Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Thu Nov 23 2023

Valid 12Z Sun Nov 26 2023 - 12Z Thu Nov 30 2023

...Overview...

A broadening and fairly deep upper trough will dominate the
pattern over the central and eastern U.S. through much of next
week, with some embedded shortwaves and frontal systems. One
surface low tracking north through the western Atlantic is likely
to brush the Northeast around Monday and enhance precipitation
there, behind which westerly flow and chilly conditions will
promote lake effect snow through the first half of the week. The
pattern in the West is more complex and uncertain, as energy could
linger over the Southwest while upper ridging builds in the
Northwest early in the week but then model spread increases with
when/how these features could interact with an upstream trough
coming through the eastern Pacific. This overall pattern should
lead to relatively dry and cool conditions across the lower 48.


...Guidance Evaluation/Predictability Assessment...

Model guidance is reasonably agreeable early in the forecast
period Sunday-Monday as energy digging through south-central
Canada causes troughing to expand across the east-central U.S. and
ridging comes into the Northwest. At the surface, a nor'easter
looks to track quickly north through the Atlantic with a path
trending farther west over the past day, potentially over eastern
parts of New England, which would increase precipitation potential
in the Northeast. The 12Z models maintain this track along with a
slightly slower trend in the newer 12Z GFS/ECMWF. After that the
details of the trough get more uncertain based partly on the
placement of an upper low within it in Canada by Tuesday/day 5.
GFS runs have been farthest east toward the St. Lawrence River
with the centroid of the low likely combining with other energy,
while the CMC runs are west of James Bay. Shortwaves within the
trough show a lot of variability and this affects the frontal and
surface wave positions Tuesday-Thursday, for more uncertainty than
desired.

Speaking of uncertainty, model solutions continue to vary over the
West first with energy pulled west into the Southwest U.S. and
then how upstream Pacific energy (in the form of a closed low that
opens up as it progresses east) may interact with it. ECMWF runs
have the Southwest energy tracking far enough west that it
interacts more with the Pacific feature by around Tuesday compared
to other guidance, though the 12Z CMC has started trending in this
direction. Overall the ridge that begins over the Northwest should
be gradually weakening as it pushes east with a more troughy
pattern for the West, but the specifics are yet to be determined.

The WPC forecast was based on a blend of the 00Z/06Z GFS and 00Z
ECMWF, CMC, and UKMET early in the period. Increased the
proportion of the GEFS and EC ensemble means as the period
progressed, amid increasing spread. Additionally, increased QPF in
the Great Lakes compared to the NBM given the low bias it has with
lake effect precipitation.


...Weather/Hazards Highlights...

Some moisture may pool near a frontal boundary and wave of low
pressure near the Gulf Coast on Sunday for some shower potential
over the Gulf Coast states. Meanwhile a separate low developing
off the Carolinas and lifting northward through the western
Atlantic is trending toward a track closer to or over New England
and is likely to spread some precipitation to the Northeast. Snow
is possible in the interior regions and especially in the higher
elevations. Areas closer to the coast may see a brief bout of
heavy rain, and totals have increased from the previous forecast
given the low track. However, this rain should be moving quickly
so the Days 4-5 Excessive Rainfall Outlooks remain with no risk
areas. Behind this system, cold flow will support areas of lake
effect snow that may persist through at least midweek. Generally
dry conditions are likely elsewhere, with perhaps some light
precipitation perhaps coming into the West by midweek after a dry
period and some moist return flow into the Gulf Coast around
midweek as well.

Much of the lower 48 should see near to below normal temperatures
especially during the first half of next week. Decent coverage of
highs and lows 10-15F below climatology can be expected from the
Great Basin into the central/southern Plains early in the week,
shifting into the eastern U.S. Tuesday-Wednesday while the
west-central U.S. slightly moderates. An exception to the below
average temperatures may be across the northern Plains, where by
Tuesday and beyond lows in the 20s and highs in the 40s are
expected, which are around 5-15F above normal. Lows of a few
degrees above average and highs near average in Florida early next
week are likely to cool Tuesday and beyond after a frontal
passage.


Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Fri Nov 24 2023

Valid 12Z Mon Nov 27 2023 - 12Z Fri Dec 01 2023

...Overview...

Upper troughing will generally dominate the pattern over parts of
the central and eastern U.S. through much of next week, with some
embedded shortwaves and frontal systems. A western Atlantic
surface low on Monday should quickly track through/close to New
England with some moderate to locally heavy precipitation likely
there. Behind this, favorable westerly flow and chilly conditions
sets the stage for enhanced lake effect snows through at least the
first half of the week. An initial ridge over the West to start
the week should be replaced by mean troughing as a couple of
shortwaves slide through, with a possible stronger one reaching
the Southwest by the end of the week promoting some ridge building
downstream across the Gulf Coast/Southeast. With the exception of
the Great Lakes, and then perhaps some wet weather entering the
south-central U.S. by Thursday, the weather forecast across the
CONUS looks fairly quiet for the entire medium range period.


...Guidance Evaluation/Predictability Assessment...

While the pattern on the synoptic scale showing troughing in the
eastern half of the U.S. through at least the middle of next week
is in good agreement, the details of individual shortwaves
affecting frontal and surface wave positions still have lower
confidence than desired. The first forecast challenge with this is
the timing and placement of a nor'easter affecting New England at
the start of the period Monday. Most of the models in the 00/06Z
cycle sped up the surface low a tad, putting it over eastern
Massachusetts 12Z Monday. The GFS runs, UKMET, and many GEFS
members all showed this, while the CMC and CMC ensemble were even
a bit faster, but the ECMWF suite was a bit slower, especially the
deterministic 00Z EC. Since even the ECMWF-based machine learning
models were faster than the EC, the WPC forecast favored the
faster trend, most like the middle ground GFS/GEFS solutions. The
newer 12Z ECMWF is faster than its 00Z run but still not quite as
fast as other guidance. The low should track quickly into Canada
Monday-Tuesday. After that there is increasing uncertainty in the
details within the trough pattern.

Meanwhile across the West, fortunately models have converged
somewhat on the pattern, with southwest U.S. energy that was
complicating the pattern appearing weaker now. So the timing of
the upper low tracking east from the Pacific has improved though
the GFS runs are farther south than other guidance, and then
establishes troughing across the West. By the latter part of the
week though, additional energy coming into the West shows
increasing spread.

Similar to the previous shift, used a blend of mainly
deterministic models early in the forecast period but with some
ensemble mean component to smooth out individual model
differences. Gradually increased the percentage of ensemble means
in the overall blend to half throughout the week. For the most
part, this maintained fairly good continuity with the previous WPC
forecast, though some changes in fronts were made. In terms of
QPF, or lack thereof, the only major change from the NBM was to
increase quite a bit QPF in the Great Lakes given the low bias it
typically has with lake effect precipitation.


...Weather/Hazards Highlights...

A surface low looks to track northward near or over New England on
Monday. This could spread some moderate to locally heavy
precipitation into parts of New England with a quick push of heavy
rainfall in coastal areas, but with a general lack of instability
and highly anomalous moisture combined with how fast it will be
moving, probabilities for any adverse flooding/flash flooding
impacts are less than the threshold for even a Marginal Risk on
the Day 4/Monday Excessive Rainfall Outlook. In the higher terrain
of northern New England, this precipitation should fall as snow.
Behind this system, favorable westerly cold flow over the still
relatively warm Great Lakes should promote a period of
accumulating lake effect snow especially downwind of Lakes Erie
and Ontario on Monday-Tuesday. Models are coming into better
agreement on a favorable lake effect event, but given the small
scale of the heavy snowbands, minor shifts in location of the
bands would provide considerably different conditions and
accumulations over slightly different areas. Lake effect snow will
be possible deeper into next week as well but with likely lighter
amounts. Elsewhere across the country, conditions look mostly dry
through next week though some light precipitation may accompany a
shortwave into the West around midweek. Precipitation chances
along the Gulf Coast region to perhaps as far north as the Middle
Mississippi Valley may begin to increase again by late week as a
warm front lifts northward through the Gulf of Mexico.

Much of the lower 48 should see near to below normal temperatures
through the first half of next week. Decent coverage of highs and
lows 10-15F below climatology can be expected from the Four
Corners states into the central/southern Plains and
Mississippi/Ohio Valleys on Monday, shifting into the eastern U.S.
Tuesday-Wednesday while the west-central U.S. moderates back
toward normal. However, the interior West looks to cool back down
during the latter half of the week due to troughing aloft. An
exception to the below average temperatures may be across the
northern Plains, where by Tuesday and beyond lows in the 20s and
highs in the 40s are expected, which are around 5-15F above
normal. By Thursday and especially Friday, southerly flow off the
warm Gulf of Mexico looks to ramp up and increase temperatures,
especially lows, in the south-central U.S. Lows of a few degrees
above average and highs near average in Florida on Monday are
likely to cool Tuesday-Wednesday after a frontal passage, before
slowly rebounding.


Tate/Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1225 PM EST Sat Nov 25 2023

Valid 12Z Tue Nov 28 2023 - 12Z Sat Dec 02 2023

...Overview...

An upper low over/near Hudson Bay will allow for general troughing
with some embedded shortwaves Tues-Thurs across the Great Lakes
and Northeast. This pattern would support a multi-day period of
enhanced lake effect snows, ongoing from the end of the short
range period, locally heavy at times. An initial ridge over the
West on Tuesday should be replaced by mean troughing as a closed
low drops into the Southwest with a reinforcing amplified
shortwave/additional embedded low Friday-Saturday. This should
promote ridge building downstream across the Southeast and
eventually the East Coast by next weekend. Much of next week
should be relatively quiet and dry, with the exception of the
Great Lakes and increased moisture and precipitation moving into
the south-central U.S. by late in the week.

...Guidance Evaluation/Predictability Assessment...

The operational guidance is in reasonable agreement regarding the
evolution of the synoptic pattern across the lower 48 through day
5. The ensembles capture the days 6 and 7 pattern well. The main
area of uncertainty is the descending trough from British Columbia
on day 5. A general model blend consisting of the 00z
ECMWF/CMC/UKMET and 06z GFS was used for days 3-5, then a majority
ensemble mean (00z CMCE/ECE and 06z GEFS) blend was used for days
6 and 7.

...Weather/Hazards Highlights...

Favorable westerly cold flow over the still relatively warm Great
Lakes will continue to promote a multi-day period of accumulating
lake effect snow especially downwind of Lakes Erie and Ontario,
with the best chance for possibly heavy totals on Tuesday. Models
show agreement on a favorable lake effect event Monday into
Tuesday (and possible additional beyond), but given the small
scale of the heavy snowbands, minor shifts in location of the
bands would provide considerably different conditions and
accumulations over slightly different areas. Elsewhere across the
country, conditions look mostly dry through next week though some
light precipitation may accompany a shortwave into the West around
midweek. Precipitation chances along the Gulf Coast region to
perhaps as far north as the Middle Mississippi Valley look to
increase again by late week as a warm front lifts northward
through the Gulf of Mexico bringing possibly at least locally
heavy rainfall.

Much of the south-central to Eastern U.S. should see near to below
normal temperatures through the first half of next week with
decent coverage of highs and lows 10-15F below climatology. The
west-central U.S. should be moderating back toward normal mid week
with the interior West cooling back down during the latter half of
the week due to troughing aloft. An exception to the below average
temperatures may be across the northern Plains, where by Tuesday
and beyond lows in the 20s and highs in the 40s are expected,
which are around 5-15F above normal. Above average temperatures
should spread into the Mississippi Valley/Midwest and the Gulf
Coast around next Friday-Saturday as upper ridging builds over the
region.

Kebede/Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Sun Nov 26 2023

Valid 12Z Wed Nov 29 2023 - 12Z Sun Dec 03 2023

...Overview...

An upper low over/near Hudson Bay will allow for general troughing
with some embedded shortwaves through Thursday across the Great
Lakes and Northeast. This pattern would support some ongoing lake
effect snows, locally enhanced in spots. An upper low off the
California coast on Wednesday should shift inland across the
Southwest with its shortwave energy eventually shifting east into
the central U.S. bringing potential for widespread moderate to
heavy rainfall across parts of the south-central U.S. into the
Southeast. The next system moves into the West Thursday-Friday
promoting amplified troughing shifting across the West next
weekend and a building ridge downstream across the Southeast.

...Guidance Evaluation/Predictability Assessment...

The operational deterministic models are in good agreement
regarding the evolution of the synoptic pattern through day 5. The
ensemble means resolve the days 6 and 7 pattern reasonably well
too. Thus, a general model blend consisting of the 00z
ECMWF/CMC/UKMET and 06z GFS were utilized through day 5. There's
some variance between the deterministic models regarding the
mid-level trough moving through the Intermountain West and Rockies
on day 4/5. This results in some QPF differences over the central
Gulf Coast into the Lower/Middle Mississippi Valley on those days
with the suggestion of a second axis of QPF maxima emerging over
the Mississippi Valley. The NBM diverges from the last several
runs of the GFS, the 00z CMC, 00z ECE and 06z GEFS with respect to
QPF downwind of the Lower Great Lakes overnight Wednesday into
Thursday so adjustments were made to increase amounts there.

...Weather/Hazards Highlights...

Favorable westerly cold flow over the still relatively warm Great
Lakes will continue to promote some locally enhanced lake effect
snows through Thursday especially downwind of Lakes Erie and
Ontario, but not nearly as heavy as is expected in the short range
period. Elsewhere, shortwave energy into the mid-south and a warm
front lifting through the Gulf of Mexico will allow for increasing
moisture and instability across the Gulf Coast/Southeast region,
with increasing confidence in some widespread moderate to heavy
rainfall beginning Thursday and lasting into next weekend. Amounts
and timing remain quite uncertain still, but there was enough
support for a marginal risk on the Day 5 (Thursday) Excessive
Rainfall Outlook from eastern Texas into the Lower Mississippi
Valley. Out West, precipitation will increase Thursday into next
weekend as a shortwave or two move through the region, with some
precipitation reaching southward into California and the
Southwest. The heaviest precipitation (coastal rain and mountain
snow) will be across the Washington/Oregon coast and Cascades next
weekend.

The East Coast should start out with below normal temperatures on
Wednesday, but should moderate back towards normal by Thursday.
Mean troughing across the interior West should allow for cooler
than normal temperatures lasting into next weekend. Elsewhere,
there is signal for a period of above normal temperatures across
the Northern Plains on Wednesday, with daytime highs 10-15 degrees
above average, but as ridging builds into the Midwest and Eastern
U.S. later this week, above normal temperatures should expand with
time with much of the Northern Plains to Midwest and Southeast
above normal by next weekend.

Kebede/Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
142 PM EST Mon Nov 27 2023

Valid 12Z Thu Nov 30 2023 - 12Z Mon Dec 04 2023

...Overview...

An upper-level shortwave initially located over the Southwest on
Thursday will travel across the Midwest and into the Northeast by
this weekend. Periods of moderate to heavy rainfall will accompany
this feature and are forecast to impact parts of the South,
Southeast, and Mid-Mississippi Valley. Meanwhile, a couple
quick-moving shortwaves will move into the West late this week,
promoting amplified mean troughing that shifts from the West into
the Central U.S. this weekend into early next week. As the pattern
amplifies, upper ridging will build downstream over the East
Coast. This setup will favor wet, unsettled weather in the West
and bring another round of potentially heavy rainfall to the Gulf
Coast and Southeast over the weekend.


...Guidance Evaluation/Predictability Assessment...

Model guidance is in very good agreement on the upper-level
pattern for the medium range time period. The highest uncertainty
continues to surround activity moving into the West late in the
period. There is a significant amount of run to run variability
with the timing/location of the western shortwaves, but there is
an overall consensus on a more amplified and active pattern. For
the afternoon forecast package, a purely deterministic blend was
used for days 3-5 with slightly more weight on the GFS and ECMWF
than the CMC and UKMET. For days 6-7, the deterministic GFS/ECMWF
were blended with their ensemble means to smooth out some model
differences.


...Weather/Hazards Highlights...

Upper-level shortwave energy will move over the Mid-South while a
warm front will lift north out of the Gulf of Mexico late this
week, which will support widespread moderate to heavy rainfall in
the South and Lower Mississippi Valley. There is a Marginal Risk
of Excessive Rainfall for this area on Thursday-Thursday night.
Models have consistently shown the heaviest rainfall amounts in
the vicinity of the eastern Texas Gulf Coast and western Louisiana
Gulf Coast, which may be where the highest risk of flash flooding
will be located. Heavy rain may linger across the Central Gulf
Coast and into the Southeast on Friday, and there is a Marginal
Risk of Excessive Rainfall for this area. Amplified troughing
shifting out of the West this weekend will provide a renewed surge
of moisture and support widespread moderate to heavy rainfall
across the Southeast. Out West, precipitation will increase
Thursday through the weekend as a couple shortwaves move through
the region, with some precipitation reaching southward into
California and the Southwest. The heaviest precipitation (coastal
rain and mountain snow) will be across the Washington/Oregon coast
and Cascades next weekend with generally lighter amounts farther
inland.

The interior West should stay cooler than normal underneath
reinforced troughing through the weekend. Temperatures for the
rest of the country should be closer to normal late this week, but
trend warmer with time across the Northern Plains into the Midwest
and eventually the East by the weekend. Daytime highs could be
5-10 degrees above average across this region. The Western U.S.
may finally trend back towards normal, or above normal in some
places, early next week as the trough moves east and ridging
builds back in.


Santorelli/Dolan


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw