Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Sun Jan 26 2020

Valid 12Z Sun Jan 26 2020 - 12Z Tue Jan 28 2020

...Multiple storm systems to cause wet and snowy conditions across the
Pacific Northwest & northern Rockies...

...Scattered snow showers to stick around in northern and central
Appalachians, Great Lakes...

...A pair of low pressure systems will bring showers and thunderstorms to
the South...

The Northwest U.S. will brace for several rounds of storms over the next
few days. Coastal ranges and valleys of the Pacific Northwest will witness
periods of rain day after day with rainfall amounts averaging 1 to 2
inches. The higher elevations of the region's coastal front could see
rainfall totals in excess of 3 inches through Tuesday. In the Cascades of
Washington and Oregon, snow will pile up quickly with snow amounts being
measured in feet in the higher elevations. Pacific moisture associated
with these storms will spill east into the northern Rockies where valley
showers and mountain snow is also a good bet through Tuesday.

As the weekend storm system in the East tracks through Maine on Sunday,
snow showers and lake effect snow bands will stick around across the Great
Lakes, eastern Ohio Valley, and the central and northern Appalachians
through Tuesday. The heaviest snowfall will be located downwind of Lakes
Erie and Ontario where cumulative amounts through Tuesday will range
between 6 to 12 inches. Downeast Maine will see locally heavy rainfall
early Sunday morning but drier conditions will quickly move in before
midday, although a few spotty rain or snow showers across central New
England cannot be ruled out.

Down south, a surface low pressure system will be responsible for the
development of scattered showers and thunderstorms from east Texas and the
Lower Mississippi Valley this morning, then across the central Gulf Coast
by this afternoon. While the thunderstorm threat should diminish tonight,
passing showers will continue across the Deep South tonight and eventually
reach the Southeast by Monday. The best chance for thunderstorm activity
on Monday will reside across central and southern Florida. Some of the
southern Appalachians could even see a wintry mix develop Monday morning.
By Tuesday, an upper level disturbance in the Southwest will spawn a new
storm system in the southern Plains leading to another round of showers
and thunderstorms across the South Central region.

Mullinax

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Sun Jan 26 2020

Valid 12Z Wed Jan 29 2020 - 12Z Sun Feb 02 2020

...Overview and Guidance/Predictability Assessment...

During the latter half of the week model/ensemble guidance
continues to advertise a transition to a more amplified mean
pattern with a strengthening Pacific ridge building over the
western U.S. and a downstream trough taking shape over the Plains
and east-central states. Then during the weekend there is good
agreement that upstream flow should finally begin to push
northeastern Pacific mean troughing eastward, with the trough axis
most likely reaching far western Canada and near the Pacific
Northwest coast by late Sun. As this occurs expect the western
U.S. ridge aloft to flatten while the eastern U.S. mean trough
continues eastward.

While there is good agreement and continuity with the overall
pattern evolution, forecast specifics continue to have lower than
average confidence. Not only do difficulties persist in resolving
individual shortwaves ahead of the building Pacific/western U.S.
ridge but details of how flow around the top of the ridge
splits--plus interaction among shortwaves--continue to be very
uncertain with such evolution tending to have below average
predictability beyond the short range time frame.

Early in the period the guidance has yet to settle on a particular
solution for the system expected to be over the Lower Mississippi
Valley/Gulf Coast region as of early Wed. Multi-day continuity
had shown a pronounced weaker/suppressed trend only to be followed
in the past day by at least a partial trend back to a more phased
shortwave aloft and farther north surface system as has been
suggested by recent CMC runs. The updated forecast reflects the
best consensus of 00Z-06Z guidance.

Trailing evolution provides increasing complexity with time.
There is decent clustering/continuity for amplifying western U.S.
energy to form a fairly sharp southern Rockies/northwest Mexico
upper trough with a closed or nearly closed low as of early Thu.
However after that time spread rapidly increases for energy that
drops into the central U.S., affecting ejection of the
Rockies/Mexico trough. Uncertainty then compounds as a fairly
wide envelope exists for how energy flowing around the western
ridge drops into the east-central U.S. mean trough after early Fri.

Playing a role in the Fri-Sun eastern U.S. evolution as well as
forecast specifics over and near the Northwest, there are
meaningful differences in strength/position for the western ridge
during that time. Given typical guidance error 5-7 days out in
time, an intermediate solution between stronger GFS/GEFS mean runs
and somewhat flatter ECMWF/ECMWF mean provides a reasonable
starting point. Meanwhile the 00Z CMC shows the lowest heights
over the West due to a farther west upper ridge axis and the CMC
mean does not support that idea. The 12Z CMC compares better to
other guidance. The intermediate preference reflects sensitivity
to details in the southern half of the upper trough approaching
the Pacific Northwest, with various degrees of potential
separation possibly developing between 130-150W longitude during
the weekend--another aspect of flow that has low predictability
especially 6-7 days out in time.

As for surface evolution over/near the eastern U.S., surface low
plots still show very little clustering during Sat-Sun but with a
little more density from the eastern Gulf through the western
Atlantic. Especially around Sat the 00Z ECMWF becomes flatter
than most other solutions with eastern U.S. flow aloft so its well
offshore surface system appears questionable--though the new 12Z
run persists with such a scenario. The 06Z GFS matched up well
with the 00Z ECMWF mean for timing/track close to WPC continuity
so those three were emphasized for the weekend mass field
forecast. The 06Z GEFS mean has a similar idea but 12 hours
later. Thus far the GFS has been fairly consistent for
timing/latitude of the surface low but have varied between
near-coastal tracks (06Z and 12Z runs) and more inland ones.


...Weather/Hazard Highlights...

The forecast pattern evolution will maintain a focus for periods
of heavy rain and mountain snow over the Pacific Northwest,
especially over favored terrain along the Cascades and near the
coast. Best confidence for highest five-day totals is over
Washington while subtle differences in strength of the upper ridge
building over the western U.S. and overriding shortwaves will
affect the extent to which significant activity reaches into
northwestern Oregon. Expect some of this moisture to produce
heavy snow in the extreme northern Rockies as well, but again with
some uncertainty over southward extent. Shortwave energy dropping
through the West around midweek will produce mostly light/moderate
precipitation over the central/southern Rockies.

Farther east expect the weakening system over the Lower
Mississippi Valley/Gulf Coast on Wed to produce mostly
light-moderate rainfall over parts of the southern tier early in
the period. One or more of the shortwaves amplifying downstream
from the building western U.S. ridge aloft may produce some
precipitation (mostly rain) over the southern Plains around Thu,
followed by expansion of precipitation over parts of the South and
East from Fri into the weekend as surface low pressure evolves
ahead of upper troughing. Model/ensemble spread and inconsistency
with specifics at the surface and aloft--not to mention
uncertainty in thermal profiles--keep confidence low in
determining coverage and amounts of precipitation as well as
precipitation type. From the probabilistic perspective, best
potential (but still moderately low in absolute terms) for some
snow extends from the Appalachians through parts of the
Mid-Atlantic and Northeast.

For the period as a whole, expect warmest temperature anomalies
across the northern tier with highs averaging 10-20F above normal
and morning lows 15-25F above normal. The western U.S. will see a
pronounced warming trend as ridging builds overhead, leading to
increased coverage of plus 10-20F anomalies. By next weekend a
trend toward westerly flow aloft/downslope flow at low levels
should promote increasing warmth over an expanding portion of the
High Plains. The central High Plains may see readings as high as
10-25F above normal next Sun. Locations from the central-southern
Rockies through the extreme southern tier may see a day or so of
near to slightly below average temperatures.

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
247 AM EST Sun Jan 26 2020

Valid 00Z Mon Jan 27 2020 - 00Z Mon Feb 03 2020

Light to moderate trades are expected to resume through Sunday as
a frontal boundary in the vicinity of Hawaii dissipates. A
somewhat drier air mass spreading across the state through early
in the week should result in fairly sparse windward shower
activity. Models show consensus that another cold front should
approach Hawaii by late in the week as a shortwave amplifies
across the north central Pacific. Model/ensemble agreement on this
system is a bit above average given the day 5-7 time frame, with
the cold front likely to move into the western Hawaiian Islands
Thu night or Fri morning, and then sweeping east across the state
through Fri night/Sat. As the upper wave moves east, the trailing
front across Hawaii may begin to stall across the eastern islands
by next weekend. Trades should see some disruption ahead of the
front, with low-level flow veering to a more southerly direction,
and the potential for a period of northerly flow behind the front.
Deeper moisture along the front is also likely, with guidance
showing PWs increasing into the 1.2-1.5 inch range, increasing the
potential for more widespread shower activity across the state in
association with the front through next weekend.

Ryan

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1016 AM EST Sun Jan 26 2020

Day 1
Valid 16Z Sun Jan 26 2020 - 12Z Mon Jan 27 2020

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

Chenard

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sun Jan 26 2020

Day 2
Valid 12Z Mon Jan 27 2020 - 12Z Tue Jan 28 2020

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR COASTAL
AREAS OF THE PACIFIC NORTHWEST...

...Southwest Washington to northwest California...

Strong energy and moisture will be directed into the Pacific
Northwest resulting a vast area of precipitation. A broad area
with enhanced vertical motion will develop over the coast and
inland over the Cascades as the trough nears the coast and
eventually moves inland. During this time low-level, W/SW flow of
35 to 50 knots will be transporting PW values of 0.50 to 1 inch
nearly orthogonal to the terrain; which will help sustain several
hours of moderate to the heavy rainfall. Model guidance continues
to focus 1 to 3 inches all along the Washington to northern
California coast with local maxes of 4 to 6 inches. Rainfall rates
0.50 inch per hour or greater could quickly lead to an increased
risk for flash flooding, or debris flows in sensitive areas such
as steep terrain or burn scars. As previously noted, portions of
this region have had >150% percent of normal precipitation
resulting in saturated soils with additional rainfall quickly
becoming runoff. The Marginal Risk for excessive rainfall that was
in effect for southwest Washington to northwest California remains
valid for this issuance.

Campbell


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Sun Jan 26 2020

Day 3
Valid 12Z Tue Jan 28 2020 - 12Z Wed Jan 29 2020

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

Campbell


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
339 AM EST Sun Jan 26 2020

Valid 12Z Sun Jan 26 2020 - 12Z Wed Jan 29 2020

...Northwestern CONUS...

Days 1-3...

In a series of progressive shortwaves to impact the Pacific
Northwest, snowfall accumulations are expected to primarily affect
higher elevations of northern CA, western OR, and western WA with
the highest probabilities of heavier snow totals across the WA
Cascades due to relatively high snow levels elsewhere. # day
totals a couple of feet are expected in the WA Cascades.

The highest amounts today/tonight are forecast to be in the WA
Cascades due to the longer duration of overlapping lift/moisture.
Another surge of moisture and lift associated with the upper level
jet maxima and low level warm advection is expected to produce
another round of locally heavy snow in the northern OR Cascades
and WA Cascades Monday/Mon night. A break looks to develop on Day
3 in the WA Cascades as the upper trough moves onshore and inland,
with drying aloft developing in its wake.

Across portions of central/northern ID and northwest MT, several
inches are snow are expected as the repeating shortwaves track
through the central/northern Rockies. While no particular system
is likely to produce widespread heavy snowfall, the cumulative
affect over the next 3 days will produce considerable
accumulations for the highest elevations. The combined moisture
transport and lift extends inland into the Blue Mountains of
northeast OR. The 3 day total show potential for a foot across
the ranges of northeast OR to southern ID and northwest WY.

Lighter snowfall totals will spread southeast across areas with
upslope flow in the Great Basin and CO/NM. Snow should taper
Tuesday morning as the upper trough departs and moves east onto
the Plains.


...Lower Great Lakes/New York/northern New England...

Days 1-3...
An area of low pressure move north from Maine into New Brunswick
later today. The strong low to mid level warm advection ahead of
the low has resulted in freezing rain and drizzle over interior
portions of eastern Maine. This is expected to continue into the
morning hours and then fade during the afternoon, along with
reduced precipitation rates. Because of the short duration,
amounts will be on the lighter side today.

Strong cyclonic flow in the wake of the departing system will
bring a prolonged period of enhanced moisture and lift across both
Lake Erie and Lake Ontario. Aided by the favorable west/southwest
flow and enhancement due to topography, several inches of snow are
expected in the lee of both lakes across southwest and upstate New
York.

On Monday, amounts are lighter as a progressive front crosses the
lower lakes, with lighter snow amounts following shorter cross
lake trajectories and weaker lift. Most locations on Monday are
forecast to get an inch or two, with no heavy snow areas
identified.

...Southern Plains...

Day 3...
The models show a 700 mb wave progressing across southeastern CO
and NM and then into the OK/TX panhandles
by Tue morning, continuing east across OK into AR later Tue.
Light mixed precip is forecast along the path of the wave, with
the exception of the 00z ECMWF, which showed potential for several
inches of snow in the TX/OK panhandles, due to a strong 700 mb
wave. The probabilities for mixed freezing/frozen precip decline
as the wave moves into relatively warmer air in the lower MS
Valley.

The probability of 0.25 inches of freezing rain is less than 10
percent Days 1-3.

Petersen

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Sun Jan 26 2020

Valid 12Z Wed Jan 29 2020 - 12Z Sun Feb 02 2020

...Overview and Guidance/Predictability Assessment...

During the latter half of the week model/ensemble guidance
continues to advertise a transition to a more amplified mean
pattern with a strengthening Pacific ridge building over the
western U.S. and a downstream trough taking shape over the Plains
and east-central states. Then during the weekend there is good
agreement that upstream flow should finally begin to push
northeastern Pacific mean troughing eastward, with the trough axis
most likely reaching far western Canada and near the Pacific
Northwest coast by late Sun. As this occurs expect the western
U.S. ridge aloft to flatten while the eastern U.S. mean trough
continues eastward.

While there is good agreement and continuity with the overall
pattern evolution, forecast specifics continue to have lower than
average confidence. Not only do difficulties persist in resolving
individual shortwaves ahead of the building Pacific/western U.S.
ridge but details of how flow around the top of the ridge
splits--plus interaction among shortwaves--continue to be very
uncertain with such evolution tending to have below average
predictability beyond the short range time frame.

Early in the period the guidance has yet to settle on a particular
solution for the system expected to be over the Lower Mississippi
Valley/Gulf Coast region as of early Wed. Multi-day continuity
had shown a pronounced weaker/suppressed trend only to be followed
in the past day by at least a partial trend back to a more phased
shortwave aloft and farther north surface system as has been
suggested by recent CMC runs. The updated forecast reflects the
best consensus of 00Z-06Z guidance.

Trailing evolution provides increasing complexity with time.
There is decent clustering/continuity for amplifying western U.S.
energy to form a fairly sharp southern Rockies/northwest Mexico
upper trough with a closed or nearly closed low as of early Thu.
However after that time spread rapidly increases for energy that
drops into the central U.S., affecting ejection of the
Rockies/Mexico trough. Uncertainty then compounds as a fairly
wide envelope exists for how energy flowing around the western
ridge drops into the east-central U.S. mean trough after early Fri.

Playing a role in the Fri-Sun eastern U.S. evolution as well as
forecast specifics over and near the Northwest, there are
meaningful differences in strength/position for the western ridge
during that time. Given typical guidance error 5-7 days out in
time, an intermediate solution between stronger GFS/GEFS mean runs
and somewhat flatter ECMWF/ECMWF mean provides a reasonable
starting point. Meanwhile the 00Z CMC shows the lowest heights
over the West due to a farther west upper ridge axis and the CMC
mean does not support that idea. The 12Z CMC compares better to
other guidance. The intermediate preference reflects sensitivity
to details in the southern half of the upper trough approaching
the Pacific Northwest, with various degrees of potential
separation possibly developing between 130-150W longitude during
the weekend--another aspect of flow that has low predictability
especially 6-7 days out in time.

As for surface evolution over/near the eastern U.S., surface low
plots still show very little clustering during Sat-Sun but with a
little more density from the eastern Gulf through the western
Atlantic. Especially around Sat the 00Z ECMWF becomes flatter
than most other solutions with eastern U.S. flow aloft so its well
offshore surface system appears questionable--though the new 12Z
run persists with such a scenario. The 06Z GFS matched up well
with the 00Z ECMWF mean for timing/track close to WPC continuity
so those three were emphasized for the weekend mass field
forecast. The 06Z GEFS mean has a similar idea but 12 hours
later. Thus far the GFS has been fairly consistent for
timing/latitude of the surface low but have varied between
near-coastal tracks (06Z and 12Z runs) and more inland ones.


...Weather/Hazard Highlights...

The forecast pattern evolution will maintain a focus for periods
of heavy rain and mountain snow over the Pacific Northwest,
especially over favored terrain along the Cascades and near the
coast. Best confidence for highest five-day totals is over
Washington while subtle differences in strength of the upper ridge
building over the western U.S. and overriding shortwaves will
affect the extent to which significant activity reaches into
northwestern Oregon. Expect some of this moisture to produce
heavy snow in the extreme northern Rockies as well, but again with
some uncertainty over southward extent. Shortwave energy dropping
through the West around midweek will produce mostly light/moderate
precipitation over the central/southern Rockies.

Farther east expect the weakening system over the Lower
Mississippi Valley/Gulf Coast on Wed to produce mostly
light-moderate rainfall over parts of the southern tier early in
the period. One or more of the shortwaves amplifying downstream
from the building western U.S. ridge aloft may produce some
precipitation (mostly rain) over the southern Plains around Thu,
followed by expansion of precipitation over parts of the South and
East from Fri into the weekend as surface low pressure evolves
ahead of upper troughing. Model/ensemble spread and inconsistency
with specifics at the surface and aloft--not to mention
uncertainty in thermal profiles--keep confidence low in
determining coverage and amounts of precipitation as well as
precipitation type. From the probabilistic perspective, best
potential (but still moderately low in absolute terms) for some
snow extends from the Appalachians through parts of the
Mid-Atlantic and Northeast.

For the period as a whole, expect warmest temperature anomalies
across the northern tier with highs averaging 10-20F above normal
and morning lows 15-25F above normal. The western U.S. will see a
pronounced warming trend as ridging builds overhead, leading to
increased coverage of plus 10-20F anomalies. By next weekend a
trend toward westerly flow aloft/downslope flow at low levels
should promote increasing warmth over an expanding portion of the
High Plains. The central High Plains may see readings as high as
10-25F above normal next Sun. Locations from the central-southern
Rockies through the extreme southern tier may see a day or so of
near to slightly below average temperatures.

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