Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sun Jun 10 2018
Valid 12Z Wed Jun 13 2018 - 12Z Sun Jun 17 2018
...Pattern Overview...
Latest guidance continues to refine ideas for flow evolution over
Canada and contiguous U.S. The general consensus is for the
strong upper high forecast to be northwest of Hudson Bay as of
early Wed to collapse/merge with a ridge that develops and then
progresses ahead of energy ejecting from the upper trough moving
into western North America. Troughing should persist over the
West through the end of the period given the strength/sharpness of
the ridge over the eastern Pacific. At the same time an eastern
U.S./Canada trough should amplify during the latter half of the
week and then proceed into the western Atlantic during the
weekend. Multi-day trends seem to suggest some weakening and/or
eastward adjustment of mean ridge over the southern Plains. The
overall pattern evolution may promote a flow of tropical eastern
Pacific moisture into parts of the southwestern U.S./Rockies and
eventually a separate area of moisture into the western half of
the Gulf of Mexico.
...Model Guidance/Predictability Assessment...
As has been the case in recent days, an operational model blend
(more 06Z GFS/00Z ECMWF than 00Z UKMET/CMC) represented the most
common themes of guidance well in the 3-5 day time frame. Then
the ensemble means (00Z ECMWFens/NAEFS and 06Z GEFS) became more
useful for their stability and teleconnection support, though some
operational guidance matched up better in principle than in some
previous cycles.
One emerging trend in the guidance, reinforced in this cycle, is
for a fairly progressive ejection of initial upper low energy
initially near Vancouver Island and reaching inland by Thu. This
trend, and possible incorporation of some upper low/trough energy
to the north, is steadily increasing support for a fairly strong
low pressure system tracking into south-central Canada. The
ejection of leading energy is encouraged in part by an upstream
shortwave/possible compact low that may reach the Northwest by
around Sat and is much more defined than it was 24 hours ago.
This second bundle of energy most likely drifts into the northern
Intermountain West/Rockies by Sun with some troughing extending
southwestward. This evolution compares well with teleconnection
ideas relative to the upstream ridge's positive height anomaly
center--some broadness to the northern part of the mean trough and
only weak troughing extending over/through California. GFS/ECMWF
runs are now fairly close to this idea as well, after the GFS
trended away from a deeper CA upper low (now in the 00Z CMC) and
ECMWF pulled the southern part of the trough westward.
Exact details of the western trough will be critical for
determining timing/axis of a potential surge of deep layer
moisture from eastern Pacific T.S. Bud into parts of the
southwestern U.S. The most agreeable focus in the GEFS/ECMWF
ensembles is over southern Arizona late this week/early weekend
with precipitable water values potentially reaching 4-6 standard
deviations above normal and the 95th or greater climatological
percentiles. Somewhat westward axis in the ECMWF scenario would
lead to such anomalies extending into southern California. Some
moisture may eventually continue northeastward along the surface
front into the Rockies/Plains.
Over/near the Gulf of Mexico, surface solutions still vary widely
though a more agreeable signal is developing toward at least some
shortwave energy/moisture reaching the western half of the Gulf by
the latter half of the period. The ECMWF does not yet have a
meaningful surface reflection but the past couple runs have
noticeably increased the amount of shortwave energy over the
western Gulf. Thus the depiction of a surface trough heading into
the western Gulf continues to provide the most reasonable way to
depict potential for some sensible weather effects later in the
forecast.
Confidence is fairly low with eastern U.S. trough details by the
latter half of the period. Ensemble mean trends have been toward
more progression, likely due in part to trends upstream, though
the CMC and some recent ECMWF runs suggest energy could close off
just beyond the East Coast.
...Weather Highlights/Hazards...
There will be multiple areas of focus for enhanced rainfall
potential during the period. One will be along an initially
stalled frontal boundary over and east of the central Plains, with
emerging western U.S. low pressure pushing it north as a warm
front. While the Northwest will likely see just light-moderate
activity with the strong upper trough moving into the region,
expect precipitation to become heavier and more widespread late
week/weekend from the Southwest into parts of the Rockies as the
flow aloft may bring some moisture from T.S. Bud across the
region. Confidence is not especially high with timing/axis
specifics but compared to yesterday there is a strengthening
signal toward some rainfall enhancement from the eastern Pacific
tropical moisture. By late in the period some of this moisture
could interact with the western front extending into the Plains
and slower frontal motion could promote repeat activity/training
at some locations. Also by the weekend there is increasing
potential for some areas along the western half of the Gulf Coast
to see increasing intensity of rainfall with low level Gulf inflow
and upper level shortwave energy, even there is not a pronounced
surface reflection. Farther east, the Southeast and Mid Atlantic
will likely see showers and thunderstorms with a front providing
added focus while a system southern Canada/Maine will likely
produce an axis of enhanced rainfall over extreme northern New
England during the latter half of the week.
Well above average temperatures will progress from much of the
West on Wed into the Plains and then MS Valley. Each day should
feature an area of plus 10-20F anomalies. Upper trough moving
into the West will bring a pronounced cooling trend, with clouds
and rainfall possibly leading to much below normal max
temperatures over parts of the Southwest/Great Basin/Rockies by
the latter half of the period.
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