Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
133 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 this weekend into early 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 Rockies
across the central/northern plains from Sun through much of the
upcoming week.
...Model Guidance/Predictability Assessment...
Model consensus was sufficient during days 3-5 (Sun-Tue) to
justify a majority deterministic blend (including the 12Z
ECMWF/UKMET and 18Z GFS) as a starting point for the forecast.
Models showed good consensus that the western upper low should
linger across the northern Great Basin through much of this time
period, perhaps drifting slowly southeastward. Some differences
exist with respect to the depth of the low, but the aforementioned
blend should best represent a consensus solution. Farther east,
models showed similar timing with respect to shortwave energy
traversing the more progressive flow near the U.S. border with
Ontario/Quebec, but with some amplitude differences. The GFS has
consistently been a bit more amplified with this wave, likely the
result of a slightly weaker upper ridge farther south. The ECMWF
and UKMET both had a more limited southern extent for height falls
across the Great Lakes/Northeast. Thus, the aforementioned blend
is weighted bit more toward keeping the ridge stronger and keeping
the height falls farther north. Nonetheless, a surface cold front
should traverse the Great Lakes/Northeast Sun-Tue. 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 northward across the plains/Mississippi
Valley by Tue - around the western periphery of the southeastern
U.S. ridge.
By days 6-7 (Wed-Thu) model differences become more significant.
The consensus of the vast majority of deterministic/ensemble mean
solutions suggests that the western upper low should continue to
linger, and may even retrograde a bit to the southwest. The 12Z
ECMWF along with the 12Z ECENS/NAEFS and 18Z GEFS means all
support this idea. On the other hand, the 12Z and 18Z runs of the
GFS made a dramatic shift, quickly weakening the south
central/southeastern U.S. ridge and ejecting the upper-level
energy from the West eastward into the plains. Given the
consensus, the 12Z/18Z GFS runs were rejected. The downstream
implication of this preference is that the upper ridge should
remain stronger for longer across the south central/southeastern
states (relative to the 18Z GFS). Farther north, however, the more
progressive flow should continue, with another round of height
falls attempting to press Great Lakes/Northeast by next Thu.
Ensemble show a range of solutions with respect to the amplitude
of this trough and how much the ridge would be suppressed, but
given the pattern, would continue to be skeptical of any solutions
that weaken the ridge too quickly. Given these considerations, the
forecast for days 6-7 was based on a majority ensemble blend
including the 12Z ECENS/NAEFS means, but with a significant
component of the 12Z ECMWF continued through day 7.
...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 encounters the
southern periphery of the stronger westerlies. 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.
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