Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Apr 01 2019
Valid 12Z Thu Apr 04 2019 - 12Z Mon Apr 08 2019
...Overview...
The forecast of the overall pattern has changed little over recent
days, with a steady progression of Pacific features into the West
and continuing onward producing one or more episodes of active
weather over a decent portion of the Lower 48. Heaviest
precipitation during the Thu-Mon period should extend from the
central/northern West Coast into the northern Rockies as well as
over and slightly east of the southern Plains/Lower Mississippi
Valley. The southern third of the West has the best potential to
stay dry during the period. Parts of the High Plains may see
little precipitation as well.
...Guidance Assessment/Preferences...
Latest models and ensembles have been remarkably stubborn in
maintaining their spread for the exact details of diffuse
shortwave energy aloft and associated surface low pressure
tracking across the eastern half of the country late this week
into the start of the weekend. As has been the case in recent
days there is decent clustering for low pressure over the
south-central Plains as of early Thu. Then individual
models/ensemble members rapidly diverge for the latitude of
best-defined low pressure farther east. In forecasts valid early
Fri the one trend of note is a reduction of ensemble members in
the northern part of the spread in the Great Lakes. On the other
hand the 12Z ECMWF/CMC are in the southern part of the spread. By
early Sat there is still the potential for low pressure to be
anywhere from northern New England to southern Mid-Atlantic.
Confidence remains low for system specifics due to the sensitivity
to small scale details aloft that include a combination of initial
Plains shortwave and northern tier energy that could interact by
Fri. Preference is to minimize continuity changes until a better
consensus emerges, yielding a surface wave tracking through the
Ohio Valley and northern Mid-Atlantic. In spite of the model
spread, a multi-model blend ended up providing a reasonable
template for the desired evolution.
Farther westward the guidance has been fairly consistent in
weakening shortwave energy that crosses the Northwest and heads
into a downstream ridge, while a southern stream shortwave cross
the Southwest U.S./northern Mexico and then the southern Plains
into Mississippi Valley. A fairly strong northeastern Pacific
storm should bring a decent front (with vigorous upper shortwave
support) into the West by Sat, potentially followed a day later by
another system tracking near/northwest of Vancouver Island and
bringing a front/energy aloft into the Northwest. By the latter
half of the period there is still a rapid increase in spread for
details of flow aloft from the Pacific into the western U.S. In
forecasts valid next Mon the GFS/GEFS show the most upper ridging
over the Interior West while the past couple ECMWF runs and 12Z
CMC depict flat or even slightly cyclonic flow. The 12Z ECMWF/CMC
means show a modest ridge. Teleconnections relative to a negative
height anomaly center over the west-central Pacific in D+8 charts
and a positive center east of northern Greenland hint at a split
flow type of pattern over western North America. The established
pattern of eastern Pacific troughing and relative agreement among
the means for a western ridge ultimately lead to a preference
close to the means. Including some weight of the differing 18Z
GFS/12Z ECMWF for the full forecast domain essentially balances
out near the desired scenario over the West. The specifics of
flow over Pacific/western U.S. late in the period will affect how
energy heading into the West on Sat will ultimately interact with
the southern stream shortwave reaching the Plains during the
weekend. It will likely take a while to resolve this aspect of
the forecast. Currently the eventual combination of shortwaves is
most likely to support a surface wave tracking from the southern
Plains into Midwest during Sun-Mon.
...Weather Highlights/Hazards...
Aside from the typically variable finer details, there is good
continuity in the forecast of multiple periods of significant
rainfall/higher elevation snow from northern California/Sierra
Nevada and Pacific Northwest into the northern Rockies. Highest
five-day totals will likely be over favored terrain in the West
Coast states with somewhat less moisture reaching the Rockies.
Heaviest precipitation should occur along the West Coast during
Fri-Sun. Meanwhile the system tracking out of the Plains Thu
onward will spread a decent moisture shield across the eastern
half of the country but with continued lack of confidence in
specifics. Today there is somewhat more of a signal for heavy
rainfall potential over the Lower Mississippi Valley/Gulf Coast
region, and some convection over this area could be strong to
severe. Check SPC outlooks for latest info on severe threats.
Some snow may be possible from the central Great Lakes into New
England. Behind this system, most guidance continues to suggest a
heavy rainfall threat next weekend over the southern Plains/Lower
Mississippi Valley. This activity will be encouraged by the
approaching southern stream shortwave aloft and a healthy flow of
Gulf moisture interacting with a stalled front that should return
north as a warm front. Rainfall of varying intensity should also
spread farther northward across the central and eventually eastern
U.S.
Temperatures will vary with the progression of individual systems
but there will be a noticeable trend over time toward greater
coverage of above normal readings across the Lower 48. The one
exception should be over the northern and central parts of the
West where clouds and precipitation will tend to keep highs near
to below normal. Highest anomalies (plus 10-20F with some higher
values especially for min temps) will be most likely from the
northern half of the Plains into east-central U.S. during Sat-Mon.
A large part of the West should see plus 5-15F anomalies for min
temps as well.
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