Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Sat Sep 01 2018

Valid 12Z Tue Sep 04 2018 - 12Z Sat Sep 08 2018

...Pattern Overview and Guidance/Predictability Assessment...

An impressive mid/upper-level ridge will dominate the pattern
across the Eastern Seaboard throughout much of the forecast.
Through mid-week, 500-mb height anomalies should sit around 2 to
2.5 sigma above climatology. Gradual amplification within the
northern stream should effectively lower heights in this ridge
beyond Day 5/Thursday as a backdoor front swings down through the
northeastern U.S. Within the subtropics, the 00Z ECMWF and its
ensemble members have trended toward a more defined surface
cyclone tracking along the northern Gulf of Mexico. Compared to
previous model cycles, the 00Z ECMWF is 5 to 10 mb deeper with
support from the 00Z CMC. Based on this latest ensemble plot,
there is increasing confidence in some sort of circulation to
affect the Gulf Coast during the period. There does remain a
dearth of support from the GEFS members which has been a theme the
past few days. As this system reaches the western Gulf Coast by
Thursday, a broad inverted trough is primed to reach the Bahamas
by Day 6/Friday with the best support from the 06Z/00Z GFS and 00Z
ECMWF.

Across the western/central sections of North America, multi-day
ensemble trends have shown a tendency to push a longwave trough
farther back toward the eastern Pacific early on while being more
amplified with a system crossing central Canada. The latter was
depicted in such a manner multiple runs ago from the ECMWF where
future guidance eventually moved in this direction. Thus, value
the continuity of the ECMWF rather highly here. This amplified
flow will ultimately flatten the downstream ridge across the
eastern U.S. Farther west, the slowing trend in the guidance seems
to be supported across the board although operational run-to-run
differences are evident. Most notably, the 00Z GFS stood out on
the quicker side which suggested more reliance on the later 06Z
run. Ensemble means are in agreement with this slower scenario
although plenty of spread does exist ahead of the mean position of
the trough.

With sufficient model agreement through Day 4/Wednesday, went with
a combination of the 06Z GFS and 00Z ECMWF/CMC before gradually
incorporating ensemble mean influences. Generally took a 50/50
approach between the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF
ensemble mean through the remainder of the period. Was a bit leery
in placing too much emphasis on the 00Z ECMWF solution by Day 6/7,
September 7/8, given rather amplified flow across eastern Canada.
Thus, kept its contributions more conservative in nature at that
point.


...Weather Highlights/Hazards...

Strong ridging from the Mississippi River out to the Eastern
Seaboard will support a lengthy period of above average
temperatures. The most intense anomalies should sit around 10 to
15 degrees above climatology centered over the Great Lakes into
the northeastern U.S. Several days of daily records being broken
are likely given the warm overnight lows. Some cooling trend is
likely by Friday and into next weekend given the approach of the
backdoor front. Farther west, well below average temperatures are
likely across the middle of the country given an active
precipitation pattern accompanied by extensive cloud cover. Highs
may struggle to get out of the 60s across the north-central U.S.
on Tuesday and Wednesday.

Regarding precipitation, multiple rounds of semi-organized
convection is likely from the central U.S. northeastward into the
Great Lakes where a frontal zone will stall. While overall 24-hour
amounts are not terribly high, the multi-day nature of these
events could offer a threat for hydrologic issues at some point
during the period. Along the Gulf Coast, the mentioned tropical
disturbance could pose a threat for heavy rainfall from east to
west. Even if the system does not intensify into a named storm, it
will still bring heavy rainfall potential given the anomalous Gulf
moisture and sufficient mid/upper-level dynamics.


Rubin-Oster

WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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