Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Thu Jun 28 2018
Valid 12Z Sun Jul 01 2018 - 12Z Thu Jul 05 2018
...Overview and guidance evaluation/preferences...
Today's guidance maintains good continuity/agreement for the
expected longwave pattern across a majority of the forecast
domain. An upper low dropping southeast from the Gulf of Alaska
into southern BC and/or the Pacific Northwest will begin the
process of trough amplification near the West Coast. With time
this trough will further amplify and by next Wed-Thu likely drift
offshore as the upstream Pacific ridge retrogrades. Farther east
the guidance is also fairly consistent in depicting an upper ridge
rebuilding over much of the central U.S. and southern Rockies
after progressive shortwave energy passes through the Northern
Tier states. As for one notable change in the models/means since
yesterday, there is now a tendency to stall upper ridging over the
northern Mid Atlantic/southern New England through early next week
instead of having the ridge continue eastward and/or weaken.
Latest consensus has this ridge eventually returning westward to
become part of the very large overall ridge expected to be
centered over the Plains by the end of the period. Around the
periphery of the eastern ridge, most solutions still a broad upper
level weakness to remain off the East Coast while some amount of
upper level energy may brush the Gulf Coast.
Based on guidance clustering or divergence as the case may be, a
solution starting with mostly operational guidance (18Z GFS and
12Z ECMWF/CMC/UKMET) early followed by a transition toward a
little more than half weight of the 18Z GEFS/12Z ECMWF means
seemed to provide the best starting point for the forecast.
Within the developing West Coast trough there are still some
lingering differences for timing of the leading Gulf of Alaska
low, in particular as the new 00Z GFS has reverted to a slower
than consensus solution. By mid-late period the question is what
proportion of the initial upper low energy ejects into Canada and
when this occurs. Differences in models/ensembles suggest greater
than average uncertainty in specifics across southern Canada and
extreme Northern Tier areas especially by Wed-Thu. At that
late-period time frame guidance is generally within the bounds of
typical guidance error for the trough as it drifts offshore,
fairly consistent with the forecast position of the upstream
ridge/positive height anomalies in latest D+8 multi-day mean
charts.
Consensus changes in the upper pattern over the East have led to a
weakening of the front that reaches the Northeast/northern Mid
Atlantic by Tue and then a farther northward position by Thu.
Guidance trends toward higher heights aloft since yesterday have
been fairly unanimous but it will be worth watching to see if this
adjustment persists. Teleconnections relative to the expected
Pacific/West Coast pattern still signal the possibility of a
little more mean troughing over the Canadian Maritimes and
Northeast Coast as per earlier runs.
It remains to be seen whether the upper feature near the Gulf
Coast becomes as concentrated as in recent ECMWF runs, but
teleconnections relative to the extent of eastern U.S. ridging do
support the general tendency for above average rainfall
probabilities over that region. Farther east, the UKMET runs
bring the western Atlantic upper low/inverted trough closer to the
coast than other solutions.
...Weather threats/highlights...
The upper Great Lakes to Plains front as of early Sun will bring
areas of showers and thunderstorms into the East. Most heavy
activity should be fairly localized as upper dynamics lift into
Canada. The trailing part of the front over the Plains will lift
back north as a warm front during the period and help to focus
periods of convection, with best potential for heavier activity
over the northern Plains/upper Midwest regions. Locations along
and/or inland from the Gulf Coast may also see one or more days of
significant rainfall enhanced by upper level energy and a weak
surface reflection. Confidence is low in the specifics of these
features but the overall pattern is favorable for rainfall.
Between this activity and the eastern front, expect some periodic
convection of varying intensity in response to daytime heating.
Portions of the Northwest may see scattered and mostly
light-moderate rainfall in response to the upper trough developing
near the West Coast. Parts of the southern half of the Rockies
and vicinity may see a little scattered rainfall as well.
Guidance changes over the past day suggest potential for greater
persistence of hot/humid weather over the Northeast, though expect
this weekend and Mon to be the focus for the most extreme heat
index values. Record warm lows are possible within a fairly large
area over the East. Record highs are likely to be less numerous
and tend to be between the eastern Great Lakes and northern Mid
Atlantic/Northeast. In that time frame some readings could
locally reach 15-20F above normal. Anomalies may still reach or
exceed plus 10F over some areas in the Great Lakes/Northeast into
midweek. Meanwhile the strengthening central U.S. ridge aloft
later in the period will bring a return of well above normal
temperatures (up to plus 10-15F anomalies) to the northern
Plains/Midwest by Wed-Thu. The most persistently cool
temperatures in association with the West Coast upper trough will
be over the Pacific Northwest/northern Rockies with Mon-Tue likely
to have the coolest highs, approaching 10F below normal.
California should trend cooler after the weekend as the upper
trough amplifies.
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