Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Thu Mar 21 2019
Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019
...Overview and Guidance Assessment/Preferences...
Anomalous upper-level ridging extending from western Canada into
the north central U.S. this weekend is forecast by gradually break
down during the early to middle portion of next week. Some degree
of ridging appears likely to become reestablished from the Gulf
of Alaska north across Alaska - a position much more common in
recent months. Mean troughing should persist across the north
central Pacific, bringing a continuation of the recently seen
pattern with Pacific shortwave energy crossing the CONUS every
couple days. The reconfiguration of flow at the higher latitudes
will have some implications farther south, with less split flow
and greater likelihood of phased systems across the central U.S.
by mid to late next week. In the meantime, one significant
southern stream system remains during the medium range, affecting
the south central and southeastern U.S. Sun-Tue.
Models continue to struggle with the system crossing the southern
U.S. from Sun onward, and the potential evolution of a coastal low
along/off the Eastern Seaboard early-mid next week. In general,
guidance over the past day has trended toward a quicker eastward
progression, and somewhat less likelihood of a significant low
along the Southeast/Mid-Atlantic coast early next week. Ensemble
spread remains quite large, however. At 12Z on Tue, for example,
ensemble members range from a low still over the Mississippi
River, to others having energy already transferred to a new low
off the Southeast coast. While the consensus of solutions has
shifted faster, spread also seems to have increased over the past
few ensemble cycles. Thus, confidence is low, and would expect
continued run-to-run variability. Opted to tweak the forecast
toward a faster progression rather than making a dramatic change
given the low confidence. This resulted in a blend weighted a bit
more toward the slower solutions (12Z ECMWF/CMC) and not much
weight on the GFS which continues to lead the pack in terms of
eastward progression of the system and quickly moving any low out
into the open Atlantic early next week. Given significant
uncertainty as to how quickly any low along the Southeast coast by
the middle of next week moves out to sea or, perhaps up the coast
as the 12Z CMC and a handful of ensemble members show, maintained
at least low PoPs for portions of the Southeast/Mid-Atlantic into
midweek.
Elsewhere, models show better consensus that one weakening
shortwave should reach the West Coast late Mon, with a more
significant wave arriving late Tue, moving into the Great Basin or
Rockies Wed night-Thu. Models handle the first wave similarly, but
some larger differences emerge by the middle of next week with the
second system. Some timing differences emerge with the second
system Wed-Thu. Differences also exist as to structure of the
north Pacific ridge, with impacts on how/if the second shortwave
interacts with additional northern stream energy. In general, a
more ensemble-based approach was taken to the forecast by Wed-Thu,
with increased weight on the ECENS and NAEFS ensemble means.
...Weather Highlights and Hazards...
The southern stream system is expected to bring rain and
thunderstorms from portions of the Southern Plains and
lower/mid-Mississippi Valley to the Southeast. Locally heavy rain
is possible, but no strong signal for excessive rains is evident
in the guidance at this point given the trend toward increased
forward speed of the system. Cold temperatures on the northern end
of the precipitation shield may marginally support snow or a
rain/snow mix for some areas from the central Appalachians and
Mid-Atlantic, and perhaps into the Northeast. Meanwhile, the West
will see a couple rounds of precipitation from the incoming
Pacific systems, bringing rain and mountain snows to much of the
central/northern Rockies and from central California to the
Pacific Northwest. The arrival of the more significant system by
the middle of next week produces a stronger signal for potentially
heavy precip (rain and mountain snow) across portions of central
and northern California. Given the increasingly progressive flow
pattern across the CONUS, temperatures will be highly variable,
with much of the country seeing both above and below average
temperatures at different points during the extended period.
Ryan
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