Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1115 AM EST Sat Dec 29 2018

Valid 12Z Tue Jan 01 2019 - 12Z Sat Jan 05 2019

...Overview...

The medium range period features a possibly closed low progressing
eastward from the Four Corners region on Day 3 (Tue) to the
Southeast/Tennessee Valley by Day 6 (Fri) before becoming absorbed
into the larger scale troughing. To the north, additional energy
slides along the U.S./Canadian border underneath of a deep low
rotating over Hudson Bay. Out west, upper ridging across the east
Pacific shifts inland as an upstream shortwave trough moves
towards the Pacific Northwest. The overall pattern would bring
widespread rain to the eastern half of the nation while
precipitation again increases in the Pacific Northwest.

...Guidance Evaluation/Preferences...

A trend in both global models and ensembles is emerging towards a
slower and slightly stronger upper low progressing from the Four
Corners to the East next week. The latest run of the CMC favors a
more elongated trough type feature across the Central U.S. and as
a result is noticeably faster than the rest of the guidance and
thus was not included in the blend. There is some question as to
the location of the surface low on day 5 with the ECMWF being
noticeably farther north than the GFS (northern MS/AL vs central
Gulf Coast) but based on the ensemble low plots, somewhere in the
middle of the two is probably a good place to be at this point.
Out in the west, beyond day 5, timing of the ridge-trough-ridge
pattern in the eastern Pacific and Northwest U.S. becomes an
issue, especially in the operational models. The ensemble means
appeared to present a more agreeable (with obvious suppression)
solution this late in the period.

The blend for this cycle of the WPC medium range progs features a
majority deterministic model blend (between the 06z GFS/00z
ECMWF/00z FV3-GFS) days 3-5, with increasing weighting of the
ensemble means (GEFS/ECENS) from day 5 and beyond. Continues to
include a small bit of the 00z deterministic ECMWF through day 7
because it seemed to fit best with the ensemble means.

...Weather Highlights/Hazards...

Closed low in the Southwest should bring a second round of
precipitation to parts of eastern Arizona and New Mexico, with the
highest potential for accumulating snows confined to the higher
mountains. As this low progresses across the Southern tier, this
will promote heavy rainfall potential from the Deep South into the
Eastern U.S. next week. A surface low lifting through the
Southeast into the Mid-Atlantic is likely to provide a focus for
enhanced rainfall with the best chance for 3+ inch 5-day rainfall
totals currently extending from the central Gulf Coast into
Georgia. Moisture early in the period reaching the Great Lakes/New
England may be in the form of snow, especially with lake effect
activity behind low pressure exiting New England mid-week. In the
Pacific Northwest, expect an increase in rain/high elevation snow
by Thursday and continuing into Friday -- with some meaningful
snow totals likely in the Olympics/Washington Cascades and
portions of the Northern Rockies.

In regards to temperatures, the northern Plains can expect very
cold temperatures on Tuesday--with high temperatures reaching
15-25F below normal. By Wednesday, temperatures will rebound in
this region and could be near 10-15F above normal. The rest of the
Plains will also be much below normal on Tuesday and
Wednesday--becoming near normal by Thursday. Meanwhile, the East
Coast will be anomalously warm on Tuesday ahead of the approaching
trough/surface front, returning to near normal by Wednesday and
beyond. Along the West coast, temperatures are forecast to be
within a few degrees of normal.

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