Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Mar 29 2019
Valid 12Z Mon Apr 01 2019 - 12Z Fri Apr 05 2019
...Overview...
A ridge over mainland Alaska as of early Mon will likely split
with a part collapsing to the southeast and the rest rebuilding
northwest toward the Bering Strait and eastern Russia. Even with
this transition there is some support for maintaining cyclonic
mean flow over Canada into the northern Lower 48. Meanwhile
periodic Pacific shortwaves will bring precipitation to areas
across the western U.S. as well as parts of the central/eastern
states.
...Guidance Assessment/Preferences...
Within the fairly agreeable mean pattern there are some
significant issues with specifics. Differences in handling the
initial Alaska ridge play at least a partial role in the
divergence of solutions for flow across southern Canada and
northern U.S. Through the 12-18Z cycles there are two general
clusters, one including the ECMWF/ECMWF mean and CMC/UKMET which
show more troughing in western Canada and higher heights over
eastern Canada, versus the GFS/GEFS which keep troughing away from
southwest Canada and bring a more amplified trough across the
Upper Mississippi Valley/Great Lakes into New England Tue-Wed.
The 00Z UKMET has adjusted somewhat closer to the GFS up to the
point of the trough reaching the Great Lakes--after which point
the UKMET trends less amplified than GFS runs.
At the same time guidance suggests a shortwave reaching the
southern Plains by Mon will continue onward and generate low
pressure along or offshore the southeastern coast by Tue. The
ECMWF is in the slower part of the spread and GFS runs in the
middle to faster side of the envelope. By mid-late week, what
happens with southern Canada/northern U.S. flow will affect the
forecast of the southern stream system. For now GFS runs and some
GEFS members are in the minority with their more phased scenario
that would bring the system rapidly northward into/through the
Canadian Maritimes instead of spending a longer time over the
western Atlantic--albeit with very uncertain duration of effects
along the East Coast. Based on data through the 18Z cycle,
forecast preferences encompassing these features end up leaning
2/3 away from the GFS for southern Canada/northern U.S. flow and
an intermediate solution for the southern stream system. However
an early look at the new 00Z ECMWF shows a shift toward the GFS
scenario with both features so needless to say there is high
sensitivity in this pattern.
Looking farther westward, most solutions expect energy offshore
British Columbia to combine with a separate system tracking toward
the West Coast and produce an upper trough that progresses through
the West around midweek and the Plains/Mississippi Valley Thu-Fri.
There is still potential these features may not merge though. At
the surface low pressure will likely track into the Northwest
early in the week and then a wavy frontal system should reach the
central/east-central states Wed-Fri. Not surprisingly the
operational models develop significant differences with some
specifics at the surface and aloft by the latter half of the
period--favoring greater emphasis on the means at that time.
In the wake of this system expect a modest ridge to build into the
West Coast/Interior West while more trough energy pushes into the
eastern Pacific. Thus far GFS runs have tended to be a bit on the
aggressive side of the envelope with bringing the energy into the
ridge though the 12Z ECMWF starts to become questionably amplified
with its trough by late Fri. The ensemble means are fairly close
(joined in principle by the 18Z FV3 GFS) and merit the greatest
weight in the forecast by late in the period.
...Weather Highlights/Hazards
Expect rain/higher elevation snow to spread through the northern
half to two-thirds of the West in the Mon-Wed period, followed by
additional precipitation reaching the West Coast late in the week.
Some moisture may have a connection to lower latitudes. Highest
precipitation totals are most likely over northern
California/Sierra Nevada with enhanced activity reaching into the
northern/central Rockies. Locations across the South and East
Coast regions may see a period of rain (and possibly some snow
over higher elevations) with the system forecast to track into the
Atlantic. Wide guidance spread for track and timing keeps
confidence low with respect to precipitation coverage and
intensity. Pockets of locally moderate/heavy rainfall may be
possible. The system emerging from the West may spread
precipitation along a wavy frontal system into the
Plains/Mississippi Valley region during the latter half of the
week. Thus far signals for significant amounts are not agreeable
enough to provide meaningful confidence in specifics.
Temperatures will vary in accordance with system progression. The
South and East will be on the chilly side early in the week with
some areas of minus 10-20F anomalies. Then above normal readings
(mostly plus 5-15F anomalies) should progress from the West and
into the central/eastern states over the course of the week.
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