Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1153 AM EDT Thu Jun 14 2018
Valid 12Z Sun Jun 17 2018 - 12Z Thu Jun 21 2018
...Heavy rains possible for portions of the northwestern Gulf
Coast and portions of the central/northern Rockies/plains...
...Potential for excessive heat from portions of the Midwest to
the Mid-Atlantic and Southeast next week...
...Pattern Overview...
Progressive flow will be in place during the medium range from the
higher latitudes of Canada across Hudson Bay and into the
northeastern U.S.. Meanwhile, the flow farther south will be
rather blocked across most of the contiguous U.S.. A broad and
anomalous upper ridge is expected to persist from the Mississippi
Valley to the Southeast through much of the forecast period while
an upper low lingers across the Great Basin, trapped between upper
ridging to the west/north (from the eastern Pacific north into
western Canada) and to the east. The result will be a persistent
quasi-stationary frontal boundary draped from the central Plains
into the Midwest from Sunday into the middle of next week.
...Model Guidance/Predictability Assessment...
Overall there is fairly remarkable model agreement through the
entire medium range period (Sun-Thu). For days 3-5, consensus was
sufficient to justify a majority deterministic blend (which
includes the 06 GFS and the 00z ECMWF/UKMET) as a starting point
for the forecast. Models continue to show good agreement that the
western U.S. upper low should linger across the northern Great
Basin through much of the medium range period, perhaps drifting
very slowly southeastward. The main outlier to this was yesterdays
18z GFS run which actually quickly weakened the upper low and
ejected the energy eastward into the Plains, but the latest two
runs (todays 00z and 06z) keep the upper low strong and locked
over the Great Basin. This suggests a possible trend, hopefully,
towards that solution which is consistent with the vast majority
of models and ensemble means.
Farther east, models showed similar timing with respect to
shortwave energy traversing the more progressive flow near the
U.S. border with Ontario/Quebec days 3-5, with mostly minor
amplitude differences. Regardless, a surface cold front should
traverse the Great Lakes/Northeast Sun-Tue. Another round of
height falls attempts to cross the Great Lakes/Northeast by next
Thurs, with models and ensembles showing a range of solutions with
respect to the amplitude of this trough and how much of the
Southeast ridge would be suppressed. Given the pattern though,
would continue to be skeptical of any solutions that weaken the
ridge too quickly. The forecast for days 6-7 was based mostly on
the EC and GEFS ensemble means, with a more than usual component
of deterministic solutions (mainly the ECMWF which shows better
run-to-run continuity across the board).
Also of note during days 3-5 is a broad trough and area of
moisture/disturbed weather in the northwestern Gulf of Mexico.
Models show good consensus that this feature should drift inland
across eastern Texas by Mon and then north/eastward across the
plains/Mississippi Valley by Tue around the western periphery of
the southeastern U.S. ridge.
...Weather Highlights/Hazards...
Heavy rain is increasingly likely for portions of the western Gulf
Coast states as the broad trough/area of disturbed weather move
onshore Sun-Mon. Model solutions vary widely as to the specific
rainfall totals and the location, but multi-inch rainfall totals
appear possible, especially for portions of eastern Texas and
western Louisiana. Heavy rains will also be possible from portions
of the central/northern Rockies east across the northern plains
and Upper Midwest Sun-Mon as moisture leftover from Tropical Storm
Bud is spread northward east of the upper low and interacts with a
frontal boundary across the region. By Tue-Wed, models suggest
that favorable upper flow will continue to support the transport
of deeper moisture from lower latitudes northward into the
southern/central Rockies and adjacent High Plains, supporting the
potential for additional convection and areas of heavy rainfall,
especially in close proximity to a stationary surface frontal
boundary.
The other significant story during the medium range will be the
potentially excessive heat beneath the central/eastern U.S. upper
ridge. High temperature anomalies of +10 to +18 deg F are possible
Sun-Mon from portions of the mid/upper Mississippi Valley east to
portions of the Southeast and Mid-Atlantic. The hot temperatures
may persist into the middle of next week for portions of the
Southeast and southern Mid-Atlantic region, where upper subsidence
along with favorable downslope flow may result in temperatures
near or even just surpassing 100 degrees for a few locations. Some
areas may near or surpass record high temperatures, especially by
Sun-Mon along the East Coast. Widespread record high minimum
temperatures are expected from the Midwest to the Eastern
Seaboard, increasing the potential for heat-related illnesses.
Anomalously warm temperatures may also creep into the Pacific
Northwest by early to mid next week, on the eastern side of the
upper ridge across the Northeast Pacific. High temperature
anomalies of +10 to +20 deg F are possible Mon-Thu, with some
locations soaring well into the 90s.
Santorelli/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