Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Fri Jun 08 2018
Valid 12Z Mon Jun 11 2018 - 12Z Fri Jun 15 2018
...Pattern Overview...
The higher latitudes of North America in particular are likely to
see a significant degree of blocking during the period, with the
evolution of embedded/surrounding flow providing some significant
forecast challenges for specifics by the mid-late part of next
week. There is currently a majority scenario that has strong
western North America trough/upper low energy progressing
underneath an anomalously strong northern Canada upper high that
closes off (highest standard deviations from normal likely to be
around Tue-Wed). The southern Canada/northern U.S. energy may
combine with eastern Canada flow to yield an overall trough
covering parts of eastern Canada and U.S. by late next week.
Elsewhere, expect an upper trough to reach the West Coast around
midweek with some degree of inland progression possible thereafter
while ridging that builds over the southern Rockies/Plains during
the first half of the week should persist into Fri with a hint of
southeastward drift.
...Model Guidance/Predictability Assessment...
Even though there has been some pronounced trending with some
aspects of the forecast from the perspective of multi-day
continuity, the ensemble means have provided less run to run
variability than other individual solutions. In addition the
likely closure of an upper high for a time over northern-central
Canada lends itself to low predictability for forecast details
across much of southern Canada and northern contiguous U.S. As a
result the overall forecast preference places the greatest focus
on the ensemble means. An operational model blend (06Z GFS/00Z
ECMWF and to a lesser degree 00Z CMC/UKMET) compared well to the
means while providing added detail days 3-4 Mon-Tue so the early
part of the forecast did not use the means per se, but subsequent
divergence of operational runs led to exclusive use of the means
(more 00Z ECMWF mean than 06Z GEFS) and some continuity by days
6-7 Thu-Fri with a model-mean blend for Wed.
Most guidance as a whole has shown faster multi-day trends for the
greatest proportion of energy that should progress eastward along
the U.S.-Canadian border into midweek. However the 06Z GFS
differs from the majority in holding a greater part of closed low
energy over western Canada, while some solutions like the ECMWF
offer potential for energy to linger over central longitudes for a
time before continuing eastward. The faster trend has led to
earlier flattening of New England troughing aloft after Mon, while
keeping the compact upper low over eastern Canada--which itself
has trended noticeably slower versus previous consensus--well
north of Maine. Beyond that there is the question of how much
eastern Canada flow could retrograde and interact with flow coming
from the west. Typical biases suggest leaning away from the side
of the envelope that shows the most retrogression. Note that
CMC/CMC ensemble runs have offered very different scenarios that
seem more reflective of ideas from multiple days ago, but at least
they provide some added reservations about any specific
deterministic forecast. So after Tue overall confidence rapidly
decreases regarding details of surface lows/fronts affecting
southern Canada and the eastern half of the U.S.
Regarding the western U.S. trough, the most notable aspect of the
forecast is that compared to 24 hours ago the GFS and ECMWF have
essentially flipped, with the 00Z/06Z GFS now showing a deep/slow
solution that the 00z ECMWF had yesterday. Ensemble means have
been more consistent over recent days and arguing for an
intermediate solution, though the 06Z GEFS mean has trended
somewhat deeper in deference to its operational counterpart.
Strength/sharpness of the upstream ridge supports a fair degree of
amplitude but teleconnections relative to the ridge's positive
height anomaly center suggest the western trough should become a
little weaker and more progressive than the GFS scenario at or
shortly after the end of the forecast.
GFS/GEFS guidance still appears overdone with the surface
development/moisture heading into the Gulf of Mexico from the
western Caribbean but has been gradually trending toward other
guidance that has more consistently signaled a more suppressed
pattern.
...Weather Highlights/Hazards...
A cold front progressing eastward from the Plains, with the
trailing end stalling over the south-central Plains and eventually
lifting back north as a warm front, will likely provide a
multi-day focus for showers and storms over the central-eastern
U.S. with some rainfall locally heavy. SPC outlook currently
shows potential for severe weather over parts of the Midwest early
in the week. Other convection may be on the strong side as well.
Diurnally favored convection over the Southeast may be enhanced
early in the week by a leading front over the East and an upper
level weakness. The system moving into the West mid-late week
should bring clouds and scattered precipitation. Most activity
should be on the lighter half of the spectrum but a few pockets of
at least moderate totals cannot be ruled out.
The strong upper system departing from western North America early
in the week will bring chilly temperatures to the northern Rockies
and vicinity on Mon followed by a warming trend. Ridging aloft
over the southern Rockies/Plains will promote a broad area of warm
to hot weather over the western-central U.S. during the week.
Highest anomalies for min and/or max temperatures in the plus
10-15F range should expand from California through much of the
Intermountain West and then portions of the Rockies/Plains. On
Mon there will also be an axis of well above normal readings just
ahead of the Plains cold front.
Rausch
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be 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