Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
233 AM EDT Sat Jul 21 2018

Valid 12Z Tue Jul 24 2018 - 12Z Sat Jul 28 2018

...Southern Plains Heat Wave shifts across the Southwest/West...
...Widespread Heavy Rain and Flooding Threat over the Eastern
U.S...
...Rockies to Central Plains/Mid-MS Valley Strong Convection and
Localized Heavy Rain Threat...

...Guidance/Uncertainty Assessment and Weather Threats...

Forecast predictability remains much above normal in showing that
an amplified/closed mid-uppper level trough will linger with an
axis roughly along 85W early early-mid next week. Deep moisture
with pooled precipitable water values upwards of 2" feeding
into/ahead of the system and associated/slow moving surface
fronts/instability will fuel a prolonged pattern favorable for
cell training/repeat activity that will lead to a threat of
locally heavy/excessive rainfall centering up across the
Appalachians and Eastern Seaboard. This lead system will be
eventually surplanted by a well defined upstream trough working
across the Great Lakes and northeastern states Wed into next
weekend whose development will focus the widespread heavy rainfall
threat more into the Northeast and ern Mid-Atlantic consistent
with dynamic support/frontal progression. Flash flooding and
longer term/larger scale flooding (e.g., river flooding) is
possible.

Meanwhile, a potent mid-upper level ridge settles over the
Southwest and gradually expands up over the West next week.
Underneath the ridge, some record hot temperatures are expected as
heights peak several standard deviations above climatology.
Forecast highs are in the upper 100s and teens (110-119+) in the
desert areas (including Phoenix, Vegas) for several consecutive
days. Low temperatures upwards to 90 deg in some desert locals
will amplify the hazardous impacts of the heat. A series of
impulses rotating within and overtop the ridge offer less
predictable timings individually, but each will provide enhanced
lift. This may be especially problematic in upper diffluent
flow/boundary convergence from the central Rockies/Plains into the
mid-MS Valley where convective complexes/thunderstorms will be
fueled by inflow pooling moisture and heavy/repeat cells may lead
to a local excessive rainfall threat.

Much of this medium range forecast remains of high confidence
during this cycle, with strong agreement and clustering among the
models and ensembles regarding these well defined and
slowly/steadily evolving features. Accordingly, the WPC medium
range forecast suite was primarily derived from a composite blend
of the latest GFS/ECMWF, GEFS and ECMWF ensemble means, the NBM,
and WPC continuity.

Schichtel

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