Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Tue Jun 26 2018
Valid 12Z Fri Jun 29 2018 - 12Z Tue Jul 03 2018
...Overview and guidance evaluation/preferences...
Model guidance this morning continues to forecast for a heat wave
to spread eastward from the central Plains into the East Coast by
this weekend while general troughing will keep conditions cool
over the western U.S. Model agreement remains good regarding a
broad upper-level trough and the associated cold front moving out
of the northern Rockies into the northern Plains through Day 5
Sunday. Thereafter, models begin to show differences on how an
upper-level closed low over the Gulf of Alaska evolves as it drops
towards the west coast of Canada early next week. The ECMWF has
been the fastest on bringing cyclonic vorticity inland across
British Columbia toward northern Idaho on Day 7 Tuesday while the
GFS keeps the cyclone center farther back along the coast of
British Columbia. The ECMWF has settled into the faster solution
for the last couple of runs while the GFS continues to show
inconsistency between its deterministic solutions and the ensemble
means (i.e. the GEFS). The GEFS has been comparable to the 00Z EC
ensemble mean with a weaker surface low center around Alberta
Canada on Day 7.
Therefore, the WPC morning package was based on a general
compromise between the 06Z GFS/06Z GEFS and the 00Z ECMWF/00Z EC
mean through Day 5. More weights were then given to the 00Z EC
mean for Day 6 and 7, with 70% assigned to the 00Z EC/EC mean on
Day 7.
...Weather threats/highlights...
Low pressure tracking northeast from the Plains into Canada from
Fri onward along with its associated fronts and upper shortwave
support will likely produce convective episodes capable of
producing areas of heavy rainfall. Best signal for highest totals
is across the northern Plains/Midwest/upper MS Valley/upper Great
Lakes. Southward progression of the cold front may bring
convection into the central Plains. Some activity may be strong
to severe as well. Check latest SPC outlooks as details become
more clear in coming days. Expect showers/thunderstorms to extend
farther eastward later in the period but likely with lesser
amounts as best upper dynamics lift into Canada. Meanwhile there
should be diurnally favored convection over portions of the
South/Florida through the period with some locally heavy amounts.
It remains a question mark to what extent upper energy/surface
feature(s) flowing around the deep layer high to the north may
enhance this convection.
A heat wave is expected to spread into the eastern U.S. this
weekend and only dampened a bit early next week with a better
chance of showers and thunderstorms. Within the large area of hot
and humid weather over the central/eastern U.S., highest min/max
temperature anomalies of plus 10-20F should extend from the
Midwest through the Great Lakes/Ohio Valley and Northeast. The
heat/humidity will gradually erode from the west over the course
of the period as a Plains cold front moves into the Great Lakes.
There are many locations that may see one or more days of min
temps at least close to daily record warm values. Daily record
highs should be less numerous and most likely confined to the
Great Lakes/Ohio Valley/Northeast during Sat-Mon. The humidity
will contribute to high heat index values. Below normal
temperatures will be most common over the West and High Plains,
with the northern Rockies/High Plains likely to see one or more
days of highs 10F or more below normal Fri-Sun. Ejection of the
initial western trough aloft will help readings trend to
moderately above normal levels over the central/southern parts of
the West by early next week.
Kong/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