Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Wed Jun 20 2018
Valid 12Z Sat Jun 23 2018 - 12Z Wed Jun 27 2018
The WPC forecast during the medium range period for this weekend
through the middle of next week remains fairly consistent from the
previous forecast issuance, with above average forecast confidence
through Monday given overall synoptic scale agreement with
deterministic and ensemble mean guidance. In terms of
differences, the 00Z UKMET appeared slower with the Northeast low
and stronger with the low developing over the central plains
Sunday and into Monday. The CMC becomes more amplified with the
upper trough crossing the Great Lakes and then New England by
Monday compared to the model consensus, but it has some support
from some of the GEFS members. The EC ensemble members are
holding onto the closed upper low over that is initially over the
western High Plains longer than the GFS and CMC ensemble members,
and overall the EC mean is more in line with the deterministic GFS
and ECMWF compared to the 6Z GEFS mean. The updated WPC forecast
was derived primarily from deterministic GFS/ECMWF/CMC through
this weekend with greatest weighting to the ECMWF given less model
run-to-run variability during that time, and gradually increasing
contributions from the EC mean and some of the GEFS mean by the
end of the forecast period. The previous discussion from
overnight is appended below for reference.
...Guidance and Predictability Assessment...
Guidance overall shows above normal agreement with the flow
pattern evolution for the medium range, bolstering forecast
confidence. Nrn stream flow features a persistent central Canada
upper ridge book-ended by amplified wrn Canada/NW U.S. and ern
Canada/NE U.S. cooling troughs. A modestly progressive pattern
underneath over the CONUS has some tendency for system
amplification. Energy crossing from the Rockies to the central
U.S. shows potential for deepening and/or closing off, moving
through a region of relatively weak upper flow beneath the
Some recent guidance including the 18/00 UTC GFS runs and
yesterdays 00 UTC Canadian show potential for closing off a
Northeast trough Mon/Tue as per nrn stream energy split. However,
the last two Canadian runs backed off from flow
separation/cyclogenesis and the last few ECMWF/UKMET runs and
ensembles support more modest amplitude. The amplified central
Canadian ridge position offers some 18/00 UTC GFS support, but
they are less likely scenarios. The 12 UTC ECMWF on the other hand
may be too progressive late in the period given upstream amplitude.
Accordingly, the WPC medium range product suite was primarily
derived from a composite blend of reasonably clustered guidance
from 12 UTC GFS/ECMWF and GEFS/ECMWF ensemble means. This
maintains excellent WPC continuity.
...Weather Highlights and Threats...
Lead OH Valley trough/low energy Saturday shears northeastward
over the Northeast Sunday. Post-frontal high pressure will nudge a
moderating trailing front far down over the ern U.S. next week.
Upstream, two main amplifying shortwave troughs dig from the NW
U.S. southeastward over the Rockies to the north-central states.
These systems produce organized surface lows along a main frontal
boundary from the Plains to the MS/OH/TN valleys and the East.
Convective complexes/heavy rain focus along the front and into the
warm sector fueled by pooled moisture. A late season cooling
front/modest pcpn shows increased promise to work more earnestly
over the NW U.S. next week with supporting mid-upper level trough
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities are found at: