Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sat Apr 06 2019
Valid 12Z Tue Apr 09 2019 - 12Z Sat Apr 13 2019
...Confidence increasing that a significant late-season storm will
impact portions of the Northern Plains and Midwest with heavy rain
and snow mid to late next week...
...Overview and Guidance Assessment/Preferences...
A moderately progressive northern stream early in the medium range
will become more amplified over time, with split flow becoming
increasingly prominent through the extended period. One southern
stream shortwave is expected to cross the Southeast U.S. Tue-Wed,
with another crossing from the West Coast to the Four
Corners/Rockies during the same time frame. The second of these
waves is forecast to quickly amplify as it approaches the central
U.S. late Tue into Wed, with support increasing for a significant
late-season cyclone across the central U.S. by Wed-Fri. Additional
shortwave energy is expected to drop southward along the West
Coast Wed-Sat, eventually consolidating into an anomalous upper
trough (500 hPa height anomalies of -2 to -3 standard deviations)
across the southwestern states.
A multi-model deterministic blend was used as a forecast starting
point during days 3-5 (Tue-Thu), including the 06Z GFS and 00Z
ECMWF/UKMET/CMC. Guidance has shown a trend toward more separation
of streams across the eastern U.S. Tue-Wed, keeping a somewhat
less amplified and more distinct southern stream system that
crosses the Southeast and moves east into the Atlantic. This trend
was reflected across a range of model/ensemble solutions and is
well-represented by a general model blend. Farther west, models
handle the shortwave crossing the Western U.S. Tue-Thu similarly,
with only minor differences in timing and/or amplitude. Models
have shown significant difficulty over the past couple days
handling this system as the surface low deepens rapidly across the
Central Plains Wed and then lifts northeastward. Solutions have
varied between keeping a more open upper wave and quickly moving
the surface low eastward, and a more closed upper low with a
deeper surface low that progresses east much more slowly. There
was a large reduction in spread during the last ensemble cycle,
with solutions overwhelmingly trending toward the idea of a
deeper/slower system. Interestingly, the UKMET has been the most
consistent deterministic solution, showing a solution similar to
the current consensus for at least the past three runs. Given
these considerations, and solutions which were better centered
within the ensemble consensus, the 00Z ECMWF/UKMET solutions were
weighted a bit more relative to the GFS/CMC, especially by
Wed-Thu.
Spread still increases with respect to the evolution of this
system during days 6-7 (Fri-Sat), and its potential interaction
with the northern stream as it moves into Canada. Given
significant continued run-to-run variability from Fri onward,
opted to lean much more heavily on ECENS/GEFS ensemble means
during that time frame. This preference holds along the West Coast
as well, where models have struggled to deal with the specifics of
a series of shortwaves dropping southward across the West
Coast/Great Basin. There is some consensus, however, that energy
should eventually consolidate into a broad and anomalous
upper-level trough across the Southwest by Fri-Sat.
...Weather Highlights/Hazards...
Confidence has increased that a deep late-season storm system
should track from the Central Plains to the Midwest/Great Lakes
during the Wed-Fri time period. The upper-level shortwave will
initially bring rains and potentially heavy mountain snows to much
of the Rockies Tue into Wed. After the system emerges into the
Plains, models show improved consensus that significant band of
dynamically forced precipitation is likely to the north of the low
track, from portions of eastern Wyoming and the Dakotas east to
the Upper Midwest. A variety of precipitation types are likely,
but sufficient cold air should be in place to support a band of
moderate to heavy snow. Some areas near the precipitation
transition zone will likely see the potential for both heavy rain
and snow. Additionally, the deep nature of the low (central
pressures likely in the mid 980-mb range), will likely result in
the potential for strong and gusty winds across a large area of
the central U.S. Farther south, limited moisture return and
potential for a capping inversion will limit precipitation
coverage ahead of the system across the Southern Plains, although
given the strong and dynamic nature of the low pressure system,
any isolated to scattered convection that develops could be
strong. As the low moves into Canada by Fri-Sat and the cold front
sweeps into the eastern U.S., widespread rains are likely from the
Ohio Valley to the Mid-Atlantic and Northeast. Farther west, the
series of shortwaves diving south along the West Coast Wed-Sat
will support continued scattered and periodic rounds of rain and
mountain snow for much of the West.
Warm temperatures are expected ahead of the deep low pressure
system Tue-Wed from the Plains to the lower/mid-Mississippi
Valley, where high temperatures of 5 to 15 deg F above average are
likely. In the wake of this low pressure system, a colder polar
air mass will overspread areas from the northern Rockies to the
Upper Midwest, with high temperatures as cold as 15 to 25 deg
below average expected. A broad area of cool temperatures is
expected across the interior western U.S. by late next week as
upper troughing amplifies overhead.
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