Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EDT Mon Mar 18 2019
Valid 12Z Thu Mar 21 2019 - 12Z Mon Mar 25 2019
...Overview and Guidance Assessment/Preferences...
Split flow pattern is expected to persist across North America
during the extended period. Ensemble guidance shows good consensus
that a strong and blocking upper-level ridge axis initially across
the High Plains of the U.S. and Canada will very slowly shift
eastward over the next week. With the strongest positive height
anomalies remaining centered across north central Canada, this
will continue to split incoming energy from the Pacific into
northern stream components directed northward toward Alaska, and
southern stream components that traverse the southern/central
U.S., undercutting the ridge. Downstream of the ridge, the
Northeast will be subject to continued mean troughing, reinforced
by a series of energetic shortwaves. Some guidance suggests that
perhaps this northern stream amplified ridge/trough pattern may
begin to weaken by early next week, but confidence in that is
relatively low at this time given increased spread among the
ensemble guidance as to the intensity and duration of the upper
ridge.
Model depictions are now relatively similar with respect to the
upper low/trough crossing the Four Corners Thu-Fri and exiting
into the Plains Fri night-Sat. Farther east, solutions differ more
significantly with respect to small scale northern stream
shortwave energy traversing the larger mean trough, and the
potential for development of a surface low off the Eastern
Seaboard Thu night-Fri. The majority of solutions keep any wave of
low pressure relatively weak off the U.S. East Coast, delaying
significant intensification until the feature nears the Canadian
Maritimes where it potentially interacts with additional shortwave
energy. Finally, models show general consensus that an another
shortwave/frontal system should reach the West Coast Fri-Sat, with
relatively minor timing and amplitude differences through Sat.
Based on these factors and an assessment of moderate confidence, a
blend of the ECMWF/GFS/UKMET was used a basis for the forecast
during days 3-5.
Timing differences become more significant as the second Pacific
shortwave moves inland by Sun, with the GFS persisting in a much
quicker progression relative to the slower ECMWF/CMC and ensemble
means. The same holds true regarding yet another potential system
approaching the West Coast. The GFS continues to be quicker than
the consensus of other guidance in bringing another amplified
trough/upper low to the West coast by Sun night-Mon. Given these
differences, the GFS was excluded from the forecast during days
6-7, with the forecast based on a blend of the ECMWF and the
ECENS/GEFS ensemble means.
...Weather Highlights and Hazards...
Rain and high elevation snows are expected across the
southern/central Rockies Thu-Fri as the leading Pacific system
passes the region. Increasing moisture return ahead of the system
will increase coverage of showers and some thunderstorms across
the southern/central High Plains by Friday. The next system will
bring another round of rain/snow to Pacific Northwest to the
Rockies Fri-Sat, with further increased potential for rain and
thunderstorms, some of which could be heavy, from the
southern/central Plains to the Mississippi Valley through the
weekend. As the upper ridge moves slowly east, above average
temperatures are expected to develop by late this week across the
Midwest, gradually spreading east across the Great Lakes to the
Eastern Seaboard through the weekend. Maximum temperatures ranging
from 5 to 15 deg F above average are forecast, with minimum
temperatures approaching 20 deg above average for some areas
across the Midwest and Mississippi Valley.
Ryan
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