Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Thu Sep 30 2021


30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2021

The update to the October monthly temperature and precipitation outlook 
released in mid-September primarily considered and utilized the latest ensemble 
model guidance at multiple time scales. Modifications to the precipitation 
outlook are more substantial than that for temperature in the update.

The MJO is less coherent now than observed in late August and early September. 
Anomalous tropical convection appears to be taking on a more interannual, 
somewhat more stationary flavor in recent weeks with enhanced convection 
primarily remaining across the eastern Indian Ocean and Maritime continent 
region with suppressed convection preferred over the western Hemisphere. This 
suggests the equatorial Pacific atmospheric and oceanic state is slowly 
transitioning to La Nina conditions. The MJO did not play a substantial role in 
the outlook. Current anomalous soil moisture conditions also did not play a 
substantial role due to the time of the year.  

One factor that is likely to have some impact in the eventual October longwave 
pattern and so the monthly mean temperature and total precipitation amounts is 
the degree to which recurving tropical cyclones currently in the western 
Pacific will be entrained and affect the mid-latitude flow over the Pacific 
Ocean and North America. Tropical cyclone activity is likely to remain enhanced 
in the western Pacific and October is a peak month for recurving typhoons into 
the mean westerly flow. These conditions consequently considerably increase 
uncertainty in the updated October outlook.

Dynamical model guidance across time scales supports elevated odds of 
above-normal monthly mean temperature for most of the CONUS with the greatest 
odds in the northern Plains, upper Mississippi Valley and Great Lakes. Warmer 
than normal temperatures are favored from medium-, extended- and 
subseasonal-range model guidance in this region. Changes from the previous 
mid-September outlook include the addition of favored above-normal temperatures 
for parts of the Southeast and Gulf coast where Equal-Chances (EC) was 
previously indicated. More troughing and cooler temperatures in the most recent 
model guidance supports the addition of a small area of favored below-normal 
temperatures in the Pacific Northwest and a reduction in forecast coverage of 
favored above-normal temperatures in the Southwest. Elevated odds for 
below-normal temperatures are increased in coverage for areas in Alaska as 
troughing in the mean remains most likely.

For precipitation, the updated October outlook remains similar to the 
mid-September release for the Pacific Northwest and Alaska. For the remainder 
of the CONUS, however, the forecast indicates considerable differences in some 
areas from that released earlier in the month. Model guidance favors troughing 
at times entering the CONUS and impacting portions of the western and central 
U.S.. This pattern and evolution should create a favorable environment in these 
regions during periods of the first half of October for elevated odds for 
above-normal monthly total precipitation amounts from eastern areas of the 
Southwest northward and eastward to include much of the High and Great Plains 
as well as much of the Mississippi Valley. Confident predictions from extended- 
and subseasonal-range ensemble model guidance for below-normal precipitation 
supports elevated odds for below-normal precipitation for the Northeast and 
parts of the mid-Atlantic. There remains, however, concerns for potential 
tropical development in the northwest Caribbean Sea and Gulf of Mexico and 
north-northeast movement in October during the second peak of the Atlantic 
hurricane season. This conflicts with some ensemble model support for a dry 
start to October for parts of the far eastern Southeast, lower Atlantic 
seaboard and the eastern Gulf coast, thus EC is forecast for the region in 
place of the favored above-normal precipitation depicted in the original 
mid-September outlook. 

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****               Previous mid-month discussion below                  *****
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The October 2021 outlook is prepared with a backdrop of most likely developing 
La Nina conditions in the equatorial Pacific Ocean both in the ocean and 
atmosphere. The MJO, although not as coherent and well defined as during the 
months of July and August remains a factor in the outlook as does antecedent 
soil moisture conditions and coastal sea surface temperature anomalies. 
Statistical, hybrid and dynamical model based forecast guidance contribute 
significantly to the October outlook. 

For temperature, the outlook depicts favored above-normal monthly mean October 
temperatures for much of the contiguous U.S. with the region stretching from 
the Southwest eastward to cover the Great Plains and extending across the Ohio 
Valley, Great Lakes, mid-Atlantic and Northeast. The greatest odds are 
generally in the center of this highlighted region for a variety of factors 
such as antecedent surface moisture departures from normal, prospects for 
potential developing La Nina impacts as well as dynamical model forecast 
information. Although the eventual evolution of the MJO and so its impacts on 
the U.S. are uncertain given its current state, the MJO would favor 
above-normal temperatures for much of the central and eastern contiguous U.S. 
during the first two weeks of October.

Favored unsettled conditions and associated enhanced cloudiness and rainfall 
increase uncertainty near the Gulf Coast and so Equal-Chances (EC) is depicted 
in this region as well as in the Pacific Northwest where conflicting signals in 
climate forecast tools and information are present. 

Favored troughing to the north of Alaska and anomalous north to south 
meridional flow supports elevated odds for below-normal temperatures for 
portions of interior and southeast Alaska and the Alaska Panhandle. This is 
supported by much of the available dynamical model guidance as well as 
potential impacts from developing La Nina conditions over the next six weeks. 
Above-normal sea surface temperatures in nearby coastal waters for the 
Aleutians and southwest Alaska as well as some dynamical model guidance support 
modest odds for above-normal temperatures for southwest Alaska and the 
Aleutians. 

The October precipitation outlook is influenced by potential background La Nina 
and MJO impacts in some areas as well as both statistical forecast tools and 
dynamical model guidance. Elevated odds for below-normal October total 
precipitation amounts is depicted for a region from California eastward to 
include the Southwest, the central Rockies and the central and southern Great 
Plains -- based on potential La Nina impacts and support from much of the 
dynamical model guidance. 

The most consistent signal in the forecast tools is for elevated probabilities 
for above-normal precipitation for the Pacific Northwest as depicted by model 
guidance and consistent with La Nina impacts for this time of the year. This 
signal extends northward to include the Alaska Panhandle. Anticipated anomalous 
north to south meridional flow supports and is the basis for a small, generally 
weak tilt in the odds for below-normal precipitation for the south-central 
portion of the state. 

Across the eastern CONUS, a region of elevated odds for above-normal 
precipitation is depicted from the Gulf coast, eastern seaboard and for parts 
of the Great Lakes and Northeast. The southern areas of this highlighted area 
are primarily based on potential tropical cyclone related impacts linked to the 
climatological secondary peak in tropical cyclone activity in October for areas 
in the northwest Caribbean Sea and Gulf of Mexico. In addition, a potential 
factor later in the month could be the MJO influence of above-normal 
precipitation in this region linked to MJO RMM phase 8 through 2. The forecast 
in this area is at odds for much of the dynamical model guidance where near- to 
below-normal monthly precipitation amounts are forecast, thus there is 
considerable uncertainty in the forecast in this area. 

The forecast for above-normal precipitation further north in the mid-Atlantic, 
much of the Great Lakes and Northeast is primarily related to long term 
positive precipitation trends and supplemented by signals from some of the 
dynamical model forecast output.       

FORECASTER: Jon Gottschalck

The climatic normals are based on conditions between 1991 and 2020, 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 21 2021

These outlooks are based on departures from the 1991-2020 base period.
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