Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
138 AM EDT Wed May 29 2019
Valid 12Z Sat Jun 01 2019 - 12Z Wed Jun 05 2019
...Overview and Guidance Evaluation/Preferences...
A split flow pattern is expected to be in place across North
America on day 3 (Sat). Flow associated with an active North
Pacific upper-level jet will split as it approaches the West Coast
of North America where a persistent broad upper-level low will be
in place near California. An upper ridge axis initially across
western Canada is expected to shift eastward to central Canada and
the north central U.S. by Sun-Mon. Farther east, northern stream
energy will periodically encounter a persistent upper low
initially in the vicinity of Hudson Bay, and gradually drifting
eastward through the forecast period. These shortwaves will
amplify as they dig into the Great Lakes and Northeast, traversing
the broad cyclonic flow. Meanwhile, slower progressing southern
stream energy will periodically eject from the California upper
low, reaching the central U.S. and potentially interacting with
amplifying northern stream energy. Meanwhile, the upper-level
ridge which has been persistent across the Southeast and eastern
Gulf of Mexico as of late will begin a slow retrogression westward
this weekend into next week. This will not significantly change
the feed of warm and moist air northward into the central U.S.,
but it will allow for temperatures across the Southeast to return
much closer to seasonal norms.
A blend of the 12Z ECMWF/UKMET and 18Z GFS solutions was used as a
basis for the forecast during days 3-5 (Sat-Mon). These solutions
showed some timing/amplitude differences with the shortwave/low
pressure system crossing the Great Lakes/Ohio Valley/Northeast
during this time period, but the blend represented a good
consensus. Differences also exist in how shortwave energy is
handled traversing the North Pacific jet, reaching British
Columbia and the Pacific Northwest by Mon. During days 6-7
(Tue-Wed), a rapid succession of shortwaves traversing from the
North Pacific along the U.S./Canada border becomes the primary
forecast concern. Given the relatively high frequency nature of
this flow and amplifying differences/inconsistencies among
solutions, a lean heavily toward ECENS/GEFS ensemble means was
preferred by days 6-7. This approach supported the idea of a
frontal system moving into the north central U.S. by Tue and into
the Great Lakes by Wed, with the trailing cold front crossing the
Plains and stalling across the Rockies/Great Basin.
...Weather Highlights and Hazards...
Areas of showers and thunderstorms with locally heavy rainfall are
possible across portion of the Southern and Central Plains Sat
north of a weakening surface front across south Texas. Showers and
thunderstorms may become more widespread across these areas
Sun-Tue, with continued heavy rain potential, as another frontal
boundary arrives and stalls, providing additional
focus/enhancement of convection. This same frontal boundary will
also bring scattered to numerous showers and thunderstorms to
areas from the Midwest and Ohio Valley to the Mid-Atlantic and
Northeast Sat-Sun, with locally heavy rainfall a possibility. The
arrival of yet another frontal system across the north central
U.S. Tue-Wed may bring another round of increased showers and
thunderstorms to the Northern and Central Plains. Farther west,
periodic showers are expected from the Great Basin to the
central/northern Rockies as a couple weak southern stream
shortwaves traverse the region Sat-Sun.
Temperatures across much of the Southeast are expected to remain
above average, although not to the extent seen in recent days.
High temperatures are forecast to be several degrees above average
into early next week, before perhaps falling close to seasonal
norms by Wed. Farther west, the Pacific Northwest will initially
be quite warm, with highs expected to be 10 to 15 deg above
average with the upper ridge in the vicinity. These temperatures
should moderate slightly by early next week, but above average
temperatures are expected to continue as northern stream shortwave
energy fails to lower heights substantially across the region.
Across the lower Great Lakes, Mid-Atlantic, and Northeast, passage
of a cold front may bring a brief period of below average
temperatures Sun-Tue, with high temperatures across portions of
those areas several degrees below average.
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