Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
215 PM EDT Wed Jun 12 2019
Valid 12Z Sat Jun 15 2019 - 12Z Wed Jun 19 2019
...Overview and Guidance/Predictability Assessment...
The models and ensembles remain fairly similar and consistent with
the large scale pattern evolution from the weekend through the
first half of next week. From early Sat into Mon expect low
amplitude broadly cyclonic mean flow over the lower 48, supported
by an upper ridge that builds into western Canada and mean ridging
over the eastern Pacific (consisting of a leading ridge initially
nearing the West Coast and a much stronger ridge that builds
farther west). Once this stronger ridge becomes established it
appears likely that troughing will amplify over the west-central
U.S. by next Tue-Wed. Guidance continues to show a heavy rainfall
potential that is most pronounced from the south-central Plains
into the Ohio Valley and central Appalachians--along and south of
a front that settles into a northeast-southwest orientation over
the eastern two-thirds of the country from Sun into early next
week. Some areas within this threat region will be very sensitive
to any significant rainfall in light of ongoing flooding or wet
ground.
Aside from still-oscillating timing for the wave/frontal system
crossing the Lower Great Lakes/New England, the updated forecast
maintained reasonable continuity by starting with an operational
consensus on day 3 Sat and then incorporating gradually more 06Z
GEFS/00Z ECMWF ensemble mean input such that day 7 Wed consisted
almost entirely of the means.
The combination of the upper low over the eastern half of Canada
and individual smaller scale shortwaves tracking along the
U.S.-Canada border continue to cause difficulties in resolving
specifics at the surface over the northeastern quadrant of the
lower 48. On the positive side, solutions are slowly becoming
less divergent for the upper low itself. However the shortwaves
are fairly small in scale and it may take well into the short
range time frame to achieve good clustering and continuity.
Latest 06Z/00Z GFS runs were still faster than most other models
and the GEFS means with a leading surface wave Sunday, so opted to
stay nearer to the larger consensus around the 00Z ECMWF or 06Z
FV3-GFS (the last run in parallel before the switch with the 12Z
cycle today). Even with these differences and variability there
is still good agreement in principle that the trailing front will
stall over the central U.S. by Sun and possibly the Northeast
toward midweek.
Farther west, in the Sat-Tue time frame, typical depth/timing
differences arise for the upper trough expected to deepen over the
west-central U.S. days 6-7 Tue-Wed. Relative to typical errors at
that time frame the differences do not appear great enough to
favor one side of the spread or the other for timing. An
additional complication that comes into play is the upper low
tracking just south of Alaska, with the ECMWF and its ensembles
generally quicker than the GFS/GEFS members. Each ensemble system
has trended stronger with the Pacific ridging off California, but
its shape differs between the ECMWF ensemble and GEFS mean--the
former being elongated east-west while the latter oriented more as
a circle. Without some conflicting trends downstream, preference
was to trend in between the means late in the period to lean away
from the various low-confidence specifics offered by individual
model runs. The deepening upper trough should help to push the
stalled Plains boundary northward as a warm front toward the
latter part of the period.
...Sensible Weather Highlights/Threats...
For the time being not a lot has changed with the heavy rainfall
threat from the south-central Plains into the Ohio
Valley/Appalachians, aided by moist flow from the Gulf interacting
with a wavy/stalling northeast-southwest front and multiple
shortwaves embedded in flow aloft. The best consensus of guidance
maintains highest potential for greatest five-day totals from
Oklahoma/southeast Kansas into the Ohio Valley. There are some
hints that this favored area could extend into parts of the Lower
Great Lakes but historically northern biases for some convective
events suggest less confidence in that idea. More localized
significant activity may extend into the Mid-Atlantic/Northeast.
The overall forecast over the West is fairly consistent as well,
with shortwaves aloft promoting showers/thunderstorms that should
produce highest totals in the vicinity of the northern-central
Rockies. Some activity could extend back into parts of the Great
Basin/Sierra Nevada. Specifics of flow aloft become more
uncertain toward next Tue-Wed, thus lowering confidence in
location/coverage of rainfall by then. Energy that amplifies into
the west-central U.S. should begin to spread more
showers/thunderstorms into the northern-central Plains from later
Mon into Wed. The Florida peninsula will likely see
diurnally-enhanced convection through the period.
The West will continue to see above normal temperatures through
the weekend with plus 5-15F anomalies for highs extending from the
northern half of California into the interior Northwest/extreme
northern Rockies. Such anomalies for morning lows should cover a
greater portion of the West. Expect gradual moderation in
temperatures next week, especially over the Northwest. The
developing west-central trough aloft may bring below normal
temperatures to the northern/central Rockies by midweek. Over the
eastern half of the country readings should be fairly close to
normal for highs but near to above normal for lows. Depending on
the exact surface pattern parts of the Mid-Atlantic could see one
or more days of above normal highs. Greatest uncertainty in
temperatures next week will be in the West due to the timing of
height falls and eastward extent of the upper ridge perhaps into
California rather than remaining offshore.
Fracasso/Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain from portions of the south-central Plains to the Ohio
Valley, Sat-Tue, Jun 15-Jun 18.
- Flooding occurring or imminent across portions of the
Mississippi and Missouri River Basins, and South Carolina.
- Flooding likely across portions of the Northern and Central
Great Basin, and the Central Plains.
- Excessive heat across portions of south Texas, Sat-Wed, Jun
15-Jun 19.
- Much above normal temperatures across portions of the Northern
Great Basin, Sat-Sun, Jun 15-Jun 16.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml