047
FXUS06 KWBC 191917
PMDMRD
Prognostic Discussion for 6 to 10 and 8 to 14 day outlooks
NWS Climate Prediction Center College Park, MD
300 PM EDT Thu September 19 2024

6-10 DAY OUTLOOK FOR SEP 25 - 29 2024

A strong anomalous ridge stretching from northeastern Canada southward and
southwestward into much of the central and eastern Contiguous United States
(CONUS) is expected to remain approximately in place through the 6 to 10 day
period while weakening. The strength of this ridge will have peaked just prior
to this period, but even so, 500-hPa height anomalies are forecast to average
almost 24 dm over the mouth of the Davis strait. This is slightly weaker and
farther east than yesterday, indicating that this feature will be in the
process of deamplification when the forecast period starts. To the east and
south, a mid-level trough is expected to be in place over the east-central
CONUS, with an axis stretching from approximately the Great Lakes southward
through the Lower Mississippi Valley and central Gulf Coast region. Models have
trended stronger with this feature over the past several days, and that trend
continued today. The Canadian ensemble mean is strongest with this feature,
nearly closing off a 500-hPa circulation center over northern Missouri. The
other models show lesser negative height anomalies centered slightly farther
east, and do not indicate the potential for a closed mid-level low to form. The
lower amplitude solution is preferred, as depicted by most of the guidance.
Its more consistent with yesterdays models, whereas the Canadian ensemble
mean represents a rather dramatic shift. However, with the models trending
progressively stronger, the Canadian ensemble mean solution cant be discounted.

Farther southeast, models depict an area of near- or below-normal 500-hPa
heights over or near the southeastern CONUS. Individual members of the
ensembles show the potential for tropical or subtropical cyclone development.
Confidence is increasing in the development of a tropical system later this
period or during week-2, but confidence in the placement and timing of any
system that might develop is low. Also, with the aforementioned trough over the
interior eastern CONUS looking stronger, the possibility of some interaction
between the two features increases uncertainty further. Meanwhile, farther
west, the models are in decent agreement showing a mid-level ridge from the
western CONUS into the Canadian Prairies, with the largest height anomalies
near the Canadian border, a little stronger than yesterday. Elsewhere, there is
a strong mid-level trough with its axis through western Alaska at the start of
the forecast period. A significant shortwave moving through the longwave trough
is expected to rotate northeastward and weaken while the longwave trough
reintensifies near western Alaska, increasing the odds that more than one storm
system will affect southeastern Alaska. But the models are very inconsistent
regarding the location, strength, and timing of any storms that form after days
6 to 8. Overall, these changes result in a somewhat less amplified pattern
across North America by day 10, with the strong mid-level ridge centered in
northeastern Canada deamplifying more quickly than shown yesterday. Near normal
500-hPa heights are forecast for Hawaii, with a weak cyclonic circulation west
of the state slowly dissipating through the period.

Despite uncertainties in the evolution of unsettled weather in the Southeast,
the tools derived from the ensemble means are in fair to good agreement, but
with some notable exceptions. In addition to differences in the model
solutions, some additional uncertainty comes from inconsistencies between the
raw and bias-corrected temperature and precipitation output on one hand, and
the calibrated and reforecast guidance on the other. There is general agreement
that above-normal temperatures will prevail across most of the CONUS, with the
best chances for above-normal temperatures stretching from the Great Basin into
the northern Plains under the mid-level ridge axis. Meanwhile, over the South
Atlantic region and adjacent areas, the surface high pressure over the
northeastern CONUS is not as pronounced as yesterday due to the stronger trough
west of the Appalachians. This results in less of a cooler east to
northeasterly surface wind, which was expected to keep temperatures close to
normal in concert with more unsettled weather than usual. As a result,
above-normal temperatures are now slightly favored across this region. The
reforecasts are again warmer here than the raw and bias-corrected tools, but
less weight has been given to the cooler solutions given the expected surface
wind configuration. But more clouds and precipitation than usual over the
Southeast is still expected to keep temperatures closer to normal than in most
other parts of the CONUS. Across Alaska, considerable uncertainty in the
temperature pattern remains, but in Southeast Alaska, the big differences
between the colder raw and bias-corrected tools and the much warmer reforecast
tools is less pronounced today. Todays somewhat cooler reforecast tools give
more confidence to a colder solution in eastern Alaska, so slightly enhanced
odds for subnormal temperatures are extended eastward to cover most of the
state, with the best odds for below-normal temperature over western Mainland
Alaska near the longwave trough axis. Meanwhile, warmer than normal weather is
favored over the northwestern half of Hawaii, consistent with the consolidation
forecast and ensemble mean output.

In the southeastern CONUS, there is a lot of uncertainty regarding the
formation and evolution of surface low pressure systems, including the
increasing likelihood of one or more tropical or subtropical cyclones. However,
most potential scenarios would result in heavy rain affecting at least part of
the Southeast. Chances for abnormally wet weather exceed 50 percent in the
South Atlantic region westward to near the central Gulf Coast, but the high
degree of uncertainty, especially regarding any tropical development, cant be
overemphasized. The likelihood of unusually heavy precipitation also exceeds 50
percent in southeastern Alaska, where multiple impactful storm systems are
expected during the period. Elsewhere, with the stronger mid-level trough west
of the Appalachians, unusually moist air should be steered farther north than
expected yesterday, enhancing chances for surplus precipitation northward up
the Atlantic Seaboard into lower New England. Farther west, the mid-level ridge
over the western to north-central CONUS should inhibit precipitation
development, resulting in a large area with increased chances for subnormal
precipitation extending as far south as the southern Rockies. The western Gulf
Coast is an area of uncertainty due to some indications that unsettled weather,
including potential tropical systems, could be pulled farther west than
expected at this time. Meanwhile, weak cyclonic mid-level flow increases the
odds for surplus precipitation over western Hawaii while the consolidated
forecast along with most deterministic models lean toward drier than normal
conditions in the Big Island.

The official 6-10 day 500-hPa height blend consists of 10% of Today`s 0z GFS
Ensemble Mean centered on Day 8, 10% of Today`s 6z GFS Ensemble Mean centered
on Day 8, 10% of Today`s GFS Superensemble Mean centered on Day 8, 45% of
Today`s 0z European Ensemble Mean centered on Day 8, and 25% of Today`s 0z
Canadian Ensemble Mean centered on Day 8


FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: Below average, 2 out of 5, with
fair agreement on the mid-level pattern offset by increased uncertainty later
in the period as the mid-level features deamplify, along with the high degree
of uncertainty regarding tropical development in or near the Southeast and
storm systems affecting Alaska later in the period. Also, there are areas where
raw and derived temperature and precipitation tools are in poor agreement.

8-14 DAY OUTLOOK FOR SEP 27 - OCT 03, 2024

Deamplification of the mid-level pattern is expected during week-2, with mostly
a zonal pattern covering the North Pacific Ocean and North America by the end
of the period. This expectation is a little tentative today, as there are some
models showing an amplified 500-hPa pattern toward the end of the period, but
there is little agreement on the strength and location of individual features.
Given broad inconsistencies, more zonal mid-level flow is favored, though with
a little more uncertainty than yesterday given the increased disagreement
between model solutions. Presently it looks like the moderately amplified
pattern in place at the start of week-2 should evolve into broadly near- or
above-normal 500-hPa heights, with weak mid-level ridging over northern and
western North America while a very weak mid-level trough settles into the
eastern CONUS. The mid-level trough over or near Alaska is expected to linger a
little longer than yesterday, consistent with the ensemble means, but this
doesnt create significant changes to the forecast mean temperatures and
precipitation there. However, most features across North America and adjacent
areas are expected to be subtle at this time, with resulting 500-hPa heights
much closer to normal than at the start of the 6- to 10-day period. Still, the
favorable set-up for storm development will continue at least through the
beginning of week-2 in the Southeast, including possible tropical or
subtropical development. One or more significant storms are also expected to
affect south-central and southeastern Alaska, but the evolution of individual
systems is highly uncertain.

This pattern should keep above-normal temperatures over most of the CONUS,
especially across the west-central to north-central CONUS under a weak but
persistent mid-level ridge. Meanwhile, slightly enhanced odds for warmer than
normal conditions continue over westernmost Hawaii. The only areas with
increased chances for subnormal temperatures is the Alaskan Mainland,
especially in western parts of the state where subnormal 500-hPa heights will
persist the longest.

Enhanced chances for wet weather continue in southeastern Alaska downstream
from a variable mid-level trough. This favors more storminess than normal
across that region, but the evolution of individual storm systems is highly
uncertain at this time. Similarly, tools maintain unsettled weather over the
southeastern CONUS, including the potential of tropical cyclone development,
but the day-to-day evolution of individual features is highly uncertain.
Enhanced chances for above-normal rainfall still exceed 50 percent in the South
Atlantic region. Precipitation guidance derived from the deterministic models
bring increased chances for surplus precipitation northward, so the area with
enhanced chances for wet weather extends northward into the Northeast and lower
New England. Guidance remains inconsistent in the central Gulf Coast region,
but odds lean toward near- or above-normal precipitation in the official
forecast because there are some indicators raising the possibility that at
least some of the storminess expected in the Southeast could extend farther
west. Meanwhile, drier than normal weather is favored across a large area
covering most of the central and western CONUS under weakening mid-level
ridging. Slightly increased odds for above normal rainfall remain in place over
westernmost Hawaii, but deterministic model output shows more rainfall than
yesterday in eastern parts of the state, thus the enhanced odds for dryness
have been removed.

The official 8-14 day 500-hPa height blend consists of: 30% of Today`s GFS
Superensemble Mean centered on Day 11, 40% of Today`s 0z European Ensemble Mean
centered on Day 11, and 30% of Today`s 0z Canadian Ensemble Mean centered on
Day 11


FORECAST CONFIDENCE FOR THE 8-14 DAY PERIOD: Below average, 2 out of 5, due the
inherent uncertainties involved in forecasting a mid-level pattern change, the
potential for a more amplified solution with high uncertainty in the nature and
location of individual features, some areas of disagreement among the
temperature and precipitation tools, significant uncertainty on the evolution
of the wet pattern in the Southeast and perhaps farther west along the Gulf
Coast, and uncertainties in the daily evolution of the wet pattern expected in
southeastern Alaska.

FORECASTER: Rich Tinker

Notes:


Automated forecasts are issued on Saturday and Sunday. Occasionally manual
intervention is necessary to address quality control and consistency issues. In
these cases, forecasts are manually drawn but a full discussion is not issued.


The notation for the categorical forecast indicated on the maps is the same as
that in the tables: A-above   N-near normal   B-below


The temperature map shows regions with > 33% chance of being warmer (orange,
"A"), colder (blue, "B"), or close to (unshaded, "N"). Historical average
values for the calendar period of the forecast (dashes, "f").  Labels on the
shaded lines give the probability (> 33%) of the more likely category (B or A).
 Probability of N is always < 40%.


The precipitation map shows regions with > 33% chance of being wetter (green,
"A"), drier (tan, "B"), or close to (unshaded, "n"). Historical median values
for the calendar period of the forecast (dashes, "inches").  Labels on the
shaded lines give the probability (> 33%) of the more likely category (B or A).
 Probability of N is always < 40%.


In the southwest and other climatologically dry regions - there will be a
greater than 33.3% chance of no precipitation and occasionally even a normal
(i.e. Median) value of zero - especially during the dry seasons.  In such cases
a forecast of near normal is effectively a forecast of little or no
precipitation.


The climate prediction center uses 1991-2020 base period means as reference in
the climate outlooks.

The next set of long-lead monthly and seasonal outlooks will be released on
October 17.


Analogs to the 5 day mean observed pattern centered 3 days ago (D-3)
for the region from 20N to 70N latitude and 175E to 60W longitude
include the 5 day periods centered on the following dates:
20050905 - 20040920 - 20070923 - 19880919 - 19831001


Analogs to the 7 day mean observed pattern centered 4 days ago (D-4)
for the region from 20N to 70N latitude and 175E to 60W longitude
include the 7 day periods centered on the following dates:
20040919 - 20070922 - 20050905 - 19830930 - 19850916


6-10 DAY OUTLOOK TABLE
Outlook for Sep 25 - 29 2024

STATE      TEMP PCPN   STATE      TEMP PCPN   STATE      TEMP PCPN
WASHINGTON  A    B     OREGON      A    B     NRN CALIF   A    B
SRN CALIF   A    N     IDAHO       A    B     NEVADA      A    B
W MONTANA   A    B     E MONTANA   A    B     WYOMING     A    B
UTAH        A    B     ARIZONA     A    B     COLORADO    A    B
NEW MEXICO  A    B     N DAKOTA    A    B     S DAKOTA    A    B
NEBRASKA    A    B     KANSAS      A    N     OKLAHOMA    A    N
N TEXAS     A    N     S TEXAS     A    N     W TEXAS     A    N
MINNESOTA   A    B     IOWA        A    B     MISSOURI    A    N
ARKANSAS    A    A     LOUISIANA   A    A     WISCONSIN   A    B
ILLINOIS    A    N     MISSISSIPPI N    A     MICHIGAN    A    N
INDIANA     A    N     OHIO        A    A     KENTUCKY    A    A
TENNESSEE   A    A     ALABAMA     A    A     NEW YORK    A    A
VERMONT     A    A     NEW HAMP    A    A     MAINE       A    A
MASS        A    A     CONN        A    A     RHODE IS    A    A
PENN        A    A     NEW JERSEY  A    A     W VIRGINIA  A    A
MARYLAND    A    A     DELAWARE    A    A     VIRGINIA    A    A
N CAROLINA  A    A     S CAROLINA  A    A     GEORGIA     A    A
FL PNHDL    A    A     FL PENIN    A    A     AK N SLOPE  B    A
AK ALEUTIAN B    N     AK WESTERN  B    N     AK INT BSN  B    A
AK S INT    B    A     AK SO COAST B    A     AK PNHDL    B    A



8-14 DAY OUTLOOK TABLE
Outlook for Sep 27 - Oct 03, 2024

STATE      TEMP PCPN   STATE      TEMP PCPN   STATE      TEMP PCPN
WASHINGTON  A    B     OREGON      A    B     NRN CALIF   A    B
SRN CALIF   A    N     IDAHO       A    B     NEVADA      A    B
W MONTANA   A    B     E MONTANA   A    B     WYOMING     A    B
UTAH        A    B     ARIZONA     A    B     COLORADO    A    B
NEW MEXICO  A    B     N DAKOTA    A    B     S DAKOTA    A    B
NEBRASKA    A    B     KANSAS      A    B     OKLAHOMA    A    B
N TEXAS     A    B     S TEXAS     A    B     W TEXAS     A    B
MINNESOTA   A    B     IOWA        A    B     MISSOURI    A    B
ARKANSAS    A    N     LOUISIANA   A    A     WISCONSIN   A    B
ILLINOIS    A    B     MISSISSIPPI N    A     MICHIGAN    A    B
INDIANA     A    N     OHIO        A    N     KENTUCKY    A    A
TENNESSEE   N    A     ALABAMA     N    A     NEW YORK    A    N
VERMONT     A    N     NEW HAMP    A    N     MAINE       A    N
MASS        A    N     CONN        A    N     RHODE IS    A    N
PENN        A    N     NEW JERSEY  A    N     W VIRGINIA  A    A
MARYLAND    A    A     DELAWARE    A    A     VIRGINIA    A    A
N CAROLINA  N    A     S CAROLINA  N    A     GEORGIA     N    A
FL PNHDL    A    A     FL PENIN    A    A     AK N SLOPE  N    A
AK ALEUTIAN B    N     AK WESTERN  B    N     AK INT BSN  B    A
AK S INT    B    A     AK SO COAST B    A     AK PNHDL    B    A

                           LEGEND
TEMPS WITH RESPECT TO NORMAL     PCPN WITH RESPECT TO MEDIAN
A - ABOVE   N  - NEAR NORMAL     A - ABOVE   N - NEAR MEDIAN
B - BELOW                        B - BELOW

THE FORECAST CLASSES REPRESENT AVERAGES FOR EACH STATE. NORMAL
VALUES - WHICH MAY VARY WIDELY ACROSS SOME STATES - ARE
AVAILABLE FROM YOUR LOCAL WEATHER SERVICE FORECAST OFFICE.

FOR ADDITIONAL INFORMATION SEE MESSAGE FXUS06 KWBC - ON AWIPS AS
PMDMRD.

$$