146
FXUS06 KWBC 042051
PMDMRD
Prognostic Discussion for 6 to 10 and 8 to 14 day outlooks
NWS Climate Prediction Center College Park, MD
300 PM EST Wed December 04 2024

6-10 DAY OUTLOOK FOR DEC 10 - 14 2024

Most model guidance from this morning indicates that a change in the 500-hPa
pattern will start to evolve during this period. Todays guidance is in
somewhat better agreement on the mean pattern than for the past few days, and
there are some individual features that are handled similarly. But run-to-run
consistency is still poor, and there are differences in some of the details
that would significantly affect the sensible weather, and there are some
non-trivial differences between yesterdays manual 500-hPa blend and todays.
In addition, teleconnections are highly inconsistent. There are two primary
features that all guidance reflects to some extent: a ridge with positive
500-hPa height anomalies in the central North Pacific, and another in the
central North Atlantic. The dynamical models, including the ensemble means,
show these features prominently, and the anomaly centers are within 5 deg.
latitude/longitude of each other. The only exception is the Canadian ensemble
(CMCE) mean, which contains these features, but somewhat farther south in the
North Atlantic, and somewhat farther east in the North Pacific. Assuming the
CMCE mean is an outlier and using consensus locations from the other dynamical
models, the resulting teleconnections are not merely inconsistent, but nearly
180 degrees out of phase. This supports the idea that the regime is unstable
and transitional, and along with the other factors, makes this a low confidence
forecast. Due to some agreement in the mean pattern, however, it is slightly
more confident than over the past two days.

Relative to yesterday, the 6-10 manual 500-hPa blend shows some significant
differences. The mid-level ridge in the North Atlantic is slightly sharper, and
the maximum positive anomalies are about 10 degrees farther west than
yesterday. Meanwhile, a broad and weak mid-level trough is actually a little
farther east (and therefore sharper), with its axis along the Mississippi
Valley. To the west, the ridge in the central North Pacific is a little weaker
and flatter, and is centered almost 20 degrees farther to the west-southwest.
However, positive height anomalies from this relatively flat ridge stretch
farther eastward than yesterday (into the High Plains, as opposed to the
Intermountain West). Finally, a significant difference that has major
implications for observed conditions is near the western Canada coastline.
Yesterday, the North Pacific mid-level ridge was centered just offshore, where
height anomalies exceeded +150 dm. With this feature now considerably farther
west, model consensus places a weak trough there, with lesser 500-hPa height
anomalies than surrounding areas (under +30 dm). This change has a significant
impact on the forecast across Alaska, and to a lesser extent the central and
northern West Coast.

Despite the better pattern agreement, the raw dynamical model temperature and
precipitation output, and the derived tools imposing reforecast and
bias-correction adjustments, remain at odds with each other in some areas.
Because of this, the run-to-run inconsistency, and conflicting teleconnection
guidance, the enhanced chances favoring either extreme of temperature or
precipitation do not reach as high as usual, reflecting the uncertainty. But
there are a few locales with commonality and some consistency over the past few
days, and the best chances for significant deviations from normal weather are
in these areas. Above-normal heights near the East Coast favor warmer than
normal conditions, especially in New England, which has greater 500-hPa
positive height anomalies. Due to the mid-level trough axis shifting slightly
east from yesterday, enhanced chances for warmth are restricted relatively
close to the coast. Any remaining snowpack from the recent storms near the
Great Lakes and Appalachians would work against this warming trend, so the
highest chances were reduced slightly from the consolidation and most of the
model output tools. Another area of relative consistency is the northern and
central High Plains. Much of the surface pressure guidance places a mean high
pressure center over the northern Rockies, which could produce strong
downsloping winds to the east of the higher elevations, favoring above-normal
temperatures. Elsewhere, warmth is somewhat favored in the Far West under
positive 500-hPa height anomalies. In contrast, a brief cold snap is likely in
the wake of the mean mid-level trough late in the period and early week-2,
keeping 5-day mean temperatures near or slightly below normal in the southern
Plains and part of the Lower Mississippi Valley.

Meanwhile, with the mean mid-level trough a little farther east in the eastern
CONUS, a potential storm track along or near the East Coast is also shifted
eastward slightly, limiting enhanced chances for surplus precipitation to areas
from the Appalachians eastward. Farther west, the relatively low mid-level
height anomalies off the southwest Canada coast reflect a weak trough.
Downstream from the trough axis under cyclonic mid-level flow, increased
chances for wet weather cover the Pacific Northwest and northern Intermountain
West. A drier regime is more likely over the central and southwestern CONUS due
to generally above-normal heights and its location downstream from a broad flat
ridge and upstream from a mean mid-level trough, where subsidence is favored.
And this is one of their drier times of the year climatologically.

In broad southwesterly mean mid-level flow, warm and wet weather is favored
over Alaska, especially across the southern Mainland and southeastern areas.
However, the expected area of relatively low mid-level heights southwest of
Canada will weaken this set-up, reducing the chances for wet and warm weather
relative to yesterday. Warm and wet weather is also favored for Hawaii, with
the best odds for warmth in the northwestern islands, and better chances for
surplus precipitation along the northeastern tier of the island chain, with
lower odds to the west.

The official 6-10 day 500-hPa height blend consists of 45% of Today`s 6z GFS
Ensemble Mean centered on Day 8, 25% of Today`s 0z European Ensemble Mean
centered on Day 8, 20% of Today`s operational 0z ECMWF centered on Day 8, and
10% of Today`s 0z Canadian Ensemble Mean centered on Day 8


FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: Below normal, 2 out of 5, with
better mean pattern agreement among the models damped by conflicting
teleconnections, inconsistent temperature and precipitation tools, and
run-to-run model inconsistency.

8-14 DAY OUTLOOK FOR DEC 12 - 18 2024

Models are in fair agreement regarding the mean week-2 mid-level pattern, but
still, contradicting teleconnections, impactful differences in the details of
the 500-hPa height field in the guidance, run-to-run inconsistency, uncertainty
in the raw and statistically-adjusted temperature and precipitation tools from
the ensembles, and the apparent transitory nature of the pattern lead to a
forecast with a little higher confidence than yesterday, but lower than for
most forecasts. Strong mid-level ridging persists in the central North Pacific
and North Atlantic. Relative to the 6-10 day mean, the area of maximum height
anomalies is somewhat farther south in the North Atlantic while some weakening
and retrogression occurred in the North Pacific. Above-normal but declining
500-hPa heights stretch northward from the central North Pacific across most of
Alaska. The area of relatively low 500-hPa heights off the southwest Canadian
coast evolves into a more typical and positively-tilted mid-level trough
reaching from off the west-central North American coast northeastward through
Hudson Bay. To the south, positive 500-hPa height anomalies continue to expand
westward from the central North Atlantic ridge, covering most of the CONUS by
the end of the period.

With above-normal 500-hPa heights across the vast majority of the CONUS and
Alaska, most of these locations have at least slightly enhanced chances for
above-normal temperatures. The best chances for anomalous warmth are across the
north-central CONUS (consistent with most of the temperature tools derived from
the ensembles), the northeastern CONUS (closer to the ridge axis in the North
Atlantic), and southern Alaska (under broad but weakening southwesterly flow).
Lesser chances are expected over the southern and western CONUS, and the
northern tier of Alaska. Hawaii is also favored to experience warmer than
normal conditions with a flat 500-hPa ridge just north of the island chain. The
remnants of a brief intrusion of cold air will get scoured out of the southern
Plains and Lower Mississippi Valley early week-2, with temperatures expected to
rise quickly over the next couple of days to above normal. However, the
residual cold air early in the period lowers the odds for a warm week-2 in the
mean. Conditions are expected to change little in Hawaii, with above-normal
temperatures anticipated, consistent with the consolidation but tempered
slightly by raw output from the ensemble means, which are not quite as bullish
with the increased temperatures.

Given the transitional regime and the flattening of the 500-hPa flow, the
forecast favors neither abnormally wet nor dry conditions for week-2 as a
whole. The mid-level trough in the East has dissipated, reducing the odds for
an active storm track. Unusually wet weather is most likely in the Pacific
Northwest just upstream from the developing mid-level trough, and across the
Great Lakes and some adjacent areas, as depicted by many of the precipitation
tools derived from the ensembles. Subnormal precipitation is favored in the
Southwest, but the eastward extent of subnormal precipitation is inconsistent,
with some tools restricting it to the Southwest and others favoring dryness as
far east as the lower Ohio Valley. Given all the uncertainty, increased odds
for drier than normal conditions is restricted to areas of agreement among the
tools, with neither unusually wet nor dry weather expected farther east. The
trough to the southeast of Alaska should start breaking down the persistent
warm and wet pattern across Alaska, reducing the odds of both for the week-2
period. Farthest removed from the center of the mid-level ridge, a warm week-2
is no longer favored across northern Alaska. Hawaii remains in a regime
generally favorable for precipitation, but the wet weather odds are higher
along the  northeastern tier of the island chain because of the islands
orography.

The official 8-14 day 500-hPa height blend consists of: 35% of Today`s 6z GFS
Ensemble Mean centered on Day 11, 40% of Today`s 0z European Ensemble Mean
centered on Day 11, and 25% of Today`s 0z Canadian Ensemble Mean centered on
Day 11


FORECAST CONFIDENCE FOR THE 8-14 DAY PERIOD: Below normal, 2 out of 5, with
better agreement in the mean pattern offset by inconsistent teleconnections,
areas of disagreement in the temperature and precipitation tools, and the
uncertainty inherent to a transitory pattern with relatively small mid-level
height anomalies across most of the country.

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
December 19.


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:
20021130 - 19581212 - 19551128 - 19561125 - 20041213


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:
20021128 - 19581212 - 19561125 - 19601217 - 20041214


6-10 DAY OUTLOOK TABLE
Outlook for Dec 10 - 14 2024

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



8-14 DAY OUTLOOK TABLE
Outlook for Dec 12 - 18 2024

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

                           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.

$$