Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Thu Aug 19 2021
Valid 12Z Sun Aug 22 2021 - 12Z Thu Aug 26 2021
...Henri to intensify to Hurricane status and threaten parts of
New England by Sunday and Monday...
...Overall weather pattern...
A rather active upper level pattern will be in place across the
northern tier states and southern Canada with several well
developed shortwaves moving through. A building upper level ridge
will continue to build across the southern tier states and the
northern Gulf of Mexico, and the core of the upper high is
expected to retrograde westward through early next week.
Meanwhile, Henri is expected to come uncomfortably close to
eastern New England by late Sunday and into Monday with gusty
winds and heavy rainfall possible.
...Model analysis and preferences...
Models and ensembles are initially in good overall synoptic scale
agreement across most of the continental U.S. on Sunday.
Noteworthy differences become apparent by Tuesday across the
north-central U.S. with a strong shortwave perturbation near the
Canadian border, with the GFS indicating a considerably faster
solution than the better clustered CMC/ECMWF/ECENS, and the GFS is
also faster than its ensemble mean. The GFS is also stronger with
a trough building near the West Coast by next Thursday and is out
of phase by having a ridge over the Upper Midwest compared to the
other guidance. Taking these factors into account, the WPC
fronts/pressures forecast was primarily derived from a non-GFS
blend, although some of the 18Z GEFS mean was incorporated by the
second half of the forecast period.
In regards to Henri, the main factor determining its eventual
track near New England will be the strength of the blocking upper
ridge over eastern Canada and a weak upper low over the
Mid-Atlantic region. The trend over the past 48 hours has been
farther west and this brings the storm very close to Cape Cod
owing to a stronger ridge to the northeast. The 00Z UKMET is the
farthest west over New England, whilst the ECMWF still maintains a
weaker storm by the time it reaches this region.
...Sensible weather and potential hazards...
Bad weather is becoming more likely for eastern New England by
Sunday afternoon as Henri approaches from the south and comes
close enough to produce heavy rain, strong winds, and coastal
flooding from Rhode Island to southern Maine. It is important to
note that future adjustments in the track can be expected, and the
impacts will become more predictable in the days ahead. The
National Hurricane Center has the latest information pertaining to
this storm.
Elsewhere across the continental U.S., amplified shortwave troughs
will steadily progress from the Pacific Northwest to the
north-central U.S., and then the Great Lakes next week, and these
will lead to the potential for scattered to perhaps numerous
showers and storms, some of which could be strong or severe where
convective complexes develop. In the temperature department,
highs are expected to be about 5-15 degrees below normal from the
Northern Rockies to the Northern Plains, and up to 10 degrees
above normal for much of the central and southern U.S. that will
be under the influence of the upper ridge, along with plenty of
humidity.
Hamrick
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