Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
108 AM EDT Tue May 08 2018
Valid 12Z Fri May 11 2018 - 12Z Tue May 15 2018
...Pattern overview...
Ridging is expected to remain prevalent along the North American
west coast through much of the period, periodically flattened or
undercut by Pacific shortwave energy. Meanwhile, a broad area of
negative height anomalies will persist through the medium range
northeast of Hudson Bay. The area of broad cyclonic upper flow
between the large scale western ridge and the Hudson Bay trough
will be favorable for amplification of any Pacific shortwave
energy that can break through the ridge, and this appears to be
the case from Fri onward across the Great Basin/Rockies.
Meanwhile, the western Atlantic subtropical ridge will attempt to
exert its influence northward into the Southeast and portions of
the Mid-Atlantic states.
...Guidance/uncertainty assessment/preferences...
The amplifying trough/upper low across the West will be the most
significant forecast problem during the medium range.
Models/ensembles begin the medium range with a reasonably good
consensus as the system is beginning to amplify across the
northern Great Basin on Fri. From Sat onward is when some
significant predictability issues emerge, primarily related to
whether the system lingers for longer as a closed upper low across
the Great Basin, or begins to more quickly eject east into the
plains. By Sun morning, deterministic solutions range from the 12Z
CMC which still has the upper low entirely over the Great Basin,
to the 12Z ECMWF/UKMET which were over the Rockies. The
deterministic guidance does not tell the full story, however.
Ensemble spaghetti plots reveal a substantial number of ECENS
members that have the system over the central plains or even the
Mississippi valley by the same time (as was shown by the 00Z ECMWF
run last night). A closer look at the ECENS members seems to
reveal a relatively rare bimodal distribution of members, with a
number of members also keeping the upper low over the Rockies or
even the Great Basin Sun morning (making the mean likely
unrealistic).
The differences in ECENS members only grow with time from Sun
onward, and the more quickly progressive members result in phasing
with northern stream energy and much lower heights by Mon-Tue
across the Great Lakes/Northeast. Meanwhile, the overwhelming
majority of GEFS and CMCE members fall closer in line with the
deterministic guidance, showing a slower progression, and a
resultant stronger ridge maintaining itself in the eastern U.S. by
early next week. Finally, additional Pacific shortwave energy
appears likely to reach the Northwest or western Canada by next
Tue, once again eroding (to some degree) the ever resilient
western ridge. Model solutions continue to vary widely as to the
exact character of this energy, ranging from a consolidated upper
low to a more elongated trough. Preferred to stay close to the
ensemble means at this time.
Given these considerations, the WPC forecast was initially based
on a heavily deterministic multi-model blend, including the 12Z
ECMWF/CMC and 18Z GFS during days 3-4. From day 5 onward,
weighting of deterministic solutions was gradually reduced, with
significant weight placed toward the 12Z NAEFS mean by days 6-7.
...Sensible weather/threats highlights...
An often stationary frontal boundary extending from the central
Rockies and adjacent plains to the Great Lakes/Northeast Fri-Mon
will serve as a focus for scattered to numerous showers and
thunderstorms over a large area. Models continue suggest the
potential for areas of heavy rain across portions of the central
Rockies Fri-Sat as the vigorous upper trough amplifies to the
west, and a surface low develops along the front across the
central plains, enhancing low-level easterly upslope flow. A
number of deterministic solutions show the potential for localized
multi-day totals of at least a few inches, likely in favored
upslope areas. Farther east, broad southerly flow across the
plains will transport moisture north and lift it across the
frontal boundary, and models show potential for multiple rounds of
convection along the front Fri-Sun across portions of the
Midwest/Great Lakes and perhaps even into portions of the
Northeast, with the potential for areas of heavy rainfall.
Confidence in the specific locations of any heavy rain is
relatively low at this time, and would likely be determined by
mesoscale/convective processes. Farther south, a dissipating
frontal boundary will focus increasingly widespread shower and
thunderstorm activity across the southern/central Florida
Peninsula from late this week into early next week. Areas of heavy
rainfall will be possible given the multi-day nature of the event
and high Pwat values (+2 to +3 standard deviations).
High temperatures are expected to be 10 to 20 deg F above average
across much of the southern tier on Fri, with the center of
greatest anomalies shifting east to the lower Mississippi valley,
Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F
anomalies). A few record high temperatures are possible.
Ryan
Associated WPC medium range forecasts of 500 mb heights, surface
systems, weather grids, quantitative precipitation forecasts, and
winter weather outlook snow/sleet 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