Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sat Jul 14 2018
Valid 12Z Tue Jul 17 2018 - 12Z Sat Jul 21 2018
...Overview...
The flow pattern across the contiguous U.S. will become a bit more
amplified during the medium range period. Expect two separate
upper ridges, centered near California and the southern Plains as
of early Tue, to merge with the more dominant western center
tracking across the Four Corners and then settling over the
southern Rockies/Plains (while also strengthening/expanding) by
Fri-Sat. Meanwhile one or more upper waves originating from the
eastern Pacific will be rounding the upper ridge at the start of
the period and should gradually carve out a trough across the
eastern U.S. A leading upper trough will cross eastern North
America early-mid period. Heights aloft will fall across the
Pacific Northwest and eventually eastward across the Northern Tier
from midweek onward as a northeastern Pacific upper trough with
embedded low digs southeastward and then continues eastward.
...Guidance Evaluation and Preferences...
Most of the latest model/ensemble guidance agrees well with the
overall evolution aloft during the period. Canadian ensembles are
a bit of an exception as they are somewhat slower with the
northeastern Pacific into Canada upper trough/low--counter to the
general trend of the GEFS/ECMWF means toward slightly faster
progression. Operational runs have varied on timing though, and
the full envelope of GEFS/ECMWF members would suggest a
significant degree of uncertainty on specifics across the western
half of the northern U.S./southern Canada. There is also a
reasonably high level of uncertainty in exactly how the Thu-Sat
upper trough over the East will evolve with corresponding effects
at the surface. Both the GFS and ECMWF easily fit into the
expected mean pattern but the deep/closed 00Z ECMWF aloft
generates a rather strong Great Lakes surface low by day 7 Sat
while the 00Z-06Z GFS weak/open wave depiction yields a very
diffuse surface pattern. Ultimately prefer an intermediate
solution between these two extremes, which provides reasonable
continuity while awaiting better guidance agreement. Overall a
blend among the 00Z/06Z operational models represented consensus
well for days 3-5 Tue-Thu. With the increasing detail uncertainty
later in the period the forecast transitioned toward a blend of
operational guidance with the 06Z GEFS/00Z ECMWF means and WPC
continuity.
...Weather Highlights/Threats...
Showers and storms with locally heavy rain will accompany a cold
front from the Northeast/Mid-Atlantic/Appalachians to the
Southeast Tue-Wed. The trailing end of the frontal boundary will
likely stall from the Southeast into the southern/central Plains,
which could provide an ongoing focus for locally heavy showers and
thunderstorms through later next week. Monsoonal moisture will
also produce scattered afternoon/evening thunderstorms from the
southern Great Basin/Southwest to the southern/central Rockies
through next week. Expect the best potential for locally heavy
rain to be early in the week as the building upper ridge should
gradually reduce the coverage/intensity of convection by late
week/weekend. There is decent agreement in the guidance that
shortwave energy emerging into the Plains should encourage some
organized convection/rainfall over the northern half of the Plains
Tue-Wed. However after that time the uncertainty in details at
the surface and aloft holds confidence lower than desired for the
rainfall forecast from the eastern Plains into the Great
Lakes/Appalachians. The general evolution toward upper troughing
should at least promote some increase in shower/thunderstorm
activity.
The combination of relatively cooler air behind the initial
eastern U.S. cold front and clouds/rainfall ahead of the potential
feature emerging over the Plains/east-central U.S. should lead to
an area of below normal temperatures from the northern-central
Plains into the East. Some locations may see one or more days
with minus 5-10F anomalies. Hot temperatures across the Pacific
Northwest (some highs greater than 10F above average over interior
areas) should moderate by later next week as a Pacific cold front
moves inland. Evolution aloft will keep warm mins over a large
area covering the West and into the northern Plains, while the
strengthening southern Rockies/Plains ridge aloft by Fri-Sat will
lead to somewhat more focus on the southern Plains and vicinity
for highest max temperature anomalies.
Rausch/Ryan
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