Extended Forecast Discussion...Amended
NWS Weather Prediction Center College Park MD
147 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 crosses the Great Lakes & New
England...
Synoptic Overview
~~~~~~~~~~~~~~~~~
A strengthening Rex Block near the 165th meridian West should lead
to downstream ridging across the eastern Pacific and troughing
across the Northwest. Farther east, relatively strong mid-level
height anomalies persist across the North Atlantic and Southeast
Canada through the period, keeping the flow pattern blocked across
the region and the polar jet stream suppressed, with a pseudo Rex
Block forming across southeast Canada and the East.
Guidance Assessment/Choices
~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is relatively good agreement through Saturday before the
27/12z ECMWF becomes a decided outlier in showing greater
progression with systems across the Lower 48. As the operational
ECMWF appears out of tolerance with its own ensemble mean in
southwest Canada, it is likely too progressive with the eastern
trough downstream.
For pressures, winds, and 500 hPa heights, a compromise of 12z
Canadian, 12z UKMET, 18z GFS, and 12z ECMWF was useful into
Saturday. Thereafter, used declining percentages of the 12z ECMWF
and bulked up the percentages of the 12z NAEFS mean & 12z ECMWF
ensemble mean. This led to greater progression of a front moving
into the West from Friday onward but otherwise maintained good
continuity with day shift.
The weather, dew point, temperature, cloud, and precipitation
chance grids were more evenly distributed between the above
deterministic and ensemble mean guidance. An early draft of the
precipitation forecast is composed of a blend of the 12z Canadian,
12z ECMWF, 18z/00z GFS, and the 00z National Blend of Models for
days 4-5, with days 6-7 an even blend of the 12z Canadian, 18z/00z
GFS, and the 00z National Blend of Models.
Weather highlights & Threats
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Post-Tropical Cyclone Alberto's circulation aloft, merged with an
incoming mid-level disturbance from the Westerlies, is expected to
drop southeast into the Mid-Atlantic States, setting the stage for
heavy rains in the northern Mid-Atlantic states. Abundant
tropical moisture spilling into the southern and eastern states
will keep rainfall chances above climatology through much of the
week; heavy rains are possible near the central Gulf Coast this
weekend near/ahead of an approaching cold front. Farther
upstream, highly divergent flow aloft in advance of the western
U.S. trough will foster the development of late season rainfall
for the Northwest and the northern Rockies, with the heaviest
precipitation expected near North Dakota.
Heat will dominate the forecast throughout much of the week,
outside the influence of upper troughs such as the Northwest and
Northeast. The hot spot should be the Southern Plains where
numbers approach 110 degrees along the lower Rio Grande while
century degree readings also encompass much the region surrounding
the Red River. A heat wave with forecast record high temperatures
and record high minima in the southern Plains and western Gulf
Coast is expected through the Medium Range period. Surrounding
locations get in on the action as widespread 90s are expected
across the central Plains as well as the lower Mississippi Valley.
Roth/Ryan/Rubin-Oster
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