Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
218 AM EDT Wed Sep 12 2018

Valid 12Z Sat Sep 15 2018 - 12Z Wed Sep 19 2018

...Hurricane Florence could bring catastrophic flash flooding and
significant river flooding to portions of the Carolinas and
Southern Appalachian states this weekend into early next week...


...Hurricane Florence...

Powerful Hurricane Florence is most likely to make landfall as a
major hurricane along the southern half of North Carolina or
northeastern South Carolina on Friday. The models and ensembles
continue to waver on the expected landfall location (or
near-landfall) and timing of when Florence may slow down or
stall/meander just inland. In spite of ongoing guidance spread for
track/timing specifics, confidence is gradually increasing that an
extreme rainfall and flooding event may unfold along and
north/east of the track of Florence, aided by inflow off the Gulf
Stream. Currently expect an area of 15-25 inch accumulations with
isolated local maximum totals to 35 inches.
Upslope/terrain/coastal enhancement would exacerbate the
seriousness of the rainfall/flooding threat. These forecast
rainfall amounts have rarely been recorded in the past century and
can result in flooding where it has not been observed from past
events. The areas of greatest impact and highest rainfall will be
refined over the next couple of days (in the short range) but the
signal for an extreme/catastrophic event has been more consistent.
This will be a multi-day and multi-faceted event that will change
through time as Florence is forecast to be a coherent entity
through at least next Tuesday (though in weakened form). Please
consult the National Hurricane Center for the latest information
on Florence as well as your local NWS forecast office.


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

Evolving upper ridging near Alaska will continue to favor
positively tilted troughing from the Pacific Northwest/eastern
Pacific into central Canada/Hudson bay. Upper ridging is forecast
to stretch from the Four Corners region northeastward into New
England and into the NW Atlantic, helping to guide a weakened
Florence westward near the NC/SC border per the latest NHC track
forecast.

The ensemble trend in the PacNW has been decidedly slower and for
a stronger shortwave or closed low near coastal Washington that
will slide east-northeastward and drag a well-defined cold front
through the High Plains/Upper Midwest next week. This front, as
currently forecast, would then pick up the remnants of Florence as
it lifts northward.

Used a blend of the recent 12Z guidance with more weight toward
the ECMWF and its ensemble mean by the end of the period due to
their better handling of upstream flow over the Pacific. The
initial blend was adjusted for the NHC track of Florence as needed
out through midweek.


...Weather Highlights/Hazards...

Regarding precipitation away from Florence, portions of Texas
(most likely the southern half of the state) may see significant
rainfall in association with a surface trough/upper low emerging
over the southern Gulf, crossing between short range and medium
range. Consult latest NHC tropical weather outlooks as conditions
could become more favorable for development as the feature heads
into the western Gulf. Slower guidance trends with the West Coast
upper trough have delayed the drying trend over the Northwest,
with light to moderate activity likely to persist through most of
the weekend. Temperatures over/near the northern Rockies may be
cold enough to support some higher elevation snow but the best
snow potential will be to the north of the US/Canadian border.

Aside from a modest delay per guidance trends with the upper
pattern, expect the emphasis for coolest temperatures (highs 5-15F
below normal) to transition from the West Coast states into the
northern Plains. Over the latter area there may be decent coverage
of minus 10-15F anomalies for highs by next week (air temperatures
in the 50s). Otherwise most of the central/eastern states should
see above normal temperatures, especially for overnight mins.
Warmest morning lows versus normal (some 15F+ anomalies) should
extend from the central Plains into upper Great Lakes, ahead of
the northern Plains front, and over middle/northern latitudes of
the East as Florence pulls in warm/humid air. Some lows may be
near or exceed record warm values on one or more days. Maximum
temperatures would be held down near Florence and its cloud shield.

Fracasso/Rausch


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