Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Mon Aug 19 2019
Valid 12Z Thu Aug 22 2019 - 12Z Mon Aug 26 2019
...Overview/Guidance/Predictability Assessment ...
The days 3-7 fronts/sea level pressures/500 mb heights were
derived from a blend of the 06z GEFS Mean and 00z ECMWF Ensemble
Mean (with the 00z NAEFS clustering well with these solutions),
with less weighting on the operational 00-06z GFS and 00z ECMWF.
...Eastern US...
From late this week into the weekend expect an upper level trough
to cross the Great lakes and northeast, with the trough departing
either late Sunday or early next week. The leading cold front
reaching the East as of early Thu will likely stall over the
Southeast while the trailing part of the front over the Plains
returns as a warm front (though with slower multi-day trends).
For the northeastern upper trough there is decent consensus in
principle. However shortwave differences between models and in
consecutive runs lead to some spread/variability in specifics of
low pressure forecast to track just north of New England and the
trailing wavy front that sinks through the Mid-Atlantic into the
Southeast.
The 00z ECMWF was an outlier in closing a 500 mb low and
developing a stronger wave of low pressure along the front in the
mid Atlantic. The vast majority of ECMWF ensemble members are
more progressive in moving the trough off the coast and don't
develop a 500 mb low, which is a rare occurrence in Aug.
Consequently, the ECMWF Ensemble Mean is preferred over the
operational 00z run.
...Western US...
Guidance is starting to converge on the evolution of the 500 mb
trough crossing the northern Rockies on Thu and northern Plains on
Fri, deamplifying as it moves east towards the upper MS Valley
Sat. The 00z Canadian global was a high amplitude outlier, with
the good clustering of the GFS/ECMWF and their respective
ensembles preferred.
Low amplitude mean flow should prevail from the North Pacific into
western-central parts of the northern CONUS/southern Canada, with
a trend toward some troughing moving out of the northern Rockies
this weekend and into the northern Plains early next week. The
latest ECMWF/CMC ensemble means have been more in agreement with
their depiction of the trough relative to the latest GEFS mean
runs that are somewhat slower, so this aspect of the forecast
remains to be resolved. Here the intermediate NAEFS forecasts
would be helpful.
The ECMWF/GFS/Canadian have waves at the base of the trough in the
central Plains to the upper MS Valley that differ in timing and
location early next week, so predictability of that aspect of the
forecast is low.
...Weather Highlights/Threats...
The front dropping through the Mid-Atlantic into the Southeast
continues to serve as a likely focus for areas of showers and
thunderstorms, with potential for highest totals along and just
inland from the southern Mid-Atlantic/Southeast coast.
A separate wave along the trailing front in the Plains will result
in other clusters of showers/storms in parts of the
central/south-central Plains into Mississippi Valley. Moisture
return from the Gulf and possible low level trough should enhance
showers/storms in the Gulf Coast this weekend and into the
southeast early next week.
Areas from the northern Plains east into the Mississippi Valley
may see one or more episodes of convection with fronts reaching
the region.
Expect above normal temperatures to be most persistent from
southern Oregon and the northern two-thirds of California into the
southern High Plains with upper ridging in place during the
period. Scattered locations within this area may see max and/or
min temperatures at least 10F above normal. The central/northern
Plains into Great Lakes seeing the best opportunity for below
normal highs from Thu into Sat, moderating thereafter.
Petersen
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