Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1250 AM EDT Sat Jul 14 2018
Valid 12Z Tue Jul 17 2018 - 12Z Sat Jul 21 2018
...Overview...
The flow pattern across the contiguous U.S. will become a bit more
amplified during the medium range period with an upper-level ridge
expanding from the Great Basin to the southern Rockies, and a
series of upper waves gradually carving out a trough across the
Great Lakes. Heights are also expected to fall across the Pacific
Northwest by late next week as an upper low digs south from the
Gulf of Alaska toward the region.
...Guidance Evaluation and Preferences...
A majority deterministic blend including the ECMWF/GFS/CMC was
initially preferred during days 3-5 (Tue-Thu). Models have settled
toward a somewhat slower/more amplified solution with the
trough/upper low passing south of Hudson Bay and moving east into
Quebec during this time frame. The same generally holds true for
additional shortwave energy crossing the north central U.S.
Wed-Thu, which then looks likely to amplify further across the
Great Lakes/Ohio Valley Fri-Sat. Farther west, models continue to
show a compact but energetic shortwave diving southeast across the
Gulf of Alaska toward British Columbia Tue-Wed, with heights
falling across the U.S. Pacific Northwest by later in the week.
Some timing differences remain, but in general guidance has
trended toward somewhat slower/more amplified solutions here as
well. Given the increase in spread, majority weighting was shifted
to ensemble means for days 6-7 (Fri-Sat), including the ECENS and
NAEFS means.
...Weather Highlights/Threats...
Showers and storms with locally heavy rain will accompany a cold
front from the Appalachians to the
Southeast/Mid-Atlantic/Northeast Tue-Wed. The trailing end of the
frontal boundary is expected to stall from the Southeast to the
southern/central plains, which could also focus locally heavy
showers and thunderstorms through later next week. Monsoonal
moisture will also produce scattered afternoon/evening
thunderstorms from the southern Great Basin/Southwest to the
southern/central Rockies through the next week, with locally heavy
rain possible especially early in the week (the building
upper-level ridge should gradually reduce the coverage of
convection by later next week).
In the wake of the cold front, high temperatures are expected to
be 5 to 10 deg below average across the central/northern plains
and portions of the Midwest/Great Lakes through much of next week.
Hot temperatures across the Pacific Northwest (highs 5 to 10 deg
above average) should moderate some by later next week as a
Pacific cold front moves inland.
Ryan
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
http://www.wpc.ncep.noaa.gov/heat_index.shtml