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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
830 AM EDT Thu Aug 15 2019


30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2019

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.

An updated monthly outlook... for Sep will be issued on Sat August 31 2019

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

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