Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Mon Jun 04 2018
Valid 12Z Thu Jun 07 2018 - 12Z Mon Jun 11 2018
...Heat wave from the Desert Southwest to the south-central U.S...
...Pattern overview...
Two primary synoptic-scale features will be focal points during
the period spanning June 7-11, each of which will be rather
anomalous relative to climatology. A sprawling upper ridge across
the south-central U.S. as well as a deep-layered cyclone off the
British Columbia coast are forecast to feature anomalies on the
order of 2 sigma. This particular regime is expected to change
very little during the period of interest, generally sustaining an
omega block setup as lower heights across eastern North America
fill out this signature. The largest changes are expected during
the Day 5-7, June 9-11, timeframe as an impressive longwave trough
accelerates through the Pacific Northwest. Ultimately, this should
help erode the northern extent of the upper anticyclone over the
south-central states. Across the southeastern U.S., a persistent
mid/upper-level weakness will linger in the vicinity of Florida
for much of the period. This area should be juxtaposed by a rather
healthy moisture plume as evidenced by the 1.50 inch and above
precipitable water values.
...Model guidance/predictability evaluation/preferences...
Starting across eastern North America, a conveyor belt of
low-amplitude shortwaves should migrate across Quebec while
potentially grazing northern New England. A more amplified system
will likely take shape by late in the weekend into the following
week which ultimately shifts a wavy frontal zone south/east toward
the Carolinas by 11/1200Z. Overall differences are not terribly
significant here with the global guidance suite all in favor of
this evolution. Likewise, there is solid agreement regarding the
position of the weakness across the southeastern U.S. into the
eastern Gulf of Mexico. Eventually the signal shears out over the
weekend although multiple embedded vorticity centers are likely to
still spin about the region in its wake.
Moving to the center of the nation, a stout upper ridge will be a
fixture for the foreseeable future with the 588-dm height contour
of the mid-level heights extending up toward the Northern Plains
at times. Like yesterday, the 12Z CMC seemed to be more suppressed
with this ridge which is directly in response to the creation of
sizable perturbations rounding the ridge axis. Otherwise,
solutions appear to be within tolerance into the weekend as
eventual upstream Pacific height falls begin to dent the northern
extent of the anticyclone center. Much of this of course is based
on the level of Pacific amplification which remains somewhat
nebulous at this time.
Over the West Coast, a semi-permanent upper low remains fixed off
the British Columbia coast. Eventually a notable shortwave rounds
the base of this trough with potent height falls accelerating
across the Pacific Northwest by Saturday. Ensemble spaghetti plots
vary with the depth of this trough as well as the temporal
evolution. It appears the GFS/ECMWF have taken turns with being
more anomalous than the consensus solution as the trough moves
across the Pacific Northwest potentially down to northern
California. More specifically, it was the 00Z GFS yesterday and
the 12Z ECMWF. Such solutions have gained some company with the
00Z CMC joining it, albeit in a much slower fashion.
While many uncertainties exist in the pattern, most are on the
mesoscale revolving around the "ridge rollers" which cannot be
adequately forecasted at this time. As such, felt a
multi-operational model blend would suffice through Day 4/Friday
utilizing a combination of the 18Z/12Z GFS with the 12Z
ECMWF/UKMET solutions. Thereafter, removed the 12Z UKMET in favor
of a consensus of the operational GFS/ECMWF models in conjunction
with their respective ensemble means. The blend became fairly
heavy in ensemble use by the end of the forecast with slightly
more weighting toward the 18Z GEFS mean.
...Weather highlights and hazards...
The heat wave will be the main story with readings 100 degrees
plus anywhere from the Desert Southwest eastward along the Mexican
border into the state of Texas. Departures from climatology should
be around 10 degrees although slightly higher numbers can be found
farther north into the remaining Great Plains states and into the
Intermountain West/Rocky Mountain chain. Widespread 80s and 90s
are likely over these locations accompanied by temperature
anomalies in the 10 to 15 degree range. Regarding cool spots,
Thursday should prove to be near or slightly below climatology
across the East Coast with such conditions persisting into the
weekend across Florida given immense cloud cover and rainfall.
Elsewhere, deep upper troughing over the Pacific Northwest will
hold temperatures around 5 to 10 degrees below average from
Saturday onward.
It should remain intermittently wet over the southeastern U.S.,
particularly Florida, given a stagnant upper trough/weakness in
conjunction with an anomalous moisture feed. Back toward the
middle of the country, multiple convective complexes are likely to
erupt north of the ridge with propagation downstream likely toward
the Middle Mississippi Valley and into the Great Lakes/Ohio
Valley. Details would be fuzzy at best this far into the forecast
but expect bouts of heavy rainfall accompanied by discernible
severe weather threats. Elsewhere, it should prove to be quite wet
over the Pacific Northwest given the deep cyclonic flow overhead.
Strong forcing with the trough and eventual frontal passage could
spread considerable early/mid June precipitation to the region.
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