Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1144 AM EDT Wed Jun 13 2018
Valid 12Z Sat Jun 16 2018 - 12Z Wed Jun 20 2018
...Tropical moisture to bring potentially heavy rains to portions
of the Southwest and northwestern Gulf Coast...
16Z update...
00Z/06Z models/ensembles remain in good agreement from the
previous cycle and overnight WPC forecast. A refresh with a blend
of the latest GFS/ECMWF and their ensemble means paired well with
the demise of (current) Tropical Storm Bud in northwestern Mexico
and the general trough/ridge pattern over the lower 48. Questions
lingering in the forecast lie mostly in the west regarding how far
south the upper low may sink into California next Tue/Wed (ECMWF
and its ensemble members were farther south than the GFS/GEFS).
For now, a blended solution was prudent given a lack of trend and
a tendency for the models to falter in blocky patterns.
Fracasso
...Pattern Overview...
The breakdown of a blocking ridge across Hudson Bay will allow for
more progressive northern stream flow to take hold from the
Canadian prairies into the Great Lakes and Northeastern U.S.
Meanwhile farther south, the flow will become more amplified and
less progressive as an anomalous upper ridge axis builds off the
North America West Coast, and subtropical ridging migrates
eastward from the southern/central plains to the Ohio/Tennessee
Valleys and the Southeast. This will favor a relatively amplified
and slowly-evolving flow pattern across the CONUS, with a
persistent and perhaps retrograding trough/upper low across the
interior West. The western trough will transport moisture
associated with the remnants of now Tropical Storm Bud into the
southwestern U.S. and eventually into the Plains, where a
lingering stationary frontal boundary will meander through most of
the medium range period. Additionally, a slow-moving trough/area
of disturbed weather moving west across the Gulf of Mexico is
expected to approach the western Gulf Coast states by Sat-Sun,
potentially lingering for a few days due to the blocky pattern
surrounding the feature.
...Model Guidance/Predictability Assessment...
The WPC forecast was initially based on a heavily deterministic
multi-model blend, including the 12Z ECMWF/UKMET and 18Z GFS
during days 3-5 (Sat-Mon). Models consensus has continued to
improve with respect to the timing of the breakdown of the
blocking upper high across Hudson Bay, and the developing
progressive higher latitude flow in its wake. Farther south, as
the ridge amplification occurs off the West Coast, models have
continued a slow trend toward more trough/upper low amplification
across the West. Farther east, models have trended toward a more
progressive solution with respect to shortwave energy expected to
amplify off the East Coast on Sat, with most solutions now taking
this feature east and out to sea more quickly than in recent days.
Additionally, models show good consensus that the upper ridge
initially centered across the lower/mid-Mississippi Valley should
show a slow migration eastward toward the Ohio/Tennessee Valleys.
Improved model consensus and resultant forecast confidence during
the day 3-5 time frame was sufficient to justify majority use of
deterministic solutions.
During days 6-7 (Tue-Wed) the pattern remains largely stable with
very little in the way of significant changes. Models (with the
exception of the 12Z CMC) have shown a trend over the past day
toward retrograding the western U.S. upper low by Tue-Wed,
bringing it to California by the middle of next week (a solution
that the GFS showed led the other models on). Meanwhile, the slow
eastward migration of the Mississippi Valley upper ridge should
continue, and more progressive flow should persist farther north
with another shortwave crossing the Great Lakes/Northeast. Models
show a range of solutions as to how far south into the ridge the
northern stream flow/height falls will be able to push across the
Mid-Atlantic and Ohio Valley by the middle of next week, with the
GFS a bit more aggressive with this relative to the ECMWF.
Nonetheless, given the evolution toward a seemingly stable upper
pattern, predictability seems slightly above average for the time
frame. A majority of ensemble means (12Z ECENS/NAEFS) was used as
a forecast basis for days 6-7, but a substantial component of the
deterministic ECMWF/GFS was continued through the end of the
forecast period.
...Weather Highlights/Hazards...
A weakening surface front and moist air mass will support
scattered showers and thunderstorms through the weekend,
especially during the afternoons and evenings, across the
Southeastern states, with the potential for locally heavy
rainfall. A broad trough/area of disturbed weather moving
west-northwest to northwest across the Gulf of Mexico is also
expected to bring widespread and potentially heavy rains to
portions of TX and LA from the weekend through early next week,
with moisture potentially spreading north across the southern
plains as well. Farther north, a shortwave, surface low pressure
system crossing the northern plains and Upper Midwest (along with
a potential influx of moisture from the eventual remnants of
Hurricane Bud) will produce areas of convection with potentially
heavy rainfall. The frontal boundary appears likely to stall for a
period of time from the central plains to the Upper Midwest,
resulting in several days with shower/thunderstorm potential, and
potential flooding threat for some areas. Meanwhile, moisture
associated with the remnants of Hurricane Bud should begin to
spread into the southwestern U.S. and the Rockies by the weekend,
bringing the potential for areas of heavy rainfall. Interactions
with a stationary frontal boundary draped from the Great Basin
into the Central Rockies could lead to an enhancement of heavy
rainfall potential across the central and northern Rockies.
Temperatures will also become a bigger story during the next week
as the upper ridge drifts east across the Mississippi Valley
toward the Ohio/Tennessee Valleys and the Southeast. High
temperature anomalies of +10 to +18 deg F are expected Sat-Sun
across areas from the central plains to the Ohio Valley and Great
Lakes. By Mon-Tue as the ridge shifts east, these temperatures
anomalies will reach the Eastern Seaboard. Widespread record high
temperatures are not anticipated, many locations will near record
values, and a few records are possible. High dew points in
addition to the hot temperatures will make for dangerous heat
index values for some areas, particularly for urban areas where
warm overnight lows will remain warmer than surrounding rural
areas.
Ryan
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