Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 AM EDT Tue Jun 26 2018
Valid 12Z Fri Jun 29 2018 - 12Z Tue Jul 03 2018
...Overview and guidance evaluation/preferences...
Energy flowing around a strong eastern Pacific ridge will help to
eject an amplified trough forecast to be over the West as of day 3
Fri. Cyclonic mean flow across the Northern Tier states will
trend broader and less amplified as this occurs. By the first
half of next week guidance agrees in principle that the Pacific
ridge will begin to retrograde and set the stage for downstream
trough reloading that should align farther west than with the
preceding trough. Farther east/south, a strong eastern ridge
aloft will likely contain a high center that should track from the
Ohio Valley northeastward into or near southern New England.
Ridging across the South will likely build across the southern
Plains/Rockies early next week as the Pacific/West Coast pattern
adjustment begins.
Guidance trends over the past day have dramatically lowered the
spread for what happens to the initial western trough. The
ECMWF/ECMWF mean have backed off a bit on the timing of height
falls reaching the Great Lakes region by early next week but a
faster progression in the GFS/GEFS/CMC mean played a much greater
role in achieving the current clustering. In spite of this better
agreement in principle there is still meaningful spread for
details of the ejecting energy and to a greater extent for
upstream flow feeding into the mean trough.
In addition guidance still shows a variety of possible solutions
for an upper low that is currently near the Aleutians. Most
solutions indicate this feature will become the core of the trough
that reloads near the West Coast around the end of the period. As
has been the case in recent runs the ECMWF is on the fast side of
the spread (but at least with the 12Z run slower than its
predecessor) with the Aleutians low. GFS runs are keeping the low
offshore through day 7 Tue while the CMC has adjusted from a
compromise in the 12Z run to being closer to the GFS in the new
00Z run. An average of the ensemble means offers a reasonable
intermediate approach given current guidance spread. Once the
retrograding Pacific ridge settles into place, teleconnections
with the associated positive height anomalies suggest the axis of
the developing West Coast trough should ultimately drift a little
offshore.
The forecast remains ambiguous to the south of the upper high over
the East. There continues to be a general signal that some energy
flowing around the high may reach the Southeast but with poor
agreement/consistency for specifics. The 12Z ECMWF was stronger
than most other guidance with this energy by late in the period.
For days 3-5 Fri-Sun guidance agreement was sufficiently good to
allow for an operational model blend (18Z GFS/12Z ECMWF and lesser
weight of the 12Z UKMET/CMC). Decreasing confidence in varying
details of the operational runs led to incorporating 50-70 percent
total weight of the 18Z GEFS and 12Z ECMWF/NAEFS means by days 6-7
Mon-Tue. The primary adjustment in the forecast was farther
eastward progression of the Plains front into the upper MS Valley
and Great Lakes mid-late period.
...Weather threats/highlights...
Low pressure tracking northeast from the Plains into Canada from
Fri onward along with its associated fronts and upper shortwave
support will likely produce convective episodes capable of
producing areas of heavy rainfall. Best signal for highest totals
is across the northern Plains/Midwest/upper MS Valley/upper Great
Lakes. Southward progression of the cold front may bring
convection into the central Plains. Some activity may be strong
to severe as well. Check latest SPC outlooks as details become
more clear in coming days. Expect showers/thunderstorms to extend
farther eastward later in the period but likely with lesser
amounts as best upper dynamics lift into Canada. Meanwhile there
should be diurnally favored convection over portions of the
South/Florida through the period with some locally heavy amounts.
It remains a question mark to what extent upper energy/surface
feature(s) flowing around the deep layer high to the north may
enhance this convection.
Within the large area of hot and humid weather over the
central/eastern U.S., highest min/max temperature anomalies of
plus 10-20F should extend from the Midwest through the Great
Lakes/Ohio Valley and Northeast. The heat/humidity will gradually
erode from the west over the course of the period as a Plains cold
front moves into the Great Lakes. There are many locations that
may see one or more days of min temps at least close to daily
record warm values. Daily record highs should be less numerous
and most likely confined to the Great Lakes/Ohio Valley/Northeast
during Sat-Mon. The humidity will contribute to high heat index
values. Below normal temperatures will be most common over the
West and High Plains, with the northern Rockies/High Plains likely
to see one or more days of highs 10F or more below normal Fri-Sun.
Ejection of the initial western trough aloft will help readings
trend to moderately above normal levels over the central/southern
parts of the West by early next week.
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