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Tools Used (see Discussion for explanation)
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Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Sep 16 2021
30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2021
The October 2021 outlook is prepared with a backdrop of most likely developing
La Nina conditions in the equatorial Pacific Ocean both in the ocean and
atmosphere. The MJO, although not as coherent and well defined as during the
months of July and August remains a factor in the outlook as does antecedent
soil moisture conditions and coastal sea surface temperature anomalies.
Statistical, hybrid and dynamical model based forecast guidance contribute
significantly to the October outlook.
For temperature, the outlook depicts favored above-normal monthly mean October
temperatures for much of the contiguous U.S. with the region stretching from
the Southwest eastward to cover the Great Plains and extending across the Ohio
Valley, Great Lakes, mid-Atlantic and Northeast. The greatest odds are
generally in the center of this highlighted region for a variety of factors
such as antecedent surface moisture departures from normal, prospects for
potential developing La Nina impacts as well as dynamical model forecast
information. Although the eventual evolution of the MJO and so its impacts on
the U.S. are uncertain given its current state, the MJO would favor
above-normal temperatures for much of the central and eastern contiguous U.S.
during the first two weeks of October.
Favored unsettled conditions and associated enhanced cloudiness and rainfall
increase uncertainty near the Gulf Coast and so Equal-Chances (EC) is depicted
in this region as well as in the Pacific Northwest where conflicting signals in
climate forecast tools and information are present.
Favored troughing to the north of Alaska and anomalous north to south
meridional flow supports elevated odds for below-normal temperatures for
portions of interior and southeast Alaska and the Alaska Panhandle. This is
supported by much of the available dynamical model guidance as well as
potential impacts from developing La Nina conditions over the next six weeks.
Above-normal sea surface temperatures in nearby coastal waters for the
Aleutians and southwest Alaska as well as some dynamical model guidance support
modest odds for above-normal temperatures for southwest Alaska and the
The October precipitation outlook is influenced by potential background La Nina
and MJO impacts in some areas as well as both statistical forecast tools and
dynamical model guidance. Elevated odds for below-normal October total
precipitation amounts is depicted for a region from California eastward to
include the Southwest, the central Rockies and the central and southern Great
Plains -- based on potential La Nina impacts and support from much of the
dynamical model guidance.
The most consistent signal in the forecast tools is for elevated probabilities
for above-normal precipitation for the Pacific Northwest as depicted by model
guidance and consistent with La Nina impacts for this time of the year. This
signal extends northward to include the Alaska Panhandle. Anticipated anomalous
north to south meridional flow supports and is the basis for a small, generally
weak tilt in the odds for below-normal precipitation for the south-central
portion of the state.
Across the eastern CONUS, a region of elevated odds for above-normal
precipitation is depicted from the Gulf coast, eastern seaboard and for parts
of the Great Lakes and Northeast. The southern areas of this highlighted area
are primarily based on potential tropical cyclone related impacts linked to the
climatological secondary peak in tropical cyclone activity in October for areas
in the northwest Caribbean Sea and Gulf of Mexico. In addition, a potential
factor later in the month could be the MJO influence of above-normal
precipitation in this region linked to MJO RMM phase 8 through 2. The forecast
in this area is at odds for much of the dynamical model guidance where near- to
below-normal monthly precipitation amounts are forecast, thus there is
considerable uncertainty in the forecast in this area.
The forecast for above-normal precipitation further north in the mid-Atlantic,
much of the Great Lakes and Northeast is primarily related to long term
positive precipitation trends and supplemented by signals from some of the
dynamical model forecast output.
FORECASTER: Jon Gottschalck
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.
An updated monthly outlook... for Oct will be issued on Thu September 30 2021
These outlooks are based on departures from the 1991-2020 base period.