Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1139 AM EDT Sat Mar 30 2019
Valid 12Z Tue Apr 02 2019 - 12Z Sat Apr 06 2019
...Overview...
A short range Alaska ridge aloft (rebuilding into northeastern
Russia next week) and then another ridge building into western
Canada by late week will tend to favor cyclonic mean flow across
southern Canada and into the northern CONUS east of the Rockies.
Beneath the ridges expect a continued progression of Pacific
shortwaves into the Lower 48. The series of features will likely
support low pressure tracking off the East Coast Tue-Wed, a
trailing system moving from the West Coast into the eastern U.S.
Tue-Sat, and then a front reaching the West Coast to start the
weekend.
...Guidance Assessment/Preferences...
Models and ensembles continue to agree on the large scale
evolution but not for embedded important details, including
whether any stream interaction may occur over the eastern half of
the country. A blend focusing more on latest operational models
early in the period and about 3/5 toward the ensemble means by day
7 Sat captured the most agreeable aspects of the forecast while
accounting for significant model/ensemble member spread for some
features.
The system expected to track over the western Atlantic from off
the southeastern coast Tue onward is still problematic with
track/timing spread leading to a variety of potential outcomes for
the East Coast states or at the least the coastal areas. 00Z-06Z
suite of guidance continued to slowly come together toward a
consensus but remains several hundred miles apart N-S and E-W,
east of the NC coast early Wednesday. Sensitivity to the short
term upstream flow evolution will continue to hamper
forecastability for at least another day or two, and would expect
more shifts in the guidance. By Wed the character of a trough
crossing the northeastern U.S. may play a part in system
evolution/track--with current guidance split on how much
interaction will occur.
An upstream trough near the West Coast still has some question
marks about specifics of a couple separate pieces of energy making
up the feature. As the upper trough continues its way across the
country the models/means agree fairly well for its existence and
timing but there are a lot of differences in exact distribution of
embedded energy at any particular point in time. This leads to
ongoing difficulties in determining the path and strength of
best-defined low pressure over the central/eastern states during
the latter half of the week. North-south spread in sfc low track
remains from AL/GA northward to Michigan as the ECMWF ensembles
were farther south than the GEFS ensembles. 06Z GFS/FV3-GFS
offered reasonable clustering near the 00Z ECMWF for the morning
update, but the pattern remains inherently less predictable than
average amid the lower amplitude flow.
During the latter half of the period the models and ensemble
members diverge for the particulars of eastern Pacific shortwaves,
with fairly low confidence for specifics. There is a general
theme of some leading shortwave energy heading into a developing
western North America mean ridge, followed by a potentially more
energetic trough nearing the coast by early Sat. At the same time
a southern stream shortwave may reach the southern
Rockies/northern Mexico to start the weekend. Majority ensemble
mean weighting (06Z GEFS and 00Z ECMWF ensemble mean) will
hopefully reduce future timing/location shifts.
...Weather Highlights/Hazards...
The forecast remains fairly well on track for areas of rain and
higher elevation snow across the northern half to two-thirds of
the West due to the system crossing the region Tue-Wed, with
favored terrain seeing the highest amounts. Another round of
moisture should spread into the West during the Thu-Sat period
with precipitation extending from the northern/central West Coast
into the northern Rockies. Within this area of late week
rain/snow the northern California-Sierra Nevada region currently
shows the best potential for highest totals. The system tracking
out of the West around midweek will likely spread a broad shield
of precipitation across the eastern half of the Lower 48 but with
below average confidence for intensity at any particular location.
Pockets of locally heavy activity are possible but so far there
is coherent signal for their location and timing. Some of the
precipitation may be in the form of snow over far northern areas.
Behind this system another area of rain may develop by Sat in
response to an approaching southern stream shortwave. Finally,
Tue-Wed trends for the western Atlantic system continue to lower
potential precipitation amounts/westward extent along the East
Coast, but maintained at least some chance of rain after 00Z
Wednesday from the Outer Banks northward to coastal New England
for now.
The progressive pattern over most of the Lower 48 will keep
temperatures variable, but with a general increase of above-normal
temperature coverage with time. This is especially the case for
min temps. Some localized anomalies of minus 10-15F are possible
over the southern tier on Tue followed by a warming trend. The
unsettled pattern over the West should keep morning lows well
above average most days.
Fracasso/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