Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1207 PM EDT Sun Aug 18 2019
Valid 12Z Wed Aug 21 2019 - 12Z Sun Aug 25 2019
...Overview...
Deep low pressure pushing into southeast AK Tuesday night
amplifies a ridge up the Canadian Prairies which in turn shunts a
deep low pressure system on the southern shore of Hudson Bay on
Wednesday across Quebec through Friday. Shortwave energy rounding
this eastern Canadian low brings a trough across the Great Lakes
Wednesday night into Thursday and across the northeast/New England
Thursday night. A cold front ahead of this trough shifts east from
MI Wednesday and off the New England coast Thursday.
The shortwave energy associated with the Pacific system will move
into western Canada and the Pacific Northwest Wednesday and serve
as the leading edge of a fairly zonal flow regime extending from
the North Pacific (south of mean troughing over the Gulf of Alaska
through the period) across the northern tier states and provincial
Canada that should cross the Great Lakes Saturday. Weak troughing
persists over the Northeast U.S. into the weekend. Broad Ridging
over the southwest states persists through the period. Meanwhile
there is a moderate clustering of guidance that shows upper level
energy drifting north through the western Gulf of Mexico which is
on pace to reach the Gulf Coast Friday night.
...Guidance/Predictability Assessment...
The guidance blend for Days 3-5 emphasizes operational guidance
(00Z ECMWF/06Z/00Z GFS/00Z UKMET/00Z CMC in order from most to
least weight) provides a reasonable starting point for the
forecast. Within the eastern upper trough there are still smaller
scale detail differences that will affect progression of the
leading cold front and which maintains uncertainty. There are also
meaningful differences for the leading energy that progresses into
the northern U.S./southern Canada, with a blend approach providing
the most stable solution while awaiting any confident adjustments.
The blend for days 6 and 7 increases 06Z GEFS/00Z ECMWF mean
weight (60 percent total by day 7/Sunday) as models/ensemble
members continue to differ for specifics within the main band of
westerlies. Relatively small scale keeps confidence low for the
western Gulf energy aloft while the forecast maintains continuity
for the possible surface reflection.
...Weather Highlights/Threats...
A focus for precipitation will be the cold front that pushes into
the East and Great Plains late week and settles over the
Southeast, with the trailing part of the front extending back into
the central Plains and returning northeast as a warm front. The
best signals for highest rain totals along the front are currently
over southern half of the Mid-Atlantic and the central Plains
centered on KS/MO. The energy aloft/surface reflection expected to
lift northward over the Gulf may bring areas of heavy rainfall to
parts of the central-western Gulf Coast by the weekend. The rest
of the Southeast/Gulf Coast region should see diurnal convection
with greater influence from localized boundaries. A cold front
approaching/reaching the Northwest Wed-Thu directs a weakening
atmospheric river into northern parts of the Pacific Northwest
though uncertainty remains with the magnitude of precipitation.
One or more frontal systems looks to produce scattered convection
over the northern plains.
Expect most of the West and southern High Plains to see above
normal temperatures during the period. Anomalies may reach or
exceed plus 10F at some locations over the West with the some
potential for reaching daily records any of the days. The cold
front pushing into the East/South will provide a cooling trend
after Wednesday. The northern/central Plains will see a couple
days of below normal highs before warming over the weekend.
Jackson
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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