Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Tue Jun 11 2019
Valid 12Z Fri Jun 14 2019 - 12Z Tue Jun 18 2019
...Overview and Guidance/Predictability Assessment...
Guidance agrees fairly well with the idea of broad and low
amplitude cyclonic mean flow downstream from strengthening ridging
that rebuilds into western Canada and the eastern Pacific. Within
this pattern there continue to be multiple embedded uncertainties
that provide challenges for resolving important specific details
and significantly lower forecast confidence. The most prominent
question marks involve the upper low forecast to be over or near
northern Ontario as of early Fri, multiple weak shortwaves flowing
through the West and continuing eastward, and the character of
western U.S. troughing aloft by early next week. Although there
is ample opportunity for best consensus to change, guidance has
established a fair degree of persistence in suggesting that the
overall pattern evolution may be favorable for another episode of
heavy rainfall from the south-central Plains to locations at least
as far east/northeast as the Ohio Valley and central Appalachians.
Some areas will be highly sensitive to any significant rainfall
given ongoing flooding.
The solution spread for the upper low near northern Ontario on Fri
still differs enough to cause havoc with the forecast of frontal
progression across the northern U.S. The 00Z guidance suite
mostly separated into two camps--the GFS/GEFS vs the
FV3-GFS/ECMWF/UKMET/Canadian (and Canadian/ECMWF ensembles).
Overall they still suggest that the low/trough aloft would remain
far enough north that any corresponding cold front would not push
extremely far south over the eastern half of the lower 48--in
contrast to one or more runs from various global models over
recent days. A blend of the cluster around the 00Z FV3-GFS/ECMWF
provided a good representation of the most likely
scenario--quicker with the lead trough in the northeast Friday and
farther east with the upper low over Ontario (compared to the
slower and farther west 00Z/06Z GFS, respectively). This majority
cluster would have a general area of low pressure track from the
northern Plains toward eastern Canada or New England with a
trailing front reaching down to and stalling over the
south-central Plains during the weekend into early next week.
Over the eastern Pacific/western U.S. solutions continue to
diverge after Sat, as weak shortwave energy reaches the coast
followed by a strengthening of the eastern Pacific ridge aloft.
Ensemble means continue to show stronger ridging (near +2.5 sigma
at F180 which is impressive) along 135W west of northern
California next Tuesday). For forecasts valid around day 5 Sun
multi-day trends seem to leaning more in the ECMWF direction
versus some previous days' GFS runs that had held back or
deflected the energy. Teleconnections relative to the agreeable
core of positive height anomalies off the Pacific Northwest favor
the weaker trough contrary to earlier deeper deterministic runs.
...Sensible Weather Highlights/Threats...
It will take more time to determine the details but guidance as a
whole continues to signal a threat for heavy rainfall from the
south-central Plains northeastward, with the most probable
locations for highest five-day totals between Oklahoma/southeast
Kansas and the Lower Ohio Valley. Isolated significant amounts
may extend into the Mid-Atlantic/Northeast but confidence is low.
Some areas within the potential threat area will be very sensitive
to additional rainfall due to ongoing flooding or recent wet
conditions. Multiple ingredients should contribute to areas of
enhanced rainfall: establishment of a multi-day period of low
level flow from the Gulf, potential stalling of a wavy
northeast-southwest oriented front, and shortwaves aloft
progressing out of the West. While over the western states these
shortwaves will encourage episodes of showers and thunderstorms of
varying intensity from the Great Basin and possibly Sierra Nevada
into the northern-central Rockies. The Florida peninsula should
see diurnally-enhanced showers/storms for most of the period.
Expect above normal temperatures to persist over a majority over
the West through the period. The primary axis of plus 5-15F
anomalies for highs should extend from the northern half of
California into the interior Northwest/extreme northern Rockies.
Coverage of similar anomalies for morning lows will likely be more
expansive. The eastern U.S. will see a cool Friday with some
areas 5-15F below normal before a quick rebound (some morning
record lows in the middle-Mississippi Valley are possible). From
the weekend into next week most areas over the eastern half of the
country should see readings within a few degrees on either side of
normal. Some low-confidence pockets of greater anomalies (mainly
for warm lows or cool highs) will depend on location of
clouds/precipitation on any particular day.
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 Southern/Central Plains to the
lower Ohio Valley, Sat-Mon, Jun 15-Jun 17.
- Flooding occurring or imminent across portions of the
Mississippi and Missouri River Basins, and western North Carolina.
- Flooding likely across portions of the Mississippi and Missouri
River Basins, northern South Carolina, and parts of the
Central/Northern Great Basin.
- Excessive heat across portions of south Texas, Sat-Tue, Jun
15-Jun 18.
- Much above normal temperatures across portions of the interior
Northwest, Fri-Sat, Jun 14-Jun 15.
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