Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
237 AM EDT Thu Jul 12 2018
Valid 12Z Sun Jul 15 2018 - 12Z Thu Jul 19 2018
...Overview...
As an elongated upper trough slowly departs off the Eastern
Seaboard, a sharpening trough will zipper across central Canada
reaching the vicinity of Hudson Bay by Monday. Such height falls
should gradually translate eastward possibly exiting the eastern
U.S. by Day 7/July 19. In its wake, the synoptic pattern remains
more nebulous in nature as guidance vary in which phase of the
sinusoidal flow will be favored. It does appear some form of
longwave trough should be positioned over western North America
toward the end of the forecast. To the south of the active
mid-latitude pattern focusing along the international border with
Canada, a prominent 594-dm mid-level ridge should become fixed
over the southwestern U.S. throughout the period.
...Guidance evaluation and preferences...
There do not appear to be any significant issues with the handling
of the slow moving trough lifting away from the Gulf Stream. This
region is continuing to be monitored by the National Hurricane
Center for tropical cyclone development with a 50 percent chance
of such occurrence the next 5 days. The 12Z UKMET is the most
bullish with cyclogenesis depicting a sub-1000 mb wave lifting
toward the eastern tip of Nova Scotia by Monday evening.
Meanwhile, the other region of only modest differences is the
southwestern U.S. with a 2 sigma above climatology mid-level ridge
setting up for the foreseeable future. The past few runs of the
GFS are a bit more expansive with the 594-dm height contour
relative to other guidance.
A pronounced upper trough and the attendant height falls will
slide eastward from central North America toward the Eastern
Seaboard during the period. Relative to yesterday, the models have
trended a bit slower with frontal progression with a position
likely offshore of New England by Day 7/July 19. Ensemble
spaghetti plots from the 12Z/00Z cycles would suggest the ECMWF
members are slower as a whole although its pattern has looked a
bit wonky upstream. So have not put tremendous stock in its
solution suite. Plenty of run-to-run variability in the guidance
though as this large-scale upper trough marches toward eastern
North America by mid/late next week.
The most contentious sector of the map is the northeastern Pacific
eastward into western North America where the guidance are at odds
with one another. Phase differences are immediately evident in
operational and ensemble model comparisons. During the last three
runs, the ECMWF ensembles continue to aggressively shove a trough
into British Columbia. Investigating where this system comes from,
it currently lurks near the far western Aleutians where the ECMWF
sustains it as an open wave into northern British Columbia while
the GFS shears it apart over the Yukon Territory. Suffice to say,
this difference leads to significant phase differences on the map.
The previous GFS runs and 12Z UKMET favor troughing into western
Canada on Tuesday while the past 3 ECMWF cycles build heights
ahead of another compact upper low. Still difficult to trust the
ECMWF, particularly its operational run given how much it departs
from the ensembles by the Day 4/5, July 16/17 timeframe.
During the forecast, kept a 30 percent contribution of some
variation of the 12Z ECMWF/ECMWF ensemble mean throughout the
period. Did not want to completely discount it but the forecast
was more heavily based on the 18Z/12Z GFS, 18Z GEFS/12Z NAEFS
ensemble means, with some early minimal contributions from the 12Z
CMC/UKMET. A complete ensemble approach was deemed necessary
beyond Wednesday given a breakdown of the pattern.
...Weather highlights/Threats...
While upcoming temperatures should remain rather close to
climatology during the forecast period, there are a few exceptions
to this rule. Sunday should prove to be a rather cool summer day
across the Northern Rockies/Great Plains down into western
Nebraska with highs 10 to 15 degrees below average. This would
suggest highs in the 70s before rebounding by early next week as
the cold advection ceases. Elsewhere, well above average readings
should be commonplace across the Pacific Northwest with a building
upper ridge. High temperatures will likely be 10 to 15 degrees
above climatology from Sunday through Tuesday as readings soar
into the 90s inland with 70s closer to the Pacific coast.
Wet conditions should prevail over the Four Corners region up into
the Central Rockies although coverage will be dependent on how
expansive the 594-dm ridge becomes. Warm mid-level temperatures
underneath this ridge should stunt more vertically developed
convection. Farther east, an expansive region of showers and
thunderstorms is expected in advance of the central North American
trough. While the better focus should occur closer to the height
falls, abundant instability would promote widespread convection
within the vast warm sector.
Rubin-Oster
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes 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
http://www.wpc.ncep.noaa.gov/heat_index.shtml