Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1140 PM EDT Sun Sep 12 2021
Valid 12Z Thu Sep 16 2021 - 12Z Mon Sep 20 2021
...Heavy Rain From Tropical Storm Nicholas May Continue Into
Thursday...
...Overview and Guidance Evaluation/Preferences...
Weak flow around Nicholas may allow heavy rain from the system/its
remnants to continue over portions of the western Gulf Coast into
Thursday but the system is expected to be absorbed/dissipated by
late in the week. Follow the National Hurricane Center advisory
products for the latest information on Nicholas.
General agreement among 12Z global deterministic/18Z GFS of a deep
central Canadian low shifting east with the next low/trough
approaching the Pacific Northwest Saturday. The 12Z ECMWF diverges
from consensus and its ensemble mean with a much less amplified
wave than the GFS/CMC and the ensemble means by Day 5. There is
decent ensemble mean agreement particularly with the 18Z GEFS and
12Z ECENS means Days 5-7 with the deepening mean trough aloft
along the West Coast late this week into the weekend while western
Atlantic/eastern U.S. ridging (or at least well above normal
heights) deflects the westerlies across the northern tier U.S. and
southern/eastern Canada.
Another feature to note is a wave/weak area of low pressure
possibly forming north of the Bahamas by Thursday and track
northward, off the East Coast. The 18Z GFS remains the strongest
with possible tropical development of this system with great
divergence in overall guidance. A fair degree of uncertainty
persists for specifics of flow aloft reaching the East Coast from
late week into the weekend but the majority scenario has enough
westerly flow to push the system and its rainfall away from most
if not all of the Northeast. This system requires close monitoring
as minor changes could have significant implications for any rain
that may fall across an already saturated coastal
Mid-Atlantic/Northeast.
...Weather/Hazard Highlights...
Nicholas could maintain its definition long enough to support a
heavy rain threat through mid to late week, though there is
uncertainty in the eventual track of the system, how long it will
maintain tropical characteristics and heavy rainfall threat before
becoming absorbed into the weak flow and dissipate. Continue to
monitor the latest NHC advisory products.
Remnants of Nicholas may allow some enhanced rainfall over the
eastern US depending on the track of the remnants and the
associated plume of tropical moisture. Mostly lighter rainfall
looks to accompany frontal passages through the central parts of
the CONUS through this weekend. With the next frontal progression,
a slower and more wavy front could produce heavier rainfall over
the Upper Mississippi Valley/Great Lakes while rainfall intensity
would be less with a faster front. Meanwhile moderate to locally
heavy rainfall looks to reach the Pacific Northwest Friday into
the weekend with some snow possible in the highest elevations of
the Washington Cascades.
Evolution of the large scale pattern will favor cooler
temperatures settling over the West Coast states, with some highs
10-15F below normal over the Northwest by the end of the week/next
weekend, along with steadily increasing coverage of plus 10F and
greater anomalies for max/min readings over the Plains and
east-central U.S. Locations to the south of an initial
northeastern U.S. front will also see well above normal
temperatures from midweek onward. Any clouds/moisture with the
system forecast to track over the western Atlantic will be a wild
card with respect to high temperatures near the East Coast.
WPC
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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