Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sat Jun 29 2019
Valid 12Z Tue Jul 02 2019 - 12Z Sat Jul 06 2019
...Overview and Guidance/Predictability Assessment...
Downstream from a strong Rex block covering Alaska and the
central/east-central Pacific, the pattern over the lower 48 will
be close to the summertime average with progressive flow across
the northern states and a southern ridge aloft that will become
more expansive with the departure of an initial central-southern
Plains weakness after midweek. Eastward progress of a central
Canada upper low will be fairly slow. The pattern will favor a
couple wavy fronts meandering over parts of the northern half of
the country and a focus for convective systems from just east of
the northern half of the Plains to central Appalachians and
possibly East Coast.
One of the primary forecast challenges will continue to be timing
the ejection of the upper trough/closed low that starts the period
near the Pacific Northwest coast on Tue. This will depend in part
on exact evolution of energy over/offshore British Columbia in
response to the strong ridge/upper high building into mainland
Alaska. Confidence will likely be low for a while since evolution
of energy to the east/south of a closed upper high tends to be one
of the lowest predictability situations for extended-range
synoptic scale forecasts. A compromise for Pacific Northwest
trough ejection looks reasonable at this time, between the faster
00Z ECMWF and slow/deep 00Z CMC. As for the elongating
BC/northeastern Pacific energy, teleconnections relative to the
Alaska ridge's positive height anomalies do offer support for an
embedded upper low that could settle near Haida Gwaii per the 00Z
ECMWF/ECMWF mean and 06Z GFS--but there's high sensitivity in
specifics depending on exact location of the positive height
anomaly center. The main guidance discrepancy elsewhere involves
the 00Z GFS becoming a slow extreme with the Canadian upper low
during the latter half of the period.
The updated forecast maintains the approach of starting with an
operational model consensus with eventual incorporation of the 06Z
GEFS/00Z ECMWF means by mid-late period (reaching 60 percent total
weight by days 6-7 Fri-Sat, while keeping some 06Z GFS/00Z ECMWF
input) given typically increasing detail uncertainty. The one
notable adjustment in the majority cluster is a southward trend
for the front reaching the Great Lakes toward the end of the
period.
...Weather Highlights/Threats...
One wavy surface front extending across the northern U.S. Tue-Thu
and a trailing front dropping into the northern tier from Canada
will provide a multi-day focus for showers/thunderstorms across a
broad area from just east of the northern Rockies through the
Great Lakes/Ohio Valley into the central Appalachians, with some
activity possibly reaching the East Coast at times. Some storms
may be strong to severe but with low confidence in determining any
favored areas this far in advance. Some rainfall will likely be
heavy and best potential for highest five-day rainfall totals
should be over the northern Plains/Upper Midwest. Locations along
the southern Rockies/High Plains may see some showers/storms at
times, parts of the southern Plains/western Gulf Coast may see
some rainfall with the upper weakness over the southern Plains as
of Tue, and scattered showers/storms will be possible over the
Southeast/Florida.
High temperatures of 5-10F or so above normal over the Southeast
could get close to daily record values at a few locations on one
or more days. Humidity should be sufficient to produce fairly
high heat index values, with some max heat index values possibly
reaching at least 105F. Potentially hazardous heat/humidity could
extend as far north as the central Mid-Atlantic. Meanwhile the
northern High Plains should see below normal highs most days,
perhaps by 10F or so at a few locations. The front(s) to the
south should push farther south late week into the weekend,
extending the cool highs toward the central High Plains by
Fri-Sat. Expect the Pacific Northwest and northern California to
see a fairly rapid change from below normal highs to plus 5-12F
anomalies with the eventual departure of the initial upper trough.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml