Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Wed Sep 30 2020


30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2020

The updated October 2020 temperature and precipitation outlooks are adjusted 
from the mid-October outlook by utilizing short-, medium- and extended-range 
dynamical model guidance in concert with ongoing antecedent conditions, 
background La Nina and MJO conditions and the most recent subseasonal and 
monthly integrated dynamical model guidance. 

The updated temperature outlook has been adjusted to increase the coverage and 
considerably increase the probabilities for above-normal temperatures for much 
of the western half of the CONUS and parts of Alaska. Favored above-normal 
monthly mean temperatures remain forecast for New England, the northern Great 
Lakes and the Florida Peninsula. Forecast support in these areas are consistent 
and strong signals from dynamical model guidance across multiple time scales 
and to a lesser extent surface states such as close proximity to above-normal 
ocean / lake surface temperatures and dry surface moisture states. Model 
guidance supports considerable ridging to remain in place across western North 
America with substantial positive 500-hPa height departures. 

The main difference in the updated outlook for temperature is where troughing 
downstream of western CONUS ridging is to be quite strong for approximately the 
first week or so of October supporting the passage of strong cold fronts across 
the eastern and southern CONUS supplying ample below-normal temperatures 
primarily for the lower Ohio Valley, Tennessee Valley and parts of the 
Southeast and Gulf Coast. The dilemma for the forecast in these areas is that 
will this cool signal (below-normal temperatures) remain by the end of the 
month, when statistical and dynamical model guidance supports more mild, 
moderate temperatures for much of the CONUS during the remaining portion of 
October as a more zonal, positively anomalous 500-hPa height pattern evolves. 
The updated outlook does depict that below-normal October monthly mean 
temperatures are most likely for some of the above mentioned areas in the 
Southeast corner of the CONUS.    

For precipitation, there are not too many changes from the mid-October outlook. 
The main difference is a substantial increase in coverage and probabilities for 
below-normal precipitation across the interior three-quarters of the CONUS. The 
significant precipitation expected at the very end of the month of September is 
currently forecast to shift off the eastern seaboard by the start of the 
forecast period. Even so, below-normal precipitation is forecast are more 
modest odds closer to the eastern seaboard and for New England as some 
troughing in the mean remains into early October in that area. The favored 
large-scale pattern in the mean during October is forecast to be a ridge-trough 
pattern stretching from the western to eastern CONUS which is dry pattern for 
much of CONUS as surface high pressure is likely to prevail more often than 
not. 

Another change based on short- and medium-range forecast guidance is an 
increase in odds for above-normal precipitation for the Florida Peninsula as a 
stationary front and interaction with tropical moisture favors potentially high 
precipitation totals during the first several days of October.

Above-normal precipitation remains favored for the Pacific Northwest and areas 
in Alaska based on model guidance as well as it consistency with background La 
Nina conditions. The only change in Alaska is the removal of favored 
below-normal precipitation for the south-central coast of Alaska where lack of 
strong signals from dynamical model guidance across time scales tempers this 
typical La Nina response.    

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********** Previous Forecast discussion from mid-October is below ************
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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.

The next monthly outlook...for Nov ... will be issued on Thu Oct 15 2020

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