Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Sun Dec 30 2018

Valid 12Z Wed Jan 02 2019 - 12Z Sun Jan 06 2019

...Overview...

Medium range period features a closed low in the southern High
Plains day 3 (Wed) progressing eastward towards the Mid-Atlantic
by day 5 (Fri) while additional waves of energy slide through near
the U.S./Canadian border in the northern stream. Out West, the
first shortwave crosses into the Pacific Northwest by late week,
with another deeper (possible closed low?) trough arriving by the
weekend. This pattern would favor heavy rainfall from the Gulf
Coast to the Northeast, and increasing precipitation in the
Pacific Northwest/West coast.

...Guidance Evaluation/Preferences...

The biggest question in the medium range remains the closed low
progressing eastward across the South and eventually lifting into
the Mid-Atlantic and off the Northeast coast by the weekend. Even
as early as day 3, timing differences start to arise with the UK
Met remaining the slowest/strongest (over the Southwest) and the
CMC/FV3-GFS on the faster side (over the central/southern Plains)
with the latest ECMWF/GFS runs sitting somewhere in the middle.
After this time, things really take a turn for the worst as the
the 06z GFS lifts the energy quickly northeastward opening up into
more of an open wave type of structure, while the ECMWF/UK met
hold strong to a slower closed low. Looking at ensemble spaghetti
plots, there remains a slight trend towards a slower/stronger
solution like has consistently been depicted run after run by both
the ECMWF and the UK Met. After day 3, the GFS is wildly
inconsistent run to run with evolution of this system. Also
concerning, is that the deterministic GFS is even faster than its
ensemble mean which also would give a bit more credibility to the
slower solutions. Therefore, WPC prefers an early period blend
heavily weighted towards the ECMWF, with increasing percentages of
the ensemble means later in the period. This also provides a
forecast close to that of previous shift continuity.

Out in the west, differences in the details and slight timing
differences arise with the second trough approaching the west
coast at the end of the period. Given a day 6-7 forecast, a blend
of the ECENS/GEFS mean sufficed with some small percentage of the
ECMWF just given its better run-to-run continuity across the
board.

...Weather Highlights/Hazards...

Closed low progressing in the South will promote heavy rainfall
potential from the Lower Mississippi Valley to the Mid-Atlantic
and Northeast during the mid to later half of this week. A surface
low lifting through the Southeast into the Mid-Atlantic is likely
to provide a focus for enhanced rainfall with models continuing to
show the heaviest axis of rainfall setting up from Mississippi to
the central Atlantic coast. Much of the precipitation associated
with this storm should be in the form of rain, but some changeover
to snow is possible on the far northern edge of the precipitation
shield though accumulation potential at this point (from roughly
the mid-Mississippi Valley to the northern Mid-Atlantic) remains
low. The best chance for accumulating snows will be across
northern New England as the surface low deepens along the
Northeast U.S. coast. Out West, rain/mountain snows move into the
Pacific Northwest by Thursday, with more precipitation arriving
into the West Coast/California/eventually the Southwest next
weekend.

The Four Corners region to the Southern Plains will be the cold
spot in the beginning of the medium range period as afternoon
highs are forecast to be 15-25F below normal. Northern tier states
into the East transition to above normal with the highest
anomalies (15-20F above normal) expected in the Northern Plains
Thur-Fri. The West should stay at or within a few degrees of
normal through the medium range period.

Santorelli


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