Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Fri Jun 28 2019
Valid 12Z Mon Jul 01 2019 - 12Z Fri Jul 05 2019
...Overview and Guidance/Predictability Assessment...
Today's guidance continues to show a fairly typical summer pattern
over the lower 48. The northern tier states will see progressive
and low amplitude flow to the south of a central Canada upper low
while ridging will broaden over the southern U.S. by late in the
week after a southern into central Plains weakness fills/shears
out. Energy within an upper trough near the Pacific Northwest
coast should eject inland as a mid-late week amplifying Alaska
ridge encourages shortwave energy to settle over/just offshore
British Columbia. The combination of an active upper jet over the
northern states and wavy fronts over the northern half of the
country will promote multiple episodes of convection and locally
heavy rainfall from the northern Plains through the Midwest and
into parts of the Great Lakes/Ohio Valley.
The most prominent model discrepancy still involves the upper
trough initially near the Pacific Northwest coast. 00Z-06Z GFS
runs are pretty much on their own in weakening the trough much
more quickly than consensus that includes multiple GEFS mean runs.
Established consensus brings the trough only gradually inland and
suggests that pronounced ejection/weakening will occur only after
early Thu. The majority cluster would keep heights across the
northern Plains into the Great Lakes higher than GFS runs, thus
keeping greatest convective potential farther north than the GFS.
However the tendency for some convection to stray south of where
modeled by global guidance does keep the window open for
significant activity to extend south of the preferred non-GFS mass
field cluster. Northern U.S. shortwave details, as well as
specifics of exactly how Pacific Northwest energy ejects during
the latter half of the week, have low predictability due to the
features' small scale. The updated forecast uses input from the
00Z ECMWF/UKMET/CMC and 06Z GEFS mean Mon into early Wed and then
transitions to a blend of the 00Z ECMWF/CMC and 06Z GEFS/00Z ECMWF
mean.
...Weather Highlights/Threats...
During most of next week a broad area from the northern Plains
through parts of the Great Lakes and Ohio Valley should see
multiple episodes of showers/thunderstorms with some activity
possibly strong to severe and capable of producing locally heavy
rainfall. Currently the best signal for highest rainfall totals
over the five-day period exists over the Upper Midwest. Some of
the rainfall from this activity may extend toward the East Coast.
Shortwaves within progressive northern U.S. flow aloft and two
surface fronts--one settling across the north-central U.S. and a
second that drops from Canada into the northern Plains--will
provide ample focus for these anticipated convective systems.
Meanwhile areas from the western Gulf Coast into the
southern-central Plains may see showers and storms aided by an
upper level weakness and some low level inflow of Gulf moisture.
There may be somewhat more focus for rainfall along and just
inland from the Gulf Coast but otherwise confidence is fairly low
for pinpointing greatest coverage/intensity. Expect diurnally
favored convection over Florida and parts of the Southeast.
High temperatures over the Southeast of 5-10F above normal may not
challenge too many daily record highs but the air mass may be
humid enough to produce some potentially dangerous heat index
values. Max heat index values could reach at least 105F at some
locations. Elsewhere areas from the northern High Plains into the
southern Plains will tend to see slightly below normal highs
during the period while temperatures over the northern half of the
West Coast states should trend warmer late in the week as the
Pacific Northwest upper trough weakens/departs. Much of the lower
48 should see near to above normal morning lows most days next
week.
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