Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Aug 20 2020


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.

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.

An updated monthly outlook... for Sep will be issued on Mon August 31 2020

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