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Official 90-day Outlooks are issued once each month near mid-month at 8:30am Eastern Time. Please consult the schedule of 30 & 90-day outlooks for exact release dates.

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HOME> Outlook Maps>Seasonal Forecast Discussion
 
Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD
300 PM EDT Tue Aug 31 2021


30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2021

During the latter half of August, equatorial Sea Surface Temperatures (SSTs)
have remained near to below average across most of the tropical Pacific Ocean.
The latest weekly SST departure in the Niño 3.4 region was -0.3C. This weekly
value of the Niño 3.4 index is little changed from the most recent three month
observations of the Niño 3.4 index (corresponding to the May-June-July season)
which stood at -0.4C. Equatorial Pacific SSTs are generally forecast to remain
near to below normal with La Niña conditions becoming more likely as we
progress through the upcoming fall. Meanwhile, a weak Madden-Julian Oscillation
(MJO) signal has stalled over the Indian Ocean since Mid-August. The
combination of the most recent evolutions of the El Niño Southern Oscillation
(ENSO) and the MJO would tend to favor above normal 500-hPa heights across the
western and northeastern CONUS and mean troughing near the Mississippi Valley
during the month of September. Additionally, the current ENSO/MJO evolution
would favor below normal 500-hPa heights over the Gulf of Alaska. Uncertainty
in the evolution of the MJO during the month of September is high, contributing
to the potential of a highly variable and incoherent pattern across much of the
forecast domain for the upcoming month. WPC short term and CPC week-2 and weeks
3-4 outlooks, dynamical model guidance, trends , and analysis of antecedent
boundary layer conditions were used to complement the background ENSO and MJO
signals where applicable.

Uncertainty in the temperature outlook is high, as MJO composites indicate a
weak signal and dynamical model guidance forecasts a highly variable pattern
across much of the CONUS as the month of September progresses. Therefore, equal
chances of above, below, and near normal temperatures are indicated for a large
portion of the CONUS from the Rockies to the East Coast. Above normal
temperatures are favored west of the Continental Divide due to support from MJO
composites and dynamical model guidance. A very slight tilt toward above normal
temperatures is indicated for the Northeast based primarily on MJO composites
and CFSv2 guidance. However, the magnitude and coverage of this signal were
reduced relative to the mid-month outlook as shorter term signals indicate the
potential for a relatively cool pattern early in September. Warmer than normal
conditions are slightly favored for the Florida Peninsula, consistent with
recent trends . Conversely, there is a slight tilt toward below normal
temperatures for parts of the Tennessee and Lower Mississippi Valley,
consistent with dynamical model guidance and mean troughing over the
east-central CONUS suggested by ENSO/MJO composites. Statistical and dynamical
model guidance generally support a mild pattern across southern Alaska.
Therefore, enhanced probabilities of above normal temperatures were expanded
across southern Alaska relative to the mid-month outlook. The greatest
probabilities of above normal temperatures are indicated over parts of
southwestern Alaska, due, in part, to above normal SSTs in adjacent waters.

Above normal precipitation is favored from the western Gulf Coast Region
northeastward to parts of the East Coast, consistent with mean troughing over
the east-central CONUS indicated by ENSO/MJO composites. Copious rainfall is
expected early in the month for much of the Mid-Atlantic states associated with
the remnants of Ida, greatly increasing the probabilities of above normal
precipitation for the month of September as a whole. Above normal precipitation
is also favored for parts of the Southwest as dynamical model guidance supports
influxes of monsoonal moisture early in the month. A drier than normal pattern
is favored for the northwestern CONUS consistent with shorter term WPC
forecasts and monthly CFSv2 guidance. ENSO/MJO composites as well as a storm
system predicted early in the month support a slight tilt toward above normal
precipitation for southeastern Alaska.

*******************************************************************************
****** Previous mid-month discussion below ******
*******************************************************************************

The September 2021 outlooks are issued against the backdrop of El Niño Southern
Oscillation (ENSO) Neutral conditions and an ongoing strong Madden-Julian
Oscillation (MJO) event. The most recent three month observations of the Niño
3.4 index (corresponding to the May-June-July season) indicated mean sea
surface temperature (SST) anomalies of -0.4C.  This represents a weakening
relative to the La Niña event experienced during the past winter when the
average index value during the December-January-February season was near -1.0C.
Equatorial SSTs are generally expected to remain near to below average with a
re-emergence of La Niña conditions possible within the next three months and a
La Niña Watch is currently in effect. Additionally, an ongoing strong MJO event
is currently propagating eastward over the Indian Ocean with the amplitude of
the Wheeler-Hendon index exceeding 2.5. The current evolution of these ENSO and
MJO signals would tend to favor below normal 500-hPa heights across much of the
eastern Contiguous United States (CONUS) and south of Alaska. Signals from the
MJO and ENSO conflict across western North America as MJO composites favor much
higher 500-hPa heights relative to that produced by ENSO composites. It should
be noted that the sample size of these composites are relatively small, thus,
reducing the reliability of the signal. Analysis of the most recent monthly and
subseasonal dynamical model guidance, recent trends , as well as contributions
from current environmental conditions were also utilized in the production of
these outlooks.

Above normal temperatures are favored for much of the western CONUS and
Northern Plains due to good agreement among dynamical model guidance and MJO
composites. Enhanced probabilities of above normal temperatures are largely
absent farther to the south and east across the remainder of the Plains, most
of the Mississippi Valley, and the Southeast, consistent with potential
troughing over the eastern CONUS suggested by MJO and ENSO composites. Below
normal temperatures are favored for the western and central Gulf Coast Region,
the Lower Mississippi Valley, and eastern sections of the Southern Plains,
supported by dynamical model guidance from the CFSv2 and Copernicus Climate
Change Service (C3S). Agreement is generally good among dynamical model
guidance for warmer than normal conditions across the Great Lakes and
Northeast. Thus, above normal temperatures are favored for these regions.
However, probabilities are tempered due to potential troughing across the
eastern CONUS suggested by MJO and ENSO composites. Uncertainty is high across
most of Alaska due to weak or conflicting signals among dynamical model
guidance. Therefore Equal Chances (EC) of above, near, or below normal
temperatures are indicated for most of the state. However, above normal
temperatures are weakly favored for parts of coastal southwestern Alaska due,
in part, to above normal SSTs in adjacent waters.

The potential for troughing may promote an active pattern across much of the
central and eastern CONUS. This is supported by MJO and ENSO composites which
favor above normal precipitation from parts of the Gulf Coast northeastward to
the Great Lakes and Mid-Atlantic states and the CFSv2, which depicts a wet
pattern across most of the southern two-thirds of the CONUS. However,
probabilities are tempered due to lack of support from the NMME and C3S
dynamical model tools. Conversely, below normal precipitation is favored across
much of the West as dynamical model guidance generally supports a dry pattern,
particularly from the Great Basin to the Northern Rockies and Northern High
Plains. A wet pattern is favored farther to the north across southeastern
Alaska due to good dynamical model support and agreement with MJO composites.
ENSO composites, as well as dynamical model guidance from the NMME and CFSv2
favor a dry pattern for parts of western Alaska, where elevated chances of
below normal precipitation are indicated.

FORECASTER: Scott Handel

The climatic normals are based on conditions between 1991 and 2020, 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 16 2021

These outlooks are based on departures from the 1991-2020 base period.
$$

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