Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Prognostic Meteorological Discussion (PMD)

Issued by National Weather Service (NWS)

FXUS02 KWBC 241600

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Wed Apr 24 2019

Valid 12Z Sat Apr 27 2019 - 12Z Wed May 01 2019

...16Z Update...

Large-scale changes were not made to the medium range forecast
suite during this issuance. Much like the previous issuance,
products were based on a multi-model deterministic blend early in
the period, with much more emphasis on the EC and GEFS means late.
More specifically, on day 3/Sat, differences remained regarding
the trough pulling away from the Northeast U.S., with the 00Z and
06Z GFS slowing up the trough with more vorticity present farther
south. Decided that the 06Z parallel GFS (FV3) was a better fit
with this feature than the operational runs; it clustered better
with the 00Z EC, UKMET, and CMC. This blend kept lower
precipitation in New England compared to the west GFS. Secondly,
the 00Z ECMWF came in stronger with the shortwave crossing the
Great Lakes/Ohio Valley (mentioned below), more in line with the
other guidance, so consensus is growing there. The 00Z UKMET,
while usable until day 4/Sun, is more energetic with its shortwave
moving through the northwestern to north-central U.S. on Sun,
which spins up into a strong upper low in the Northern Plains and
develops a surface low there on Mon. Ensembles do not support this
evolution, keeping the shortwave progressive instead, so excluded
the 00Z UKMET from the forecast at day 5/Mon. Model run-to-run
variability continues to be large regarding how a trough moving
through the Northwest will merge with the Eastern
Pacific/California originally cutoff low toward Tue/Wed, so stuck
with the means for now.

Please see the previous discussion for details on sensible weather.


...Previous discussion...

...Overview and Guidance Preferences...

Models suggest the potential for at least some degree of pattern
change across the CONUS during the medium range. Initially
separate streams with a progressive northern stream and less
progressive southern stream appear set to consolidate by the
middle of next week. This is expected to occur as the northern
stream undergoes amplification in response to a persistent
upstream Rex Block extending from Alaska to Hawaii. Amplification
of an upper trough across the western U.S. by the middle of next
week is expected along with an expansion of the subtropical ridge
northward into the Southeast and southern Mid-Atlantic.

A negatively tilted upper trough is expected to be in place on day
3 (Sat) across the Northeast, with a phased surface low pressure
system expected to be deepening as it pulls away from New England.
Farther west, the northern stream remains active with one compact
but energetic shortwave crossing the Midwest/Great Lakes Sat-Sun
and yet another diving into the Pacific Northwest/northern
Rockies. Meanwhile, a cutoff upper-level low should approach
California by Sun night/Mon before moving inland and interacting
in some fashion with the broader upper trough amplification across
the West by early-mid next week.

The forecast during days 3-5 (Sat-Mon) was based on a multi-model
deterministic blend including the 12Z ECMWF/UKMET and 18Z FV3-GFS.
Model solutions differed on the evolution of the negatively tilted
upper trough and deepening surface low as they pull away from the
Northeast Sat-Sun, but a blended approach appeared best at this
time. Preferred the GFS/FV3/UKMET idea of a more amplified
shortwave crossing the Great Lakes/Ohio Valley Sat-Sun over the
weaker/more suppressed ECMWF solution. Farther west, consensus has
improved with respect to amplified shortwave energy reaching the
Northwest/northern Rockies Sat/Sun, but some timing differences
remain, and models also differ as to whether the wave quickly
moves east into the Northern Plains by late Sun or shears out with
more energy remaining across the interior Northwest. The 12Z UKMET
suggested that a compromise between these ideas was feasible and
thus this was the preference. Ensembles do provide a fair degree
of support for the idea of keeping a significant portion of the
shortwave energy back across the interior West into early/mid next
week, with the arrival of additional energy helping to lower
heights and amplify a broader upper-level trough. The evolution of
the upper trough will also impact what happens farther south as
the initially cutoff Pacific energy moves inland across the Four
Corners region, with the eventual forward speed of this system as
it begins to become absorbed tied to the degree of northern stream
amplification. While models differ as to exactly how this occurs,
ensemble means agree in principle, thus leaned heavily toward
ECENS and GEFS means by days 6-7 (Tue-Wed).

...Weather Highlights/Threats...

Widespread rain is expected across New England on Sat as the
deepening low pressure system moves away from the region. Low
pressure moving from the Central/Northern Plains to the
Midwest/Great Lakes/Northeast Sat/Sun should be accompanied by
showers and thunderstorms, with the potential for locally heavy
rainfall. A few areas on the northern fringe of this precipitation
area may see the potential for snow or a rain/snow mix,
particularly portions of northern Wisconsin and Michigan on Sat.
Through the reminder of the forecast period, into the middle of
next week, the focus for scattered to numerous showers and
thunderstorms will shift back to the central U.S., where a
stationary surface front along with arrival of additional
upper-level shortwaves will focus multiple rounds of showers and
thunderstorms. Models currently show the greatest potential for
heavy rainfall across areas from the Central Plains to the Upper
Midwest Mon-Tue. The central/northern Rockies will also see an
increase in precipitation (rain and mountain snow) by early to mid
next week as the upper trough amplified across the West and
additional southern stream Pacific energy arrives.

Below average temperatures are expected through much of the
forecast period from the northern Rockies to the Upper Midwest and
Great Lakes, where high temperatures of 5 to 15 deg F below
average may be common. Above average temperatures initially in
place from California to the southern Rockies and adjacent Plains
should shift east to the Southeast and southern Mid-Atlantic
regions by early to mid next week as the western trough amplifies
and ridging builds into the Southeast.


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: