Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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 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 18Z
GEFS/12Z ECMWF ensemble mean input such that day 7 Wed consisted
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.
12-18Z guidance generally trended slower than the prior 12-hourly
cycle. Latest GFS runs are faster than most other models and GEFS
means with a leading surface wave. 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 the latest GFS/GEFS
cluster has adjusted to consensus, away from a deeper California
trough that had been forecast. Then 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 12Z ECMWF bringing it into
the northwestern U.S. by next Wed while other models hold it
farther back. Preference was to trend toward 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.
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
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