Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Wed Jul 11 2018
Valid 12Z Sat Jul 14 2018 - 12Z Wed Jul 18 2018
Amplified flow should remain confined to the international border
with Canada throughout the period. Broad longwave troughing will
track from Alberta to the vicinity of Hudson Bay by Tuesday with
the southern extension of height falls slipping across the Great
Lakes and Ohio River valley. In its wake, renewed amplification is
likely across western Canada into the Pacific Northwest by Day
6/7, July 17/18. Elsewhere, while an elongated upper trough
lingers over the western Atlantic, an expansive 594-dm ridge is
likely to unfold over the Desert Southwest while a weaker
anticyclone is expected across the southeastern U.S.
...Guidance evaluation and preferences...
Starting with the upper trough sitting over the western Atlantic,
the National Hurricane Center has the region in a 50 percent
threat for tropical cyclone development during the next 5 days.
The 18Z GFS was most aggressive with this scenario depicting a
couple of closed isobars surrounding the circulation. Eventually
this elongated negative height anomaly lifts northeastward as
heights build along the East Coast.
Looking to the active pattern along the Canadian border, models
are initially in decent agreement ejecting lower heights across
the province of Alberta on Saturday. Issues with timing and
overall placement become more numerous as the trough closes off
and attains a rather formidable appearance. The 12Z ECMWF, and now
the 00Z UKMET are well northeast of the recent GFS/CMC solutions.
Multi-cycle comparisons have shown forecasts bouncing around a bit
so confidence begins to diminish from Day 5/Monday onward. In
spite of the model spread, it appears reasonable to believe a
slow-moving baroclinic zone should be crossing the I-95 corridor
by late Tuesday/early Wednesday next week.
Where much of the forecast spread looms is with the next trough
approaching western North America beginning early Tuesday. Most
notably, the 12Z ECMWF and a fair share of its ensemble members
trending significantly quicker with this feature. While perhaps a
trend, it also has no support from all of the available 00Z
guidance. Its downstream effects would favor a shortwave across
western Canada where other models build heights. This appears to
be a contentious region of the map so will continue to stay away
from this outlying 12Z ECMWF suite in favor of its preceding cycle.
There is a fairly unanimous signal in the guidance favoring a
marked uptick in mid-level heights late in the weekend into next
week. 594-dm heights are supported from the Desert Southwest into
the Four Corners region with the GFS runs being the most
The medium range preference initially favored a combination of the
18Z/12Z GFS and previous 00Z ECMWF (10/0000Z) through Day 4/Sunday
before gradually increasing ensemble influences into the picture.
By the middle of next week, had little confidence in any one
solution so took a multi-ensemble blend approach utilizing the 18Z
GEFS/12Z NAEFS/00Z ECMWF ensemble means.
While many locations across the nation can expect temperatures
near to above climatology, there are a few areas which will be on
the cooler side. An active southwestern U.S. monsoonal circulation
should keep these locations wet given daily thunderstorm chances
with accompanying enhanced cloud cover. In a general sense,
expected highs should be around 5 degrees below climatology.
Meanwhile, Sunday should prove to be a rather cool day for July
standards over the Northern Rockies into the adjacent Northern
Plains as well as portions of Nebraska. 10 to 15 degree departures
will keep highs in the 70s given cold advection in the wake of the
Regarding rainfall threats, as mentioned, the Four Corners region
will remain wet due to monsoon considerations with activity
gradually reaching the Central Great Basin and circulating into
the Central Rockies. Farther east, widespread convection is
anticipated along the slow moving front which will migrate from
the middle of the nation toward the Eastern Seaboard from Day 4-7,
July 15-18. There is also a signal for enhanced thunderstorm
chances across the southeastern U.S. as high precipitable water
air pools across the region.
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at: