Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1204 PM EDT Sat Jun 16 2018
Valid 12Z Tue Jun 19 2018 - 12Z Sat Jun 23 2018
...Heavy rains possible along the western Gulf Coast and into the
northern/central Rockies and Plains...
...Pattern overview and Guidance/Predictability Assessment...
The medium range period (Tue-Sat) starts out with a closed low
over the western U.S., with the guidance now showing a faster
weakening of the upper low and ejection of short wave energy into
the Plains and the Mississippi Valley by the end of the week.
Strong ridging over the Pacific Northwest will retreat eastward as
a shortwave enters the West Coast by Friday. Meanwhile, the
northern periphery of the persistent downstream ridging across the
East/Southeast is eroded by northern stream cyclonic flow across
the Great Lakes and the Northeast, which should finally suppress
the oppressive heat and humidity expected this weekend and early
next week.
Overall, the models and ensembles show a reasonably similar larger
scale pattern over the next week, but smaller mid-scale embedded
features still offer a bit of forecast spread. This cycle of the
WPC medium range suite used a blend of the latest runs of the
GFS/ECMWF with their ensemble means. Significant contribution from
deterministic runs were included days 3-4, with increasing
ensemble contribution by the end of the forecast period to try and
de-emphasize the less predictable smaller scale features which
still remain highly uncertain. The solution maintains reasonably
good WPC continuity. One difference from previous shift forecast
was at the end of the period with a slightly faster progression of
troughing into the Plains/MS Valley which may help spin up a wave
along the boundary, with the nearly stationary boundary across the
Plains now beginning to drop slowly south/eastward.
...Weather Highlights/Threats...
A long standing and wavy frontal boundary remains quasi-stationary
from the central Plains to the Ohio Valley and Mid-Atlantic.
Periods of locally excessive convection will focus along this
boundary and in the warm sector aided by early period tropical
moisture influx from the Gulf of Mexico. WPC medium range 48-hour
QPF shows the heaviest potential along the immediate TX coast due
to a potential tropical wave/disturbance. Moisture streaming
northward intersects the wavy boundary to the north to produce
possibly several inches of rainfall across the central Plains and
back into the northern/central Rockies. As the trough and front
shifts slowly eastward days 5-7, the best rainfall will focus
along the surface boundary and to the north of any potential wave
development across the upper Midwest.
Temperature wise, ridging across the Southeast should weaken in
response to cyclonic flow in the northern stream and the western
U.S. trough sliding eastward. Afternoon highs above normal are
still likely heading through next week across much of the East,
but not nearly as extreme as the late weekend/early week heat
expected. Just a handful of near record max temps are expected
Tuesday, but there is a higher probability of near or record
breaking high minimums, as overnight temps hover in the 70s. Well
above normal temps expected across the Pacific Northwest early
week as well with daytime highs well into the 90s across interior
regions, which would come close to records. Across the
Intermountain West and Central states, temps should be below
normal underneath upper level troughing and cloudy/rainy
conditions.
Santorelli
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