Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Thu Jun 07 2018
Valid 12Z Sun Jun 10 2018 - 12Z Thu Jun 14 2018
...Pattern overview...
Guidance continues to suggest an evolution from an omega block
appearance over Canada and about the northern half of the
contiguous U.S. to a possible rex block configuration over Canada
as northern U.S./southern Canada flow cuts through the initial
ridge. At this time the precise details of this transition and
the eventual shape of the pattern by the middle of next week are
still fairly uncertain though. Meanwhile there is an improving
signal toward a mean trough settling along or slightly inland from
the West Coast by next Wed-Thu. A weakening mean ridge over the
southern Plains early in the period will rebuild over the southern
Rockies/High Plains and then possibly expand or shift eastward.
The western periphery of an Atlantic/Caribbean ridge aloft will
gradually build westward over the course of next week.
...Model guidance/predictability evaluation/preferences...
Into about day 5 Tue a consensus blend (mostly operational
guidance, including 06Z GFS/00Z ECMWF/00Z CMC/00Z UKMET, with a
little ECMWF mean) best represents the most likely forecast
scenario, as the core of a strong western North America trough
heads into west-central Canada and energy east of Hudson Bay
descends within a persistent mean trough. Within this eastern
mean trough, the 06Z GFS is closer to the majority cluster over
the past couple days versus the farther westward and southward 00Z
GFS/UKMET which bring the center of a compact upper low close to
the northern tip of Maine.
By later in the period guidance has generally maintained the idea
(which was fairly new 24-36 hours ago) that northern U.S. flow
will continue eastward and ultimately lead to a low amplitude
pattern across the northern states by next Wed-Thu while an
ill-defined axis of lower heights prevail over southern Canada,
south of a closed high. Confidence is low in the specifics
though, favoring a blended approach. Upstream the previously
chaotic model/ensemble plots show improved clustering today toward
a trough settling near the West Coast late in the period. Overall
the means best depicted this idea yesterday and had teleconnection
support from surrounding anomaly centers in the D+8 means.
However the deep/amplified ECMWF lies near the extreme side of the
ensemble envelope so only a small minority of its forecast could
be incorporated by late in the period. Farther southeast, an
average of the GEFS/ECMWF means is reasonable for resolving
differences in specifics of the southern Rockies/Plains ridge
while the GFS/GEFS still appear to show premature development and
progression of a tropical system over the northwestern
Caribbean/Gulf of Mexico. The historically enthusiastic CMC as
well as the NAVGEM seem to remain as a more realistic "extreme
bound" that is much more suppressed than the GFS/GEFS. Thus GFS
forecasts had to be excluded from the favored blend late in the
period. The later part of the period trended toward more ensemble
mean input, from the 00Z ECMWF mean and a split of 00Z GEFS/NAEFS.
...Weather highlights and hazards...
A wavy frontal boundary extending from the East Coast through the
Ohio Valley and into the northern Plains will provide a focus for
showers/storms over multiple days, with some rainfall locally
heavy. Convection may also be strong in some cases. Current SPC
outlook indicates that parts of the northern Plains may have
favorable ingredients for severe potential during the latter half
of the weekend. The western part of this front will connect to
the strong system emerging from the West and continued progression
of the western front will likely bring further convection to the
northern Plains into the MS Valley before possible weakening. The
Southeast will see diurnally favored convection most days with
activity enhanced by localized boundaries and possibly the front
to the north. Departure of the strong weekend system will lead to
a drier trend over the Northwest for the first half of next week.
Then the arrival of upper troughing should bring some
precipitation but with generally light amounts.
The northern half of the Intermountain West and northern Rockies
will see a day or two of highs at least 10F below normal early in
the period as strong upper troughing passes over the area. At the
same time warm sector highs/lows are likely to reach 10-20F above
normal over and near the northern-central Plains Sun-Mon.
Rebuilding of the upper ridge over the southern Rockies and
vicinity will promote an expanding area of plus 5-15F anomalies
for min/max readings over the West by midweek, eventually
spreading into parts of the High Plains. Expect mean troughing
aloft to keep the Northeast/Mid Atlantic on the cool side through
at least early next week. Flattening of the mean pattern should
lead to near-normal temperatures near the East Coast by next
Wed-Thu.
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