Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Tue Apr 23 2019
Valid 12Z Fri Apr 26 2019 - 12Z Tue Apr 30 2019
...16Z Update...
No major changes were needed to the overnight medium range
forecast package. Still expect an upper low to move through the
eastern Pacific this weekend, into California on Mon, and into the
Four Corners or Central Plains by Tue. This feature seemed to be
handled reasonably well by most deterministic and operational
models, though by Sun the 00Z CMC propagates that low faster than
consensus, so chose to exclude that from the forecast package,
going with a blend of the 00Z and 06Z GFS, 00Z GEFS, and the 00Z
ECMWF and its mean. The 06Z GFS does strengthen a surface low in
the Plains more than we were prepared to put in our forecast yet.
In the northern stream, shortwaves are expected to traverse the
northern tier of the U.S. throughout the period, with some model
differences in strength and timing. As discussed overnight, a
shortwave on Sat-Sun has come into better agreement, but a
different shortwave moving through the Great Lakes region on Sun
is stronger in the GFS/GEFS than in the EC suite, which leads to
slower movement and the GFS suite keeping the feature back on Mon,
while the EC progresses the energy more. A compromise seems like
the best solution at this point.
For more details, see the previous discussion below.
Tate
...Previous Discussion...
...Overview and Guidance Preferences...
Upper-level flow across the CONUS is not expected to change
significantly during the extended forecast period. A progressive
northern stream and less progressive southern stream, similar to
the pattern seen in recent weeks, is expected to persist.
Models/ensembles do hint at potential northern stream flow
amplification from the North Pacific into Canada by next weekend,
however. Development of a broad Rex block extending from Hawaii to
Alaska is expected, which will reinforce upper troughing centered
west of Hudson Bay. Conceptually, this setup seems to favor
potential southward transport of polar air into central Canada and
the north central U.S., and this bears out when teleconnections
associated with the Pacific Rex block are examined.
Began the forecast during days 3-5 (Fri-Sun) with a multi-model
deterministic blend (12Z ECMWF/UKMET/CMC/18Z GFS). Models have
trended slightly more amplified with northern stream shortwave
energy crossing the Great Lakes on Fri, resulting in a quicker
progression of energy initially across the Southeast, and more
phasing between the two streams. The resulting phased low pressure
system should deepen as it quickly lifts into the Canadian
Maritimes by Sat. Consensus has improved around an energetic
shortwave expected to dive southeastward into the Pacific
Northwest and northern Rockies Sat-Sun, as another cutoff southern
stream upper low approaches California.
Beyond Sun, there remains a significant question as to how much
heights fall from the northern Rockies to the Northern High
Plains. Upstream ridge amplification across the north Pacific
should favor the tendency for cyclonic upper-level flow and
potential trough amplification across the Northwest, and this is
the direction models seem to be trending at the moment. Current
model trends suggest that the amplifying trough across the
Northwest by early next week should begin to absorb the initially
cutoff southern stream feature as it moves into the Four Corners
region, more rapidly kicking it eastward into the Southern/Central
Plains by Tue. Ensemble spaghetti plots reveal a fair amount of
spread involving the trough amplification as well as the evolution
of the flow across Canada (with implications for whether colder
polar air is able to spread south into the north central U.S.).
Thus, confidence is below average by early next week, and a trend
toward heavier weight of ensemble means was favored in the
forecast. Regardless of the specifics of the western trough,
ensembles show broad agreement that subtropical ridging should
gradually build into the Southeast and southern Mid-Atlantic
regions by early next week.
...Weather Highlights/Threats...
The shortwave and surface low pressure system traversing the
Southeast Fri-Sat should be accompanied by scattered to numerous
showers and thunderstorms, with the potential for at least locally
heavy rainfall. As the phased low pressure system develops across
New England late Fri into Sat, widespread rain is expected,
perhaps even mixing with snow across interior portions of far
northern New England. Farther west, a low pressure system moving
from the Northern/Central Plains to the Great Lakes Sat-Sun should
bring rain to much of the Midwest, with the potential for some
areas of snowfall on the northern edge of the system, from
northern Minnesota east into northern Wisconsin and Michigan. A
stalled frontal boundary across the central U.S. in the wake of
this system will continue to generate areas of showers and
thunderstorms into early next week from the Central Plains to the
Midwest, with further enhancement possible by Tue as the next
shortwave approaches. Rain and mountain snow will become more
common across much of the interior western U.S. by the weekend
into early next week as well, as upper troughing amplified
overhead.
Above average temperatures are expected Fr-Sun from California and
the Great Basin east to the High Plains. High temperatures of 10
to 15 deg F above average are expected to be common. Early next
week, warm temperatures are expected to spread eastward to the
Southeast/southern Mid-Atlantic as the subtropical ridge builds.
At the same time, falling heights across the northern tier should
bring below average temperatures from the northern Rockies to the
Upper Midwest from Sun onward.
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 the Central Plains and the Middle
Mississippi Valley, Tue, Apr 30.
- Heavy rain across portions of the Northeast, Sat, Apr 27.
- Flooding possible across portions of the Southern Rockies, the
Great Lakes, the Northeast, the Southern Plains, and the Upper
Mississippi Valley.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley, the Northern Plains, the Tennessee Valley, the
Northern Great Basin, the Middle Mississippi Valley, the
Northeast, the Upper Mississippi Valley, the Great Lakes, and the
Ohio Valley.
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