Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Sun Mar 17 2019
Valid 12Z Wed Mar 20 2019 - 12Z Sun Mar 24 2019
...Overview and Guidance Assessment/Preferences...
Latest guidance continues to anticipate a split flow regime over
western North America, with a persistent western Canada mean ridge
deflecting periodic bundles of shortwave energy into the western
Lower 48 and the rest into or around mainland Alaska. Aside from
some specifics there has been decent agreement that the leading
feature coming into the West will form an upper low near the
California coast around midweek and then gradually weaken as it
reaches the central U.S. Another shortwave should reach the West
by early next weekend but there is greater uncertainty for
specifics of this feature and upstream flow. Mean troughing will
prevail over eastern North America mid-late week and then
gradually shift into the western Atlantic. Within this trough
there is still a lot of guidance spread/run-to-run variability
with individual shortwaves and thus low confidence in forecast
specifics.
Compared to the past couple days guidance seems to be better
clustered for the leading system moving into/through the West and
eventually reaching the Plains. A consensus approach seems
reasonable given the low predictability of embedded small scale
impulses that will play a role in determining exactly where the
upper low center will be at a particular point in time. Upstream
details rapidly diverge during the latter half of the period.
There are essentially two clusters of guidance, the GFS/GEFS which
stray faster with North Pacific flow from earlier in the period
versus most other solutions (ECMWF/ECMWF mean/CMC/UKMET/NAVGEM and
to some degree the CMC mean) that are slower. As a result by late
in the period the GFS cluster already has another strong trough
nearing the West Coast by day 7 Sun and rapidly weakens the
shortwave reaching the coast the previous day. On the other hand
other solutions are slow enough upstream to allow more of the day
6 Sat energy to move into the southern half of the West on Sun.
Prefer to lean 3/4 toward the ECMWF cluster given the more
pronounced guidance signal toward that scenario and the fact that
it would represent somewhat better persistence of the mean pattern
that has become established.
Confidence remains well below average for details over the
central/eastern U.S., including possible surface low development
off the East Coast. The closed upper high forecast near or north
of the northern Montana border as of early Wed is continuing to
cause havoc with the forecast of shortwave energy over the
Plains/Mississippi Valley at that time. The 00Z ECMWF/NAVGEM are
in the minority today, holding more energy over the north-central
Plains on Wed versus established consensus. Further complicating
the forecast, there are various possibilities depicted for
amplification/timing of Canadian energy that may drop into the
eastern U.S. with some potential for interaction/phasing to occur.
The 06Z FV3 GFS is on the extreme side of guidance regarding
associated surface development and effects along parts of the East
Coast but there is in general a stronger signal for surface
development just close enough to the coast to merit close
monitoring. Note that the 12Z/16 ECMWF contributes to this
signal, with an initial Plains/MS Valley shortwave more similar to
the current majority. Additional shortwaves may feed into the
mean trough aloft behind the possible late week Atlantic
development but should reflect merely as cold fronts at the
surface.
Due to lower confidence in the 00Z ECMWF east of the Rockies
early-mid period, the primarily operational model blend weights
that solution somewhat lower than would normally be the case.
Then by next weekend the forecast phases out the less probable GFS
over the Pacific/western U.S. while emphasizing the 00Z
ECMWF/ECMWF mean/CMC and modest weight of the 06Z GEFS mean.
...Weather Highlights and Hazards...
The southern stream system initially heading into the West will
bring rain and mountain snow to most locations from California
eastward into the central/southern Rockies mid-late week. Most of
the precipitation may stay just north of the Desert
Southwest/southern Arizona though. Typically favored terrain
should see the highest totals with this system. Expect the
central/southern Plains to see greater coverage and intensity of
showers/thunderstorms during the late week into weekend time frame
as moisture increases ahead of the western system. Some areas of
heavy rainfall are possible. The next system reaching the West
Coast late Fri-Sat will likely focus rain and high elevation snow
over central/northern areas with lighter activity extending
farther south. Areas near the central West Coast may see a brief
period of moderate-heavy precipitation. Confidence decreases in
coverage/areas of greater focus over the West by next Sun. To the
east of the Rockies, some rainfall may accompany a weakening
wave/frontal system over the Plains/east-central U.S. early in the
period. Frontal passages may bring one or more episodes of mostly
light rain/snow to the Great Lakes/Northeast. Specifics of
potential surface low development off the East Coast continue to
be a major uncertainty around Thu-Fri. The most probable scenario
is for any system to track far enough offshore to keep most
precipitation away from East Coast locations. However there is
still an ongoing lesser threat for more meaningful effects that
require monitoring.
The overall pattern should keep daytime highs somewhat below
normal over the central/southern latitudes of the West, though
clouds/precipitation will keep lows on the warmer side of normal
(by 10-15F in some cases). Aside from periodic cool pushes into
the Northeast, there should be expanding coverage of above normal
temperatures to the east of the Rockies during Fri-Sun, with some
plus 10-20F anomalies possible over parts of the
Plains/Mississippi Valley.
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