Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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 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.
Compared to yesterday the solution spread is not quite as dramatic
for the upper low near northern Ontario on Fri, but models still
differ enough to cause havoc with the forecast of frontal
progression across the northern U.S. Even the ensemble means have
adjusted somewhat, holding the feature farther west than indicated
24 hours ago. However they have been the most consistent in
suggesting 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. Based on
data available through the 18Z cycle a model consensus
transitioning toward a blend of the 18Z GEFS/12Z ECMWF means with
lingering input from the corresponding GFS/ECMWF runs provided a
good representation of the most likely scenario. 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. New
00Z guidance still suggests high uncertainty over the upper low of
interest and frontal progression.
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.
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. Then for the
rest of early next week the GFS/GEFS become noticeably deeper than
most other guidance with the trough centered over California.
Teleconnections relative to the agreeable core of positive height
anomalies off the Pacific Northwest in latest D+8 multi-day means
favor the weaker trough seen in the non-GFS majority. Thus the
aforementioned blend tilts a bit in the ECMWF/ECMWF mean
direction, but more toward its mean as the operational run may
build the Pacific ridge a little far eastward.
...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. Some significant amounts may
extend into the Mid-Atlantic/Northeast. 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--including 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. 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.
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