Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
127 AM EDT Tue Apr 14 2020
Valid 12Z Fri Apr 17 2020 - 12Z Tue Apr 21 2020
...Overview and Guidance/Predictability Assessment...
The extended range period begins on Friday with a shortwave moving
down the Rockies into the Plains, and crossing the Ohio Valley on
Saturday. Models continue to struggle quite significantly both
with timing and intensity of this feature, even as early as day 4.
The latest run of the ECMWF (12z/Apr 13) slowed down quite a bit
with the shortwave compared to its previous couple of runs, which
looked closer to the faster, more amplified, and more consistent
GFS. The Canadian looks more like the GFS (slightly less
amplified), while the UKMET sides with the ECMWF. Ensemble members
exhibit just as much spread and so the ensemble means are
expectedly flatter with this wave, though its position may split
the difference between the GFS/CMC and the ECMWF/UKMET camps and
should be a good starting point.
Meanwhile, models continue to hone in on a closed low by day 3
just off the Southern California coast. This low will emerge into
the Southwest and eventually the Southern tier states this
weekend, with potential for frontal wave development across the
Southeast/southern Mid-Atlantic by Monday. The latest runs of the
GFS/ECMWF are fairly close with evolution of this feature,
although still exhibit high run to run variability so increased
weighting was given to the more agreeable and stable ensemble mean
guidance. The latest run of the Canadian tries to phase this
energy with an additional northern stream shortwave, resulting in
a much more amplified and more northerly position, which seems
well out of line with the rest of the guidance.
Behind the aforementioned shortwave, upper level ridging will
follow into the Western and eventually Central U.S. while another
possible closed low meanders off the West Coast embedded within
the larger scale Eastern Pacific/western Canada ridge. Energy
rotating around the base of this trough should send another
frontal boundary into the Southwest U.S. which reaches the
Southern Plains by day 7/Tuesday.
Across the board, this cycle of the WPC medium range progs favor a
blend of the deterministic GFS/ECMWF with the ECENS/GEFS means
with a bit more weighting towards the deterministic runs in the
beginning of the period for better definition of features. This
fits well with previous WPC continuity as well.
...Weather/Hazard Highlights...
As the unsettled eastern Pacific closed low/trough enters the
Southwest/Four Corners region this weekend, it should begin to tap
a return of downstream Gulf of Mexico moisture to fuel showers and
thunderstorms back into the forecast over the South. Thereās
potential for heavy rainfall associated with this system, mainly
from the lower Mississippi Valley into the Southeast, although
given model variability, there remains low confidence in any kind
of specifics at this time.
Farther north, almost daily showers can be expected this weekend
and into Monday across the Ohio Valley/Mid-Atlantic/Northeast
initially associated with the northern stream shortwave
Friday-Saturday, and then as the main cold front moves through
Sunday-Monday ahead of the larger scale troughing. Things should
dry out across much of the East by Tuesday as the cold front exits
into the Atlantic. Moisture should also return to the West Coast
states early next week as another upper trough lingers offshore.
For Temperatures across the CONUS, locations from the Rockies to
the Midwest and the Northeast will begin the period much below
normal, though quickly should moderate this weekend and into early
next week back towards normal. Meanwhile, temperatures should
trend warmer out west and eventually into the Plains states as
upper ridging follows behind the late week California closed low.
Santorelli
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