Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Mar 31 2019
Valid 12Z Wed Apr 03 2019 - 12Z Sun Apr 07 2019
...Overview...
Today's guidance remains fairly agreeable and consistent in
showing a progression of shortwaves from the Pacific into and
across portions of the Lower 48, with a corresponding series of
surface systems. The northwest periphery of strong low pressure
off the East Coast on Wed may have some influence on New England's
coastal areas as the system tracks northeastward. Then shortwave
energy over the West on Wed will make steady progress and push a
wave/frontal system across the central/eastern states during the
latter half of the week. The northern part of incoming Pacific
energy Thu-Fri should weaken as it heads into a developing western
Canada ridge but the southern part will likely continue across the
southern U.S. and northern Mexico. Then a fairly strong shortwave
should reach the West Coast around Sat with continued progression
thereafter, perhaps followed by yet more energy brushing the
Pacific Northwest late next weekend. The combination of these
systems will likely bring precipitation to most of the CONUS at
some point during the Wed-Sun period, with the southern third or
quarter of the West the most likely area to stay dry.
...Guidance Assessment/Preferences...
Model/ensemble clustering has improved dramatically for the storm
off the East Coast as of early Wed, and with somewhat of a
westward adjustment from 24 hours ago after that time. However
note there are some smaller but locally significant differences
that persist even in the short-range time frame. Otherwise
guidance continues to agree fairly well with the existence of each
feature in the Pacific/CONUS stream but displays considerable
spread/uncertainty for specifics. This favors keeping an approach
that emphasizes components of the operational models during the
first half of the period followed by a transition toward more
ensemble mean input as operational details increasingly diverge.
The shortwave and associated low pressure reaching the Plains
around Thu agrees fairly well in the guidance up to that time.
However after early Thu solutions rapidly go astray for exactly
how energy will be distributed within the overall trough
aloft--along with strength and track of best-defined surface low
pressure. Fast moving northern tier energy may also feed into the
upper trough as it reaches the eastern U.S., providing added
complexity to the forecast. This type of evolution tends not to
be easily resolved in the extended time frame. By early Fri
ensemble members are still widely varied north-south (northern
Gulf Coast states into the Upper Great Lakes). At that time the
12Z ECMWF is in the southern third of the spread while the best
density of models/members is between the Ohio Valley and Lower
Great Lakes. Established consensus has generally held within that
area but with some north-south waffling from day to day. Further
lowering of confidence near the East Coast by Sat favors a
model/mean blend that is fairly close to continuity.
Farther upstream a model/mean blend represents established
consensus with little continuity change for the southern stream
Pacific shortwave that eventually heads into the southern Plains,
as well as for the vigorous shortwave reaching the West Coast
around early Sat. Models and ensemble spaghetti plots show a more
rapid increase in detail spread over the entire eastern Pacific
through Plains area by next Sun. This favors increasing weight of
the similar 18Z GEFS/00Z ECMWF means toward the end of the
forecast period. Note that the 12Z ECMWF leans a bit to the
strong/slow side of the spread for the energy over the West by Sun.
...Weather Highlights/Hazards...
The series of systems affecting the West will bring at least two
or three periods of enhanced rain and high elevation snow. Expect
highest totals for the five-day period to be along favored terrain
from northern California/Sierra Nevada through the Pacific
Northwest. Moisture will extend through the Intermountain West
and northern-central Rockies, with some significant precipitation
over these areas as well but with less extreme totals relative to
areas near the coast. The system emerging into the Plains on Wed
and reaching the East Coast toward the end of the week will spread
a broad shield of precipitation across the eastern half of the
country. There is still less than desired agreement on areas of
heaviest rainfall but at least localized pockets of moderate to
heavy rainfall may be possible over parts of the sensitive eastern
Plains/Mississippi Valley region and then into the Northeast. The
northern periphery of the moisture shield may contain snow with
best potential from the north-central Plains into New England.
Today's guidance is showing a stronger signal for heavy rainfall
potential over parts of the southern Plains and possibly into the
Lower Mississippi Valley from about Fri night through the weekend.
This activity should be encouraged by the combination of an
approaching southern stream shortwave aloft and low level Gulf
inflow interacting with a front expected to stall near the Gulf
Coast. During the day Wed most precipitation with the western
Atlantic storm should be mostly confined to areas near the New
England coast.
Steady progression of systems across the Lower 48 will provide
some day-to-day variability of temperatures. Within this pattern
the general evolution of mean flow will likely be toward more
above average heights aloft with time, corresponding to increasing
coverage of above average temperatures from midweek through the
weekend. The areas of clouds and precipitation accompanying each
system will tend to favor greater warm extremes for morning lows
with some areas of plus 10-20F anomalies. In terms of five-day
averages the Interior West into the Plains should see warmest
temperatures relative to climatology. Plus 10F or greater
anomalies for highs are most likely over the parts of the Plains
Fri-Sun.
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