Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Sat Jun 29 2019
Valid 12Z Tue Jul 02 2019 - 12Z Sat Jul 06 2019
...Overview and Guidance/Predictability Assessment...
Relatively progressive northern stream flow will be squeezed
between a building upper ridge across the southern half of the
CONUS, and increasingly blocked flow at higher latitudes, from the
North Pacific and Alaska east across Canada. This will keep an
active upper-level jet in place from the Pacific Northwest to the
Midwest, Great Lakes, and Northeast through the medium range. A
wavy surface front will persist from the northern/central Plains
to the Great Lakes through much of the forecast period along the
northern periphery of the upper ridge, and will be reinforced by
the arrival of another polar front by the middle of next week. A
broad weakness in the upper ridge is also expected to initially be
in place across the southern Plains, with little in the way of
movement until a somewhat more significant shortwave traverses the
northern Plains/Midwest by late next week.
Model consensus was sufficient initially to use a multi-model
deterministic blend (ECMWF/GFS/UKMET/CMC) during days 3-5
(Tue-Thu). Models showed timing differences with shortwave energy
digging into the Pacific Northwest during this time frame, and a
consensus approach was preferred. By days 6-7 (Fri-Sat), model
differences continue to gradually increase as the aforementioned
shortwave moves east across the northern Plains/Midwest,
potentially interacting with additional shortwave energy diving
south around the broad upper low near Hudson Bay. Additionally, as
a blocking upper ridge strengthens across Alaska by late next
week, this should favor some degree of troughing/negative height
anomalies downstream of the ridge across western Canada and
perhaps the Pacific Northwest, but models are highly variable in
their solutions. Given the spread, leaned heavily toward
ECENS/GEFS ensemble means by days 6-7, with a bit more weight
toward the GEFS by day 7 as it showed an upper pattern more
consistent with what would be expected given teleconnections from
the Alaska ridge.
...Weather Highlights/Threats...
The wavy surface front extending from the northern Rockies to the
Great Lakes, and a second cold front arriving later in the week,
will focus areas of showers and thunderstorms across multiple
days, with heavy rainfall possible. Showers and thunderstorms
should spread east across portions of the Appalachians and
Mid-Atlantic regions by later next week. Meanwhile, high
temperatures of 5 to 10 deg F above average across the Southeast
and southern Mid-Atlantic will combine with a fairly humid air
mass to produce potentially hazardous heat index values over 100
degrees through much of next week.
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