Unusually hot weather expected to continue well into autumn

Weather

The Climate Prediction Center’s new monthly outlooks predict hotter than normal weather will continue, not only all summer, but through much of autumn. The precipitation outlook indicates an equal chance for above normal or below normal rainfall during the three month period August through October. Below are all outlooks for the next year, followed by a discussion of the reasoning behind the predictions.

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS 

The August-September-October (ASO) 2020 temperature outlook favors above normal
temperatures across all of the contiguous U.S. and Alaska. The ASO
precipitation outlook favors above normal precipitation over southern portions
of Alaska, including the Aleutian Islands and the Alaska Panhandle. Above
normal precipitation is also favored for much of the Northern Great Plains and
Upper Mississippi Valley, and all areas southeast of an approximate line that
extends from southern New Jersey to southwestern Louisiana. There are elevated
odds of below normal precipitation from southern Idaho and eastern Nevada
across much of the Rockies to the south-central Great Plains region. Equal
chances (EC) of below, near and above normal precipitation are indicated for
northern Alaska, and for all remaining areas of the contiguous U.S.

BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
Note: For Graphical Displays of the Forecast Tools Discussed Below See:
http://www.cpc.ncep.noaa.gov/products/predictions/90day/tools/briefing

CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS

Oceanic and atmospheric observations across the equatorial Pacific indicate
ENSO-neutral conditions persisted into July. Observed sea-surface temperatures
(SSTs) for the previous 30 days showed positive anomalies west of 170W and
negative anomalies anomalies from about 150 degrees W longitude eastward to
near the coast of South America. 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 rapidly since earlier this spring, though
they have since recovered to a value that is now slightly negative. 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.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The CPC SST Consolidation for the Nino 3.4 region indicates an SST anomaly
declining to near -1.0C (well within La Nina territory) during the autumn,
being significantly skewed by the colder outlying CFS solution. In contrast,
the other inputs to the CPC SST Consolidation (the canonical correlation
analysis (CCA), constructed analog (CA), and Markov statistical models) remain
well within the Neutral tercile (predicted SST anomaly range between -0.5C and
+0.5C). The CPC/IRI consensus forecast favors ENSO-neutral conditions through
the Northern Hemisphere summer, with a possible La Nina developing this autumn
(50%-55% chance). There are comparable odds (50% chance) of a La Nina this
winter.

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) and the Calibration, Bridging,
and Merging (CBaM) version of the NMME. Current soil moisture conditions played
a role in the ASO temperature outlook, primarily across the southern Rockies
and adjacent High Plains region. The seasonal consolidation tool, which
includes dynamical model input from the NMME and various statistical tools such
as regressions of observed temperature and precipitation on the CPC Nino 3.4
SST consolidation, was used. The likelihood of either a neutral or weak cold
event this autumn played a role in both temperature and precipitation outlooks.
During the 2021 warm season decadal trends were the primary source of climate
signals.

PROGNOSTIC DISCUSSION OF OUTLOOKS - ASO 2020 TO ASO 2021

TEMPERATURE

Above normal temperatures are likely across the entire CONUS and Alaska during
ASO with the largest probabilities (60% or greater) over western Alaska, the
southwestern CONUS including California, the Northeast, and southern Florida,
based on excellent agreement among dynamical and statistical forecast tools,
including a strong decadal signal. The primary change made from the previous
outlook (valid for ASO 2020) includes a reduction of probabilities for above
normal temperatures over the northwestern quarter of the CONUS, due in part to
the expected development of a weak La Nina. A secondary change made from the
previous outlook involves a slight reduction in probabilities for above normal
temperatures from the Northern Great Plains southeastward across much of the
Middle Mississippi Valley, as supported by many of the dynamical and
statistical tools. Soil moisture was also a consideration for the updated ASO
temperature outlook, especially over the Southwest and Southern High Plains
region. With the delay in the onset of the Southwest summer monsoon favoring
increased chances of above normal temperatures, pre-existing drought across the
region is expected to worsen, in part due to reduced evapotranspiration and its
impact on local moisture recycling. In SON and OND, the continuing trend in
delayed sea ice formation near Alaska's North Slope supports odds of at least
70% for above normal temperatures in that area. Across the northwestern CONUS,
probabilities for above normal temperatures continue to gradually decline from
previous respective outlooks, as supported by various tools and aligned with
the expected development of a weak La Nina this autumn. The bridging component
of the CBaM tool (which uses statistical relationships between NMME Nino 3.4
forecasts and U.S. surface temperature and precipitation) also picks up on a
cooler solution across the Northwest. In the Northeast, strong trends favoring
above normal temperatures in ASO and SON begin to weaken in OND. By NDJ and DJF
2020, the weak tilt towards above normal temperatures over eastern and southern
portions of Alaska, and the north-central CONUS, give way to Equal Chances
(EC), in part due to increased temperature variability at this time of year,
and increased odds of La Nina. The outlooks from JFM to MAM 2021 are based on a
general consensus of statistical and dynamical models and tools, and to some
degree on La Nina composites, even though the odds for a La Nina this winter
stand at 50%. During these three seasons (JFM to MAM 2021), there are increased
chances for below normal temperatures at various times across northern border
states between the Upper Great Lakes region and the coast of Washington state.
Probabilities for above normal temperatures are greatest (at least 50%) over
the southern CONUS. For the remaining seasons, AMJ through ASO 2021, the
temperature outlooks are based primarily on decadal timescale trends .

PRECIPITATION

During ASO, above normal precipitation amounts are favored across southern and
much of western Alaska, and the Northern Plains and adjacent portions of the
Upper Mississippi Valley, due to a general consensus of dynamical and
statistical guidance, and expected influences from a possible weak La Nina.
Above normal precipitation amounts are also indicated over the southeastern
portion of the CONUS, about as far north as Delaware and as far west as western
Louisiana. This is supported by the CFS and the Final CON tool (which uses the
CCA, CA, ENSO-OCN tool, and dynamical NMME guidance). This broad signal for
abnormal wetness is also consistent with long-track tropical cyclones that
often develop over the deep tropics of the eastern Atlantic during August and
September and eventually go on to influence the Southeast CONUS. Below normal
precipitation amounts are favored from eastern Nevada and southern Idaho
southeastward into the south-central Plains. This is based largely on the Stat
CON tool (which uses the CCA, CA, and ENSO-OCN tools, but does not use
dynamical NMME guidance) with some contribution from longer-term trends . In SON
and OND, the continuing trend in delayed sea ice formation in the vicinity of
the Beaufort Sea, resulting in a prolonged period of open water and available
moisture, increases the odds for above normal precipitation across the North
Slope region. Enhanced odds for wetter than usual conditions across southern
Alaska and portions of the northwestern and north-central CONUS are consistent
with the idea of a developing La Nina. The wet signal across the Southeast
weakens rapidly from ASO to SON. The area of increased odds for below normal
amounts of precipitation is predicted to expand east-southeastward with time,
from ASO to OND. For NDJ and DJF 2020, a consensus of dynamical and statistical
guidance indicates a broadening area of above normal precipitation across the
northern CONUS, with elevated odds of below normal precipitation across most of
the southern tier of states. This pattern is reminiscent of a typical cold
season La Nina, given somewhat more information now than was available last
month. This northern wetness/southern dryness pattern is expected to persist
during JFM and FMA, with the dry signal gradually disappearing over the
Southeast. At the longer leads, MAM through ASO 2021, the tilt in the odds
towards above normal precipitation across parts of the central and eastern
CONUS is consistent with decadal trends . The area of favored below normal
precipitation for parts of the Southwest during MAM 2021 is consistent with
decadal trends . Elsewhere, Equal Chances (EC) of below, near, and above normal
seasonal accumulated precipitation amounts are favored.

FORECASTER: Anthony Artusa

The Climatic normals are based on conditions between 1981 and 2010, following
the World Meterological Organization convention of using the most recent 3
complete decades as the climatic 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.

For a description of of the standard forecast tools - their skill- and the
forecast format please see our web page at
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools.html
(Use Lower Cas e Letters)
Information on the formation of skill of the CAS forecasts may be found at:
http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Outlook/outlook.shtm
l
(use lowercase letters)

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