Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Mon Aug 31 2020


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.

--------------- Previous 30-day discussion follows below ---------------

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.

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.

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.

FORECASTER: Anthony Artusa

The climatic normals are based on conditions between 1981 and 2010, following
the World Meteorological Organization convention of using the most recent 3 
complete decades as the climate reference period.  The probability anomalies
for temperature and precipitation based on these new normals better represent
shorter term climatic anomalies than the forecasts based on older normals.

The next monthly outlook...for Oct ... will be issued on Thu Sep 17 2020

These outlooks are based on departures from the 1981-2010 base period.
$$