Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
946 PM EDT Mon May 28 2018
Valid 12Z Fri Jun 01 2018 - 12Z Tue Jun 05 2018
...Record heat expected across the southern Plains & western Gulf
Coast...
Overview
~~~~~~~~
The expected trough-ridge-trough mean pattern compares favorably
to that expected from teleconnections relative to the core of
positive height anomalies which multi-day means show over the
northern Pacific near 150-160W longitude. This provides some
degree of confidence in the large scale forecast but
spread/variability for individual features within the mean flow
suggest a fair amount of uncertainty in the specifics. Meanwhile
relatively strong above normal height anomalies persisting across
the North Atlantic and Southeast Canada through the period will
correspond to blocky flow with the pattern possibly taking on the
shape of a Rex Block for a time across southeast Canada and the
East.
Guidance Evaluation/Preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The guidance shows better agreement than seen this time yesterday.
The main issue tonight is how quick/strong a pair of shortwaves
or closed low will be as they track across south-central Canada
Sunday and the Northwest on Monday. In both cases, the 12z ECMWF
was a strong/quick outlier. The system moving through
south-central Canada is likely to be weak as it moves over the top
of ridging in southeast Canada and in the Northwest it flies in
the face of its own ensemble mean, so used it minimally beyond
Sunday morning.
A compromise of the 12z Canadian, 12z ECMWF, 18z GFS, and 12z
UKMET proved useful for the pressures, 500 hPa heights, and winds
for Friday into Sunday. Thereafter, use an increasing percentage
of the 12z ECMWF/12z NAEFS ensemble means and rely somewhat less
on the 12z ECMWF than the 12z Canadian and 18z GFS
deterministicly. For the remainder of the weather grids, used a
more even blend of the deterministic and ensemble mean guidance.
An early draft of the days 4-7 QPF uses a four way blend of the
12z ECMWF, 12z UKMET, 18z GFS, and 00z National Blend of Models as
a starting point. If the 00z GFS proves useful, it could be
included into the mix.
Weather Highlights and Threats
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
An upper low/trough will likely drop southeast into the
Mid-Atlantic States and Carolinas, setting the stage for wet
weather for the Mid-Atlantic and Southeast states. A general
trough over the Northwest composed of a few active shortwaves
should generate some late season rainfall over northern parts of
the West late this week and locally heavy convection over the
northern and central Plains. This vorticity aloft may play a role
in rainfall distribution/intensity over the East early next week.
The dominant focus of the temperature forecast will be on a
multi-day southern Plains heat wave whose influence may extend at
times westward into the Rockies and eastward into the lower MS
Valley and western Gulf coast. Hot afternoon readings in the
100-110F range should be most common in the southern High
Plains/lower Rio Grande, with Saturday setting up to be the
hottest day. It is likely that record values for max/warm min
will be reached/exceeded on multiple days, with some of the
record-breaking heat also extending for a time farther
north/northeast. There should be some southward suppression of
the heat in Texas with time from Sunday into Monday as a front
settles southward, though it should rebound by Tuesday. On the
other hand, mean troughing aloft will tend to keep highs near to
below normal near the West Coast and by the weekend/early next
week over the East.
Roth/Rausch
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be 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