Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Thu Aug 30 2018

Valid 12Z Sun Sep 02 2018 - 12Z Thu Sep 06 2018

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

The main fixture in the synoptic-scale pattern will be a strong
positive height anomaly centered over the eastern U.S. The 00Z GFS
advertises mid-level heights around 2 to 2.5 sigma above
climatology with 594-dm values extending from the Mississippi
River eastward into the western Atlantic by mid-week. This is
rather well established in the model guidance with only modest
differences between them. Underneath this dominant ridge, models
continue to hint at some sort of tropical disturbance tracking
westward through the Bahamas and perhaps into the Gulf of Mexico.
As has been the case the past couple of days, the ECMWF solutions
have favored a weak surface reflection with some of the ECMWF/CMC
ensemble members supporting this. Meanwhile, the GFS/GEFS shows
little indication of such a scenario unfolding as surface ridging
sinks toward the southern Gulf of Mexico.

Most of the uncertainty resides across the western/central parts
of North America. The guidance continues to exhibit poor
run-to-run continuity with plenty of differences between model
suites. Ultimately, a wave train of shortwaves are expected to
slide from the northeast Pacific downstream into western North
America. Some of these impulses could amplify more than others but
details are somewhat nebulous. Eastward migration of these systems
will carry shortwave energy into central Canada possibly down into
the north-central U.S. Some degree of shearing is anticipated
given the extent of downstream mid/upper-level ridging in place.
Further instances of shortwave progression across similar regions
are likely throughout the period. A few models stand out as more
amplified in nature, most notably the 00Z ECMWF with the lead wave
by Day 5/Tuesday and the 06Z GFS given its depiction of the
following system. Meanwhile, the 00Z UKMET is flatter to start
with which is perhaps too subdued in nature. Overall, ensembles
should play a major role in forecast production for a good chunk
of next week.

The preference leaned heavily on a multi-operational model
consensus to start with featuring a mixture of the 00Z
GFS/GFS-parallel/ECMWF/CMC solutions. Thereafter, gradually
increased influences of the 00Z GEFS/ECMWF ensemble means with
these models encompassing 80 percent of the blend by Day 6/7,
September 5/6. Confidence is highest with the persistent ridge
over the eastern U.S. while likely a bit below average for areas
to the west.


...Weather Highlights/Hazards...

Upper ridging will keep conditions quite warm generally east of
the Mississippi River with the most anomalous readings expected
north of 40N latitude. Forecast highs are around 5 to 10 degrees
above early September climatology. Such departures should be even
higher for overnight lows as minima sit around 10 to 15 degrees
above average. On Tuesday and Wednesday morning, some daily
records may be broken given the warmth overnight. This would
generally focus across the Ohio Valley and along the Eastern
Seaboard. Regarding precipitation, the best potential for heavy
rainfall will reside anywhere from the Central Plains up through
the Middle Mississippi Valley and into the Upper Great Lakes. This
region is juxtaposed with where a frontal boundary should stall
during the period. Elsewhere, enhanced tropical moisture
accompanying the tropical wave could lead to heavy rainfall
anywhere along the central to eastern Gulf Coast. Elsewhere, some
return of moisture to the Four Corners should lead to daily rounds
of thunderstorms to the region.


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