Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Sun May 20 2018
Valid 12Z Wed May 23 2018 - 12Z Sun May 27 2018
...Guidance and Predictability Assessment...
Most models and ensemble means are gradually trending closer
together for the upper low expected to close off over the eastern
Pacific around midweek. However there are still meaningful timing
differences as this feature drifts toward the West Coast and
possibly inland. Consistent with prior runs, ECMWF/CMC-based
solutions show greater eastward progression than the GFS/GEFS.
UKMET runs have been holding the system back but generally faster
trends over the past day or so in most other guidance (including
18z/00z GFS runs being faster than the 12z version) seem to favor
leaning away from the slow side of the spread. While these trends
recommend leaning toward the ECMWF/CMC side of the envelope, the
fairly strong ridge that builds downstream over the West provides
lingering concern that the fast scenario could be just a little
too fast, or a little more deflection could occur such as
suggested by the 00z GFS. Based on data through 18z the
preference was to lean 2/3 toward the ECMWF-ECMWF mean scenario
relative to the 18z GFS/GEFS mean, with more ensemble mean
emphasis later in the period.
Downstream there is still better than average agreement regarding
the ejecting western U.S. energy that will likely lead to a
shortwave trough crossing the northern Plains/southern Canada and
heading into the upper MS Valley/Great Lakes late week through the
weekend. The greater uncertainty that comes into the picture late
in the period is with separate shortwave energy reaching western
Canada by Thu. Models/ensembles still differ on timing and
amplitude of this energy as it progresses into/over the western
Canada mean ridge, and then on how much the ridge rebounds after
shortwave passage. Thus by next weekend confidence is low in how
much this energy may amplify and interact with the shortwave
crossing the northern states. An intermediate solution should
provide the best starting point as North Pacific troughing should
promote a fairly rapid rebound of the western Canada ridge and
thus allow for some downstream troughing--but perhaps not quite to
the depth of recent GFS runs. The same approach applies for
depicting the overall area of surface low pressure associated with
the upper dynamics.
Over the East, during the first half of the period there are still
differences in shortwave details for cyclonic flow across the
Northeast along with corresponding spread for the southward extent
of the cold front that pushes southward and then stalls. The
shortwave details of concern have fairly low predictability so
prefer an average of guidance at this time. Farther south there
is still a lot of spread regarding a potential feature that could
track out of the northwestern Caribbean, including influence from
an increasingly well defined southern U.S./Gulf of Mexico upper
trough whose center may settle near the central Gulf Coast region.
Aside from the past couple ECMWF runs adjusting westward, the
forecast is generally in a holding pattern as GFS/GEFS runs remain
on the east side of the spread and CMC runs show their
characteristically aggressive development. Confidence in any
particular solution for such a feature is minimal but there is a
slightly more coherent signal that a weak surface low could evolve
under the upper trough/low by next weekend--albeit with different
ways of getting there.
A forecast blend tilted 60-70 percent toward the 12z ECMWF/ECMWF
mean relative to the 18z GFS/GEFS mean through the period (more
operational model input early, ensemble means late) provided a
starting point that generated only a modest nudge to continuity.
...Weather/Threats Highlights...
Shortwave energy ejecting out of the West and continuing eastward
across the northern Plains toward the Great Lakes will spread
rainfall from the Great Basin/northern Rockies through the Plains
and into the East. Showers/storms may be locally heavy. Trailing
Pacific upper low approaching and possibly reaching the western
states may generate another episode of unsettled weather toward
late week/weekend. Coverage/intensity of activity will depend on
exact path and timing of the upper low. Meanwhile expect
diurnally favored convection over the Southeast during the period.
Evolution of any defined Caribbean/Gulf surface system that could
promote significant rainfall enhancement remains very uncertain,
but the overall pattern with a deepening upper trough centered
near 90W longitude will likely promote a heavier trend to rainfall
with time over the Southeast and vicinity. Toward the end of the
period and beyond the interaction of this moisture with a front
approaching from the central U.S. will have to be monitored.
Expect above normal temperatures to prevail over a majority of the
lower 48 during the period. Anomalies exceeding plus 10F for min
and/or max readings are most likely from the Northwest into the
northern-central Plains and OH/TN Valleys. Such anomalies may
extend into parts of the southern Rockies/Plains by mid-late
period as well. Scattered daily records for max/warm low may be
possible. Exceptions to this warmth will be Florida with below
normal highs due to clouds/rainfall and California and vicinity
with the upper low/trough approaching from the eastern Pacific.
Cyclonic flow aloft may keep New England near to slightly below
normal for a time as well.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook 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
Rausch