Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 AM EDT Sun Jun 03 2018
Valid 12Z Wed Jun 06 2018 - 12Z Sun Jun 10 2018
...Heat wave from the Desert Southwest to the south-central U.S...
...Pattern overview...
Throughout the forecast, an ever persistent mid/upper-level ridge
will maintain control of the pattern with a general position over
the south-central U.S. With an area of lower heights remaining
over the West Coast, many such impulses are forecast to act as
"ridge rollers" as they track downstream toward the Great Lakes
and into the Ohio Valley. These features should eventually adjoin
the mean upper trough across the northeastern U.S. The overall set
up will likely change very little into next weekend based on the
latest guidance suite.
...Model guidance/predictability evaluation/preferences...
An initial closed low over southern New England will be
accompanied by a modest area of low pressure off the Delmarva
coast. This well clustered system is forecast to eject east of
Nova Scotia by Thursday morning. Meanwhile the focus shifts back
toward a notable perturbation in the flow straddling the southern
border of Manitoba mid-week. The 12Z ECMWF does stand out as being
too robust with its mid-level reflection as other solutions are
more subtle in nature. Consequently, the 12Z ECMWF becomes
slower/deeper while various other pieces of guidance absorb this
system into the mean northeastern U.S. trough. One place the
solutions do agree is with the poleward expansion of the sprawling
upper ridge across the south-central states. Where they disagree
is with mesoscale details of evolving impulses off the upstream
higher terrain. At this forecast juncture, it is too early to
resolve such spatial scales at this time. It is safe to say
multiple convective episodes will be likely north and east of this
ridge with activity congregating from the Northern Plains
downstream into the Great Lakes/Ohio Valley regions.
Across the eastern Pacific, a seasonably strong upper cyclone
accompanied by 500-mb height departures on the order of 2 standard
deviations below climatology will be a key player in the forecast.
While the strongest forcing/height falls should swing through
British Columbia and Alberta, there are still many operational
models and ensembles favoring a marked lowering of heights over
the Pacific Northwest. More specifically, the 00Z GFS depicts such
a solution, albeit with rather poor run-to-run model continuity.
Details vary enough to shift more toward an ensemble-based
approach into next weekend. The consensus does appear to favor an
establishment of an omega block with negative height anomalies
anchoring the western/eastern sections of North America.
With reasonable model agreement through Day 4/Jun 7, favored a
multi-operational model consensus utilizing a combination of the
18Z/12Z GFS with the 12Z ECMWF/UKMET. By the following day,
started incorporating the 18Z GEFS/12Z ECMWF ensemble means into
the picture with slowly decreasing use of the 18Z/12Z GFS and 12Z
ECMWF. Given the anomalous looking nature of the 12Z ECMWF early
on, tried to minimize its presence to some degree.
...Weather highlights and hazards...
The major weather story will be the abundant heat filling out the
Desert Southwest eastward into the Southern Plains. This is in
response to the persistent upper ridge holding strong over the
region. Forecast highs will be in the triple digits over many of
these regions, particularly the California, Arizona, and Nevada
deserts as well as the state of Texas. Temperature anomalies
should maximize over the Southern Plains with departures around 10
to 15 degrees above climatology. On the contrary, an upper trough
departing the northeastern U.S. will keep temperatures on the
cooler side for Wednesday/Thursday as readings hold into the 60s
across much of New England while lows drop into the 40s.
It should prove to be an unsettled start to the period with cool
cyclonic flow maintaining showers over the northeastern U.S. on
Wednesday while thunderstorms are likely across the southeastern
U.S. along the advancing cold front. Farther upstream, a series of
convective complexes will evolve north of the upper ridge with
hefty rainfall amounts likely from the Northern High Plains
eastward into the Middle Mississippi Valley and eventually toward
the Ohio Valley. While excessive rainfall concerns are somewhat
unknown, some convectively driven hazards are likely with any such
complex. Across the Pacific Northwest, it could prove to be quite
wet if the trough amplifies as much as suggested. While extreme,
the 00Z GFS shows impressive coverage of QPF for early/mid June.
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
Rubin-Oster