Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1127 AM EDT Sat Sep 08 2018

Valid 12Z Tue Sep 11 2018 - 12Z Sat Sep 15 2018

...Florence likely to impact a portion of the East Coast next
week...


...15 UTC Update...

Models/ensembles continue to show above average consensus on the
amplified flow pattern across North America during the extended
period. With respect to the eventual track of Florence, confidence
continues to grow that an impact somewhere along the Southeast or
Mid-Atlantic coastline will occur by mid to late next week. Among
the global deterministic guidance, the 00Z ECMWF was closest to
the official NHC forecast track through 12 UTC on day 5 (Thu),
keeping what is forecast to be Hurricane Florence a couple hundred
miles off the South Carolina coast by that time. Please refer to
products issued by the NHC for the latest information on Florence.
Elsewhere, the other significant question by late next week is the
degree of amplification of upper shortwave energy off the Pacific
Northwest coast. As mentioned overnight, the ECMWF remains the
most amplified solution, but has been very consistent in this,
while the GFS has shown rather significant fluctuations from run
to run. At this time will lean toward the ECMWF and ensemble means
here.

Given initially excellent consensus among the guidance initially,
a multi-model deterministic blend including the
ECMWF/UKMET/CMC/GFS was used a basis for the WPC forecast across
the CONUS on days 3-5 (Tue-Thu). From day 6 onward, increasing
weight was placed toward the ECENS/NAEFS means along with
continued substantial weighting of the ECMWF given its proximity
to the NHC track for Florence earlier in the period.

Ryan


Previous Discussion (issued at 0532 UTC)

...Pattern Overview...

Strong upper ridging over the northern Pacific and the
northwestern Atlantic off the New England coast will favor the
maintenance of a positively tilted trough along the west coast of
the lower 48. Current tropical storm Florence is forecast to
strengthen to a major hurricane between the Bahamas and the Outer
Banks next week and is likely to impact at least part of the
Southeast/Mid-Atlantic starting as early as late Wednesday but
perhaps still into next weekend. Please consult the National
Hurricane Center for the latest information.

...Guidance/Predictability Assessment...

Overall, the amplified pattern remains well-handled by the
ensemble guidance but with uncertainty atop the ridge in the
Pacific that translates toward the Pacific Northwest. To the east,
the strength and shape of the blocking upper high will dictate
where Florence will track, but the slowly narrowing ensemble
consensus appears to take the system into the Southeastern US but
the range of possibilities still extends from Florida northward to
at least the Mid-Atlantic which includes a chance of recurving
just offshore. With broad WSW flow from mid-continent to the
Atlantic, the typical acceleration of a transitioning tropical
system may be delayed as the longwave pattern remains rather
stuck. A blend of the deterministic models sufficed for the early
portion of the period but then transitioned to an 18Z GFS/12Z
ECMWF/18Z GEFS blend to account for the coordinated preference in
the east but also to dampen the perhaps overzealous ECMWF off the
Pac NW with subsequent/reinforcing troughing Thu/Fri.

...Weather Highlights/Hazards...

Biggest impacts will be in the east with the forecast taken
verbatim. Heavy rain, damaging winds, coastal flooding, storm
surge, etc. are real possibilities for coastal and then inland
locations. The heavy rain threat will continue inland with a
continued tropical fetch on the system's east side. A track inland
without a defined acceleration into the westerlies has the
potential for a large area of 5-10"+ inches of rain over a few
days. Many factors will determine location/amounts/timing but that
won't be resolved for a few more days at the least.

A second area of rain will exist over the northern and
northwestern Gulf Coast under a weakness aloft. With plenty of
moisture to work with, several inches of rain are possible over
the period.

Temperatures will generally be cooler than average in the
Northwest under the trough but near to above average elsewhere
(especially for minimum temperatures which may be near record warm
values in the east).


Fracasso


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