Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Tue Jun 05 2018
Valid 12Z Fri Jun 08 2018 - 12Z Tue Jun 12 2018
...Heat to continue for the Desert Southwest to the south-central
U.S...
...Pattern overview...
The primary fixture in the forecast will be a sprawling upper
ridge spread across the Desert Southwest eastward to the
mid-South. Rather impressive mid-level height anomalies on the
order of 2 sigma above climatology will be fixed over this region
on Day 3/Friday. Eventual northern erosion of this anticyclone is
expected given robust troughing moving through the Pacific
Northwest during the weekend. An embedded compact upper low or
strong open wave will evolve out of this negative height anomaly
with a position over the Upper Intermountain West on Sunday
morning. This is where the mentioned upper ridge will partially
erode although many solutions favor it rebuilding by early next
week once the Pacific system lifts into central Canada. Across the
East Coast, a broad weakness should linger over the Florida
Peninsula through part of the weekend before gradually shearing
out. Meanwhile, the mid-latitude flow significantly amplifies over
the northeastern U.S. by Sunday onward which will act to solidify
the eastern extent of a developing omega block regime.
...Model guidance/predictability evaluation/preferences...
With the 12Z CMC being more aggressive with some of the "ridge
rollers" traversing the north-central U.S., it displays a more
suppressed solution. This has been an issue over the course of the
previous couple of days. Otherwise, the GFS/ECMWF/UKMET model
suites have been supportive of building heights over the weekend
in advance of the amplified northwestern U.S. flow. As the
upstream shortwaves are able to cross the Great Plains, there are
a variety of solutions at hand with mesoscale differences quite
abundant. the 12Z CMC stood out with a powerful upper low across
western Kansas on 10/1200Z which it has since backed off on.
Eventually the upper ridge should begin to expand its spatial
coverage into early next week with some spread amongst the
guidance. By Day 7/June 12, this positive height anomaly will
likely situate from the Desert Southwest eastward into the
Southern/Central Great Plains.
There has become fairly widespread agreement in carrying a rather
significant upper trough into the Pacific Northwest/northern
California this weekend with the core of lower heights crossing
western Montana by midday Sunday. This will draw a strengthening
surface cyclone up along the Alberta/Saskatchewan border prompting
a significant increase in warm advection across the Great Plains.
Where models have struggled is with run-to-run continuity,
particularly when considering the 00Z/18Z solutions with upper low
positions either over southwestern Saskatchewan or extreme
northern Alberta, respectively, on 12/0000Z. It is safe to say the
12Z GFS solution is equally problematic with height contours
offering stark differences from the ensemble spaghetti plots.
Over the eastern U.S., models offer a bit more certainty to the
medium range pattern. Global guidance agree on maintaining a
mid/upper-level weakness around Florida through Saturday before
gradual erosion into a more randomly oriented array of vorticity
centers. To the north, ensembles favor a sharpening in the upper
trough as heights lower into New England on Sunday. While the 12Z
CMC was within this grouping, its 00Z run shifted toward a grossly
different solution which appears outlying. Additional shortwaves
within the longwave trough will help encourage further
amplification in the eastern U.S. flow, thus shifting the surface
front down to the Carolinas by next Tuesday.
With similar output from the 18Z GFS and 12Z ECMWF/UKMET through
Day 5/Sunday, favored a three-way blend of such solutions with
more weighting toward the 18Z GFS/12Z ECMWF. Thereafter, quickly
shifted toward a greater influence of the ensemble means, 18Z
GEFS/12Z ECMWF/NAEFS, into the picture given growing model
uncertainties. While synoptic-scale differences do exist, there
are plenty of mesoscale influences which will be impossible to
resolve at this time-scale.
...Weather highlights and hazards...
Given the extended influence of ridging from the Desert
Southwestward eastward into the Southern Plains, expect above
average temperatures to persist as readings of 105 to 110 degrees
are likely over the California, Nevada, and Arizona deserts.
However, the more impressive departures from climatology will
focus across the Northern Rockies into the Northern/Central Plains
with readings in the 10 to 15 degree range through Sunday. These
numbers should come down a bit as a frontal zone sweeps through
the region early next week. The cool spot in the nation will be
along the West Coast, generally focused over the Pacific
Northwest, as temperatures should be 5 to 10 degrees below
climatology this weekend. This supports highs only in the upper
50s along western sections of Washington/Oregon. Along the East
Coast, persistent troughing should allow numbers to stick a bit
closer to average for early/mid-June.
Multiple mesoscale complexes will likely evolve from "ridge
rollers" tracking from the Northern Plains downstream into the
Great Lakes/Ohio Valley region. Of course these will be difficult
to predict given their mesoscale nature. A particular focus is
likely along a meandering west-east boundary where better
low-level convergence will overlap enhanced moisture transport.
Wet conditions are expect this weekend across the eastern U.S. as
stronger dynamics descend from eastern Canada. Farther south,
conditions should remain intermittently wet across Florida,
especially this weekend with the weakness lingering overhead. Out
west, an anomalously strong upper trough will spread abundant
rainfall to the Pacific Northwest, particularly across the
Cascades. Such conditions should lift north-northeastward into
western Canada as conditions dry out over the northwestern U.S.
Rubin-Oster
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