Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
211 AM EDT Tue Jul 17 2018
Valid 12Z Fri Jul 20 2018 - 12Z Tue Jul 24 2018
...Dangerous heat wave expected for the southern plains by later
this week...
...Overview...
Model consensus remains high that a pattern change will be ongoing
at the start of the medium range (Fri) across the contiguous U.S.,
with an upper-level ridge expanding form the Great Basin to the
southern plains, and amplification of a trough across the Great
Lakes/Ohio Valley. Expect a surface low pressure system to
accompany the amplifying upper trough across the Midwest/Great
Lakes Fri through the weekend, with a surface frontal boundary
trailing into the southern plains. Additional shortwave energy
crossing the Pacific Northwest and western Canada Fri-Sat will
bring a Pacific cold front into the northern Great Basin and
Rockies, reaching the plains by next Tue.
A look at hemispheric teleconnections shows support for this
pattern evolution across North America, further increasing
confidence in the impending pattern change. An anomalous and
persistent Rex block across eastern Asia favors upper ridging
across the southwestern and south central U.S., with a relatively
strong vortex across the Arctic, and an equatorward extension in
the form of favored troughing across Hudson Bay and the Great
Lakes. Given the persistent nature of the aforementioned Rex
block, would expect that the upcoming pattern shift across North
America will not be a transient one.
...Guidance Evaluation and Preferences...
Models showed good agreement with respect to the large scale flow
pattern changes described above, and differences were largely
confined to timing/amplitude differences with individual features.
Thus, a multi-model deterministic blend (including the
ECMWF/GFS/UKMET/CMC) served as a good basis for the forecast
through day 5 (Sun). One of the bigger areas of disagreement among
the guidance is in the Pacific Northwest by early-mid next week,
with some question as to how much influence the upper ridge will
continue to exert, and whether additional Pacific shortwave energy
will be able to lower heights once again (as shown by the GFS). By
this time period (days 6-7) weighting of ensemble means
(ECENS/NAEFS) was increased to comprise a slight majority of the
forecast, to account for moderate spread among the guidance by
that time.
...Weather Highlights/Threats...
The frontal system crossing the Great Lakes/Ohio Valley Fri-Sat
will be accompanied by scattered to numerous showers and
thunderstorms, some of which could produce locally heavy rainfall.
A lingering stationary/warm front will also produce convection
across the Southeast. The strongest signal among the guidance for
heavy rainfall continues to be along the immediate Southeast
coastline Fri-Sat, in close proximity to the frontal boundary. The
upper trough is expected to linger across the eastern U.S. into
early next week as the frontal boundary begins to wash out,
keeping showers and thunderstorms a possibility for much of the
East. Farther west, the frontal system crossing the northern
Rockies Fri-Sat will gain access to deeper moisture by the time it
reaches the northern plains Sun-Mon, bringing locally heavy
convection to the central/northern plains.
The heat across the southern plains will be one of the bigger
stories during the medium range. With the upper ridge building
overhead and a strong subsidence inversion likely in place,
temperatures will soar well past the century mark for many areas.
High temperatures are expected to be 5 to 15 deg F above average
across much of the southern plains from Fri into next week, with a
number of record high temperatures and record high minimum
temperatures potentially in jeopardy. High dew points will combine
with the hot temperatures to produce dangerous heat index values
over 110 deg F for many locations. Low temperatures near 80 deg
for many areas will add to the potentially hazardous impacts of
the heat.
Ryan
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