Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sun Aug 18 2019
Valid 12Z Wed Aug 21 2019 - 12Z Sun Aug 25 2019
...Overview and Guidance/Predictability Assessment...
Guidance continues to show a fairly vigorous upper trough crossing
the northeast quadrant of the lower 48 after a short-range
northeastern Pacific system helps to amplify the flow to its east.
The shortwave energy associated with the Pacific system will move
into North America after midweek and serve as the leading edge of
a flow regime that has low amplitude mean flow extending from the
North Pacific across the northern tier states and southern Canada.
An initial Rockies-Plains upper ridge should weaken/retrograde a
bit during the period. By next weekend there is some suggestion
of at least flat ridging that builds off the West Coast--which
teleconnections say should eventually lead to a mean trough over
the eastern half of the country. Meanwhile there is a moderate
clustering of guidance that shows upper level energy drifting
north through the western Gulf of Mexico and reaching the Gulf
Coast by the latter half of the period.
During the first half of the period a primary emphasis on
operational guidance (12Z ECMWF/18Z GFS/12Z UKMET/12Z 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 will take more time to be
resolved. 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 rest of the period
increases 18Z GEFS/12Z ECMWF mean weight (60 percent total by day
7 Sun) as models/ensemble members continue to differ for specifics
within the main band of westerlies. The majority guidance cluster
shows a trough reaching the northern Plains and vicinity by next
Sun, a reasonable lead-in to the eastern trough that
teleconnections say should ultimately develop in response to the
positive height anomalies that recent D+8 multi-day mean charts
show off the West Coast. Relatively small scale keeps confidence
low for the western Gulf energy aloft while the forecast maintains
continuity for the possible surface reflection--a trough per 17Z
WPC-NHC coordination.
...Weather Highlights/Threats...
One focus for convection will be the front that pushes into the
East mid-late week and settles over the Southeast, with the
trailing part of the front extending back into the Plains and
returning northeast as a warm front. The best signals for highest
totals along the front are currently over southern half of the
Mid-Atlantic and the central Plains. 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 should bring a brief period to northern parts of
the Pacific Northwest. One or more frontal systems may produce
scattered convection over the far northern tier.
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 best
potential for reaching daily records around midweek. The cold
front pushing into the East/South will provide a cooling trend
after Wed. The northern/central Plains will see a couple days of
below normal highs early in the period followed by parts of the
Great Lakes/Northeast. Locations to the south should be close to
normal by Fri-Sat. The Great Lakes/Ohio Valley may see a warming
trend by Sun though.
Rausch
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