Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
108 AM EDT Mon Jun 11 2018
Valid 12Z Thu Jun 14 2018 - 12Z Mon Jun 18 2018
...Pattern Overview...
A transition still looks in order for the for the flow pattern
across North America over the next week. A blocking upper high
initially in place northwest of Hudson Bay should break down, with
somewhat more progressive flow taking hold by early next week from
the Canadian prairies/U.S. northern plains to New England. Farther
south, however, blocking is expected to increase, with anomalous
upper ridging likely to develop by the weekend near the coast of
British Columbia, and ridging shifting east from the
southern/central plains toward the Ohio Valley. This pattern will
favor development of an amplified and slow-moving upper trough
across the interior West by late in the week. Height falls digging
into the Northeastern U.S. by late in the week should show some
degree of amplification as well, but the speed of eastward
progression into the Atlantic Ocean is somewhat unclear at this
time as the feature will lie at the interface between more
progressive northern stream flow and the blocking tendencies
farther south.
...Model Guidance/Predictability Assessment...
The WPC forecast during days 3-5 (Thu-Sat) was based on a
multi-model deterministic blend including the 12Z ECMWF/GFS/UKMET.
This blend represents a consensus solution, favorably resolving
relatively minor timing/amplitude differences with shortwave
energy digging across the Northeast/Mid-Atlantic, and with the
upper trough beginning to amplify across the Pacific Northwest
during that time period. The GFS has continued to hang onto the
potential for low pressure development in the Gulf of Mexico by
Sat, but has trended toward the ECMWF (less organized and farther
west) with each consecutive run.
From day 6 (Sun) onward, spread begins to increase with the two
areas of upper trough amplification - across the interior west and
along/off the Mid-Atlantic/Northeast coast. Significant question
remains as to how deep a potential upper low across the Great
Basin will be, and if/how quickly the energy will move east across
the Rockies by day 7 (Mon). Ensemble spaghetti plots for the past
couple runs show a trend toward more amplification and a slower
progression, and this seems to be supported conceptually by the
evolution toward more flow blocking and the anomalous ridge
amplifying off the West Coast. The 12Z ECENS mean was less
progressive than the deterministic ECMWF, as was the GFS. Farther
east, there is a general consensus that the Northeast/Mid-Atlantic
upper trough should move offshore, but confidence is low in the
degree of amplification that will occur. The 12Z ECMWF was the
most amplified deterministic solution, closing off an anomalous
upper low east of New England that persists into day 7. Due to
lower confidence in this area, a mix of ensemble means is
preferred. Majority weighting toward the 12Z ECENS/NAEFS ensemble
means was used in the forecast for days 6-7 to account for
increased spread, and to reflect a somewhat more amplified and
slower progression of the western trough than shown by the
deterministic 12Z ECMWF.
...Weather Highlights/Hazards...
A weakening surface front and moist air mass will support
scattered to numerous showers and thunderstorms through the
weekend, especially during the afternoons and evenings, across the
Southeastern states, with the potential for locally heavy
rainfall. A broad trough/area of disturbed weather moving westward
across the Gulf of Mexico is also expected to bring the potential
for more widespread rains to portions of the western Gulf Coast
states by the weekend. Farther north, a shortwave, surface low
pressure system crossing the northern plains and Upper Midwest
will produce areas of convection with potentially heavy rainfall.
The frontal boundary may slow or stall for a period of time
Sat-Sun from the central plains to the Upper Midwest, resulting in
several days with shower/thunderstorms potential, and potential
flooding threat for some areas. Confidence is low at this time as
to the specifics of what areas will see the heaviest rain,
however. Finally, moisture associated with Tropical Storm Bud
should begin to spread into the southwestern U.S. and the Rockies
by the weekend, bringing the potential for areas of heavy
rainfall. Interactions with a stationary frontal boundary draped
from the Great Basin into the Central Rockies could lead to an
enhancement of heavy rainfall potential. Some model solutions
even suggest the possibility that the moisture from Bud could
spread east into the central/northern plains, further enhancing
heavy rain potential along the frontal boundary.
Temperatures will also become a bigger story during the next week
as the upper ridge expands across the central U.S. and then drifts
east into the Ohio Valley. High temperature anomalies of +10 to
+18 deg F are expected Thu-Sat across areas from the central
plains to the Mid-Mississippi Valley, with highs approaching 100
degrees for some areas. By Sun-Mon, the greatest temperature
anomalies will shift a bit east from the Mid-Mississippi Valley
into the Ohio Valley. A few record high temperatures are possible.
High dew points in addition to the hot temperatures will make for
dangerous heat index values for some areas.
Ryan
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be 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