Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Sat May 26 2018
Valid 12Z Tue May 29 2018 - 12Z Sat Jun 02 2018
...Subtropical Storm Alberto expected to bring heavy rainfall to
portions of the Gulf Coast and Southeast...
...Overview/Synoptic Assessment...
The flow pattern across North America will initially be somewhat
blocky early next week. Active/more progressive flow across the
northern Pacific will gradually translate downstream, however,
with the northern stream flow across the U.S. and Canada becoming
at least somewhat more progressive by later next week. Subtropical
Storm Alberto is forecast to be onshore across the Gulf Coast
states by day 3 (Tue), with the surface low moving north to the
Tennessee Valley by Wed, and the Ohio Valley by Thu. Please refer
to advisories issued by the National Hurricane Center for the
latest information and forecast for Subtropical Storm Alberto.
Farther north and west, a shortwave is expected to move from the
central/northern Rockies Tue morning into the northern
plains/Upper Midwest by Wed morning. By Thu as this shortwave
crosses the Great Lakes, some degree of interaction may occur with
the remnant circulation from Alberto across the Ohio Valley. By
next Fri-Sat, it remains unclear whether Alberto will be fully
absorbed into the northern stream or not, an outcome which will
likely depend heavily on the specific amplitude of the northern
stream wave. Models show some possibility for the combination of
northern stream energy and remnants of Alberto to form a cutoff
low near the Eastern Seaboard by next Fri-Sat.
An upper trough is expected to reach the West Coast by Wed-Thu,
with height falls reaching the Great Basin and perhaps the
northern Rockies by Thu-Fri. Additionally, an upper low traversing
the north Pacific/Gulf of Alaska is expected to move southeastward
toward the Pacific Northwest by next Fri-Sat.
While the northern stream becomes a bit more active by late next
week, guidance suggests with some degree of consensus that a ridge
should begin to build across the southern plains. Despite the
trend toward a more progressive northern stream, this appears a
bit tenuous, with a number of solutions showing the potential to
for the flow to move back toward a blockier setup by next weekend,
dependent to some degree of the amplitude of the ridging that can
develop across the central U.S., and whether an upper develops
near the East Coast.
...Guidance/Predictability Assessment...
The WPC forecast was initially based on a heavily deterministic
multi-model blend, including components of the ECMWF/GFS/CMC. The
00Z ECMWF solution seemed to fit most closely with the official
NHC forecast track for Alberto. Ensemble weighting was increased
from day 5 onward given some uncertainties in the amplitude of
shortwave energy moving from the northern plains to the Great
Lakes, and the degree of interaction with Alberto. Uncertainty
increases further by days 6-7 as model solutions differ
significantly as to what happens with the western U.S. trough. The
GFS/CMC have been persistent in developing either a highly
amplified trough or cutoff low across the Great Basin/Southwest,
while the ECMWF has been persistent in keeping the system more
progressive and quickly moving the energy east toward the northern
Rockies and northern plains. The 00Z ECENS/06Z GEFS means seems to
represent a reasonable compromise between the two scenarios, and a
middle ground solution is preferred at this time. Thus, weighting
of these ensemble means was boosted further by next Fri-Sat.
...Weather/Threats Highlights...
Subtropical Storm Alberto is expected to produce widespread heavy
rains generally near and east of the forecast track. Models
continue to show multi-inch rainfall totals possible during the
medium range across portions of the Gulf Coast states, Southeast,
and Tennessee Valley. ECENS/GEFS ensemble members continue to
suggest two precip maxima during the Tue-Thu time frame, on
extending from portions of Mississippi and Alabama north to
Tennessee and portions of Kentucky, and another across the
southern Appalachians, upslope flow along with the deep tropical
moisture could enhance rainfall rates. The northern extent of
heavy rainfall potential is a bit uncertain at this time, and will
depend on the exact track of Alberto and the degree of interaction
that occurs with the northern stream system. Additionally, the
northern Rockies and central/northern plains could see the
potential for a couple rounds of convection and locally heavy rain
through the period, with a couple shortwaves potentially
traversing the region, and a surface frontal boundary lingering in
the vicinity.
Above average temperatures are expected through much of the medium
range across the central U.S. Max temps on Tue could reach 10-15
deg F above average from the Upper Midwest to the Great Lakes.
From Wed onward, the southern plains will become the focus for the
greatest temperatures anomalies, with highs potentially reaching
10 to 20 deg F above average, and a number of record high
temperatures potentially in jeopardy.
Ryan
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