Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
227 AM EDT Thu Jul 04 2019
Valid 12Z Sun Jul 07 2019 - 12Z Thu Jul 11 2019
...Overview and Guidance/Predictability Assessment...
Downstream of a Rex block across the North Pacific/Alaska,
moderately progressive flow will pass between a building upper
ridge across the CONUS southern tier, and a mean upper trough near
Hudson Bay. A series of convection focusing shortwaves will
traverse the zone of stronger flow, across the U.S. northern tier.
This will act to push a cold front southeastward off the Eastern
Seaboard on Thu, with the trailing end of the front expected to
stall from the Southern Plains east to off the Southeast U.S.
coast. Meanwhile, shortwave energy reaching the Pacific Northwest
this weekend may separate into a more progressive northern
component and a slower-moving shortwave crossing the Great
Basin/northern Rockies through early next week, and the Northern
Plains/Midwest by the middle of next week.
A multi-model deterministic blend including the
ECMWF/GFS/UKMET/CMC was used initially during days 3-4 (Sun-Mon).
Model solutions were reasonably comparable to one another during
this time frame, and a consensus/blend approach was favored.
Starting on day 5 (Tue), deterministic solutions began to diverge
substantially on the evolution of the western shortwave, and how
much of the energy gets quickly sheared east in the stronger
westerlies versus lingering in a deeper/slower western trough. In
general, the trend has been to keep a somewhat more amplified and
slower wave, with a less significant component shearing east in
the polar jet. This idea has relatively strong support from the
ensemble means, but as mentioned, deterministic solutions were
much more variable. The solution most closely clustered with the
ECENS/GEFS means was the 12Z CMC. Meanwhile, the 12Z run of the
ECMWF was nearly out of phase with its own ensemble mean, moving
much more of the energy eastward quickly. As a result of these
differences, the forecast during days 5-7 (Tue-Thu) was based
heavily on a blend of the ECENS/GEFS means and the deterministic
12Z CMC/GEM global.
...Weather Highlights/Threats...
The frontal boundary crossing the central/eastern U.S. through
much of the forecast period will focus areas of scattered to
numerous showers and thunderstorms across a large area, including
portions of the Central/Northern Plains, Ohio/Tennessee Valleys,
and the Southeast/Mid-Atlantic through the middle of next week.
Models show a signal for areas of heavy rainfall along the
immediate Southeast U.S. coastline from Sunday onward. A renewed
threat of more widespread convection may develop across the
Northern Plains and Upper Midwest by Mon-Wed as the next
shortwave/frontal system cross the region. Temperatures should be
relatively close to seasonal norms across much of the CONUS
through the medium range. Cooler temperatures (highs 5 to 8 deg F
below average) are forecast across portions of the West early next
week as the upper-level shortwave crosses 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