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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