Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Fri Sep 07 2018

Valid 12Z Mon Sep 10 2018 - 12Z Fri Sep 14 2018

...Heavy rainfall to exit the Northeast early next week...

...Hurricane Florence looms off the East Coast later in the week...


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

At the start of the period, the extratropical remains of Gordon
will be in the process of pushing eastward across Lake Erie. Broad
mid-level southwesterly flow should usher the system toward
southern Quebec by the following day. As this exits the map
domain, the focus shifts toward the eastern Pacific and West Coast
as a positively-tilted longwave trough sits over the region during
Day 5-7, September 12-14. This configuration will afford
significant height rises across the central/eastern U.S. with
anomalies around 2 to 2.5 sigma under the core of the upper ridge.
To its south, models are hinting at some broad weakness across the
western Gulf Coast. Lastly, influences of Hurricane Florence may
reach the Eastern Seaboard during the middle to latter part of
next week. While oceanic impacts will be the primary issue,
eventual coastal to inland effects are possible depending on the
ultimate track.

Multi-day ensemble spaghetti plot comparisons have shown only
modest changes in the guidance with respect to the amplified
synoptic-scale flow. While individual members exhibit some
variations in the pattern, operational and ensemble guidance
afford a fairly confident forecast, particularly across the
western U.S. A general consensus of solutions is warranted
although the 00Z UKMET did seem to be a hair quick ejecting the
mean trough downstream. On the other hand, the forecast track of
Hurricane Florence remains highly volatile beyond Day 5/Wednesday.
Potential landfall could be anywhere from the eastern Florida
Panhandle up to the Mid-Atlantic late next week while other
scenarios turn the circulation north and east before reaching the
coast. While focus is on the individual cyclone track, impacts far
away from the center are likely given the expected increase in
strength the next few days. Please visit the National Hurricane
Center website for more information on Florence.

A general blend of operational solutions was utilized for the
first couple of days of the period. This was comprised of the 06Z
GFS and 00Z GFS-parallel/ECMWF runs while slowing adding ensemble
means into the picture thereafter. Did keep a modest amount of the
06Z GFS/00Z GFS-parallel in the mix throughout given it was the
closest match to the official track of Hurricane Florence.


...Weather Highlights/Hazards...

Initially, heavy rainfall will remain an issue as Gordon and the
attendant frontal boundary intercept abundant tropical moisture.
This should primarily focus over New England before winding down
later on Tuesday as mid-level heights begin to rise. Over the
Pacific Northwest, persistent onshore flow within the mean trough
will keep precipitation in the forecast during much of the period.
Farther east, a couple waves of low pressure should spread a
shield of rainfall across the north-central U.S. before lifting
into central Canada. Elsewhere, some lingering heavy rainfall
threat is possible near the upper low across the western Gulf of
Mexico. This would focus over the western/central Gulf Coast
although some spread exists in the guidance. And along the Eastern
Seaboard, the primary impacts will be driven by the track of
Hurricane Florence. Flooding rains, high winds, rough surf, among
others would all be possible but it is too early to pinpoint exact
impacts given the model uncertainties.

Regarding the temperature forecast, cool weather should prevail
from northern California up into the Pacific Northwest given the
trough overhead. Forecast anomalies should sit around 5 to 10
degrees below climatology during much of the week. Downstream into
the middle of the country, building heights will support abundant
warmth for several days. Farther east, the temperature forecast
will be highly dependent on the track of Florence but warm
conditions are likely initially given the upper ridge in place.


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