Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1050 AM EDT Fri Jun 29 2018
Valid 12Z Mon Jul 02 2018 - 12Z Fri Jul 06 2018
...Extended period of above average temperatures expected for much
of the central and eastern U.S...
...Heavy rain threat for the northern Gulf Coast...
...Overview...
Models and ensembles remain in good agreement next week with the
upper pattern that will feature an expanding upper high that will
likely stretch from southern California northeastward to the
Appalachians. This will support well above average temperatures
over the eastern Great Lakes/Northeast/Mid-Atlantic with daily
record highs likely to be broken. One upper low will linger over
the Pacific Northwest and another off the Southeast coast with yet
another to the southwest of the upper high over the Gulf Coast. A
surface frontal boundary will lie astride the north/northwest side
of the expanding 594-597dm (maybe 600dm?) upper high -- roughly
from the Pacific/Interior West through the northern Plains into
the Great Lakes.
...Guidance evaluation/preferences...
Similar to the overnight issuance, the latest deterministic models
formed a good starting point for the forecast through a majority
of the period. The forecast for days 3-6 (Mon-Thu) was based on a
blend of the 00Z ECMWF/UKMET and 06Z GFS. Ensemble mean (00Z
ECENS/NAEFS) weighting was gradually increased from days 5-7 to
account for some increasing spread among the guidance. Although,
spread with respect to large scale features was relatively low for
that time period. A few points of contention existed by later in
the forecast period. The first was the track/timing/strength
differences with upper low off the Southeast coast, and a
compromise solution was favored at this time. Another was the
amplifying upper low off the Pacific Northwest by Wed-Thu as
energy slides south on the eastern side of the North Pacific
ridge. Models have shown a trend toward more amplification of this
feature, with the 06Z GFS leading the way. Given the amplified
ridging across the North Pacific and the central/eastern U.S.,
this trend makes sense conceptually, and was included in the
latest forecast update. Finally, the timing/amplitude of shortwave
energy crossing central Canada next Thu-Fri has implications for
frontal timing/position across the CONUS northern tier. A trend
among some of the guidance toward somewhat greater amplification
here resulted in a faster frontal timing across the northern
plains/Midwest for days 6-7 relative to the overnight issuance.
...Weather threats/highlights...
Ascendant upper high will force precipitation around its
circulation, firstly in the Northwest in conjunction with
southwest flow ahead of the upper low atop the surface boundary.
This will extend eastward across Montana into the Plains and Great
Lakes as the surface low wanders through southern Canada. A lead
front into the Northeast next Mon/Tue will do little but knock
temperatures a few degrees and perhaps touch off a shower/storm as
it dissipates by Wednesday. The Southeast into Florida may see an
increase in precipitation should the Atlantic upper low wobble
westward toward the coast later in the week. Finally, the most
significant precipitation may lie along the Gulf Coast from LA/MS
westward to the upper Texas coast as a weakness drifts westward.
ECMWF/UKMET seemed a bit aggressive/wet but with plenty of
moisture to tap (PW anomalies about +2 to +3 sigma) heavy rainfall
is certainly a possibility.
Temperatures will be near/above records (15 to 20 degrees above
average) in the Northeast early next week with a slow downward
trend (but still above average) by late in the week as the core of
the upper high moves southwestward. High temperatures are expected
to be at least 5 degrees above average for most of the contiguous
U.S. through much of next week, with widespread 90s for many
areas. Meanwhile, cooler than average temperatures are expected
for the Northwest and also along the Gulf Coast (especially max
temps) given extensive cloud cover and rain.
Fracasso/Ryan
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