Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Tue Jul 10 2018
Valid 12Z Fri Jul 13 2018 - 12Z Tue Jul 17 2018
...Overview, guidance evaluation and preferences...
Model guidance this evening shows good overall agreement on the
evolution of the synoptic pattern through next Tuesday although
the spread has increased since yesterday. The main uncertainty
will be across the Pacific Northwest eastward into the northern
Plains this weekend into early next week as a shortwave trough is
forecast to move along or just north of the U.S.-Canadian border.
The GFS has introduced a digging and higher amplitude shortwave in
the last few runs while the ECMWF keeps a less-amplified
positively-tilted trough mainly north of the border. The GEFS on
the other hand shows a less-amplified mean solution comparable to
the EC mean. The ECMWF has actually shown a tendency for
increasing the 500 mb height over the northwestern U.S. while the
GFS shows the opposite. A general compromise between the 18Z GEFS
and the 12Z EC mean mixing in with their deterministic solutions
was used to handle this area from Day 5 through Day 7
(Sunday-Tuesday), which maintains good continuity with the
previous WPC package.
Otherwise, models continue to show excellent agreement on a
subtropical high dominating the lower 48 later this week into next
week.
...Weather highlights...
Although the dominating subtropical high will bring heat across a
large portion of the country through early next week, there will
be bouts cool air intrusion from Canada following occasional
frontal passages across the northern tier states. A cold front
will initially bring showers and thunderstorms across the northern
Plains into the Great Lakes Friday but as the front is forecast to
become more diffused with time, more isolated coverage in the
Northeast will be the result during the weekend. Meanwhile, the
next front will move out of Canada this weekend with scattered
showers and thunderstorms along/ahead of its boundary. Cool air
in its wake will break the heat wave across the northern Rockies
and is forecast to drop temperatures more than 10 degrees below
normal across the northern Plains early next week. Ahead of the
front, temperatures will be about 5-10 degrees above average (80s
and many 90s), and initially near or above 100 over portions of
the interior Pacific Northwest/Great Basin on Friday. But
triple-digit heat is expected to continue in parts of interior
California.
From the southeastern U.S. westward along the Gulf Coast, a
dissipating front will keep a chance of showers and thunderstorms
in the vicinity through the weekend. Monsoonal moisture will
linger across the Four Corners area where locally heavy downpours
are possible through early next week. Temperatures will generally
be near or below average in the Southwest under limited solar
insolation and a daily chance of showers/storms.
Kong/Fracasso
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