680
FXUS07 KWBC 301900
PMD30D
Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Mon Sep 30 2024


30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2024

The updated October 2024 temperature and precipitation outlook is adjusted by
the availability of medium- and extended-range forecast model guidance within
the validation period (i.e. first 2 weeks of October) and the most recent
available subseasonal forecast model guidance through the month of October.



The temperature outlook for monthly mean temperatures has become considerably
more clear as compared to the mid-month outlook. The previous outlook had an
above-average amount of "Equal-Chances" (EC) forecast coverage than is typical
for monthly outlooks (forecast rationale outlined below in the previous
discussion). Forecast ridging and positive 500-hPa height anomalies are
consistently forecast during the short-, medium- and extended-range time scales
for much of the western and northern CONUS. Above-normal temperatures remain
favored for the Southwest, central Rockies, Northeast, mid-Atlantic and the
Florida Peninsula as in the mid-month outlook, but odds have increased and
forecast coverage more widespread to include the Pacific Northwest, northern
Rockies, northern Plains and Midwest. Lower 500-hPa heights, frontal activity,
enhanced cloudiness and precipitation are more likely over this period in the
Southeast U.S. so EC remains forecast in this region.



Although the MJO has evolved generally as forecast in mid-September till now,
the response to the mid-latitudes (as defined through MJO composites) is not
consistent with the overwhelming medium- and extended-range model guidance
noted above. This is not overly surprising for the time of year as noted in the
mid-month discussion.



The forecast for Alaska remains largely unchanged with enhanced odds for
above-normal temperatures for northeast Alaska and enhanced odds for
below-normal temperatures (forecast coverage reduced) for southern Alaska.



For precipitation, generally only minor forecast changes are made, and the
outlook continues to be a quite dry one overall. The favored ridge-trough
pattern across the CONUS noted above supports a large area of favored
below-normal monthly total precipitation amounts stretching from the Far West
eastward across the Plains to the Ohio Valley and southward to Texas.
Probabilities are greatest for areas of the south-central High Plains and Great
Plains. The outlook is supported by short-, medium-, extended- and
subseasonal-range model guidance.



Favored above-normal precipitation remains in the new forecast from the
previous mid-September outlook for the extreme Southeast including Florida,
although for a smaller area, and for southwest Alaska. The forecast in Alaska
also now includes an enhanced likelihood of above-normal precipitation for the
south coast of Alaska and the Alaska Panhandle as well as favored below-normal
precipitation for northern Alaska from the Brooks Range to the North Slope.



An additional change in the updated outlook is the removal of the favored
wetter than normal conditions originally forecast for the Northeast CONUS -
where model guidance now opposes positive (wet) October precipitation trends.



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***********   Previous discussion from the middle of September     ***********

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The October temperature and precipitation outlooks are characterized by low
forecast probabilities and outlook coverage - not atypical for an October
monthly outlook. Ongoing climate conditions in mid-September indicate a
continued slow trend toward potential La Nina conditions later this autumn, a
better organizing MJO as well as anomalous coastal sea surface temperatures
(SSTs) - primarily for waters in proximity to Alaska. Although not impactful in
a predictability context this late in the year, soil moisture continues to
decrease in many areas across the CONUS, with the ongoing impactful drought in
parts of the Ohio Valley and western mid-Atlantic continuing to worsen.



Preparation of the October temperature and precipitation outlooks primarily
utilized subseasonal dynamical and statistical model guidance, potential
teleconnection associated with an active MJO, long term trends as well as in
some places coastal SSTs. Any potential impacts from slowly developing La Nina
conditions are not considered in the October outlook to any large degree.



The temperature outlook favors above-normal temperatures for the southwestern
CONUS, the central and southern Rockies, and southern Texas, consistent with
the majority of subseasonal dynamical model guidance and most of the NMME and
C3S participant models. Strong warmer-than-normal temperature trends and
forecast model guidance elevates chances for above-normal monthly mean
temperatures for the Northeast, mid-Atlantic and parts of the eastern Great
Lakes.



There is a large area of "Equal Chances" or EC forecast from the Pacific
Northwest eastward to include the northern and central Plains, Mississippi
Valley and Southeast. The above average coverage of EC in the outlook in these
areas is associated with highly varying dynamical model guidance on the
subseasonal time scale. This includes the latest ECMWF extended range forecast
which depicts weak signals in this region over the course of the month of
October.



Although associated impacts from anomalous tropical convection are less likely
in autumn then during winter and early spring, that potential still exists and
is at odds with much of the dynamical model guidance. Uncertainty in any
mid-latitude teleconnection from the MJO further complicates the forecast in
this area as MJO lagged composites of 200-hPa height and 850-hPa temperature
for MJO RMM phases 6 and 7 support troughing and favored below-normal
temperatures for parts of the western and central CONUS during the first half
of October. A Multiple Linear Regression (MLR) forecast tool utilizing the MJO,
ENSO and long-term trends targeting the first two weeks of October indicates
elevated odds for below-normal temperatures in parts of the north-central
CONUS.



For Alaska, anomalous SSTs and some forecast guidance supports favored
above-normal (below-normal) temperatures for the northern Alaska (Southwest
Alaska) portions of the state.



The October precipitation outlook favors below-normal precipitation for an area
from the Southwest U.S. and central Rockies eastward to include much of the
central Plains and parts of the central Mississippi Valley. This forecast is
primarily based on a high degree of agreement in bias-corrected and calibrated
dynamical model guidance from the NMME and C3S model suites as well as the
ECMWF extended range forecast.



Elevated odds for above-normal precipitation is forecast for the central and
eastern Gulf coast, parts of the Southeast, mid-Atlantic coast and much of the
Northeast. Long term wetter-than-normal precipitation trends (i.e., OCN) and
some subseasonal dynamical model guidance support the forecast in the Northeast
and mid-Atlantic areas. For the Southeast and Gulf coast areas, the outlook is
driven by potential tropical moisture entering this region of the CONUS
associated with the MJO entering RMM phases 8 and 1 during early October.
Dynamical model guidance from some sources also supports this forecast, mainly
for Florida and extreme Southeast coastal areas. In addition, a small area of
enhanced odds for wetter-than-normal conditions is forecast for northern
Washington state indicated in some forecast tools.



For Alaska, model guidance is generally consistent with wet conditions for
areas in western and northern Alaska, so above-normal precipitation slightly
favored in those portions of the state. Large uncertainty associated with
highly conflicting forecast tools any potential MJO impacts made EC the prudent
forecast for most of the Ohio Valley and much of the Midwest at this time.

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 17 2024

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