Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
147 AM EST Mon Jan 07 2019

Valid 12Z Thu Jan 10 2019 - 12Z Mon Jan 14 2019

...Overview...

A relatively amplified but still progressive flow regime will
persist across the CONUS during the extended forecast period. 500
hPa D+8 charts show a persistent mean trough just off the U.S.
West Coast, with ridging across the U.S. and Canadian Plains, and
mean a mean trough across New England. Amplified systems should
reach the West Coast every couple days, with some splitting of
energy into northern/southern stream components as the waves move
into the Rockies/central U.S. Models continue to suggest the
potential for some degree of northern/southern stream phasing
across the eastern U.S. by the weekend, with potential initial
surface low development in the northern Gulf of Mexico, and energy
transferring to a new low off the Southeast coast by Sat night/Sun.


...Guidance Evaluation/Preferences...

A deterministic multi-model blend (ECMWF/GFS/UKMET) was used as a
forecast starting point on days 3-4 (Thu-Fri). Relatively low
model spread and good run-to-run continuity during this time frame
supported use of this blended consensus solution. The most
significant features during this time frame are an amplified upper
trough exiting the eastern U.S., and another amplified trough
approaching the West Coast. Models did show some spread as to the
structure and intensity of the low pressure system off the West
Coast by Fri, but given that none of these solutions were deemed
outliers, a blended approach was best.

By days 5-7 (Sat-Mon), the aforementioned split flow begins to
play a larger role as the Pacific energy separates into northern
and southern stream components, while yet another amplified wave
approaches the West Coast. Southern stream shortwave energy should
cross the Four Corners region Fri and reach the southern plains
Sat, while the northern stream component initially weakens as it
encounters anticyclonic flow before amplifying once again as it
reaches the northern plains/Midwest Fri-Sat. The Sat-Sun time
period is when the complexities begin as to how these two features
will (or will not) interact, potentially resulted in a coastal low
pressure system off the Eastern Seaboard by Sun-Mon. The GFS has
for a couple runs shown a highly phased system with a relatively
intense low pressure system off the East Coast, but has also been
fairly inconsistent. The ECMWF, on the other hand, has shown a
less phased/more suppressed system that moves more quickly out to
sea, and has also been quite consistent in depicting this type of
solution (with a fair degree of support from ECENS/GEFS means as
well). Given the low predictability of such an evolution on days
6-7, opted to go heavily toward a solution more like the ensemble
means and ECMWF (and GFS from 12Z Sun). The next system
approaching the West Coast by day 6-7 is reflected by a range of
possible model solutions varying in timing as well as amplitude.
Thus, an ensemble mean approach is preferred here as well. Given
these considerations, the forecast during days 5-7 was gradually
trended to weight ECENS and GEFS ensemble means more heavily than
deterministic guidance.


...Weather Highlights/Hazards...

Expect the potential for multiple rounds of heavy precipitation
(low elevation rain and mountain snows) across the West Coast
during the forecast period as a rapid succession of Pacific
systems impacts the region. As shortwave energy reaches the
southern Plains/lower Mississippi Valley by Fri-Sat, areas of
heavy rain will be possible, with precipitation spreading east
into the Southeast/southern Mid-Atlantic by Sat night/Sun. Cold
air to the north of the system could result in areas if wintry
precipitation. The degree of winter weather potential from the
southern Appalachians to the Mid-Atlantic Sat-Sun is highly
uncertain for the reasons described above.

Above average temperatures will persist through the forecast
period across the north central U.S., with max temp anomalies in
the +10 to +15 deg F range (even warmer for some areas) through
much of the period. The coldest temperatures will be across the
Northeast, where temperatures as cold as 10 to 20 deg below
average will be possible from Fri through the weekend into early
next week.


Ryan


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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