Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Tue Mar 19 2019
Valid 12Z Fri Mar 22 2019 - 12Z Tue Mar 26 2019
...16Z Update...
The 00Z and 06Z suite of models maintained a signal for a
reasonably strong low pressure system centered near Cape Cod on
day 3 (Fri). The GFS and ECMWF and their ensembles are in good
agreement on the placement of the low, while the 00Z CMC and UKMET
maintained a more southern solution like their previous runs.
Leaned away from those models since they were not supported well
by the clustering of the lows in the ensembles. The central
pressure of the low has come into better alignment at least with
the GFS/ECMWF, with the 00Z ECMWF weaker compared to the previous
run, closer to the GFS, but uncertainty remains as agreement does
not necessarily equal correctness.
Another surface low is expected to be slow-moving across the
Southern/Central Rockies to Plains on day 3-5, before moving
eastward more quickly on day 6-7 as upper-level energy solidifies
to intensify a trough in the eastern U.S. It seems that models are
in better agreement on the eastward progression of the low than
the last cycle, with the 00Z and 06Z GFS taking a more southern
track across the Middle/Lower Mississippi Valley on day 6 and the
Southeast on day 7 than before. This is likely due to better
consensus on a stronger surface high coming into the Upper
Midwest. Details remain elusive, however, with the strength and
position of this low. The medium range suite of products leaned
toward the 00Z ECMWF mean and operational run, with components of
the 00Z/06Z GFS and GEFS mean.
See previous discussion for more details, including sensible
weather information.
Tate
...Previous Discussion...
...Overview and Guidance Assessment/Preferences...
Split flow pattern is expected to persist across North America
during the extended period. Ensemble guidance shows good consensus
that a strong and blocking upper-level ridge axis initially across
the High Plains of the U.S. into Alberta/Saskatchewan will meander
in place over the next week. This will continue to split incoming
energy from the Pacific into northern stream components directed
northward toward Alaska, and southern stream components that
traverse the southern/central U.S., undercutting the ridge.
Downstream of the ridge, the Northeast will be subject to
continued mean troughing, reinforced by a series of energetic
shortwaves. Some guidance suggests that perhaps this northern
stream amplified ridge/trough pattern may begin to weaken by early
next week, but confidence in that is relatively low at this time
given increased spread among the ensemble guidance as to the
intensity and duration of the upper ridge.
Model guidance has made a dramatic shift over the past 24 hours
with respect to a low pressure system along/off the Eastern
Seaboard. At this time last night, the consensus among the
majority of guidance was for a track farther east, with
significant deepening delayed until the system reached the
Canadian Maritimes. Each consecutive run of the guidance, this
morning and this evening has generally shifted the track west,
with the ECMWF and GFS now showing a significant surface low just
east of Cape Cod Fri morning (and the CMC/UKMET much farther
southeast). Clustering of ensemble members around this idea is
quite good among ECENS and GEFS members, with a secondary cluster
farther southeast/offshore comprised primarily of Canadian
ensemble members. Clustering around these solutions remains
generally good into Fri night-Sat as the surface low moves
northeastward, with some modest increase in spread. Farther west,
a more stable consensus continues with respect to a
shortwave/upper low moving from the Four Corners region to the
Plains Fri-Sat. Solutions were generally well-clustered with
respect to the timing and intensity of this feature during days
3-4. By day 5 (Sun), as the shortwave moves into the Midwest,
solutions differ as to how quickly it will weaken, but there is
general consensus that the feature should gradually shear out as
it encounters a col between the northern/southern stream flow and
the upper ridge. Farther west, additional Pacific shortwave energy
is forecast to reach the West Coast Sat, and then split, with a
more significant southern component also moving east across the
Four Corners region Sun. Models differ on their handling of this
splitting as well, with the ECMWF persistent in showing the
strongest southern component. The complex nature of this process
lends to low predictability and confidence in this aspect of the
forecast. Based on these considerations, the forecast during days
3-5 was based primarily on a blend of the ECMWF and GFS, with some
influence from their respective ensemble means as well.
By days 6-7 (Mon-Tue) the second Pacific shortwave should reach
the Southern/Central Plains in some form (solutions are similar in
timing even if they differ on the amplitude), likely promoting
development of a surface low. Ensemble members show some degree of
clustering even through day 7 with respect to this system, and the
ECMWF was generally well-centered in the spread. The GFS, on the
other hand, seems to suffer its prevalent bias and accelerate the
system eastward far more quickly than consensus. Differences also
emerge in the northern stream by early next week, with different
degrees of troughing across New England. Given some degree of
persistent ridging upstream, would likely lean toward a somewhat
more amplified trough for New England as shown by the ECMWF and
18Z/00Z GFS runs. The GFS has come more in line with consensus
with another system off the West Coast early next week, now
showing a closed upper low linger offshore rather than an open
wave rushing inland as it showed last night. Based on these
factors, the forecast during days 6-7 was based primarily on a
blend of the ECMWF and the ECENS/GEFS ensemble means.
...Weather Highlights and Hazards...
The low pressure system off the Northeast coast early Fri is
expected to produce widespread precipitation for New England. Rain
will be likely close to the coast, with snow possible for the
interior. Marginal thermal profiles will make the precipitation
type across New England highly sensitive to the eventual track of
the surface low. Showers and thunderstorms should increase in
coverage across portions of the Southern and Central Plains
Fri-Sat ahead of the leading Pacific system. By Sun-Mon, as the
second Pacific shortwave reaches the central U.S., rain and
thunderstorms should spread east into the Mississippi/Ohio
Valleys, Gulf Coast, and portions of the Southeast. Confidence is
low as to the degree of moisture return and the strength of the
surface low, but at least locally heavy rain appears possible.
Across the West, the passage of these systems will bring a couple
rounds of elevation-based rain/snow from the West Coast the Great
Basin and Rockies. Precipitation may increase once again for
northern California early next week ahead of the next Pacific low
pressure system.
Meanwhile, the Midwest will see the greatest potential for above
average temperatures through the medium range. High temperatures
of 5-15 deg F above average are expected Fri-Sat, with above
average temperatures expanding eastward by Sun-Mon.
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
Hazards:
- Heavy rain across portions of California and the Pacific
Northwest, Tue, Mar 26.
- Flooding possible across portions of the Northern/Central
Plains, the Tennessee Valley, the Upper/Middle Mississippi Valley,
and the Great Lakes.
- Flooding occurring or imminent across portions of the
Northern/Central Plains, Mississippi Valley, the Tennessee Valley,
the Great Lakes, and the Ohio Valley.
- Flooding likely across portions of the Northern/Central Plains,
the Upper/Middle Mississippi Valley, and the Great Lakes.
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