Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EDT Fri Jul 27 2018

Valid 12Z Mon Jul 30 2018 - 12Z Fri Aug 03 2018

...Hot temperatures over the West gradually moderating...
...Heavy rainfall threat extending from the central Plains/MS
Valley to Southeast/Eastern U.S...

...Pattern Overview and Weather Highlights/Threats...

Energy dropping through the Plains as of early next week will
carve out another eastern U.S. upper trough that should achieve
its greatest depth around midweek before trending weaker.  This
feature will bring another multi-day threat of convection/heavy
rainfall to parts of the eastern half of the U.S..  The most
agreeable guidance signal for highest 5-day rainfall totals exists
over the Southeast.  Locations from the central/south-central
Plains into MS Valley are most likely to see enhanced rainfall
during the first half of the period, as the upper trough is in the
process of amplifying and Plains to Great Lakes surface low
pressure evolves.  Some areas of moderate-heavy rainfall may
extend into parts of the Great Lakes/Appalachians/Mid-Atlantic. 
One notable difference between this episode and the preceding
multi-day event is that the upcoming trough aloft should be
aligned a few degrees longitude farther west due to the proximity
of a strong Atlantic ridge.  This should allow for some of the
precip shield to extend into areas that did not experience heavy
rainfall over the past week.  However other locations will be very
sensitive to any meaningful additional activity.  The pattern will
promote below normal temperatures from the Plains into the
Southeast with greatest negative anomalies of minus 5-15F most
likely over parts of the Plains early in the week.

A strong western U.S. ridge aloft will flatten somewhat as a
couple bundles of energy combine to yield an upper low that
reaches near northern Vancouver Island by early Wed followed by
some combination of splitting/progression thereafter.  By late
week this energy should support northern Plains low pressure with
a corresponding increase in convection.  Expect the coverage of
above normal temperatures over the West to decrease with time. 
Highest anomalies overall should be over the Northwest on Mon-Tue.
 The Great Basin/Southwest will likely remain quite warm for mins
through the end of the week with highs tending to be more modestly
above normal (though still hot on an absolute basis).  The
Northwest should trend below normal for highs by the latter half
of the week. 


...Guidance/Uncertainty Assessment...

Latest guidance maintains good agreement in the overall pattern
evolution during the period but shows some embedded uncertainties.
 There are modest differences in the strength of the western
Atlantic ridge, affecting specifics of the east-central U.S.
trough.  On most days the CMC/CMC mean are slightly weaker than
the majority with the ridge thus allowing eastern U.S. trough
energy to extend a little farther eastward.  Thus far guidance has
been somewhat inconsistent regarding potential interaction of
amplifying Canadian flow and the core of the eastern U.S. trough
by Wed.  Farther westward, solutions are still in the process of
resolving the details of an upper low that reaches near/just north
of Vancouver Island by Wed, composed of energy from two separate
streams.  What happens to this feature by days 6-7 Thu-Fri is
probably the lowest-confidence part of the forecast. 
Possibilities range from a fairly cohesive low/shortwave whose
timing varies by solution to some energy ejecting ahead while
lingering energy remains near the Pacific Northwest.  At least the
models/means share the common idea of a general areas of surface
low pressure over/near the northern Plains late in the period.

Based on reasonable pattern agreement and lower predictability of
most detail differences, the updated forecast reflected an
operational model blend for days 3-5 Mon-Wed.  The 18Z GFS/12Z
ECMWF had majority weighting while the blend utilized lesser input
from the 12Z UKMET/CMC.  Increasing divergence of some details
across the Northern Tier in particular led to slightly more than
half total weighting of the 18Z GEFS/12Z ECMWF means by day 7 Fri.

Rausch


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