Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
105 AM EDT Sat May 05 2018
Valid 12Z Tue May 08 2018 - 12Z Sat May 12 2018
...Pattern overview...
Persistent negative height anomalies in the north central Pacific
will support anticyclonic upper flow across western North America
through much of the medium range. Downstream of the ridge,
negative height anomalies are expected to persist in the vicinity
of Hudson Bay, with broad anticyclonic flow across areas from the
Midwest to the Mid-Atlantic/Northeast. The result will be a series
of Pacific surface fronts that generally weaken as they move into
the interior western U.S., and the potential for low pressure
development east of the Rockies across the central/northern
plains, tracking eastward across the Midwest/Great Lakes.
...Guidance and uncertainty assessment...
A multi-model deterministic blend including the 12Z
ECMWF/UKMET/GFS served as a starting point for the WPC forecast
early in the medium range, with increasing weight placed toward
the 12Z ECENS/NAEFS means after Day 5 given quickly increasing
uncertainty.
Consensus has improved somewhat early in the period (Tue-Wed)
surrounding the eastern U.S. trough. The ECMWF continues to be the
deepest solution and the slowest to lift the feature out, while
the GFS and UKMET have moved toward a deeper solution compared to
this time last night. A middle ground approach continues to be
favored per the latest ensemble means, supporting the idea of a
multi-model deterministic blend early in the forecast period.
Elsewhere, consensus is a bit better with respect to shortwave
energy crossing the northern plains/Midwest Tue-Thu and the
Northeast by Thu night. Differences surrounding this system are
largely timing related, with the ECMWF bringing the upper wave and
surface frontal system east a bit faster than the consensus based
on the latest ensemble means.
Meanwhile, models show two potential shortwave undercutting the
western ridge and moving into the northwestern U.S. The first of
these begins as an upper low off the Northwest coast on Tue, which
should begin to weaken as it approaches the coast and the broad
area of ridging. Models show some significant differences as to
how quickly the features weakens, which in turn impacts timing as
the feature moves inland. The GFS continues to lead the pack with
weakening the wave/moving it inland fastest, and a slower solution
is preferred at this time based on a consensus surrounding the 12Z
ECMWF/NAEFS means. The 12Z ECMWF was much closer to this consensus
than was the GFS. Models are in general agreement on a second
shortwave reaching the Northwest late Thu, but deterministic
solutions differ substantially. The last couple runs of the ECMWF
(along with the 12Z CMC) really amplify this feature across the
Northwest, while the GFS is much weaker moves the wave inland
fairly quickly. Given that this feature does not originate as a
deep upper low offshore like the first, and the time scale under
consideration, confidence is low as to the specific evolution by
next Fri, and an ensemble approach was favored, representing a
compromise solution.
...Sensible weather/threats highlights...
While some uncertainty remains as to the specific impacts of the
eastern U.S. trough Tue-Wed, the highest probability scenario is
for most significant activity to remain just offshore. Slight
potential remains for moisture to wrap back as far as the coast or
slightly inland. Farther west expect moisture initially over the
High Plains to progress along with surface low pressure/frontal
system into and through the northern/central Plains and eventually
reach the East. Some Plains into Great Lakes/Ohio Valley
convection could be locally heavy and/or strong. Meanwhile the
system arriving from the eastern Pacific will bring a period of
enhanced rainfall to the Pacific Northwest/northern Rockies from
mid to late next week.
The Interior West and portions of the High Plains should see the
warmest temperature anomalies during the period, plus 10-20F and
locally higher. There may be some daily records for highs/warm
lows. Eastern states will see near to slightly below readings
Tue-Wed followed by a modest rebound to warmer conditions. The
arrival of a cold front across the Great Lakes/Northeast by
Fri-Sat should usher another round of cooler temperatures into
those regions, although the magnitude of this cooler air remains a
bit murky at this point.
Ryan
Associated WPC medium range forecasts of 500 mb heights, surface
systems, weather grids, quantitative precipitation forecasts, and
winter weather outlook snow/sleet probabilities can be found at...
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4