Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sat Jul 21 2018

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

...Southern Plains Heat Wave shifts into the Southwest next week...
...Widespread Heavy Rain and Flooding Threat continues over the
Eastern U.S...
...Rockies to central Plains/Mid-Mississippi Valley localized
heavy rain threat possible...

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

The medium range period continues to advertise that an amplified
mid to upper level trough will linger through early next week with
an axis anchored across the Ohio Valley and into the Deep South.
Deep moisture and anomalous precipitable water feeding into/ahead
of the system and its associated surface front and instability
axis will prolong a pattern favorable for cell training/repeat
activity. This should continue a threat for locally
heavy/excessive rainfall with the axis for heaviest QPF centered
up the Appalachians and Eastern Seaboard. This lead system will
eventually be replaced by a well defined upstream trough working
across the Great Lakes and northeastern states Wednesday into next
weekend. The development of this trough will focus the widespread
heavy rainfall threat more into the Northeast and the eastern
Mid-Atlantic along a trailing and eventually dominant cold front.
A flash flood and longer term/larger scale flooding (e.g. river
flooding) threat is possible. 

Meanwhile, a potent mid to upper level ridge focused over the
southern Plains and back into the Southwest early next week, will
eventually shrink back and settle over the Southwest the remainder
of the week. Underneath this ridge, record heat is expected to
continue as heights peak several standard deviations above normal.
Forecast highs across much of the Southwest are in the 110 to 120
range for the desert areas (including Phoenix and Las Vegas) for
several consecutive days. Overnight low temperatures in the upper
80s to maybe even 90 will only amplify the hazardous impacts of
the heat. In addition, a series of less predictable upper level
impulses rotating overtop the ridge interacting with pooling
moisture along surface convergence boundaries may fuel strong
convection and a possibly heavy to locally excessive rainfall
threat across portions of the central Rockies/Plains and into the
middle Mississippi Valley.

The forecast confidence for this medium range period remains above
average, with strong agreement and clustering amongst the models
and ensembles for the larger/synoptic scale features. This suite
of the WPC medium range products was based heavily on a blend of
the GFS/ECMWF/UKMET through day 5, with less than majority
contribution from the latest GEFS/EC ensemble means by the end of
the period. This fits very well with previous shift continuity.

Santorelli


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