Extended Forecast Discussion...Amended
NWS Weather Prediction Center College Park MD
258 PM EDT Mon Jun 11 2018
Valid 12Z Thu Jun 14 2018 - 12Z Mon Jun 18 2018
...A weakening Bud in the eastern Pacific and an incoming
Caribbean/Gulf of Mexico tropical disturbance could lead to wet
weather across the Rockies, northern Plains, upper Midwest, and
western Gulf coast from this weekend into next Monday...
...Pattern Overview...
Coordination with the Alaska Medium Range desk led to discussion
of the downstream ramifications of the ongoing extratropical
transition of Maliksi offshore Japan, which is expected to lead to
a more amplified flow pattern than usual for mid to late June
across North America. As Maliksi swings into Alaska as a robust
system, ridging builds across the eastern Pacific which leads to a
pronounced trough across the West, which leads to a similarly
pronounced ridge/trough couplet across the Mississippi
Valley/western Atlantic respectively. A tropical disturbance
currently in the southwest Caribbean sea is expected to track
northwest into Texas on the south side of the Mississippi Valley
ridge.
...Model Guidance/Predictability Assessment...
The guidance shows reasonable agreement for much of the period.
The biggest issue involves whether or not a closed low in the
western Atlantic retrogrades into the East coast, which only the
operational 00z ECMWF shows beyond day 6/Sunday morning. While
the Canadian is favorable across the western Atlantic late in the
period, its pattern across the West and Midwest leaves a bit to be
desired. Taking that into account, a compromise of the 00z ECMWF,
00z UKMET, 00z GFS, and 00z Canadian were used for the
fronts/pressures/winds through Saturday. From Sunday onward,
phased out the 00z Canadian and 00z ECMWF solutions substituting
the 00z GFS, 00z ECMWF ensemble mean, and 00z NAEFS mean in their
place. This solution led to reasonable continuity. For the
southwest Caribbean tropical disturbance, maintained reasonable
continuity from yesterday but added a surface low as conditions
could become favorable for a convective low in the western Gulf of
Mexico, to the north of an upper level cyclone. See the latest
Tropical Weather Outlook from the National Hurricane Center for
the latest on its prospects for further development.
For the remainder of the medium range grids, a more even blend of
the ensemble means (leaning more ECMWF ensemble mean) and 00z
GFS/06z GFS solutions was utilized. For the days 4-7 QPF, the
initial template is a blend of the 00z GFS, 06z GFS, 00z ECMWF,
and 12z National Blend of Models. Should the 12z GFS look
reasonable in a couple hours, it could be utilized for the days
4-7 QPF as well.
...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 rains to portions of TX and LA from
the weekend into 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 may slow or stall for a period of
time Sat-Sun from the central plains to the Upper Midwest,
resulting in several days with shower/thunderstorms potential, and
potential flooding threat for some areas.
Moisture associated with Tropical Storm 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 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 into the Ohio Valley. High temperature anomalies of +10 to
+18 deg F are expected Thu-Sat 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 fall short of record warm/hot values through the
period. By Sunday and next Monday, 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, which would impact urban areas the greatest as they would
receive the double whammy of high afternoon temperatures and
urbanly-induced warm overnight lows.
Roth/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