Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Fri Jul 06 2018
Valid 12Z Mon Jul 09 2018 - 12Z Fri Jul 13 2018
...Overview/Guidance Evaluation/Preferences ...
A broad mid/upper-level ridge will remain fixed over a good chunk
of the nation with general 2 to 2.5 standard deviations above
climatology observed in 500-mb height fields. This 594-dm
anticyclone should periodically be disturbed by incoming Pacific
features as well as amplification within the northern stream which
generally straddles the international border with Canada. While
solutions agree on timing of the Pacific shortwave crossing into
the Pacific Northwest and southern British Columbia on Tuesday
morning, they continue to struggle across other areas of
downstream amplification. Models seem to take turns showing local
invigoration of the flow with various models exhibiting such
behaviors. Most recently, the 00Z GFS shifted back toward a
stronger upper trough sweeping through southern Ontario down into
the upper Great Lakes on Monday which was seen in the 12Z cycle.
Additionally, the 00Z GFS has struggled with a remnant upper low
spinning off the Carolinas showing slow, but general strengthening
while lifting up along the Gulf Stream. The 00Z/12Z UKMET has
depicted a similar scenario but with more eastward displacement.
Overall, the effects of these disturbances on the synoptic-scale
pattern will be to reduce the strength of the persistent 594-dm
ridge across the nation. The guidance occasionally show
re-establishment at this strength but vary in details. To support
the above assessment, a multi-model approach was utilized with
reduced influences of operational solutions beyond Day
5/Wednesday. While ensemble means sufficiently dampened forecast
details, many aspects of the pattern remain unresolvable as such
time ranges.
Weather threats/Highlights...
The upper ridge should keep numbers above average across a vast
majority of the country. In terms of forecast departures, the
Pacific Northwest and upper Intermountain West can expect
temperatures to diverge around 10 to 15 degrees from early/mid
July climatology by the middle to end of next week. This would
allow for upper 90s to thrive over much of this region with 70s
and 80s more commonplace closer to the coast and into Seattle, WA.
Meanwhile, slightly below average readings are in the forecast
from the Four Corners eastward across the Southern Plains and into
the Gulf Coast. This will be in response to enhanced cloud cover
and daily precipitation threats, particularly within the active
southwestern U.S. monsoon. Besides these locations, showers and
thunderstorms are expected across the north-central U.S. depending
on how amplified the flow becomes. Some residual downstream
precipitation threats would be likely as the frontal zone advances
and occasionally meanders. Overall, difficult to say where the
heaviest rainfall will occur during the period although model
numbers are fairly conservative in a 24-hour sense.
Rubin-Oster
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