Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM 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...

Much of the central and eastern U.S. will have a multi-day threat
of convection/heavy rainfall; leading to an elevated risk for
flooding, especially for the South Appalachian region. 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. A steady influx of
Gulf moisture feeding into this system will increase precipitation
efficiency. Areal averages over the course of 7 days are forecast
to be 2 to 4 inches, with isolated amounts exceeding 5 inches for
the Southeast and Mid-Atlantic regions. Some areas of
moderate-heavy rainfall may extend into parts of the Great
Lakes/Appalachians/Mid-Atlantic. 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.

Much of the Southwest will continue to be hot although
temperatures will be moderating through the extended periods. The
ridge over the West will become slightly flatten as a trough/low
pressure persists near the the Pacific Northwest Coast. By late
week this energy should support Northern Plains low pressure with
a corresponding increase in convection. 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...

The latest guidance continues with the synoptic pattern of a ridge
over the West, a digging trough over the central U.S. and a
retrograding ridge over the Atlantic/East Coast.

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.

Campbell/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