Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
149 AM EDT Thu May 30 2019
Valid 12Z Sun Jun 02 2019 - 12Z Thu Jun 06 2019
...Overview/Weather Highlights and Guidance
Evaluation/Preferences...
An active northern stream will flow pattern is expected to
continue, with a strong North Pacific upper-level jet splitting as
it approaches the West Coast of North America. An upper-level low
will persist in the vicinity of California through much of the
extended period, only very slowly moving eastward. Meanwhile, the
subtropical ridge will migrate westward and reposition over the
south central U.S. Farther north, an upper ridge axis initially
across central Canada should slowly move east and weaken, with
some potential for ridging to become reestablished across western
Canada and the northern U.S. Rockies by the middle of next week.
Downstream, a persistent upper low across eastern Canada will
cause incoming northern stream shortwaves to periodically amplify
as they dig into the Great Lakes and Northeast, traversing the
broader anticyclonic flow. These digging waves along with the
westward migration of the subtropical ridge will allow for heights
to fall across much of the Great Lakes/Ohio Valley and the
Mid-Atlantic/Northeast, ushering in a more comfortable air mass
with high temperatures near or several degrees below average
through much of the period.
Relatively slow-progressing southern stream shortwaves will
periodically eject eastward from the upper low, reaching the
central U.S. and potentially interacting with amplifying northern
stream energy. These waves along with return flow across a
persistent baroclinic zone may continue to support areas of
increased convective potential from the Southern/Central Plains
into the Mid-Mississippi Valley, with the potential for at least
locally heavy rainfall/flash flooding as well as strong
thunderstorms.
The WPC medium range product suite was initially derived from a
blend of the 12Z ECMWF/CMC/18Z GFS solutions during days 3-5
(Sun-Tue). These solutions handled features similarly and any
differences were favorably smoothed out by the blend. By days 6-7
(Wed-Thu), more significant differences emerged with respect to
whether upper ridging becomes reestablished across the northern
Rockies/Great Basin and western Canada (GFS/GEFS), or whether
stronger Pacific energy keeps the ridge broken down, with stronger
height falls moving inland by Thu. At this time, chose to lean
heavily toward the GEFS, and show at least some reestablishment of
the western ridge, and a slower arrival of Pacific energy by the
middle of next week. This decision was informed by an analysis of
teleconnections based on some of the stronger hemispheric 500 hPa
height anomalies persistent throughout the medium range, including
a blocking ridge across north central Asia, and negative height
anomalies north of the British Isles. Teleconnections associated
with both of these anomaly centers supported the idea of ridging
from the northwestern U.S. into western Canada, with mean
troughing across eastern Canada. Thus, the forecast for days 6-7
was based mostly on the GEFS/GFS solutions, with a minority
component of the ECENS mean included as well.
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