Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1134 AM EDT Sun May 27 2018
Valid 12Z Wed May 30 2018 - 12Z Sun Jun 03 2018
...Remnants of what is now Subtropical Storm Alberto expected to
bring heavy rain potential to portions of the Tennessee/Ohio
Valleys through the end of the week...
...Synoptic Overview and Model/Uncertainty Assessment...
While there was some suggestion yesterday that the upper flow
across North America might become a little less blocky during the
medium range, it appears that trend was not meant to be. Look at a
hemispheric perspective this morning, it now appears as if a
number of strong and persistent height anomalies will continue to
dominate the flow regime, keeping the slowing the progression of
any smaller scale systems. Alberto is forecast by the NHC to be a
tropical depression at 12Z on day 3 (Wed) across the Ohio Valley,
before becoming post-tropical as the surface low moves north into
the Great Lakes before eventually becoming absorbed into a
northern stream frontal boundary. For the latest information and
forecast for Alberto, please refer to products issued by the NHC.
A block across the northern Atlantic Ocean, comprised of negative
height anomalies east of the Canadian Maritimes and ridging south
of Greenland will prevent the combination of Alberto and the
northern stream energy from progressing eastward, and it appears
that the only place the system will have to go is southeastward,
with consensus among the guidance now that the shortwave will dig
into the Mid-Atlantic region, possibly even cutting off for a
period of time. Models also show a general consensus that a new
surface low should develop by Fri or Sat near the Mid-Atlantic
coast as the upper trough digs into the region. At this time,
model variations surrounding the evolution of these features are
not particularly large, with the primary differences by days 6-7
related to the amplitude of the eventual trough or cutoff low over
the Mid-Atlantic.
Farther west, a strong upper ridge appears set to expand across
the southern plains over the by later this week, with a ridge axis
extending north across the Upper Midwest and Great Lakes.
Meanwhile, the evolution of an upper trough along the West Coast
by Wed still remains somewhat uncertain, although there have been
some notable trends. The persistence of the blockier pattern would
seem to favor a slower progression of systems, with more potential
for amplification, and this has been a trend among some of the
models/ensembles over the past day. A sizable number of CMC
ensemble members had previously shown a slower and more amplified
upper trough across the western U.S. late this week, and this
morning a notable number of solutions have trended in that
direction, including the ECMWF, which was much more progressive
with this feature yesterday. The slowest/most amplified solution
is the 00Z CMC, which at this time cannot be ruled out given the
trend and the broader pattern. At this time, a compromise is
likely warranted, but with a bit more weight placed toward the
slower and more amplified solutions. By later Sat into Sun, the
system will likely begin to move east toward the plains, with the
degree of amplification a bit uncertain at this time given the
broad anticyclonic flow across the central U.S.
The WPC forecast was initially based on a heavily deterministic
multi-model blend including the 00Z ECMWF/CMC and 06Z GFS during
days 3-4. By days 6-7, ensemble weighting (00Z ECENS/NAEFS) was
increased to a majority of the blend, with a bit more weight
placed to the NAEFS given its somewhat slower/deeper solution with
the western U.S. system.
...Weather/Threats Highlights...
It should be a rather wet period for much of the Ohio Valley and
Great Lakes region as Alberto moves through the region on
Wednesday/Thursday. Abundant tropical moisture spilling into the
southern and eastern states will also raise rainfall chances
through much of the week. In these latter locations, it will
likely be more difficult to pinpoint where isolated heavier
amounts will occur given unknown forcing mechanisms and placement
of mesoscale features. Farther upstream, highly diffluent flow in
advance of the western U.S. trough will foster the development of
heavy rainfall over the northern Rockies into the Dakotas. This
signature slowly moves eastward into the Upper Midwest by the
weekend.
Heat will dominate the forecast throughout much of the week. While
locations along the West Coast can expect readings near or
slightly below climatology due to the deep upper trough, this will
truly be the anomaly in the pattern. Widespread temperature
records are likely from the Great Plains eastward, particularly
for the warm overnight minima. The hot spot should be the Southern
Plains where numbers may exceed 105 degrees along the Rio Grande
while century degree readings also encompass much the region
surrounding the Red River. A true heat wave is in store as several
days of such conditions are likely. Even surrounding locations
will get in on the action as widespread 90s are expected across
the central Plains as well as the lower Mississippi Valley.
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