with CFS forecasts for the week 3-4 period, and where the GEFS ensemble forecast indicates a greater probability of below normal temperatures in the week 2 and week 3-4 periods, as does the SubX MME consensus forecast for the week 3-4 period. Below normal average September temperatures are predicted to be most likely for the northern central CONUS from the Northern and Central Plains into the Great Lakes region, with troughing and greater probabilities of below normal temperatures predicted during the first two weeks of the month, and weak probabilities for above normal temperatures indicated in the CPC week 3-4 temperature outlook. Likely below normal temperatures over the Northeast in the WPC forecasts for the first week of September and in the CPC 6-10 day and week 2 outlooks, with probabilities exceeding 50 percent, give way to likely above normal temperatures in the CPC week 3-4 outlook, with probabilities exceeding 60 percent. Equal chances of above, near and below normal temperatures is indicated in the September temperature outlook for most of the Northeast region.

Predicted troughing over the Bering Sea and Aleutian Islands in dynamical model forecasts for much of the month leads to continued enhanced probabilities of above normal precipitation for western and southern Alaska including the Alaska Panhandle. Above normal accumulated precipitation is likely for a large area of the western CONUS from the Southwest region into the Central and Northern Plains, as indicated by WPC precipitation forecasts for week 1 and the CPC 6-10 day and week 2 precipitation outlooks, where moisture surges from the Eastern Pacific are expected to enhance precipitation over the Southwest. Hurricane Dorian is expected to impact Florida and parts of the Southeast Atlantic Coast during the first week of September bringing very large precipitation amounts to areas along the eventual storm track. Uncertainty in the exact storm track and uncertainty in precipitation amounts during the remainder of the month leads to moderate probabilities for above normal precipitation for the September monthly outlook. Probabilities of above normal precipitation in the September monthly outlook exceed 60 percent for the Atlantic Coasts of northern Florida, Georgia and the Carolinas. Likely above normal precipitation is indicated over the Northeast region in the September outlook, with precipitation predicted in the 7-day WPC forecast for week 1 and likely above normal precipitation predicted in the CPC 6-10 day and week 2 outlooks, along with a potential for HurricaneDorian to follow a northeastward track off of the Atlantic coast enhancing precipitation over the Northeast. Although the CPC week 3-4 outlook indicates likely below normal precipitation for the second half of the month for much of the Atlantic coast and Northeast, there is greater certainty in the precipitation outlook for the first two weeks of September.

Visual Consistency Testing.

It is useful to see how the full month forecast fits with the set of shorter forecasts that we have. These will continue to update in this article. It is important to remember that of the short term 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 so we do not have the last three days of the month represented by short-term forecasts.

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 recently so we will not need to update the commentary when the update is reissued on Friday September 6, 2019.

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 recently so we will not need to update the commentary when the update is issued on Friday September 6, 2019.

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, 2019.

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.

Summary

The purpose of this article was to present the updated Outlook for September, 2019 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. We do not have a September forecast by JAMSTEC as they only provide three-month forecasts. 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 the 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 12, 2019, IRI/CBC will issue their ENSO forecast 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 Severe Weather Article which updates in real-time to reflect NOAA Weather Headlines. All of our reports including past versions of these reports can be located via the Directory/Archive which can be accessed here.

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