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Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Mon Sep 30 2019
30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2019
The updated October 2019 temperature and precipitation outlook builds upon some
of the factors outlined in the mid-September discussion below and is modified
by the latest suite of dynamical model guidance from the short-range,
medium-range, extended-range and complete monthly forecast integrations.
The start of October is anticipated to see an amplified pattern across the
CONUS with a strong trough-ridge pattern oriented across the CONUS. Over the
first 7 days or so, this pattern is favored to shift eastward, become less
amplified and then make a transition to a more zonally oriented flow over the
CONUS. During the second week of October, a warmer pattern is favored to
develop for the eastern two-thirds and Southwest CONUS as a result of this more
zonal oriented flow and increasing coverage of positive 500-hPa height
departures along the southern tier and eastern CONUS. Residual troughing is
favored to remain across the Pacific Northwest.
A number of factors point to elevated odds for above-normal temperatures for
most of the CONUS for the second half of October. First, long term trends are
positive in nearly all areas of the forecast domain, ongoing MJO activity
favors above-normal temperatures for most of the central and eastern CONUS and
Southeast Canada and Week 3-4 dynamical model guidance from the CFS, ECMWF and
JMA favor above-normal temperatures for much of the second half of October.
Also, short-term or regime based bias corrections (i.e., based on past 45 days)
show that some model forecasts for below-normal temperatures across the CONUS
have been overestimated to some degree.
Forecast of favored below-normal temperatures for October are focused over some
areas of the Pacific Northwest to the northern High Plains. These depictions
only show modest increases in odds as compared to climatology and are related
to early in the month cold air intrusion during the first week of October that
may potentially keep mean October temperature anomalies below the lower
one-third of the historical range.
High odds for above-normal temperatures are depicted for the Southeast, lower
Appalachians and lower mid-Atlantic - a result of nearly all information
primarily pointing to above-normal temperatures and also where flash drought
conditions are ongoing and favored to continue to deteriorate during the first
10 days of October.
The updated October precipitation outlook is able to increase forecast coverage
from very low areal numbers released in the mid-September initial outlook. The
general themes, to first order, are maintained with the forecast coverage for
above-normal precipitation depicted in the initial outlook along the northern
tier of the CONUS increased with modest odds depicted for the Pacific Northwest
and Northeast and greater probabilities depicted over the northern Plains,
upper Mississippi Valley and western Great Lakes. This area of favored
above-normal precipitation is extended southward in the updated outlook as a
plume of moisture from the sub Tropics is likely to spread northward and
eastward around anticipated ridging located over the Southeast and then
interact with troughing and the general westerlies along the Canadian border.
Above-normal precipitation remains most likely for areas of the Southwest
associated with potential influence from eastern Pacific tropical disturbances
or cyclones that may increase moisture at times during the month. These odds
are on modestly increased from climatology.
Forecast guidance for the first week in October indicates very little
precipitation for much of the Southeast from Texas to the lower Atlantic
seaboard. Week 3-4 forecast tools, long term trends and MJO impacts do not
indicate with any consistency or reliability, robust signatures for
above-normal precipitation during the second half of October. Based on this
combined information below-normal precipitation is depicted for this region.
Elevated odds for above-normal precipitation remain forecast for parts of the
Florida Peninsula, however. There is high uncertainty for Alaska so the initial
outlook was modified to now favor enhanced odds for above-normal precipitation
for the west coast of Alaska.
** Previous discussion from mid-month outlook made in mid-September is below **
The initial half month lead October monthly outlook always represents a
considerable challenge in short term climate prediction. A two week lead
monthly outlook during a transition season period, such as October, integrated
over only a monthly period, historically is one in which predictability is low.
Reviewing the latest climate conditions and status of major climate modes,
there are a few items to note, however. During the summer, we entered
ENSO-neutral conditions across the Pacific Basin so that the ENSO phase did not
play any role in the October outlook. However, there is coherent subseasonal
tropical variability (MJO / atmospheric Kelvin wave activity) ongoing and is
considered in preparing the outlook. Although impacts from this subseasonal
tropical variability at higher latitudes this time of the year are tenuous and
unreliable, this forcing can play a role in October conditions for portions of
the southern CONUS - mainly related to potential enhancement of tropical
cyclone activity in the East Pacific and Atlantic basins.
It is also noted that soil moisture anomalies are substantial in some areas.
This includes large soil moisture surpluses in the northern Plains and upper
Midwest as above-normal rainfall continued through the summer months. Also, the
CONUS devoid of much drought for most of the year, has shown an increase in
drought conditions for parts of the Southeast, Texas and Southwest. The soil
moisture anomalies, although noteworthy, played a generally minor role when
preparing the October outlook.
The October temperature outlook favors above-normal monthly mean temperatures
for the entire forecast domain. The largest probabilities are depicted for the
western and northern areas of Alaska due to continued well above-normal ocean
surface temperatures, feedback due to earlier loss and lower coverage of nearby
sea ice extent, as well as dynamical model guidance from a number of sources.
Odds for above-normal temperatures are modestly elevated for the southwest
corner of the CONUS due to strong positive temperature trends during this time
of year, consistent model guidance and to a lesser extent generally drier than
normal conditions in some areas. The increase in probabilities for above-normal
temperatures from climatological values across the central portion of the
country are small. In these areas, there was considerable uncertainty as
available forecast tools and dynamical model guidance were in conflict and in
locations of historically low skill so that forecast confidence is low. A
potential cooler start to October in the Pacific Northwest and northern Rockies
along with surpluses in soil moisture in the northern Plains and Upper Midwest
tempered above-normal probabilities in these areas as well.
There is high uncertainty, low forecast confidence and so small forecast
coverage in the October precipitation outlook. Monthly total precipitation
amounts are forecast to be above-normal for small regions of the forecast
domain. This includes the Alaska Panhandle and areas of the Pacific Northwest,
Southwest, northern Plains and central and southern eastern seaboard. Model
guidance indications of northward shifted westerlies support forecasts of
above-normal precipitation for the highlighted areas along the northern tier of
the CONUS. Long term trends and the potential for an active period of tropical
cyclone activity in the Atlantic, consistent with MJO/subseasonal variability
noted earlier, tilts the odds for above-normal precipitation for the eastern
seaboard from the mid-Atlantic southward. Similar prospects for continued
enhanced tropical cyclone activity in the eastern Pacific basin increases the
potential for Gulf surges and tropical moisture to enter the far Southwest.
Some consistency in dynamical model guidance was the primary basis for the
favored below-normal rainfall highlighted area in the southern Plains.
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 17 2019
These outlooks are based on departures from the 1981-2010 base period.