Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Tue Jul 24 2018

Valid 12Z Fri Jul 27 2018 - 12Z Tue Jul 31 2018

...Heat wave lingers over the West...
...Heavy rainfall threat to spread from the Central Plains to
Mid-South then again over the East...


...Pattern Overview/Guidance Evaluation/Preferences...

A strong 594-dm mid-level ridge will remain fixed over the
southwestern U.S. for the foreseeable future thereby continuing
the period of above average temperatures. Across the remainder of
the country, an upper low just north of the Great Lakes should
eventually drift northward and fill while crossing into northern
Quebec over the weekend. On its heels, another slow moving band of
height falls is forecast to sweep across central U.S. eventually
attaining a position near the Mississippi River by Day 7/July 31.
As a persistent western Atlantic ridge begins retrograding to
areas west of Bermuda, a squeeze play of the two features is
expected aiding in more heavy rainfall somewhere across the
eastern third of the nation.

Through the weekend, the operational models have proven to cluster
rather well with minimal noise in the ensemble spaghetti plots. It
appears more of the uncertainty looms much farther upstream into
the northeastern Pacific. While solid agreement exists throughout
the period with the mean ridge over the Desert Southwest, the same
does not hold true with the next longwave digging into the
north-central U.S. by Sunday. Multi-day comparisons have shown a
slowing trend as the upper ridge across the far western Atlantic
continues to shift westward. With regards to the area of lowering
heights, the operational GFS runs have had a tendency to be east
of the ECMWF solutions. But their respective ensemble means seem
to be more stable supporting another upper low in the vicinity of
the mid-South and the corresponding ridge offshore of the East
Coast. Each comprised of 500-mb anomalies in the 1.5 to 2 sigma
range, tremendous poleward moisture fluxes would likely surge up
the spine of the Appalachians potentially toward the I-95 corridor
by next Tuesday. Elsewhere, atop the mean upper ridge across the
Desert Southwest, a weak shortwave could impact the Pacific
Northwest as shown by the ensemble means by early next week.
Otherwise, expect the corridor of strong forcing across the high
latitudes to remain over Nunavut with the southern extent of
height falls unlikely to track south of the international border
with Canada.

Given rather solid model agreement through Day 5/Sunday, utilized
a multi-operational model blend comprised of the 06Z/00Z GFS as
well as the 00Z ECMWF/CMC/UKMET. However, focused 80 percent of
this blend toward the GFS/ECMWF solutions given greater run-to-run
stability. Into next week, incorporated more of the 06Z GEFS/00Z
ECMWF ensemble means to the mix as certainty diminishes across the
central/eastern U.S.


...Weather Highlights/Threats...

Persistent cold advection across the middle of the country should
keep temperatures below climatology into the weekend. This will
particularly focus from the Northern Rockies eastward into the
Northern/Central Plains as well as much of the Upper Great Lakes
into the Middle Mississippi Valley. Expect numbers to be around 5
to 10 degrees below average for late July. Meanwhile, the warm
spot will reside along the West Coast given the persistence of
upper ridging. Although high temperatures should stay close to
climatology over the Desert Southwest as readings stay around the
lower 110s, greater departures are likely up into the Pacific
Northwest, generally 5 to 10 degrees above average.

A heavy rainfall signal continues to show up in the guidance as
the slow moving upper trough shifts focus from the central U.S.
toward the Appalachians. The initial progressive movement of this
negative height anomaly would initially mitigate the threat for
more extensive and widespread issues. However, by early next week,
models seem to agree on cutting off the upper low near the
mid-South while moisture flux anomalies rapidly rise farther
downstream. This would support the potential for heavy rainfall
over areas already being impacted during the last few days. The
threat will continue to be monitored in future shifts ahead.


Rubin-Oster

WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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