Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Fri Jun 22 2018
Valid 12Z Mon Jun 25 2018 - 12Z Fri Jun 29 2018
...Overview and guidance/predictability assessment...
The majority of model and ensemble guidance agrees fairly well
with the large scale evolution during next week. Expect upper
ridging initially over lower latitudes of the contiguous U.S. to
expand its influence over the eastern half of the country with
time, first with the departure of a Northeast/Mid Atlantic trough
and then behind an upper low/trough tracking northeastward from
the High Plains. A vigorous system entering western Canada will
continue eastward while a ridge building over the eastern Pacific
by the latter half of the week will support an upper trough that
develops along or more likely a bit inland from the West Coast.
For the days 3-5 Mon-Wed period an operational model blend (18Z
GFS/12Z ECMWF and somewhat lesser weight of the 12Z UKMET/12Z CMC)
either represented consensus or provided a good intermediate
solution in the case of more pronounced differences. The latter
situation applied to the departing East Coast trough. The 12Z
ECMWF and many of its ensemble members showed a faster departure
than other guidance while the GFS had adjusted to the slower side
of the spread. Both models have displayed some run to run
variability so a compromise appeared best. New 00Z runs thus far
seem to support that idea with the GFS trending faster than its
12Z/18Z runs but the GFS/UKMET/CMC remaining slower than the 12Z
ECMWF to varying degrees. Guidance behavior and spread holds
confidence down to moderate levels for this feature. Differences
for the system tracking from the Plains into the Great Lakes and
upstream southern Canada low pressure/front trailing into the
northern Plains-western U.S. were generally within typical
guidance error for days 3-5 forecasts.
By days 6-7 Thu-Fri the forecast incorporated 30-60 percent
ensemble guidance by way of the 18Z GEFS/12Z ECMWF mean as detail
uncertainty increased, though the operational runs were close
enough to the means to maintain some weight through the end of the
forecast. The 12Z CMC was phased out after Thu due to developing
a stronger than consensus eastern Canada/Great Lakes upper
trough--a trait that has carried into its new 00Z run. Regarding
the developing western trough, recent ECMWF runs have tended to
show the axis near the West Coast but teleconnections relative to
the upstream ridge's positive height anomaly center suggest the
trough axis could end up a little farther eastward. There are
still shortwave detail questions that lower confidence in
specifics within the more agreeable mean trough.
...Weather threats/highlights...
An area extending from the northern-central Plains into the
western Great Lakes will be one focus for heavy rainfall during
the period. The majority of convection will be supported by the
system tracking from the Plains into Great Lakes Mon-Wed. Some
moisture will eventually extend into the East but rain should
trend lighter as upper dynamics weaken/lift farther north. The
front moving into the West early in the week may bring
light/scattered precipitation to extreme northern parts of the
West and then some light-moderate rainfall over the northern
Plains. As this front stalls, it may become a more prominent
focus for convection over the northern tier states toward the end
of the week. This latter potential episode will have to be
monitored for any overlap with locations receiving significant
rainfall earlier in the week. Meanwhile some areas over the
Southeast/Florida may see one or more days of locally heavy
rainfall in diurnally favored convection. A cold front dropping
into the Southeast and eventually weakening will likely enhance
showers/thunderstorms at times.
After eastern U.S. surface high pressure departs into the Atlantic
after Tue, the expected pattern evolution at the surface and aloft
should promote an expanding area of heat/humidity over the
central/eastern states. Greatest anomalies for min/max
temperatures (plus 10-15F) by Thu-Fri should extend from parts of
the Central Plains through the MS Valley into portions of the
Great Lakes/OH Valley. Dew points reaching well into the 70's F
over some areas may contribute to high heat index values. Another
area of persistent warmth albeit with lower humidity should be
over the Great Basin/Four Corners region, with min temps tending
to be somewhat more above normal than maxes. The initial High
Plains system will lead to cool daytime highs on Mon. The
Northwest will see near to below normal highs behind the early
week front and then with upper troughing moving in late in the
week.
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