Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
122 AM EST Sat Dec 01 2018

Valid 12Z Tue Dec 04 2018 - 12Z Sat Dec 08 2018

...Cold pattern more typical of mid-winter to grip much of the
contiguous U.S. next week...


...Overview...

A cold pattern is in store for much of the CONUS during the medium
range period as mean upper-level troughing persists from the
northern Plains/Upper Midwest to New England. Upstream support for
this pattern shows no signs of waning, with a highly amplified and
multi-center blocking pattern extending from Siberia across Asia
to the North Pacific, which will maintain anomalous ridging from
western British Columbia into Alaska. The flow resulting from this
pattern will favor continued southward transport of polar/arctic
air through Canada and into the CONUS. An active northern stream
flow regime will result in a dominant storm track from central
Canada southward into the central/eastern U.S. Meanwhile a fairly
deep upper low over the eastern Pacific early in the week will
steadily move toward California through midweek, likely opening up
as the system progresses into the Southwest U.S./northwestern
Mexico late in the week. This Pacific energy may play a role in
formation of a low pressure area along a front in the northern
Gulf of Mexico by next weekend.


...Guidance Evaluation/Preferences...

Models showed reasonably good consensus during the early portion
of the medium range, and a multi-model deterministic blend
(including the ECMWF/GFS/UKMET) was used as a forecast starting
point during days 3-5 (Tue-Thu). Multiple shortwaves will traverse
an active northern stream during this period, the strongest of
which should reach the northern plains Wed night/Thu. Models
continue to show timing/amplitude differences here, but these
should be resolved reasonably well by the multi-model blend.
Farther south, consensus is also reasonably good with the cutoff
Pacific energy as it moves into California and begins to open up
by Thu.

Spread increases by days 6-7 (Fri-Sat) as the southern stream
system crosses the Southwest and reaches the southern plains/lower
Mississippi Valley. Models show the potential for some degree of
interaction/phasing with northern stream energy during this time,
but as with any such situation at this time scale, models are
struggling to resolve the most likely scenario given the large
number of potential outcomes. There appears to be some degree of
ensemble support for formation of a wave of low pressure along the
frontal boundary in the northern Gulf of Mexico by Sat, with cold
high pressure nosing into the Midwest/Northeast north of the cold
front. Farther west, additional shortwave energy will likely
approach the West Coast by next Sat, but models show spread and
run-to-run inconsistencies on timing/amplitude and character of
this energy Based on increasing spread, weighting of ECENS and
GEFS ensemble means was boosted to comprise a majority of the
forecast during days 6-7.


...Weather Highlights/Hazards...

As described above, the flow pattern will favor southward
transport of polar/arctic air into the CONUS through much of the
medium range. In a relatively rare occurrence, nearly the entire
CONUS will see below average temperatures by late next week. For
the five-day period expect the most consistently low anomalies to
be over/just north of the Great Basin and over the
northern-central High Plains. High temperatures across those areas
 will range from 10 to at least 20 deg F below average on multiple
days. Below average temps (5-10 deg F) will return to the Eastern
Seaboard by Tue-Wed, with temps dropping further by later week as
additional rounds of cold air arrive.

In terms of precipitation, this will be a relatively dry pattern
for much of the northern tier as low pressure systems traversing
the northwesterly flow remain relatively moisture-starved. Periods
of lake effect snow are possible though. The slow-moving Pacific
system will increase rain and mountain snows along the West Coast
by Tue/Wed before spreading precipitation into the Southwest and
portions of the southern Great Basin by Thu and the southern
Plains and lower Mississippi Valley by Fri, with areas of heavy
rainfall possible. Wintry weather may also be a threat at some
locations on the northern edge of the precipitation area,
depending on the precise location of the cold front expected to
drop southward through the Plains mid-late 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