Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Mon May 28 2018
Valid 12Z Thu May 31 2018 - 12Z Mon Jun 04 2018
...Record heat expected across the southern Plains & western Gulf
Coast...
...Post-Tropical Cyclone Alberto tracks from the Great Lakes into
Canada...
...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...
Already in the first half of the period there are some continuity
adjustments/spread with the combination of ejecting West Coast
trough energy and incoming flow that amplifies toward the West
Coast. There is a strong signal from the 00Z/06Z models/means
that the leading feature will track inland faster than previously
forecast. A compromise among the latest available operational
runs (06Z GFS and 00Z ECMWF/UKMET/CMC) reflects this scenario
without getting too far on the faster side of the envelope that is
represented by the 00Z GFS/ECMWF. Given the lack of consistency
in the guidance over the past couple days, a similar approach
provides the best risk-managed forecast for the incoming eastern
Pacific trough--between the deeper 00Z GFS/CMC and flatter 06Z
GFS/00Z ECMWF.
During the latter half of the forecast expect upper troughing to
deepen somewhat as it slowly settles over or just inland from the
West Coast, with a model/ensemble mean blend representing
consensus well. By day 7 Mon the 06Z GFS becomes the fast extreme
with its embedded upper low. Farther east the main question marks
will be to what extent and where eastern U.S. trough energy forms
a closed low and then how the ejecting western shortwave may
interact with the initial energy over the East. Thus far the
GFS/CMC are more aggressive with a closed/westward feature by the
weekend and also allow the upstream energy to hold the overall
feature fairly far westward into early next week.
Teleconnection-favored flow suggests the GFS cluster could be a
little too far west with the overall mean trough but the blocky
pattern could lead to a slower/less open evolution than advertised
by the ECMWF/ECMWF mean. Thus an even blend among the two
clusters would appear best at this time.
...Weather Highlights and Threats
Some combination of Post-Tropical Cyclone Alberto's energy aloft
and an approaching shortwave will likely drop southeast into the
Mid-Atlantic States, setting the stage for potential heavy
rainfall over parts of the Mid-Atlantic region and vicinity.
Uncertainty over whether and where upper flow may close off a low
tempers confidence in location/extent of any heavy rain. Abundant
moisture along/ahead of a frontal boundary will also provide a
focus for some episodes of locally heavy rainfall over
central/southern latitudes of the South and East. A shortwave
ejecting from the West Coast should generate some rainfall over
northern parts of the West late this week and locally heavy
convection over the Plains/MS Valley Fri into the weekend. This
energy aloft may play a role in rainfall distribution/intensity
over the East toward Sun-Mon. The upstream trough arriving at the
West Coast should bring some areas of precip to the Northwest.
The dominant focus of the temperature forecast will be on a
multi-day southern Plains heat event whose influence may extend at
times westward into the Rockies and eastward into the lower MS
Valley. 100-110F readings should be most common in the southern
High Plains/lower Rio Grande. 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. 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.
Rausch/Roth
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