Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1122 AM EDT Fri Jul 13 2018
Valid 12Z Mon Jul 16 2018 - 12Z Fri Jul 20 2018
...Overview...
The flow pattern across the contiguous U.S. will become a bit more
amplified during the medium range period with an upper-level ridge
expanding across the Great Basin/Four Corners region and a series
of upper waves gradually carving out a trough across the Great
Lakes/Northeast. Heights are expected to fall across the Pacific
Northwest by late next week as an upper low digs south from the
Gulf of Alaska toward the region.
...Guidance Evaluation and Preferences...
With shortwave energy passing south of Hudson Bay and skirting the
Upper Midwest/Great Lakes days 3-5, models showed a range of
solutions with respect to both timing and amplitude. The GFS and
ECMWF were generally well centered within the spread, while the
00Z Canadian/UKMET were displaced from the consensus by about next
Wednesday over western Canada (UKMET) and southeastern Canada
(Canadian). Based on the gradually increasing flow amplification
through the period, the preference was to go near or slightly
slower/deeper than the ensemble mean consensus and this was
reflected in the WPC forecast. Uncertainty increases by next
Thu/Fri with the amplitude and evolution of troughing into the
Ohio Valley (GFS most amplified) but the upstream/downstream
ridging over the southern Rockies/western Atlantic, respectively,
would support a slower/deeper trough or possibly a closed low
along 85-90W by the end of the period into next weekend. However,
models can have a tendency to overamplify the longer range and
trend weaker as lead time decreases.
Farther west, with a shortwave/upper low diving southeast across
the Gulf of Alaska mid to late next week and lowering heights
across the Pacific Northwest, there is uncertainty as to how deep
the feature will be and whether it will dig farther south or move
more quickly eastward. Ensemble means were in general agreement at
12Z next Fri on showing a mean trough axis across British Columbia
while the deterministic models (and manual forecast) mostly showed
a closed low.
The WPC forecast initially based on a multi-model deterministic
blend during days 3-5 (Mon-Wed), with most weight placed toward
the ECMWF and GFS. By days 6-7 (Thu-Fri), weighting of ensemble
means (ECMWF and GEFS) was gradually increased to comprise a
majority of the weighting.
...Weather Highlights/Threats...
Showers and storms with locally heavy rain will accompany a cold
front across the Ohio/Tennessee Valleys to the
Southeast/Mid-Atlantic/Northeast Mon-Wed. Monsoonal moisture will
also produce scattered afternoon/evening thunderstorms across from
the southern Great Basin/Southwest to the southern/central Rockies
through the next week, with locally heavy rain possible especially
early in the week (the building upper-level ridge may somewhat
reduce the coverage of convection by later next week).
Temperatures will initially be 5 to 15 deg F above average across
the Northeast ahead of the cold front early next week, with a
decrease closer to seasonal averages by mid to late next week
(which are near/at the maximum for the year). In the wake of the
cold front, high temperatures are expected to be 5 to 10 deg below
average across the central/northern plains and portions of the
Midwest through much of next week. Finally, hot temperatures
across the Pacific Northwest (highs 5 to 15 deg above average)
should moderate some by later next week as a Pacific cold front
moves inland.
Fracasso/Ryan
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes 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
http://www.wpc.ncep.noaa.gov/heat_index.shtml