Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Fri Jul 20 2018
Valid 12Z Mon Jul 23 2018 - 12Z Fri Jul 27 2018
The models and ensemble means remain in good overall synoptic
scale agreement on the pattern over the continental U.S. through
the end of next week. The 12Z CMC indicated a greater degree of
retrogression with the upper trough/closed low over the southeast
U.S. compared to the model consensus, and also faster with an
upper low near Greenland. There are modest differences regarding
the evolving upper low over south-central Canada by the end of the
week, and some of the ensemble means were incorporated for that
portion of the forecast. There has also be a trend in the
guidance over the past 24 hours regarding the upper low over the
Gulf of Alaska for the middle of next week, and this is also
resulting in some westward adjustments in the upper ridge axis
over western Canada. A blend of the deterministic GFS/ECMWF/UKMET
was used through Wednesday for pressures and fronts, and then some
of the EC and GEFS means for Thursday and Friday. The previous
discussion is appended below for reference.
...Heat Wave to Intensify Over the Southwest States Next Week...
...Heavy Rain / Flash Flooding Expected Over the Eastern U.S. Next
...Overview and Guidance Evaluation/Preferences...
It remains the case that the main mid-upper level troughs are of
medium to long wavelength and fairly slow to evolve given the
season. They are also relatively well defined, with one closed low
center expected in the east/southeast next week, eventually
replaced by another well defined upstream trough digging into the
Great Lakes by Wed-Fri. A main convective storm track remains
around a hot Southern Plains/Southwest ridge and into the
aforementioned and dynamic/unstable main troughs. The lead mean
trough position settled into the eastern/southeastern U.S. also
has ample access to a tropical moisture plume that backs in off
the Atlantic, setting the stage for a more widespread heavy
Underneath the ridge, the heat wave in the Southern Plains this
weekend expands westward across the southwest quarter of the
nation next week where some record hot temperatures are expected.
In some places like northern Mexico and southern AZ/CA, where
monsoon season is more commonly in full swing by now, these
heights peak at 3.5 standard deviations above climatology.
Standard deviations of +2.0 extend as far north as San Francisco
and Salt Lake City.
Much of this medium range forecast is of high confidence during
this cycle, with strong agreement / clustering among the models
and ensembles regarding these well defined and slowly evolving
features. Most of the more noticeable model differences are across
the very high latitudes, whereas the forecast over the CONUS shows
less spread. Accordingly, the WPC medium range forecast suite was
mainly derived from a composite blend of the latest GFS/ECMWF,
GEFS and ECMWF ensemble means, the NBM, and WPC continuity.
Significant hazards expected over broad areas of the country the
next seven days. Heat stress will be a major story in the West.
Forecast highs are in the upper 100s and teens (110-119+) in the
desert areas (including Phoenix, Vegas) for several consecutive
days. Low temperatures near or even above 80 deg for many areas
will amplify the potentially hazardous impacts of the heat. The
potential arrival of a cold front across the southern plains by
next Tue-Wed could bring some slight relief there, but the heat in
the Southwest looks to be a lengthy and potentially dangerous
Flash flooding and perhaps even longer term, larger scale flooding
(e.g., river flooding) will be possible in broad swaths of the
eastern states. The closed low dropping through the OH/TN Valleys
toward the Gulf Coast anchors a well defined trough, with
precipitable water of 2.0 inches or greater forming an axis that
overlaps with many mountainous areas in the Appalachians and New
England. These areas are prone to flash flooding, and the
combination of the synoptic support along with seasonably strong
sun angle will likely yield pockets of instability sufficient to
support an off and on heavy rainfall threat.
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at: