Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sun May 26 2019
Valid 12Z Wed May 29 2019 - 12Z Sun Jun 02 2019
...Record/Dangerous Heat expected across the Southeast especially
through Thursday...
...Episodes of heavy rain possible for portions of the central
U.S...
...Overview and Guidance Evaluation/Preferences...
Most guidance continues to show a similar pattern evolution but
there are multiple persistent uncertainties regarding medium to
smaller scale details. Across the eastern Pacific and western
North America, expect mid-latitude Pacific jet energy to encroach
upon a split flow regime (mean ridging over western Canada and
troughing over the West), ultimately leading to a trough reaching
the Pacific Northwest coast by next Sun. The flow over the
eastern half of the Lower 48 should become less amplified, as
broadly cyclonic flow becomes established over northern areas and
the northern extension of the upper ridge initially centered over
Florida flattens. This subtropical ridge will likely expand
westward across the Gulf of Mexico into northern Mexico by
Fri-Sun. The expected evolution should provide partial moderation
of the heat over the Southeast after Thu but one or more
shortwaves ejecting from the western mean trough could maintain
the threat for periods of significant rainfall over the Plains and
vicinity.
Forecast details remain complex for the system initially over the
Midwest on Wed. There is a gradually improving signal that some
shortwave energy will remain over the Midwest into day 4 Thu
(supporting a frontal wave) but it remains to be seen what
proportion will be from the feature initially over the region the
prior day (with the rest ejecting northeastward) in addition to
trailing energy that rapidly moves into the region from the
southwest. Uncertainty over amplitude of southern Canada
troughing that may extend into the Great Lakes continues to plague
the forecast as well. The 00Z UKMET is on the extreme side of the
envelope for the combined fast timing and amplitude of the trough.
A blend consisting mostly of the other available models should
provide a reasonable starting point given the ongoing potential
for detail adjustments in future runs.
By Sat-Sun the 00Z ECMWF becomes one of the stronger/faster
solutions within the ensemble spread for the upper ridge that
crosses southern Canada, with that run actually becoming an
extreme to marginal outlier solution over Ontario by Sun.
Ensemble means including from the ECMWF favor slower progression.
The combination of flow details across southern Canada/northern
tier states along with one or more shortwaves ejecting from the
western U.S. mean trough will determine the day-to-day position of
the front dropping into the northern Plains late this week and
possibly continuing southward thereafter.
Across the West the past cycle or two of guidance has become more
split as to locating the core of the mean trough on any particular
day, in contrast to prior emerging consensus that had been over
the Great Basin. Toward the end of the period the 00Z GFS once
again brings stronger troughing to the West Coast while the 06Z
GFS and other models/means are easily within typical error ranges
for extended time frames.
Based on the current array of guidance the updated forecast
started with the 06Z GFS and 00Z ECMWF/CMC with slight input from
the 06Z GEFS/00Z ECMWF means early in the period and then trended
toward a nearly even model/mean weight for next weekend.
...Weather Highlights and Hazards...
High temperatures reaching 10-20 deg F above average across much
of the Southeast should continue through Wed-Thu with readings
likely nearing or surpassing 100 deg F for many areas along/below
the fall line. These temperatures will combine with moderate
humidity levels to produce dangerous heat index values that could
briefly approach or exceed 105 deg over some areas. Daily records
for highs/warm lows will be possible. In less extreme fashion some
of this heat should spread northward into parts of the
Mid-Atlantic Wed-Thu. A front will provide a cooling trend later
in the week but temperatures over and near the Southeast may still
be 5-10 deg F above normal through next weekend. The mean trough
aloft over the West will keep the Great Basin/central Rockies on
the cool side for most of the period but with anomalies somewhat
closer to normal than the minus 10-15 F departures for highs (also
extending into the High Plains) forecast on Wed. The Northwest
will see temperatures about 5-12 deg above normal for most of the
period.
Heavy rainfall will be a threat across portions of the
Southern/Central Plains and perhaps the Mid-Mississippi Valley
during the upcoming week. The front trailing from the initial
Midwest system Wed could produce widespread showers and
thunderstorms with potentially heavy rainfall and severe weather
across portions of the Southern Plains, with localized heavier
rainfall amounts possibly extending northeast along the front into
the Mid-Mississippi Valley. One or more shortwaves ejecting from
the western trough aloft along with the northward progression of a
weakening warm front could once again enhance the rainfall
potential for portions of the Southern/Central Plains and possibly
into the Mississippi Valley Fri into the weekend. Across the West,
expect primarily terrain focused precipitation from the Great
Basin to the central/northern Rockies through much of the week
into the weekend with the passage of several upper-level
shortwaves.
Rausch/Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml