Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Wed Jun 06 2018
Valid 12Z Sat Jun 09 2018 - 12Z Wed Jun 13 2018
...Pattern overview...
Over the course of the period most guidance expects the amplified
trough-ridge-trough pattern in place at the start of the weekend
to evolve toward a more typical summertime pattern with
progressive flow across the northern tier states. However the
details of how this occurs are uncertain given significant
model/ensemble changes over the past 12-24 hours. Meanwhile upper
ridging anchored over the southern Plains as of Sat should weaken
through the weekend, and then after passage of the vigorous upper
trough through the West, rebuild with a center most likely
positioned over the southern Rockies. Farther eastward an upper
weakness over the northeastern Gulf of Mexico/Southeast U.S. will
gradually become more ill-defined with time.
...Model guidance/predictability evaluation/preferences...
From the large scale evolution perspective the greatest
uncertainty arises by the latter half of the period, involving the
strong trough energy emerging from the western U.S. and shape of
the downstream ridge. Over the past 12-24 hours GFS/ECMWF runs
and their ensemble means have changed considerably toward the idea
of allowing some of the ejecting western U.S. flow to cut through
the downstream ridge as an upper high closes off over or
north/northwest of Hudson Bay. This trend is quite pronounced
just going from the 00Z to 06Z cycle of the GEFS mean. The
multi-day trend toward faster progression of the western trough
fits this evolution. The 00Z CMC ensemble mean in particular and
perhaps to some extent operational CMC appear too slow with the
overall pattern given remaining guidance and trends, but they do
provide some reservations about going completely toward the latest
majority scenario without additional model runs to build some
degree of confidence.
As for some finer details... GFS runs appear somewhat
over-developed with a wave expected over the lower Great Lakes as
of early Sat so the forecast blend downplays that aspect of the
GFS. Also downplayed from GFS guidance is a west of consensus
track for a compact upper low that drops southward over eastern
Canada. While it fits into multi-day trends, the 00Z ECMWF
becomes faster than other solutions with northern Plains/upper MS
Valley shortwave energy by early next week. Farther upstream
model/ensemble solutions are still chaotic within a generally
agreed upon band of low amplitude progressive flow. The
combination of northern Pacific and northern Canada height anomaly
centers in latest D+8 charts seem to teleconnect with a tendency
toward modest troughing near the West Coast around or just after
the end of the forecast period. Latest GEFS/ECMWF means best
reflect this idea. Elsewhere, by the end of the period GFS runs
center the southern Rockies ridge farther east than most other
solutions. By day 7 Wed GFS/GEFS members are showing signals of
low pressure/moisture emerging from the western Caribbean. This
appears similar to their behavior leading up to Alberto, with
premature development/moisture transport. GEFS members have
already trended slower versus yesterday and the CMC--usually a
good "extreme bound" given its historical proclivity toward
tropical development--is still very suppressed through next Wed.
Other solutions show minimal development as well. Thus at this
time the probability of the GFS/GEFS scenario appears quite low.
Based on the above considerations, the forecast started with
various components of the 00Z ECMWF/00Z-06Z GFS/00Z UKMET with a
little 00Z GEFS/ECMWF ensemble input early-mid period. The latter
part of the forecast increased weight of the ensemble means while
also incorporating some elements of continuity to temper recent
changes in guidance for the evolution of the northern
U.S./east-central Canada pattern.
...Weather highlights and hazards...
The strong upper trough crossing the Northwest will bring an
episode of active weather this weekend with highest precip totals
likely to be over favored terrain in WA/OR and the northern
Rockies. Some of the highest elevations could see a little snow
from this event. The rest of the period could feature an
occasional period of rain over extreme northern areas but with low
confidence on existence/timing given the flat nature of mean flow
aloft.
The combination of shortwave energy aloft and wavy front from the
north-central Plains to East Coast will promote areas of showers
and storms/mesoscale convective complexes during the weekend, with
some locally heavy rainfall. Over the East the convective threat
will push southward along with the surface front, while the system
emerging from the West should generate another period of
convection from the northern Plains into mid-upper MS Valley, and
perhaps farther eastward, from about Sun night onward. Expect
diurnally favored convection over parts of the Southeast/FL with
some enhancement from the weakness aloft.
Expect the most extreme temperature anomalies to be associated
with the strong upper trough passing through the West during the
weekend. Readings should reach 10-20F above normal in the leading
warm sector from the eastern slopes of the northern-central
Rockies into the Plains. Some locations in the northern Plains
may see record warm mins around Sun-Mon. Cold frontal passage
will bring a day or two of minus 10-15F anomalies for highs from
the northern half of the West coast into the northern Rockies.
Rising heights over the West/southern Rockies ridging aloft should
lead to above normal temperatures over much of the West by next
Tue-Wed. Meanwhile the mean trough aloft over the Northeast and
vicinity will likely keep highs near to below normal into early
next week along the East Coast.
Rausch
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