Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Wed Aug 21 2019
Valid 12Z Sat Aug 24 2019 - 12Z Wed Aug 28 2019
...Overview and Guidance/Predictability Assessment...
Models and ensemble means continue to show the establishment of a
mean trough aloft over the northern CONUS, between an eastern
Pacific ridge that amplifies into the northeastern Pacific and
another ridge that drifts across eastern Canada. Within this mean
trough guidance becomes increasingly divergent for individual
shortwave specifics by the latter half of the period. Farther
east models/ensembles are still having some difficulty resolving
the details of energy within an initial eastern
Canada/northeastern U.S. trough which should progress eastward
according to the majority cluster. At more southern latitudes
ridging aloft should prevail over the southern half of the West
while a developing upper weakness over the Lower Mississippi
Valley and western Gulf Coast will likely split (the southern part
retrograding and the northern part lifting out to the
east/northeast).
From the weekend into the start of next week guidance agrees
decently for the gradually amplifying northern tier mean trough
aloft, with Pacific shortwave energy nearing the Pacific
Northwest/British Columbia coast as of early Sat leading to a
sharpening trough over and just east of the northern Plains by day
5 Mon. As this occurs the models and means develop strengthening
low pressure along/north of the U.S.-Canadian border with cold
front extending south/southwest. As this trough lifts
northeastward after Mon, there is considerable spread/decreasing
confidence for specifics of the northeastern Pacific ridge and
downstream energy that drops into the northern tier mean trough.
A key piece of the puzzle involves a shortwave forecast to round
the ridge Mon or later. The 12Z ECMWF looked particularly suspect
as its shortwave dampened the ridge more than other guidance
around Tue--not quite an outlier relative to the full ensemble
spread but very close. The ensemble means were similar to each
other and provided the most reasonable starting point given
operational model differences and run-to-run variability,
suggesting a shortwave reaching the northwestern U.S. by day 6 Tue
and continuing eastward thereafter.
There is still a minority cluster consisting of the 12Z
ECMWF/latest UKMET runs which show energy with the Northeast upper
trough closing off a slow-moving low. ECMWF runs have been
inconsistent with the prior cycle having no upper low while the
two runs before that had the low. Not enough ensemble members
from ECMWF/GFS/CMC show an upper low for the means to have any
more than an elongated trough, with the latter depiction likely
the best option for a single deterministic forecast at this time.
Based on the above considerations the updated forecast started
with an operational model blend on day 3 Sat and then gradually
decreased ECMWF input and increased ensemble mean weight. By days
6-7 Tue-Wed the primary emphasis was on the 18Z GEFS/12Z ECMWF
means. The blend retained a minority component of the 18Z GFS due
to reasonable comparison with the means, though it did trend a
little fast with the northern tier trough by late in the period.
...Weather Highlights/Threats...
There is a continued threat for heavy rainfall over areas from the
northern half of the Plains into the Great Lakes as upper
troughing becomes established over the northern tier states. From
the weekend into early next week the primary surface focus will be
with a cold front that pushes into the northern Plains as well as
a leading warm front. As upstream energy feeds into the mean
trough aloft, the Plains to Great Lakes part of the front may
stall for a period of time by next Tue-Wed and provide an
opportunity for training convection or repeat activity. Some
convection may be strong to severe during the period so check
Storm Prediction Center outlooks as the details become more clear
in future days.
The area encompassing the central Gulf Coast through the Southeast
and southern Appalachians/southern Mid-Atlantic will also see the
potential for areas of heavy rainfall with a combination of ample
moisture, one or more upper level impulses, and the wavy surface
front settling over the Southeast. Diurnal convection over the
Florida Peninsula may produce locally significant totals as well.
While some of the specifics of flow aloft by the middle of next
week have yet to be resolved, leading up to that point confidence
is increasing for a significant cooling trend over the northern
Plains where some locations may see multiple days with highs of
10-15F or so below normal. On the other hand areas from southern
Oregon/northern-central California into the southern High Plains
should see above normal readings for most of the period with
pockets of plus 10-15F anomalies and possibly a few daily records.
Cooler air may reach the southern Rockies/High Plains by next
Wed. Near to below normal temperatures over the eastern half of
the country during the weekend should trend toward near or
slightly above normal readings by next Tue-Wed.
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