Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Mon Dec 02 2019
Valid 12Z Thu Dec 05 2019 - 12Z Mon Dec 09 2019
...Southern tier system to affect the Lower Mississippi
Valley/Southeast late week...
...More moisture to spread into the West late this week/weekend
and the East by next Monday...
...Pattern Overview and Guidance/Predictability Assessment...
The best clustering of guidance has been stable over recent runs
in showing an eastward progression of the large scale pattern
during the period. This evolution will be highlighted by a late
week eastern Pacific trough (with embedded closed low) moving
inland by next weekend. As this occurs expect cyclonic flow over
Canada to begin interacting with at least a portion of the trough,
by next Mon ultimately leading to a mean trough extending from
Hudson Bay through the central U.S. and into northwest Mexico. At
that time an upstream ridge axis should reach near the West Coast.
D+8 multi-day means continue to highlight a core of negative
height anomalies over the North Pacific with the current day 7
consensus forecast providing a plausible lead-in to the
teleconnection-favored pattern.
For the strong eastern Pacific trough moving into the lower 48,
guidance has been showing steady improvement in clustering--by way
of the GFS trending slower with parts of the trough and recent
ECMWF runs (with some oscillation) faster than some cycles from a
couple days or so ago. An average of the GEFS/ECMWF means, with
the latest GEFS actually a little slower than the ECMWF mean
during the weekend, has provided the most stable forecast thus
far. Among data from the 12Z/18Z cycles the 12Z GFS appeared a
bit fast with leading height falls and the UKMET strayed a little
fast as well. New 00Z runs of the operational models agree fairly
well through day 5 Sat. By Sun-Mon guidance agrees on some
northern stream interaction with a combination of the Pacific and
Canadian dynamics supporting a wavy front reaching into the
Plains/Mississippi Valley/Great Lakes. There is fairly good
confidence in the front's existence but wave details and frontal
timing will take a while to resolve due to dependence on
lower-predictability shortwave details. Amplification of Canadian
flow should reflect at the surface as a separate front that drops
into the northern Plains during the weekend.
Elsewhere, there are still modest timing differences for the
weakening southern tier shortwave tracking east from the Rockies.
The UKMET has been the laggard recently with most other 12Z/18Z
solutions close together. The 00Z CMC has joined the UKMET over
the southern Plains/Mississippi Valley though. In the northern
stream one vigorous shortwave aloft will lift away from the
Northeast after Thu and an upstream amplifying shortwave will
cross the Great Lakes/Northeast Fri-Sat. Faster CMC/slower GFS
trends in the new 00Z cycle for the surface wave associated with
the second feature bring those solutions toward the 12Z
ECMWF/ECMWF mean. The new 00Z ECMWF is holding serve with its
timing. Strong southward push of the trailing cold front should
ultimately suppress the weakening southern tier system into the
eastern Gulf of Mexico by Sat.
Based on guidance comparisons/continuity the updated forecast
started with the 18Z GFS/12Z ECMWF and lesser weight of the 12Z
CMC for about the first half of the period. Then 18Z GEFS/12Z
ECMWF mean weight gradually increased, reaching 50 percent total
by day 7 Mon. Comparison of various 12Z/18Z GFS details to
consensus become more mixed by the latter half of the forecast so
GFS input was split between both runs at that time.
...Weather Highlights/Threats...
Expect relatively light southern Rockies precipitation to taper
off Thu with the responsible southern tier system bringing a brief
period of locally moderate/isolated heavy rainfall to the Lower
Mississippi Valley and parts of the Southeast Thu night-Fri before
being suppressed to the south. Moisture ahead of the eastern
Pacific system should reach the West Coast late this week and then
spread inland over the West through the weekend. Consistent
signals from the guidance point to the greatest rainfall/higher
elevation snowfall totals focusing over favored terrain in the
Sierra Nevada and across northern California, possibly extending
into the extreme southwest corner of Oregon. The Pacific
Northwest as well as windward terrain across the northern
two-thirds of the Interior West/Rockies will see less extreme but
still meaningful totals. The upper trough and leading wavy cold
front should reach far enough eastward for precipitation (mostly
rain) to expand over the eastern half of the country by next Mon.
Some activity may be locally heavy if the orientation and timing
of the upper trough allow for sufficient input of low level Gulf
moisture. Earlier in the period expect one episode of lake effect
snow to taper off to the lee of the eastern Great Lakes on Thu
followed by a modest event over the northern/eastern Great Lakes
and New England approximately Thu night-Fri night.
Locations near the East Coast should see near to moderately below
normal temperatures from late week into the weekend. New England
may see readings as much as 10-15F below normal Sat-early Sun.
Clouds/precipitation associated with the upper trough progressing
inland from the Pacific will spread above normal morning lows
across the West into the weekend while maintaining moderately
below normal highs over the Great Basin/Southwest. Cool highs
will expand into the Rockies by the start of next week. Meanwhile
expect a pronounced warming trend over the eastern half of the
country Sun-Mon, with a broad area of plus 10-20F anomalies for
morning lows and pockets of plus 10F or so anomalies for highs.
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