Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Tue Jun 12 2018
Valid 12Z Fri Jun 15 2018 - 12Z Tue Jun 19 2018
...Tropical moisture to bring potentially heavy rains to portions
of the Southwest and western Gulf Coast...
...16Z update...
00Z models/ensembles have mostly stayed the course since the
previous issuance, and a refresh of the latest guidance (including
the 00Z GFS/ECMWF/ECMWF ens mean and 06Z GFS/GEFS) offered a
reasonable starting point with few changes to the fronts/weather
grids. This included the northward progression of hurricane Bud in
the Pacific into northwestern Mexico and into Arizona (likely just
its moisture by then), the lingering of troughing in the Great
Basin, and the ridging in the Midwest/southeast/mid-Atlantic.
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 Mississippi
Valley. This will favor a relatively amplified and slowly-evolving
flow pattern across the CONUS, with a persistent trough/upper low
across the interior West and near/off the Eastern Seaboard. The
western trough will transport moisture associated with the
remnants of Hurricane 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 (Fri-Sun). Models seem to have a significantly
better handle now on the timing 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, and a slower
eastward progression (as was the thinking over recent days).
Farther east, models continue to show variation with the depth of
an upper low forecast to develop near/off the Mid-Atlantic coast
after a shortwaves dives southeastward across the Great
Lakes/Northeast Fri-Sat. Timing of the features is similar among
the various models, however, thus a blend should should represent
a consensus solution in terms of depth at this time. 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 (Mon-Tue) the pattern remains largely stable with
very little in the way of significant changes. Models now show a
general consensus that the western upper low may linger across the
Four Corners region or Great Basin into early next week while the
upper ridge over the central U.S. intensifies and then migrates
eastward toward the Mississippi Valley, and more progressive flow
persists with another shortwave crossing the Great
Lakes/Northeast. Some discrepancy exists as to the evolution of
the upper low off the East Coast, with the ECMWF eventually
absorbing the features back into the northern stream flow by Tue,
while the GFS drifts the feature farther south toward the Bahamas.
At this time, given the pattern, across most of the CONUS, would
expect the upper low to remain separated from the northern stream
flow for a longer period of time (as was even indicated by
previous ECMWF runs). Otherwise, models show typical
timing/intensity variations with these features as would be
expected for almost a week out. 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 to numerous 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.
Farther north, a shortwave, surface low pressure system crossing
the northern plains and Upper Midwest 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.
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. Some model solutions even suggest the
possibility that the moisture from Bud could spread east into the
central/northern plains, further enhancing heavy rain potential
along the stationary frontal boundary.
Temperatures will also become a bigger story during the next week
as the upper ridge expands across the central U.S. and then drifts
east across the Mississippi Valley. High temperature anomalies of
+10 to +18 deg F are expected Fri-Sun across areas from the
central plains to the Mid-Mississippi Valley, with highs
approaching 100 degrees for some areas. Forecast highs and lows
are expected to approach but mostly fall short of record warm/hot
values through the period. By Mon, the greatest temperature
anomalies will shift a bit east from the Mid-Mississippi Valley
into the Ohio Valley. 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