Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Mon Jun 25 2018

Valid 12Z Thu Jun 28 2018 - 12Z Mon Jul 02 2018

...Overview...

Once the amplified western trough/eastern ridge pattern becomes
established late this week, guidance has displayed increased
divergence over the past day or so regarding specifics of how/when
western trough energy ejects downstream.  Thus confidence has
declined for the corresponding surface evolution over the central
U.S. by the Sat-Mon time frame.  Meanwhile guidance is diverse and
variable with what type of surface reflection could evolve
near/offshore the East Coast in connection with energy that
originates from an upstream convective complex in the short range.
 Once the core of eastern ridging settles over the Ohio Valley/Mid
Atlantic region, additional energy aloft and corresponding surface
feature could evolve to the south near the Southeast
Coast/Florida. 


...Guidance evaluation/preferences...

One source of the guidance spread for ejection of western trough
energy lies with the timing of an upper low initially near the
eastern Aleutians/Alaska Peninsula.  The 12Z ECMWF/ECMWF mean were
the fastest to bring this feature around the strong eastern
Pacific ridge, likely aiding in faster ejection downstream and
resulting in a broader/more shallow mean trough across the
northern U.S. by the latter half of the period.  CMC runs are
slower with the upstream upper low but use other sheared energy
flowing around the ridge to kick out leading western trough
energy.  ECMWF-based guidance has actually trended toward faster
progression of height falls into the upper MS Valley and Great
Lakes late in the period--in contrast to the GEFS mean whose past
day of runs have trended stronger with the upper ridge covering
the northeast quadrant of the contiguous U.S.  GFS runs have been
fairly persistent in depicting the most amplified trough-ridge
pattern.  Continuity leaned about 2/3 in the GFS/GEFS/NAEFS
direction and see no reason to alter the philosophy at this time
given the further increase in guidance spread since 12-24 hours
ago and faster than consensus timing of the eastern Aleutians/AK
Peninsula upper low in the ECMWF/ECMWF mean.  In addition the 12Z
CMC mean was closer to the GFS/GEFS scenario than the CMC/ECMWF
idea.  However note that teleconnections relative to the core of
positive height anomalies over the eastern Pacific do offer
eventual potential for the eastern ridge to be somewhat less
pronounced than forecast by the GFS/GEFS.  One last note, looking
at guidance just beyond day 7 Mon suggests that early next week
may feature a retrogression of the eastern Pacific/western U.S.
pattern.  This transition may lead to a period of increased
uncertainty as well.

Spread and recent variability among models/ensembles do not
inspire confidence for the forecast near the East Coast.  The past
two ECMWF runs were wildly divergent for Atlantic low pressure
originating from a Plains convective complex.  CMC runs are
tracking low pressure off the northeastern coast while GFS runs
are quite diffuse.  Additional energy may gravitate toward the
Southeast Coast/Florida as the core of the eastern ridge aloft
reaches the Ohio Valley/northern Mid Atlantic.  Manual progs
maintain yesterday's WPC/NHC-coordinated depiction of a surface
trough/weak low near the Southeast Coast/Florida.

The first half of the forecast incorporated portions of the
12Z-18Z GFS/GEFS and 12Z ECMWF/ECMWF mean/UKMET, with less than
typical weight of the ECMWF due to its low confidence feature off
the East Coast.  The CMC was excluded due to northeastern
troughing that become fairly extreme compared to the ensemble
envelope.  The new 00Z CMC appears more reasonable with a trend
toward consensus.  The remainder of the forecast started with 2/3
total GFS/GEFS/NAEFS and 1/3 ECMWF mean.  


...Weather threats/highlights...

A system affecting the Northeast around the start of the period
will be accompanied by areas of rainfall, some of which may be
moderate to heavy.  Behind this system the focus for significant
rainfall will turn to an area encompassing the northern
Rockies/High Plains to upper Great Lakes and possibly extending
southward close to the central Plains.  A persistent frontal
boundary with one or more waves, along with shortwave energy
ejecting from the western trough aloft, will likely promote
multiple convective episodes which may produce heavy rainfall at
some locations.  Guidance spread for details aloft keeps
confidence low for the details of timing/location within a broader
pattern evolution that should support the heavy rainfall threat. 
Farther west the upper trough will bring scattered rainfall with
lighter amounts.  Meanwhile the Southeast/Florida should see areas
of locally heavy convection during the period.  Upper level energy
and possible surface reflection to the south of the deep layer
ridge over the Ohio Valley/Mid Atlantic may provide some
enhancement of this activity.

Expect a broad area of hot/humid weather across much of the
central-eastern U.S. during the period.  Highest temperature
anomalies of plus 10-20F should extend from the central
Plains/Midwest through the Northeast.  Daily records are possible
for both highs and warm lows, with warm low records likely to be
more numerous.  In addition humidity will be sufficient to yield
very high heat index values over many areas.  On the other side of
the temperature spectrum, much of the West and northern High
Plains will see multiple days with below normal highs.  The
northern Rockies and High Plains should see the most extreme
anomalies with highs as much as 10-15F below normal Fri-Sun.

Rausch

WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities are found at:

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4