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Tools Used (see Discussion for explanation)
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Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EDT Thu May 21 2020
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS
The June-July-August (JJA) 2020 temperature outlook favors above-normal
temperatures across the West, Gulf Coast States, and along the East Coast. The
largest probabilities (above 70 percent) of above normal temperatures are
centered over the Four Corners region. Equal chances of below-, near-, or
above-normal temperatures are forecast from the upper to middle Mississippi
Valley along with parts of the northern to central Great Plains. The JJA
precipitation outlook features increased chances of above normal precipitation
for much of the eastern and central U.S., while below-normal precipitation is
more likely across the Pacific Northwest, northern Great Basin, and much of the
Rockies. Above-normal temperatures are favored throughout Alaska during JJA
with increased chances of above-normal precipitation forecast for the Aleutians
and most of mainland Alaska.
During the late summer and early fall season, increased chances of above normal
temperatures expand to include the entirety of the continental U.S. (CONUS)
with the largest probabilities remaining over the Southwest. Meanwhile, chances
of above-normal precipitation remain elevated across the Midwest, Southeast,
and mid-Atlantic through August-September-October. By December-January-February
2020-2021, equal chances were introduced to the northern to central Great
Plains and Midwest. Increased chances of above normal precipitation shift from
the Southeast to the northern areas of the CONUS during
October-November-December, while chances of below normal precipitation expand
across the southern tier of the CONUS.
Equal chances (EC) are forecast in areas where the likelihood of seasonal mean
temperatures or seasonal accumulated precipitation amounts are expected to be
similar to climatological probabilities.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
Note: For Graphical Displays of the Forecast Tools Discussed Below See:
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
Oceanic and atmospheric observations across the equatorial Pacific indicate
ENSO-neutral conditions persisted into May. The observed weekly SSTs, centered
on May 13, feature average or slightly negative anomalies from 160-100 degrees
W with positive anomalies west of the Date Line. Positive subsurface
temperature anomalies (averaged across 180-100 degrees W) continued to decrease
with an eastward expansion of below-average subsurface temperatures.
Upper-ocean heat anomalies have declined rapidly since earlier this spring.
From April 17 to May 16, easterly low-level wind anomalies were present east of
the Date Line with westerly upper-level anomalies along and south of the
equator. Outgoing longwave radiation (OLR) anomalies, during this same time
period, featured suppressed convection across the equatorial central and
PROGNOSTIC DISCUSSION OF SST FORECASTS
The CPC SST Consolidation for the Nino 3.4 region indicates a steady decline
into negative anomalies to -0.5 degrees C by JAS with negative anomalies
peaking during the fall season. The SST constructed analog and Markov model
indicate a persistence of ENSO-neutral conditions through the summer and fall.
The CPC/IRI consensus forecast favors a continuation of ENSO-neutral conditions
(with about a 65 percent chance) during the Northern Hemisphere summer with
chances decreasing through the fall (to 45-50 percent).
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 JJA temperature outlook, primarily across the Great Plains and
Four Corners region. The consolidation tool, which includes NMME input and
various statistical tools, was used especially at later leads. The likelihood
that either ENSO-neutral or La Nina prevails later this summer through the fall
season played a role in both temperature and precipitation outlooks. During the
2021 warm season decadal trends were relied upon.
PROGNOSTIC DISCUSSION OF OUTLOOKS - JJA 2020 TO JJA 2021
Above-normal temperatures are favored for much of the CONUS and Alaska during
JJA with the largest probabilities forecast across the West and Northeast,
based on excellent agreement among tools including a strong decadal signal.
Probabilities of above normal temperatures were increased across the Four
Corners region from the previous outlook due to a notable trend in the latest
model guidance, consistent with an increasing signal for below normal
precipitation, and ongoing low soil moisture conditions. Conversely, high soil
moisture was one of the factors for equal chances of below-, near-, or
above-normal temperatures forecast across parts of the central U.S. this
summer. Beginning in JAS, coverage with increased chances of above normal
temperatures expanded as the effect of anomalous soil moisture wanes and
decadal trends support a larger coverage of above normal temperatures. From ASO
2020 through NDJ 2020-2021, changes from the previous outlook were to slightly
increase (decrease) probabilities of above normal temperatures across the
Southwest (Pacific Northwest to northern Rockies). These minor changes were
based on the latest consensus among dynamical and statistical tools. An
expected lack of sea ice elevates chances for above normal temperatures across
northern Alaska during the fall season. The temperature outlook at the longer
lead times is based largely on trends .
The precipitation outlooks are based primarily on the latest dynamical model
guidance, through OND, along with long-term trends . Compared to the previous
outlook for JJA, a slight tilt in the odds of below normal precipitation was
expanded south to include the Four Corners region. Consistent with the NMME and
consolidation tool, probabilities above 40 percent for above normal
precipitation were shifted south to the Gulf Coast States and the Southeast. A
large coverage of modest probabilities for above or below normal precipitation
are forecast across the CONUS due to inherent uncertainty in convective
rainfall during the summer. Elevated probabilities of above normal
precipitation were slightly increased across mainland Alaska, in part due to
the June outlook. Elevated chances of above normal precipitation across the
Southeast and mid-Atlantic were maintained longer through the fall, compared to
previous outlooks. The most notable trend in the latest dynamical model
guidance, during the late fall and next winter was an expansion of a dry signal
throughout the southern tier. Although no changes were made to the
precipitation outlook beyond DJF 2020-2021, the future evolution of ENSO
conditions will have to be closely monitored in subsequent outlooks.
At the longer leads, MJJ through JAS 2021, the tilt in the odds towards above
normal precipitation across parts of the central and eastern U.S. is consistent
with long-term trends . Also, the favored area for below-median precipitation
across parts of the Southwest during the spring 2021 is related to trends .
FORECASTER: Brad Pugh
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
(Use Lower Cas e Letters)
Information on the formation of skill of the CAS forecasts may be found at:
(use lowercase letters)
Notes - These climate outlooks are intended for use prior to the start of their
valid period. Within any given valid period observations and short and medium
range forecasts should be consulted.
This set of outlooks will be superseded by the issuance of the new set next
month on Jun 18 2020
1981-2010 base period means were implemented effective with the May 19, 2011