Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1248 AM EDT Sun Jul 21 2019
Valid 12Z Wed Jul 24 2019 - 12Z Sun Jul 28 2019
...Overview and Guidance/Predictability Assessment...
The pattern is amplified days 3-4 with the high amplitude ridge in
the southern Rockies and high amplitude eastern US trough. The
overall trend on days 5-7 is for the western ridge to drift west
across the southwest and for deamplification of the eastern US
trough. The models now show a well defined west-east ridge over
the southwest Atlantic that builds across Florida into the Gulf of
Mexico next weekend.
The northern stream wave train shows potential for a few waves
progressing across
southern Canada and the northwest, northern Plains, upper MS
Valley, and Great Lakes.
The 12z ECMWF flipped on the timing and phasing of the 500 mb wave
train. The 12z ECMWF Ensemble Mean maintained continuity, which
was preferred over the large changes in the operational run. For
example, most ECMWF ensemble members at 12z Sat have a 500 mb
ridge in MT while the 12z operational run had a trough. The mean
had a 500 mb ridge in MT Sat morning 27 Jul, which also matched up
well with the GEFS and NAEFS Means and 12z GFS.
A multi-model/ensemble mean blend (12Z ECMWF/12z ECMWF Ensemble
Mean/12z CMC/18Z GFS/18z GEFS Ensemble Mean) was used as a basis
for the forecast. Greater weighting was given to the ECENS/GEFS
ensemble means to maintain continuity and to mitigate model to
model and run to run differences.
...Weather Highlights/Threats...
The frontal boundary expected to stall off the Eastern Seaboard
through the medium range is expected to focus numerous showers and
thunderstorms. The most widespread convection and the greatest
potential for areas of heavy rainfall should occur from
eastern/coastal North Carolina southward across the coastal plain
to the Florida Panhandle Tue-Thu. Showers/storms may lessen in
coverage/intensity by late next week as the surface front washes
out. The shortwave/frontal system moving from the Pacific
Northwest to the Northern Plains/Upper Midwest should bring an
increase in shower/storm chances to the Upper Midwest by late next
week and across the Great Lakes next weekend. Meanwhile, daily
scattered storms will be possible across the central/southern
Rockies as some monsoonal moisture manages to spread into the
upper ridge.
Below average temperatures are expected across a large area in the
wake of the cold front moving off the East Coast Tue. Temperatures
are expected to gradually moderate back towards normal next
weekend. The persistence of the western Ridge leads to
temperatures gradually increasing in interior parts of
WA/OR/northern Ca and spreading across ID into western MT next Sat
27 Jul and Sun 28 July.
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