Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sun Jun 30 2019
Valid 12Z Wed Jul 03 2019 - 12Z Sun Jul 07 2019
...Overview and Guidance/Predictability Assessment...
Latest guidance continues to show a typical summertime pattern
aloft over the lower 48 during the period. Progressive and low
amplitude flow will prevail across the northern states, between
southern tier ridging (with some day-to-day fluctuations in
strength) and an upper low tracking across central-eastern Canada.
A Rex block will persist over the North Pacific/Alaska. A couple
convection-focusing fronts over the northern states during midweek
should consolidate into one boundary by late in the week, and then
push somewhat farther south by the weekend in response to the
westerlies dropping gradually southward over the eastern half of
the country.
While there is above average confidence in the overall pattern
evolution, some of the embedded features have much lower
predictability/confidence. The primary forecast problems involve
how energy will eject from the upper trough over the Pacific
Northwest and details of flow over/west of western Canada in
response to the strong upper ridge/high that builds over mainland
Alaska. Both will determine northern tier shortwave specifics
influencing convective systems and frontal waves.
Operational models have still not decided on whether the initial
Pacific Northwest trough will eject as one coherent entity or in
multiple parts, with the scale of individual impulses so small as
to yield minimal confidence in any specific solution. The
guidance spread for energy over/near western Canada by Fri-Sun has
not improved over the past day. 00-06Z GFS runs (especially the
06Z version) are farthest east while the ECMWF/CMC display some
degree of consistency in closing off a low near the British
Columbia coast. However the ECMWF mean has backed away from the
operational run's closed low. Teleconnections relative to the
Alaska ridge's positive height anomaly center are fairly sensitive
to ridge location but at least suggest holding some upper
troughing westward of the GFS runs. Thus the primary emphasis
would be to phase out GFS input by late in the period.
The latest forecast starts with a consensus model blend early and
then evolves toward a mix of 00Z ECMWF/CMC and 00Z NAEFS/ECMWF
means late. GEFS means (and more so the 06Z run) offer potential
for a stronger shortwave to produce lower surface pressures over
parts of the northern U.S. during the weekend (like the 00Z ECMWF)
but this feature has such low confidence 6-7 days out in time that
the forecast uses the NAEFS to provide a more coherent blend.
...Weather Highlights/Threats...
A fairly broad area of the lower 48 from the Rockies eastward
should see one or more days of showers and thunderstorms. Best
potential for a heavy rainfall threat would be from the northern
Plains into Midwest, near an initial wavy front and second
approaching front. Some convection will likely extend farther
east/southeast toward the East Coast with locally heavy totals as
well. The fronts should consolidate into one by Fri and then push
southward/southeastward with time, providing a larger-scale focus
for organized convection into the weekend. Strong to severe
thunderstorms will be possible over some areas but with low
predictability for any specifics this far in advance.
Expect high temperatures to be 5-10F above normal near the East
Coast mid-late week with a gradual cooling trend by the weekend.
A few locations over the Southeast could approach daily records
for highs/warm lows around midweek. Heat index values over 100F
will be possible across much of the Southeast and southern
Mid-Atlantic. On the other hand clouds/rainfall should keep
daytime highs 5-15F below average most days over the northern High
Plains. Northwestern areas should see highs 5-15F above normal
Fri-Sun after near to below normal readings on Wed due to the
upper trough over the region.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml