Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Fri Jun 22 2018
Valid 12Z Mon Jun 25 2018 - 12Z Fri Jun 29 2018
...Overview and guidance/predictability assessment...
Models and ensembles continue to agree that the split upper-level
pattern across North America will gradually become more
consolidated by the middle of next week as heights are forecast to
rise over the central and eastern U.S. First, the
positively-tilted trough digging down into New England on Monday
should move off into the Canadian Maritimes Monday night. The GFS
shows a surface low closer to the New England coast on Monday
while the GEFS centers the low farther east, which is in better
agreement with both the ECMWF and Canadian models. Meanwhile,
another compact shortwave trough/low should emerge from the
northern Rockies into the northern Plains. This system should
help the development of mesoscale convective systems over the
central Plains where excessive rainfall is possible Monday into
Tuesday. Models agree that the shortwave will lift toward the
Great Lakes and weakens will time. The intensity of the
precipitation should decrease as the system moves into the Midwest
and Great Lakes during midweek next week. By late next week,
there is excellent agreement on a Pacific front moving through the
northwest U.S. into the northern tier where moisture supply
appears somewhat lacking.
Therefore, the WPC medium range morning product suite was derived
from a composite blend of the 00Z ECMWF, 06Z GFS, and a small
portion from the 00Z Canadian model, trending toward their
ensemble means through Day 7. This maintains good WPC continuity.
...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. A warming trend is expected for the central and eastern
U.S. next week where temperatures are forecast to reach the
century mark at some locations.
Kong/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