Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Fri Aug 20 2021
Valid 12Z Mon Aug 23 2021 - 12Z Fri Aug 27 2021
...Dangerous Henri to affect New England into early next week...
...Overview...
Tropical Storm Henri is forecast to move into Southern New England
late Sunday into Monday to start the medium range period. Though
it will weaken over land before pushing into Atlantic Canada on
Tuesday, it will bring a heavy rainfall and flooding threat along
with strong winds and waves to the region. Refer to the latest NHC
products for further information on Henri. Meanwhile the large
scale pattern will be fairly typical for late summer with southern
Canada/northern U.S. flow aloft pushing along rainfall-focusing
waves/fronts across the northern half of the lower 48 while mean
ridging aloft prevails farther south and likely expands westward
with time. Western Atlantic ridging, partially responsible for
Henri's rather unusual northward track, will meander in place
between Nova Scotia and Bermuda as the remnants of Henri skirt
around its northern periphery.
...Guidance Evaluation and Preferences...
The main complexity in Henri's forecast lies in the short range as
the system interacts with an eastern U.S. shortwave/upper low.
There still remains some west-east uncertainty which will affect
the likely asymmetric rainfall distribution which may interact
with the terrain back to eastern New York and across New England.
Please see the latest National Hurricane Center for the latest
information. A GFS/ECMWF/Canadian blend was close to the earlier
forecast position when the forecast was made.
Guidance has varied with the specifics of features within southern
Canada/northern tier U.S. flow. The primary uncertainties lie with
what becomes of an upper low that ejects from southwestern Canada
Monday onward and then Pacific flow that may reach the Northwest
after midweek, with neither one particularly stable and/or
agreeable in the models over the past couple days. Most solutions
now indicate the southwestern Canada upper low should open up with
time and with moderate troughing to the south, in contrast to some
earlier runs that held onto a more persistent upper low and more
amplified trough. These differences continue to temper confidence
in frontal position over the lower 48, but a blended solution
comprising of a plurality ECMWF and minority GFS/Canadian
weighting offered a reasonable starting point amid the wavering of
solutions. Increased ensemble mean weighting for the last days of
the period where the recent GFS runs were either questionably
strong or weak (and displaced) over the East/southeastern Canada.
...Sensible Weather/Hazard Highlights...
Extending into early next week Henri will bring a threat for high
winds/waves along with some bands of heavy rainfall over and near
New England. The system should become extratropical around Tuesday
as it continues slowly northeastward. Rainfall could be locally
heavy and enhanced by the terrain. Surface waves/fronts crossing
the northern U.S. and southern Canada will interact with leading
moisture and instability to produce episodes of showers and
thunderstorms that could be locally heavy. Currently the best
potential for highest five-day totals is over and near the Upper
Mississippi Valley and Midwest in the vicinity of a wavy frontal
boundary. Central and southern parts of the East may see areas of
diurnally-favored convection within a humid and warm to hot
airmass.
Monday-Wednesday should feature a broad area of above normal
temperatures from the central-southern Plains into the
Mid-Atlantic/Northeast, especially in the wake of Henri. Scattered
plus 10-15F anomalies will be possible mainly over the
Mid-Mississippi Valley into Lower Great Lakes and more for morning
lows versus daytime highs. Even away from the areas with highest
anomalies, some daily record warm lows may be challenged. Highs
should moderate later in the week but warm lows will likely
persist over the eastern half of the country. Expect the highest
heat index values to be over the Mid/Lower Mississippi Valley and
along the Gulf Coast early-mid week where values may rise to over
110F. Westward expansion of upper ridging will promote increasing
coverage of highs 5-10F above normal over southern parts of the
West and across much of California later in the week. The Northern
Plains region will be the main area of cool temperatures with some
highs up to 10-15F below normal, mainly Tuesday-Thursday. Parts of
the Northwest will also be on the cool side early in the week in
the wake of a Pacific front under somewhat persistent upper
troughing.
Fracasso/Rausch
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