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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 Sat Aug 31 2019


30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2019

The updated monthly temperature and precipitation outlooks for September 2019
are based on the latest dynamical model guidance, WPC temperature and
precipitation outlooks for the first week of September, CPC 6-10 day, week 2,
and week 3-4 temperature and precipitation outlooks, and GEFS and CFS monthly
temperature and precipitation forecasts. There is no Madden-Julian Oscillation
(MJO) currently present, and no MJO is forecast to emerge during the next
several weeks. Therefore, the MJO is unlikely to impact the mid-latitude
circulation pattern during September.

Dynamical model forecasts for the month of September, including 700-hPa height
anomalies from the CFS monthly forecasts, generally predict troughing over the
Bering Sea, a ridge over western North America, and troughing over northeastern
North America. Forecasts from the ECMWF and NCEP GEFS ensembles, as well as the
multi-model consensus of the Subseasonal Experiment or SubX, indicate
persistence of this circulation pattern overall into weeks 3 and 4, with the
exception that a trough over the Northeast CONUS early in the month, as
indicated by the CPC 6-10 day and week 2 outlooks, is predicted to lift
northeastward, as in the CPC week 3-4 outlook.

The updated September temperature outlook continues to indicate high
probabilities of above normal temperatures over Alaska and the western CONUS,
with a persistent ridge and above-normal 500-hPa heights predicted throughout
the month. Probabilities exceed 70 percent for coastal areas of northern and
western Alaska, where sea surface temperatures remain anomalously much above
average. Probabilities exceed 60 percent for areas of the Pacific Northwest and
Central Rockies, under persistent above normal 500-hPa heights throughout the
month of September in dynamical model forecasts. Above normal monthly average
temperatures are likely for the month of September across much of Texas,
excluding the Texas Panhandle, and the Southeast region, following the CPC 6-10
day, week 2, and week 3-4 outlooks. Probabilities of above normal temperatures
are lower over Texas and Louisiana, following guidance for the month of
September from a tool that merges GEFS and CFS forecasts for weeks 1 and 2 with
CFS forecasts for the week 3-4 period, and where the GEFS ensemble forecast
indicates a greater probability of below normal temperatures in the week 2 and
week 3-4 periods, as does the SubX MME consensus forecast for the week 3-4
period. Below normal average September temperatures are predicted to be most
likely for the northern central CONUS from the Northern and Central Plains into
the Great Lakes region, with troughing and greater probabilities of below
normal temperatures predicted during the first two weeks of the month, and weak
probabilities for above normal temperatures indicated in the CPC week 3-4
temperature outlook. Likely below normal temperatures over the Northeast in the
WPC forecasts for the first week of September and in the CPC 6-10 day and week
2 outlooks, with probabilities exceeding 50 percent, give way to likely above
normal temperatures in the CPC week 3-4 outlook, with probabilities exceeding
60 percent. Equal chances of above, near and below normal temperatures is
indicated in the September temperature outlook for most of the Northeast
region.

Predicted troughing over the Bering Sea and Aleutian Islands in dynamical model
forecasts for much of the month leads to continued enhanced probabilities of
above normal precipitation for western and southern Alaska including the Alaska
Panhandle. Above normal accumulated precipitation is likely for a large area of
the western CONUS from the Southwest region into the Central and Northern
Plains, as indicated by WPC precipitation forecasts for week 1 and the CPC 6-10
day and week 2 precipitation outlooks, where moisture surges from the Eastern
Pacific are expected to enhance precipitation over the Southwest. Hurricane
Dorian is expected to impact Florida and parts of the Southeast Atlantic Coast
during the first week of September bringing very large precipitation amounts to
areas along the eventual storm track. Uncertainty in the exact storm track and
uncertainty in precipitation amounts during the remainder of the month leads to
moderate probabilities for above normal precipitation for the September monthly
outlook. Probabilities of above normal precipitation in the September monthly
outlook exceed 60 percent for the Atlantic Coasts of northern Florida, Georgia
and the Carolinas. Likely above normal precipitation is indicated over the
Northeast region in the September outlook, with precipitation predicted in the
7-day WPC forecast for week 1 and likely above normal precipitation predicted
in the CPC 6-10 day and week 2 outlooks, along with a potential for Hurricane
Dorian to follow a northeastward track off of the Atlantic coast enhancing
precipitation over the Northeast. Although the CPC week 3-4 outlook indicates
likely below normal precipitation for the second half of the month for much of
the Atlantic coast and Northeast, there is greater certainty in the
precipitation outlook for the first two weeks of September.

----------- Previous message (from August 15) is shown below ------------

The September 2019 temperature and precipitation outlooks are based on
dynamical model guidance for the month including the North American Multi-Model
Ensemble (NMME), as well as Week 4 CFS and ECMWF dynamical model forecasts,
statistical tools based on large-scale sea surface temperature patterns and
other climate forcings , as well as the influence of current soil moisture
conditions and coastal sea surface temperatures on regional temperature and
precipitation, and potential influences from modes of tropical climate
variability. ENSO-neutral conditions are currently present over the Tropical
Pacific Ocean. No discernible Madden-Julian Oscillation (MJO) signal is
currently indicated by MJO indices, and the future emergence of an MJO signal
is highly uncertain. ENSO and MJO are not likely to influence the longwave
pattern across the mid-latitudes in the next several weeks.

Dynamical model forecasts from the CFS and ECMWF indicate a slowly evolving
longwave mid-level circulation pattern from the end of August into the first
week of September, with positive 500-hPa height anomalies predicted over Alaska
and the eastern North Pacific into the Western CONUS. This circulation pattern
is predicted to persist and influence the September average temperature and
accumulated precipitation. Negative 500-hPa height anomalies and troughing are
predicted over the Northern Central CONUS during September, and dynamical model
forecasts are consistent in predicting positive 500-hPa height anomalies across
the Southern Plains, over the Southeast, and up the Eastern Seaboard into the
Northeast region.

The September 2019 temperature outlook indicates enhanced probabilities for
above normal temperatures across Alaska and the Western CONUS from the Pacific
Coast to the Rocky Mountains, under a predicted ridge and consistent with large
positive decadal temperature trends for Alaska and the Southwest region.
Probabilities for above normal temperatures exceed 70 percent for coastal
regions of Alaska, influenced by significantly above normal sea surface
temperatures around the region. Enhanced probabilities for above normal
temperatures are predicted across the Southwest region into Texas, as well as
for the Southeast, from the Appalachian Mountains eastward to the Atlantic
Coast, and across the Northeast including the northern Great Lakes region.
Probabilities for above normal temperatures exceed 60 percent for the southern
Florida Peninsula, influenced by regional above average sea surface
temperatures, and exceed 50 percent for parts of northern New England where
decadal climate trends are a significant component of seasonal climate
variability. Enhanced probabilities of below normal temperatures are indicated
for parts of the Northern Plains, due to northerly flow under a predicted
trough early in the month when the circulation is more predictable, and
supported by the negative feedback between temperature and a forecast of likely
above normal precipitation. Equal Chances of above and below normal
temperatures or EC is indicated for a large area of the Northern Central CONUS,
where cooler air flowing southward from Canada may moderate average September
temperatures.

The September 2019 precipitation outlook indicates above normal precipitation
is likely for coastal regions of Alaska including the Alaska Panhandle, due to
a predicted trough over the western Aleutian Islands and enhanced by increased
atmospheric moisture related to above average coastal sea surface temperatures.
Enhanced probabilities for above normal precipitation are indicated for a
region of the Northern Central CONUS, as storm systems are likely to develop
under a mid-level trough. There is a slight increase in the probabilities of
above normal precipitation extending into parts of Colorado and the Central
Plains, where the Southwest Monsoon circulation may draw moisture northward
from the East Pacific. Elsewhere throughout the CONUS, Equal Chances of above
and below normal precipitation or EC is indicated for the September outlook
with limited skill in a half-month lead precipitation forecast during the
summer. The September 2019 precipitation outlook will be reassessed at the end
of August when uncertainty in the dominant climate signals is reduced at
shorter lead times.

FORECASTER: Dan Collins

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.

The next monthly outlook...for Oct ... will be issued on Thu Sep 19 2019

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

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