Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Wed Jun 27 2018
Valid 12Z Sat Jun 30 2018 - 12Z Wed Jul 04 2018
...Overview and guidance evaluation/preferences...
Guidance maintains similar ideas for the large scale pattern
evolution through the period albeit with some embedded detail
spread. Cyclonic flow across the Northern Tier will broaden
during the weekend as an ejecting western shortwave progresses
east and then northeast followed by weak upstream energy flowing
around a strong eastern Pacific ridge. Then a Gulf of Alaska
upper low will drop southeastward toward northern WA/southern BC.
This feature combined with a retrogression of the Pacific ridge
will lead to development of a mean trough near the West Coast next
week. Flow on the western side of the trough may start to pull
the overall trough axis a little offshore around midweek or so.
This would be consistent with teleconnections relative to the
positive anomaly center associated with the new position of the
Pacific ridge. Meanwhile the center of the eastern U.S. upper
ridge responsible for the heat wave over the eastern half of the
country will track east-northeast from the Ohio
Valley/Appalachians. Then upper ridging will likely build over
the Plains/MS Valley/southern Rockies. Around the periphery of
these ridges there may be some diffuse upper energy near the Gulf
Coast and a western Atlantic weakness/upper low that may
eventually approach the East Coast.
Over the past day guidance has maintained fairly good continuity
in principle for Northern Tier flow during the first half of the
period. There are still persistent detail differences that are
small enough in scale to have fairly low predictability, favoring
a consensus blend at this time. Leading shortwave energy will
push an upper MS Valley-central Plains front into the Great Lakes
Sat-Mon. The front may decelerate for a time early next week as
heights aloft become nearly parallel to the front. Toward the end
of the period guidance starts to signal a hint of troughing aloft
in the vicinity of eastern Canada/New England. Spread is
consistent with detail uncertainty upstream, though
teleconnections relative to the Pacific/West Coast pattern do
favor eventual development of moderate troughing over eastern
Canada/New England. Thus the surface front reaching the East
could speed up a bit around midweek.
Recent guidance trends for the Gulf of Alaska upper low have gone
fairly strongly toward the ECMWF/ECMWF mean which have been on the
faster side of the envelope over the past couple days. New 00Z
solutions further this trend with the GFS/CMC now showing ejection
of the upper low by day 7 Wed though not quite as fast as ECMWF
runs. Guidance spread through the 18Z cycle recommended leaning
more toward the ensemble means to downplay uncertain details by
days 6-7 Tue-Wed.
Diffuse nature of energy forecast over the western Atlantic and
Gulf Coast lends itself to low predictability for specifics.
There has been a consistent signal for Gulf Coast energy in some
form over the past couple days, while the GFS/ECMWF have trended
toward holding western Atlantic energy farther eastward.
Above forecast considerations yielded a blend of operational model
guidance (18Z GFS and 12Z ECMWF/UKMET/CMC) days 3-4 Sat-Sun
followed by a steady trend toward more 18Z GEFS and 12Z
NAEFS/ECMWF mean input such that ensembles contributed 70 percent
to the forecast by day 7 Wed.
...Weather threats/highlights...
Expect potential for some heavy/strong convection between the
upper Great Lakes and central Plains early in the period, in
association with a cold front initially near this area.
Progression of the front will eventually bring
showers/thunderstorms into the East but with any heavy activity
tending to be more localized as the best dynamics lift into Canada
after the weekend. The trailing part of the front will stall over
the Plains and lift back as a warm front so there will be
potential for additional rainfall over portions of the central
U.S. after the more concentrated activity during the weekend. The
other area of focus for heavy rainfall potential will be over the
Gulf Coast/Florida, with the combination of upper level energy and
surface troughing contributing to activity.
In terms of highest temperature anomalies, the core of the heat
wave affecting the eastern half of the U.S. should extend from the
Midwest/Great Lakes into the Northeast. The greatest coverage of
plus 10-20F above normal readings will likely exist Sat-Mon,
followed by gradual moderation as the Plains front approaches.
Best potential for record highs will be from the lower Great
Lakes/upper Ohio Valley into the northern Mid Atlantic/Northeast
Sat-Sun while broader coverage of record warm lows is possible
Sat-Mon. The northern Rockies/High Plains will see below normal
highs during the weekend followed by a trend toward near normal
temperatures. The evolving pattern by late in the period will
tend to support moderately below normal temperatures along the
West Coast and over the Northwest toward midweek.
Rausch
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities 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