Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 AM EDT Tue May 28 2019
Valid 12Z Fri May 31 2019 - 12Z Tue Jun 04 2019
...Overview and Guidance Evaluation/Preferences...
Split flow is expected to persist across North America through
most of the medium range. The subtropical ridge is expected to
remain in place across the Gulf of Mexico as it begins a very slow
retrogression westward. Flow associated with an active North
Pacific upper-level jet will split as it approaches the West Coast
of North America where a ridge axis will be in place initially,
before gradually shifting eastward to the central U.S./Canada by
late in the weekend into early next week. As northern stream
energy periodically encounters a persistent upper low in the
vicinity of Hudson Bay, the energy will amplify in the broad
cyclonic flow with shortwaves digging into the north central and
northeastern U.S. Meanwhile slower progressing southern stream
energy will move from the Great Basin/Rockies to the central U.S.
and Great Lakes, where interaction with amplifying northern stream
energy is possible.
Model consensus was sufficient during the first part of the medium
range to use a multi-model deterministic blend (including the 12Z
ECMWF/CMC and 18Z GFS) during days 3-4 (Fri-Sat). The CMC was
dropped from the blend by day 5 (Sun) as some significant
differences began to emerge with how that solution handled
northern stream shortwave energy across east central Canada. A
wave of low pressure is expected to quickly cross the Upper
Midwest/Great Lakes/Northeast during this time period, with a
trailing frontal boundary extending south into the
Ohio/Mississippi Valleys and the Plains. Meanwhile, a northern
stream shortwave is expected to move into western Canada,
weakening as it moves into the ridge, with a cold front moving
into the Pacific Northwest.
By days 6-7 (Mon-Tue), models show decent consensus that the
northern stream ridge axis should shift eastward to the Northern
Plains/Midwest, allowing stronger height falls to reach the
Pacific Northwest. Meanwhile, the subtropical ridge should
gradually retrograde to the western Gulf of Mexico, keeping a warm
and moist air mass feeding northward into the south central
states. Leading shortwave energy crossing the northern Rockies Sun
should bring a low pressure system into south central Canada by
early next week with the associated frontal system moving across
the Northern Plains and Midwest. Spread among deterministic
solutions was higher for this system given the time range, but
ensemble means painted a somewhat more consistent picture in terms
of timing/placement of the surface low. Thus, majority weight was
shifted to ECENS/GEFS ensemble means in the forecast for days 6-7.
...Weather Highlights and Hazards...
Areas of showers and thunderstorms with locally heavy rainfall are
possible across portion of the Southern and Central Plains Fri-Sat
north of a weakening surface front along the Rio Grande Valley.
Showers and thunderstorms may become more widespread across these
areas Sun-Tue, with continued heavy rain potential, as another
cold front arrives from the north, providing additional
focus/enhancement of convection. 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.
This activity should be a bit more widespread Fri-Sat before
becoming more sparse Sun-Tue as the most significant shortwave
energy moves east and heights gradually rise across the region.
Above average temperatures are expected to continue across the
Southeast, although not to the extent seen in recent days. High
temperatures are forecast to be 5 to 10 deg F above average across
this area into early next week. 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.
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