Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Tue Nov 17 2020

Valid 12Z Tue Nov 17 2020 - 12Z Thu Nov 19 2020

...Unsettled weather continues over the Northwest...

...Light to moderate lake effect snow is likely downwind of the Lower
Great Lakes...

An active weather pattern over the northeastern Pacific and northwestern
U.S. will keep those areas unsettled over the next couple of days.
Precipitation is likely for the Pacific Northwest, California, the Great
Basin, and the Northern Rockies through Thursday. As a moisture plume
moves in today, moderate to locally heavy rain is expected over parts of
the Pacific Northwest into the northern half of California, and isolated
instances of flooding cannot be ruled out. The highest elevations should
see snow. Then snow levels should lower on Wednesday behind a cold front
passing through tonight. Snow amounts through Thursday morning are likely
to be over a foot for the Cascades, Sierra Nevada, and Northern Rockies.
Winter Storm Warnings are in place for northern portions of the Sierra.
Additionally, strong gusty winds are a threat for portions of the West
Coast states into Nevada, where Wind Advisories and High Wind Warnings are
in effect.

On the other side of the CONUS, lake effect showers are forecast downwind
of the Great Lakes today as a colder and more unstable air mass moves over
the warm lake waters. While light snow should end downwind of the Upper
Great Lakes today (on the U.S. side), favored areas southeast of Lakes
Ontario and Erie will likely see at least a few inches of snow. Light snow
is also forecast along the western slopes of the Central Appalachians,
with a dusting possible over some areas of the interior Northeast as well.
Most other areas of the U.S. should remain dry for the next couple days.
However, light rain showers could persist over southern Florida today as a
cold front passes through.

An upper-level ridge is expected to build across the western and central
U.S. ahead of the upper-level disturbances affecting the Northwest.
Underneath the ridge, temperatures of 10 to 30 degrees above normal are
forecast for the interior West toward the Plains, with the highest
anomalies in northern and central parts of the Plains Wednesday. Some
daily record highs could occur with this warmth. Additionally, fire danger
will be elevated across the Southern High Plains today given the warm and
dry conditions. Meanwhile in the East, temperatures will cool down to 10
to 20 degrees below normal through midweek behind a cold front. Much of
the Northeast can expect chilly highs in the 30s on Wednesday, with 20s
for highs in Maine.

Tate

Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
155 AM EST Tue Nov 17 2020

Valid 12Z Fri Nov 20 2020 - 12Z Tue Nov 24 2020

...Overview and Guidance Evaluation...

From the multi-day mean perspective guidance shows broadly
cyclonic flow with fairly brisk progression. Individual model and
ensemble member solutions indicate above average difficulty in
resolving important embedded shortwave details that will affect
sensible weather over some areas.

The first feature of note will be a shortwave moving into the West
late this week and continuing across the country thereafter,
interacting with a leading surface front initially extending from
the northeastern U.S. through the central Plains into the Four
Corners states. Compared to yesterday the guidance is showing
increased spread regarding individual impulses within the overall
trough, with differences in phasing or flow separation leading to
some very different ideas for low pressure evolution over the
eastern half of the lower 48 from late weekend into next week.
For the purposes of the deterministic forecast, prefer to lean
away from the more separated solutions aloft with stronger/slower
surface low pressure like the 12Z ECMWF/CMC given the progressive
pattern--but with the understanding that some potential for that
scenario does exist. The new 00Z ECMWF has trended away from the
12Z run's separated/slow evolution.

Upstream the ECMWF-based guidance has been trending toward other
faster and less amplified solutions for leading energy forecast to
approach/reach the West by around Sun. This reflects greater
influence of strong North Pacific flow upstream, which should
ultimately develop a broad trough aloft over the northeastern
Pacific by the end of the period next Tue, and suggests that the
positive height anomaly expected to be near 40N 170W may not be
strong enough to support a more amplified eastern Pacific/West
Coast trough aloft recommended by teleconnections. The incoming
energy around Sun will provide additional detail uncertainties as
it continues across the western/central U.S. early next week.

During the first half of the period an operational model blend
among the 12Z models and 18Z GFS provided an intermediate starting
point for significant features given lower predictability smaller
scale detail issues aloft. Then the forecast rapidly adjusted
toward primary emphasis on the ensemble means, with the 18Z GFS
providing the only operational model input after early Mon.


...Sensible Weather and Hazards...

From late this week through the weekend one area of significant
precipitation will start out as snow and lower elevation rain
over/near the central Rockies and quickly expand across portions
of the central U.S. as the upper shortwave progressing from the
West into the Plains interacts with a northeast-southwest front
and likely encourages some degree of surface wave development.
Areas from the central Plains through Midwest into central Great
Lakes continue to see the best potential for highest totals from
this event. Meaningful rain may also extend to the south of the
central Plains/Midwest. The northern periphery of the moisture
shield may contain some snow, most likely from the
central/north-central High Plains into the Upper Great Lakes if
any does fall. Precipitation coverage and intensity become
increasingly uncertain over the central/eastern U.S. by early next
week given a variety of solutions for upper shortwave details and
associated surface evolution--not just the leading system of
interest but also for upstream energy emerging from the West
toward the end of the period.

Over the Northwest, for late this week expect most rain and higher
elevation snow to be confined to the northern Pacific
Northwest/extreme northern Rockies and be of light-moderate
intensity. Then the northern half of the West Coast into the
northern Rockies will likely see a trend toward heavier
precipitation from Sun through Tue as strong Pacific flow aloft
brings a surge of moisture into the region, with highest totals
focused over favored windward terrain.

Late this week areas from Arizona and the southern Rockies
northeastward will see well above normal temperatures in the warm
sector ahead of the initial northeast-southwest front, with a
decent area of plus 10-20F anomalies along with min temps possibly
exceeding 20F above normal over parts of the central Plains.
System/front progression will bring plus 5-15F anomalies into the
East around Sun-Mon. Trailing cooler air will offer near to
moderately below normal anomalies spreading from the West into the
southern half of the Plains Fri-Mon before moderating farther east
thereafter. The West will see temperatures rebound to near or
slightly above normal by next Mon-Tue. The northern Plains should
be moderately above normal most of the period, with a late warming
trend possibly bring some highs at least 10F above normal by next
Tue.

Rausch


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


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Tue Nov 17 2020

Valid 00Z Wed Nov 18 2020 - 00Z Wed Nov 25 2020

Latest models and ensembles agree that a series of surface highs
passing to the north of the state will maintain trades of varying
strength through next Tuesday. There is good continuity/agreement
that the strongest trades will likely be during the weekend. The
pattern will support primarily windward focused rainfall but with
sporadic episodes of showers reaching other locations as well. A
compact upper low expected to linger southwest of Kauai through
midweek may provide some enhancement to rainfall before it
weakens/drops southward. Most guidance has been indicating that
an area of greater deep moisture over the state (precipitable
water of 1.50 inches or greater) should progress westward after
Wednesday with the weakening of the upper low, followed by
fluctuating moisture values the rest of the period. The 00Z GFS
is an exception to that scenario as it keeps an area of relatively
higher deep moisture over the area well beyond Wednesday. The
GEFS mean hints at the GFS idea but with a less expansive area of
highest PWATs. Weakening of the upper low should allow for
departure of the greatest moisture after midweek per
consensus/continuity. However in recent days occasional cycles
exhibiting faster ECMWF moisture progression versus the GFS have
tended to be resolved in future runs by the ECMWF trending slower.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1007 AM EST Tue Nov 17 2020

Day 1
Valid 16Z Tue Nov 17 2020 - 12Z Wed Nov 18 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

1600 UTC update

No changes made to the previous Excessive Rainfall Outlook.

Oravec

0900 UTC discussion

Additional rounds of moderate to heavy rainfall will move into the
west coast today and tonight. We will maintain two Marginal risk
areas over the region...one over the coastal ranges of northwest
CA and southwest OR, and another over the western slopes of the
Sierra in northern CA. Rainfall will move into the coastal ranges
of northwest CA and southwest OR this morning as a frontal
boundary moves southeastward across the region. The frontal zone
is pretty progressive...so the heaviest rain should only last ~6
hours...with post frontal showers generally weaker and more
scattered in nature. Rainfall rates will generally peak around
0.2"-0.4" in an hour, with some localized totals of 0.5" possible.
Another round of heavier rain is expected to move into this region
between 06z-12z Wednesday as a shortwave rounds the base of the
longwave trough. This system may be a bit more dynamic and have
slightly better (although still weak) instability to work with.
This may offset the lower PWs and result in localized 0.5" in an
hour rains. Overall, generally looking at 2-4" of rain over the
favored terrain of northwest CA and southwest OR through 12z
Wednesday.

Rainfall associated with the southward dropping front will move
into the Sierra by early this afternoon. The heaviest rainfall
axis should remain progressive off to the south, with about a 6
hour window for the heaviest rains at any one location. Again not
much if any instability to work with...however the strong low
level convergence and upslope flow should still support hourly
rainfall magnitudes of 0.3"-0.5" over this region...with rainfall
totals through 12z Wednesday of 2-4" expected. This may result in
flash flooding over the Bear and Claremont fire areas within the
North Complex burn scar, along with any other more flood prone
areas.

Chenard

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Tue Nov 17 2020

Day 2
Valid 12Z Wed Nov 18 2020 - 12Z Thu Nov 19 2020

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Southwest Oregon into northwest California...
A closed mid level low just west of the British Columbia coast
sends short wave energy across southwest OR and northwest CA
during Day 2, providing synoptic scale ascent for showers in the
broadly cyclonic flow. Showers are expected to be ongoing at the
beginning of the period, and with 00z NAM model soundings showing
local 100/250 J/KG of MUCAPE in the column, there could be a few
instances of hourly rainfall rates near 0.50 inches before 18/16z
or so.

As the first short wave moves eastward (taking the deeper moisture
with it), shower coverage is expected diminish for a time. With
the approach of the next short wave, shower activity is expected
to peak again between 19/06z and 19/12z. With some instability in
the column, there could be some heavier showers, with mid level
cooling supporting hail as well. However, as the low levels winds
shift to the west, moisture becomes more sparse, and hourly
rainfall rates are expected to remain below 0.50 inches.
Additionally lowering snow levels could also alleviate the flood
threat to some degree. After collaborating with WFOs EKA/MFR, no
excessive area was assigned. However, should the threat for
localized flooding issues increase in later model runs, a Marginal
Risk could be needed here in subsequent forecasts.

Hayes

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Tue Nov 17 2020

Day 3
Valid 12Z Thu Nov 19 2020 - 12Z Fri Nov 20 2020

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hayes

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EST Tue Nov 17 2020

Valid 12Z Tue Nov 17 2020 - 12Z Fri Nov 20 2020


...Western U.S....
Days 1-3...

An upper-level trough along with a series of shortwave impulses
rounding the base of the trough and embedded within moist deep
layer southwest flow out ahead of it will be moving inland across
the West Coast by tonight, and then continuing through the Pacific
Northwest and northern Rockies going through Wednesday and
Thursday. The influx of deeper layer Pacific moisture in
conjunction with orographic ascent, and at least moderately strong
upper-level divergent flow will favor moderate to heavy snowfall
over the higher terrain of the northern Sierra-Nevada and Cascades
initially, but then overspreading areas downstream over the
northern Rockies including the Sawtooth, Bitterroots, Tetons and
Wind River ranges. As the larger scale trough and series of
shortwave impulses move inland, the snow levels should gradually
lower with time.

The heaviest snowfall will be over the northern Sierra-Nevada and
the Cascades where 3-day snowfall totals of 1 to 2 feet are
expected, with isolated totals in the northern WA Cascades
potentially approaching 30 inches. Farther inland across the
northern Rockies, snowfall totals generally around a foot can be
expected, with locally heavier totals over the southwest facing
slopes of the Sawtooth that may approach 2 feet.


...Lower Great Lakes...
Day 1...

Deep layer cold cyclonic flow over the Great Lakes will be
reinforced early this morning by the passage of a shortwave
impulse and associated weak area of low pressure. This energy
along with a cold front will sweep quickly across the northern
Mid-Atlantic region and in the wake of this will be plenty of
lake-effect snow shower activity, with a few organized bands of
heavier squalls by midday and through the afternoon that will
likely impact areas of northwest PA and western NY. Some
accumulating snow is likely to make it south down into the central
Appalachians including southwest PA, far western MD and northern
WV, but the overall heaviest totals are likely to be across
northwest PA downwind of Lake Erie as the deeper layer cyclonic
flow, strong cold-air advection and sufficient mid-level moisture
fosters a set-up for a multi-lake fetch involving both Lake Huron
and Lake Erie. This will support the potential for a particularly
robust and organized band of heavy snowfall. In fact, the latest
WPC snowfall probabilities indicate a moderate to high risk (40 to
70%) of snowfall totals exceeding 4 inches across parts of
northwest PA. By later tonight, the low-level flow will begin to
weaken over the lower Great Lakes as high pressure advances east
across the OH Valley, and this will allow the lake-effect snow
shower activity to begin to taper down, and then gradually
diminish on Wednesday.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Orrison

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
155 AM EST Tue Nov 17 2020

Valid 12Z Fri Nov 20 2020 - 12Z Tue Nov 24 2020

...Overview and Guidance Evaluation...

From the multi-day mean perspective guidance shows broadly
cyclonic flow with fairly brisk progression. Individual model and
ensemble member solutions indicate above average difficulty in
resolving important embedded shortwave details that will affect
sensible weather over some areas.

The first feature of note will be a shortwave moving into the West
late this week and continuing across the country thereafter,
interacting with a leading surface front initially extending from
the northeastern U.S. through the central Plains into the Four
Corners states. Compared to yesterday the guidance is showing
increased spread regarding individual impulses within the overall
trough, with differences in phasing or flow separation leading to
some very different ideas for low pressure evolution over the
eastern half of the lower 48 from late weekend into next week.
For the purposes of the deterministic forecast, prefer to lean
away from the more separated solutions aloft with stronger/slower
surface low pressure like the 12Z ECMWF/CMC given the progressive
pattern--but with the understanding that some potential for that
scenario does exist. The new 00Z ECMWF has trended away from the
12Z run's separated/slow evolution.

Upstream the ECMWF-based guidance has been trending toward other
faster and less amplified solutions for leading energy forecast to
approach/reach the West by around Sun. This reflects greater
influence of strong North Pacific flow upstream, which should
ultimately develop a broad trough aloft over the northeastern
Pacific by the end of the period next Tue, and suggests that the
positive height anomaly expected to be near 40N 170W may not be
strong enough to support a more amplified eastern Pacific/West
Coast trough aloft recommended by teleconnections. The incoming
energy around Sun will provide additional detail uncertainties as
it continues across the western/central U.S. early next week.

During the first half of the period an operational model blend
among the 12Z models and 18Z GFS provided an intermediate starting
point for significant features given lower predictability smaller
scale detail issues aloft. Then the forecast rapidly adjusted
toward primary emphasis on the ensemble means, with the 18Z GFS
providing the only operational model input after early Mon.


...Sensible Weather and Hazards...

From late this week through the weekend one area of significant
precipitation will start out as snow and lower elevation rain
over/near the central Rockies and quickly expand across portions
of the central U.S. as the upper shortwave progressing from the
West into the Plains interacts with a northeast-southwest front
and likely encourages some degree of surface wave development.
Areas from the central Plains through Midwest into central Great
Lakes continue to see the best potential for highest totals from
this event. Meaningful rain may also extend to the south of the
central Plains/Midwest. The northern periphery of the moisture
shield may contain some snow, most likely from the
central/north-central High Plains into the Upper Great Lakes if
any does fall. Precipitation coverage and intensity become
increasingly uncertain over the central/eastern U.S. by early next
week given a variety of solutions for upper shortwave details and
associated surface evolution--not just the leading system of
interest but also for upstream energy emerging from the West
toward the end of the period.

Over the Northwest, for late this week expect most rain and higher
elevation snow to be confined to the northern Pacific
Northwest/extreme northern Rockies and be of light-moderate
intensity. Then the northern half of the West Coast into the
northern Rockies will likely see a trend toward heavier
precipitation from Sun through Tue as strong Pacific flow aloft
brings a surge of moisture into the region, with highest totals
focused over favored windward terrain.

Late this week areas from Arizona and the southern Rockies
northeastward will see well above normal temperatures in the warm
sector ahead of the initial northeast-southwest front, with a
decent area of plus 10-20F anomalies along with min temps possibly
exceeding 20F above normal over parts of the central Plains.
System/front progression will bring plus 5-15F anomalies into the
East around Sun-Mon. Trailing cooler air will offer near to
moderately below normal anomalies spreading from the West into the
southern half of the Plains Fri-Mon before moderating farther east
thereafter. The West will see temperatures rebound to near or
slightly above normal by next Mon-Tue. The northern Plains should
be moderately above normal most of the period, with a late warming
trend possibly bring some highs at least 10F above normal by next
Tue.

Rausch


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


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml