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