Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1032 AM EDT Sat Jun 23 2018
Valid 12Z Tue Jun 26 2018 - 12Z Sat Jun 30 2018
...Overview and guidance/predictability assessment...
The model guidance shows good agreement on a shift towards
increasing western troughing/increased ridging near the
Mid-Atlantic states by the end of the period. There are some
detail issues in regards to how strong the western trough gets and
how closed the system moving through the Great Lakes on Wednesday
might be as it moves over the top of the Southeastern ridge, but
overall agreement is so good that confidence is above average in
the forecast.
The days 3-7 pressures, winds, and 500 hPa heights are based on a
deterministic blend of the 00z Canadian, 00z ECMWF, 00z UKMET, and
06z GFS through Thursday. For Friday into next Saturday, slowly
increased the contribution of the 00z NAEFS mean and 00z ECMWF
ensemble mean to compromise 40-45% of the distribution by the end
of the period. The remainder of the weather grids are based on a
more even blend of the National Blend of Guidance, WPC continuity,
and the NAEFS ensemble mean guidance. The plan for the days 4-7
QPF is to use an even blend of the 12z National Blend of Models,
06z GFS, 00z ECMWF, and 00z Canadian guidance. Should the 12z GFS
prove useful, it could be included.
...Weather threats/highlights...
The system tracking east-northeast from the northern Plains/upper
MS Valley Tue onward will likely produce one or more episodes of
locally heavy rainfall. Best potential for areas active
convection should exist from the upper-mid MS Valley through the
Great Lakes and Ohio Valley. Some pockets of moderate to heavy
rainfall may extend into the Northeast but should generally be
more scattered as the system weakens. The trailing front
progressing across the Northern Tier will stall over the Plains
with at least a portion likely to return north as a warm front by
Friday into next Saturday. The combination of this low-level
focus and increasing tendency for shortwave impulses to eject from
the evolving western trough aloft may promote periods of
convection/rainfall--some potentially heavy--from the northern
Rockies eastward by late week/weekend. The western trough should
also bring unsettled weather to other parts of the northern half
of the West though with lighter amounts. Meanwhile expect periods
of convection over the Southeast/Florida during the period with
some heavy activity possible. A gradually weakening front over
the region will provide some added low-level focus during the
first half of the period.
The forecast is still on track for hot/humid conditions to become
established over an increasingly large portion of the
central/eastern states. Locations from the central Plains to Mid
Atlantic/Northeast will have the best potential for seeing one or
more days with min and/or max readings reaching as high as 10-15F
above normal, with possible records appearing to be confined to
warm overnight lows from Wednesday onward. Due to increased
low-level moisture within a more summer-like regime, highest heat
index values should expand from the South early in the period to a
large area centered over/near the mid MS Valley as well as
southern Mid Atlantic/Southeast. The Great Basin/Four Corners
region will see warm temperatures but with some gradual cooling
later in the period with the arrival of the western trough. This
feature should bring coolest anomalies (5-15F below normal for
highs) into the northern parts of the West by next Saturday.
Roth/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