Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 PM EST Sat Dec 21 2019
Valid 12Z Tue Dec 24 2019 - 12Z Sat Dec 28 2019
...Overview and Guidance/Predictability Assessment...
Expect a mean trough aloft to persist over the West Coast and
Great Basin for most of next week but with some progression toward
the Rockies/Plains possible by around Fri-Sat as an eastern
Pacific ridge builds toward the West Coast. Moderately
progressive flow will prevail downstream across the central and
eastern U.S. as shortwave energy ejects from the western trough
and is deflected around a broad eastern U.S. mean ridge which some
guidance suggests may be anchored by an upper high settling over
Cuba and the Bahamas by days 5-7 Thu-Sat. A persistent upper low
across the Arctic Ocean well north of Alaska along with weakly
positive height anomalies across Canada should continue to
minimize potential for any significant outbreaks of cold air
across the lower 48 through the end of the week.
There is still decent initial clustering for the leading shortwave
that rapidly weakens as it ejects from the West into the Plains
Tue-Wed. Exactly what happens after Wed has been up for debate in
the models for multiple days though. 00Z-06Z runs changed yet
again from prior solutions, now suggesting that the combination of
enough energy lingering over the northern tier and stronger
eastern ridging aloft will divert Plains frontal waviness into
Ontario by day 6 Fri--instead of a path somewhere across the
northeast quadrant of the lower 48. This leads to the trailing
front taking longer to pass through the Northeast and supports
stalling of the trailing part of the front over the Plains. Thus
far the 12Z model runs are maintaining continuity with the 00Z-06Z
cycles for this overall evolution. Two similar runs have not
meant a lot so far but hopefully the forecast projection is
starting to get near enough for future adjustments to be less
dramatic.
Over the past day the GFS has made a pronounced adjustment toward
the slower consensus for the next significant shortwave expected
to reach the West Coast by Wed-Wed night, finally allowing for
somewhat more use in the forecast than in some previous days.
Other guidance has still varied on the specifics of a potential
embedded closed low whose track would most likely be near the
California coast. As expected the ensemble means are less defined
than the operational models but show a decent signal for flow
separation and the 00Z ECMWF mean/12Z GEFS mean briefly close off
an upper low center in the vicinity of southern
California/Arizona. Faster trend of the 00Z ECMWF brought it more
in line with the means.
Models and ensembles remain varied on specifics of upstream
northeastern Pacific flow that should reach western North America
by late this week and have some influence on the trough moving
into the West. In contrast to most trends in recent guidance, the
00Z ECMWF becomes quite aggressive in how it amplifies the Pacific
energy over the West late week into the weekend. On the other
hand GFS runs are fairly strong with energy that reaches the
northern Plains by day 7 Sat--but at least they are closer to the
ensemble mean trough axis and the old 12Z/20 ECMWF. CMC runs have
been out of sync with one or more aspects of North Pacific into
North America flow. Lack of confidence in operational model
specifics would recommend a rapid increase of ensemble mean
emphasis late in the forecast.
A compromise among the 00-06Z GFS and 00Z ECMWF/UKMET best
represented the most common ideas of guidance from day 3 Tue into
day 5 Thu. The starting blend incorporated half total weight of
the 06Z GEFS/00Z ECMWF means for Fri and 80 percent for Sat.
...Weather Highlights/Threats...
The combination of a leading shortwave ejecting from the West and
some weak trailing energy will produce areas of rain and higher
elevation snow over the Southwest/Interior West and Rockies
Tue-Wed. Best potential for a brief period of enhanced activity
would be over favored terrain of Arizona on Tue. By Wed-Thu some
of the energy aloft and corresponding surface wave(s) may produce
generally light precipitation (likely more snow than rain) from
the northern Plains eastward.
Expect the next Pacific system to bring rain and high elevation
snow to parts of California and the Southwest mid-late week. Best
potential for highest precip totals currently exists over terrain
near the southern California coast but this is very sensitive to
the system's exact track which is not yet fully resolved.
Precipitation should expand and become heavier over the central
U.S. by Fri-Sat as the western trough aloft approaches and
moisture carried by low level Gulf inflow interacts with a stalled
front over the Plains. Thus far variability with progression of
the upper trough has lowered confidence in timing the onset of
enhanced precipitation. Wintry weather will be possible in the
northern/western periphery of the moisture shield. The Northwest
will see an episode of light-moderate precipitation into midweek.
After a brief break, northeastern Pacific flow should bring more
rain/higher elevation snow to the region though with uncertainty
over precise intensity and southward extent. Highest totals
should be over the Olympics.
The mean trough aloft will keep highs over a majority of the West
near to below normal with central/southern areas most likely to
see some minus 5-10F anomalies. Extreme northern areas may see
min/max readings drift a little above normal with time. East of
the Rockies, recent guidance adjustments for the pattern at the
surface and aloft result in greater persistence of central-eastern
U.S. warmth and higher peak anomalies. Highs of 10-20F above
normal should be common through the period and one or more days
with morning lows at least 20-25F above normal will be possible
from the Midwest into the Appalachians.
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