Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
211 AM EDT Mon May 07 2018
Valid 12Z Thu May 10 2018 - 12Z Mon May 14 2018
...Pattern overview...
A relatively amplified flow will be present across the northern
hemisphere during the medium range. Negative height anomaly
centers will persist across the north central Pacific and
northeast of Hudson Bay. Meanwhile, ridging will attempt to expand
northward along the U.S. and Canadian west coast. Meanwhile,
positive height anomalies will also be prevalent across the
southeastern/Mid-Atlantic states as the influence of the Atlantic
subtropical ridge grows. A series of shortwaves ejecting from the
broad north Pacific upper low will erode and/or undercut the
western ridge. These waves will have a tendency to amplify as they
cross the Rockies and enter the central U.S., in a broad region of
cyclonic upper flow in between the ridging along the two coasts.
Teleconnections based on a few of the stronger and persistent
height anomalies across the northern hemisphere support the
described North American pattern, and thus would expect this
pattern to remain relatively stable for some period of time.
...Guidance and uncertainty assessment...
The WPC medium range forecast was based heavily on a blend of
deterministic guidance, including the 12Z ECMWF/CMC and 18Z GFS,
during days 3-4 (Thu-Fri). Guidance consensus during this time
period was relatively good, with substantial improvement noted
among ensemble members over the past couple cycles, and a shift
among the GEFS/CMCE members toward the ECENS mean. This was
particularly true with respect to the rapid amplification of
shortwave energy across the Great Basin/Rockies Thu night-Sat. The
trend has been towards more amplification, as was shown by the
ECMWF and its ensemble last night.
Spread associated with the western system still increases by day 5
(Sat), with a significant degree of spread between the 12Z
UKMET/CMC which continued to dig the trough/upper low across the
Great Basin, while the ECMWF/GFS begin to move the system
eastward. Something a bit closer to the GFS/ECMWF solutions here
was preferred, although ensemble means (especially the GEFS) did
allow for a solution somewhere in between the two deterministic
camps. As the shortwave/upper low ejects east into the central
U.S. by Sun-Mon, the GFS/ECMWF solutions remained relatively close
to one another in terms of timing, but some significant spread
emerges farther north as a shortwave of Arctic origin (moving
along the western side of the Hudson Bay upper low) begins to
interact with the Pacific energy across the central
plains/Midwest. Differences in the phasing of these systems have
major implications for if and how quickly amplification would
occur and heights would fall across the Great Lakes. The past
couple runs of the ECMWF have generally been more aggressive with
amplifying a long wave trough axis from Hudson Bay south to the
Great Lakes by day 7 (Mon), while the GFS keeps the systems more
separated, and the CMC still hangs the upper low back across the
Great Basin by that time, obviously precluding any northern stream
interaction. Farther west, guidance are in general agreement that
another potential shortwave/cold front will approach the Northwest
by early next week. While most solutions agree on the existence of
this feature, deterministic models and ensemble member spread is
quite large with respect to the amplitude, which appears largely
dependent on how much energy remains consolidate in the Gulf of
Alaska versus a more open wave.
Given the quickly increasing uncertainty from day 5 onward, the
ensemble means (12Z ECENS/18Z GEFS) were given increasing/majority
weight during that time period, with 70 percent means by day 7.
...Sensible weather/threats highlights...
A frontal boundary expected to stall through much of the period
from the central plains to the Great Lakes/Northeast will produce
scattered to numerous showers and thunderstorms, with areas of
potentially heavy rainfall possible through much of the forecast
period. The greatest potential for heavy rains will focus across
portions of the central/northern plains and Midwest from Thu
onward. As moisture surges farther northeastward, heavy rains will
also be possible from portions of the lower Great Lakes to the
Northeast Sat-Sun. The trailing end of this frontal boundary will
also produce locally heavy areas of rainfall across portions of
the central Rockies, with the heaviest amounts in favored upslope
areas between the surface front and high pressure to the north.
Farther south, additional showers and thunderstorms will be
possible Sat-Sun across the southern plains ahead of the surface
front and dry line. A lingering washed out frontal boundary will
also bring increasing coverage and intensity of showers and storms
to the Florida Peninsula by Fri-Sun.
Temperatures are expected to be above average through much of the
period from the Great Basin/Southeast eastward across the
central/southern plains to the Eastern Seaboard. The greatest
temperature anomalies Thu-Fri are forecast from the Southeast to
the southern High Plains, where high temperature anomalies of +10
to +20 deg F are expected. By Sat-Sun, as the subtropical ridge
builds, the greatest high temp anomalies will shift a bit
eastward, from the southern plains to the Southeast/Mid-Atlantic,
where +10 to +15 deg F anomalies are possible. Finally, north of
the surface front, areas from the northern Rockies to the Great
Lakes will see below average temperatures through much of the
period, with highs 10 to 20 deg F below average.
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