Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Fri Sep 14 2018

Valid 12Z Mon Sep 17 2018 - 12Z Fri Sep 21 2018

...Hurricane Florence to bring catastrophic flash flooding and
prolonged significant river flooding to parts of the Carolinas and
Appalachians...


...Hurricane Florence...

Hurricane Florence made landfall this morning over southeastern
North Carolina and the National Hurricane Center forecast has the
very dangerous system gradually weakening on an arc across South
Carolina over the next couple of days before lifting roughly up
along/west of the southern and central Appalachians Monday. The
wet post-tropical system is forecast to further accelerate
northeastward over the northern Mid-Atlantic/Upper Ohio Valley
through the Northeast and Canadian Maritimes Tuesday into
Wednesday. Flooding remains the main threat owing to heavy and
persistent rainfall as it interacts with mountainous terrain and
with the threat of prolonged/training of cells. While the majority
of prolific rainfall occurs prior to medium range time scales, a
swath of enhanced rain will follow along the track of Florence
into midweek. Please consult the latest advisories and warnings
from the National Hurricane Center, the Weather Prediction Center
and local NWS forecast offices and agencies for updated
information.


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

The forecast period begins on Monday with a trough centered near
the West Coast, an upper level ridge over the southern plains, and
a broad trough over the Ohio Valley that captures the remnants of
Florence. The upper ridge expands across the south-central U.S.
and then the Southeast states by the end of the week with an
increasingly progressive flow pattern slowly developing over the
northern tier of the nation.

The 00 UTC ECMWF ensemble means was used as a starting point for
WPC surface fronts and pressures progs days 3-5 in a period of
smaller forecast spread excluding Florence. This solution meshed
best with the latest NHC track for Florence that was inserted. The
00 UTC ecmwf and to a lesser extent ECMWF ensembles though seem to
break continuity and transition to a progressive flow pattern
across the U.S. northern tier much more quickly than most other
guidance. Therefore, instead based day 6/7 WPC guidance more in
line with the seemingly more reasonable 06 UTC GEFS mean amid
growing forecast spread.


...Weather Highlights/Hazards...

Florence will spread a large area of significant rainfall
northeastward next week despite its acceleration ahead of an
approaching cold front. Flooding may be a concern along the I-81
to I-95 corridors but will be a function of forward speed. Back to
the west, as troughing finally moves out of the northwest, a cold
front will usher in cooler fall air to the High Plains and Upper
Midwest/Great Lakes Tue-Wed and then to the east coast next
Thursday as ex-Florence should race well into Atlantic Canada
then.Expect record high minimum temperatures along/ahead of the
moist track of Florence.

Meanhwile, a new area of precipitation will take shape and expand
over the northern Plains and Upper Midwest next week as waves and
attendant preciptation focus along/near the front exits the
foothills of the Rockies.

Schichtel


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