Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Sep 17 2020


30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2020

The October 2020 monthly temperature and precipitation outlooks are primarily 
based on consistent dynamical model forecast guidance within the context of 
ongoing and likely strengthening La Nina conditions as we progress into the 
autumn months. In addition, some areas of the forecast domain utilized current 
land surface states and nearby ocean surface temperatures as input.

The MJO is also considered, but played a lesser role in the outlook as the 
recent strong, coherent MJO activity has weakened some and is being disturbed 
by higher frequency coherent, subseasonal tropical variability (atmospheric 
Kelvin and equatorial Rossby wave activity) and evolving under strengthening La 
Nina conditions. Consequently, it is unclear how coherent and robust the MJO 
enhanced convective phase will emerge from the Maritime continent to the 
Pacific Ocean during the second half of September. Also, statistically 
significant impacts to higher latitudes from the MJO are often marginal during 
October. 

The October 2020 temperature outlook favors above-normal monthly mean 
temperatures for nearly all of the forecast domain except for areas in 
southeast Alaska and the Pacific Northwest where Equal Chances (EC) is 
forecast. Dynamical model forecast guidance and typical temperatures observed 
during October under La Nina conditions support this outlook. Long term 
positive temperature trends enhance odds for above-normal temperatures for 
parts of the western CONUS as does dry surface conditions. Strong negative 
trends in sea ice coverage and thickness and so warmer than normal ocean 
surface temperatures in waters surrounding Alaska also favors higher odds for 
above-normal temperatures for parts of western and northern Alaska. 

La Nina conditions tend to favor near to below-normal temperatures in parts of 
the Pacific Northwest, southeast Alaska and the Alaska Panhandle, but with 
positive long term trends in some of these areas counteracting this generally 
weak La Nina signal, EC is forecast for these areas. Long term positive 
temperature trends, dynamical model forecast guidance, and to a lesser extent 
near coast ocean surface temperatures and dry surface conditions support 
slightly higher odds for above-normal temperatures for the Northeast and 
mid-Atlantic.

For precipitation, consistent dynamical model forecast guidance and typical La 
Nina conditions favors a region of below-normal precipitation stretching from 
the Southwest and south-central Rockies eastward to include the central and 
southern Plains and Mississippi Valley to parts of the Tennessee and Ohio River 
Valley's. Potential tropical systems developing in the Gulf of Mexico and 
northwestern Caribbean and their often northeast migration supports slightly 
elevated odds for above-normal rainfall for parts of the extreme Southeast and 
the state of Florida. 

La Nina and consistent dynamical model forecast guidance favors above-normal 
monthly total precipitation amounts for parts of the Pacific Northwest and 
Alaska Panhandle. La Nina conditions typically support above-normal 
(below-normal) precipitation for the west coast of Alaska (south-central 
Alaska) respectively. 

FORECASTER: Jon Gottschalck

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 Oct will be issued on Wed September 30 2020

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