Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 PM EDT Thu Aug 19 2021
Valid 12Z Sun Aug 22 2021 - 12Z Thu Aug 26 2021
...Dangerous Henri to intensify to Hurricane status and threaten
New England by Sunday...
...Overall Pattern and Weather/Hazard Highlights...
Henri is forecast to gain strength over the western Atlantic and
threaten New England Sunday into early next week, bringing a
substantial threat for high winds/waves and heavy rainfall.
Guidance has overall come in better agreement with Henri, but the
overall blocky flow around the system and less predictable smaller
scale interactions still offer a tricky forecast. The National
Hurricane Center indicated during our 17 UTC collaboration that
Hurricane Hunter recon aircraft now investigating Henri may shed
additional light for subsequent guidance runs. Please refer to the
latest information from the NHC on Henri.
Otherwise, an active upper level pattern will be in place across
the northern tier states and southern Canada with several well
developed shortwaves moving through with development and slow
translation of well defined and wavy fronts to pool moisture and
instability. This offers broad potential for scattered to perhaps
numerous showers and storms, some of which could be strong or
severe where convective complexes develop as per SPC. Meanwhile, a
building upper level ridge underneath will continue to build
across the southern tier states and the northern Gulf of Mexico
leading to hot and humid conditions for much of the
south-central/southern states. The upper high/ridge is expected to
slowly retrograde westward next week and allow for a mainly dry
and increasingly hot West.
...Model analysis and preferences...
The WPC medium range product suite is primarily derived from the
NHC forecast for Henri and elsewhere a composite of reasonably
well clustered model and ensemble guidance along with the National
Blend of Models days 3-5 (Sun-Tue). In this preferred blend
outside of Henri, leaned strongly on the GFS/ECMWF/Canadian/UKMET
days 3-5 (Sun-Tue) and then the GEFS/ECMWF ensemble means into
days 6/7 amid growing forecast spread. In particular, recent GFS
runs have been more progressive/amplified bringing east Pacific
upper trough energies into the Northwest later next week and that
does not seem well supported by the pattern nor GEFS/ECMWF
ensembles. The resultant WPC forecast suite maintains good product
continuity.
Hamrick/Schichtel
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