Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sat May 05 2018
Valid 12Z Tue May 08 2018 - 12Z Sat May 12 2018
...Pattern overview...
Downstream from a mean upper low whose emphasis likely transitions
from the Bering Sea southeastward into the northern Pacific,
expect mean ridging to persist over western Canada through the
period. Initial ridging also in place over the western U.S.
should be eroded as an eastern Pacific system progresses/opens up
and trailing energy arrives. Farther east cyclonic flow will
prevail over Canada through the period while mean troughing over
the eastern U.S. should trend toward a more zonal flow toward next
Fri-Sat. By that time the dominant feature will likely be a mean
closed low between Hudson Bay and Greenland.
...Guidance and uncertainty assessment...
The most contentious/uncertain aspects of the forecast during the
period exist over the eastern Pacific and western U.S. Guidance
is continuing to have some difficulty in resolving specifics of
the eastern Pacific upper low that most guidance now shows opening
up as it approaches the western North America coast by Wed, with
some influence by shortwave energy immediately upstream. Compared
to 24 hours ago the majority consensus has trended farther
northwest with the track of the upper low/trough core, which
happens to be closer to GFS/GEFS scenarios from a couple days ago
when there was much wider spread. By early day 5 Thu there is
also reasonable similarity among guidance for the shortwave
immediately upstream. However after early Thu guidance rapidly
diverges over the eastern Pacific/western U.S. domain. The 00z
CMC and past couple ECMWF runs quickly sharpen/close off this
trailing energy into a western U.S. trough and compact embedded
low in contrast to GFS/GEFS solutions that in varying ways lead to
lower heights over the eastern Pacific and higher heights in the
GFS/GEFS.
Flow evolution by the D+8 time frame leads to greatest emphasis on
a strong core of negative height anomalies centered just west of
the southern tip of Greenland, associated with the upper low
expected between Hudson Bay and Greenland. Teleconnections
relative to this negative anomaly center suggest moderate western
U.S. troughing and near to slightly above average heights over the
eastern Pacific. The 00z ECMWF/CMC means and to some degree 00z
GEFS mean (thus 00z NAEFS as well) offer the best template for
this teleconnection-favored pattern by days 6-7 Fri-Sat.
Confidence is not sufficiently high to incorporate much of the
deeper operational ECMWF/CMC runs at this time.
For the most part discrepancies are less significant over the
central/eastern states. An intermediate forecast still looks best
for the upper trough initially nearing the East Coast, between the
deeper/slower 00z ECMWF-CMC and weaker/faster GFS/UKMET. A
positive trend is better agreement that any waves along the
leading frontal boundary should remain offshore. Meanwhile
through about Thu-Fri a blended approach offers good continuity
and downplays low predictability detail differences for the
eventual interaction of amplifying Canadian flow aloft/leading
cold front and the system tracking eastward from the Plains.
Toward the end of the period the aforementioned issues with
upstream flow will begin to play a role in low pressure/frontal
positions over the central-eastern U.S. Thus forecast confidence
declines considerably after Thu-Fri.
Based on the above considerations, forecast input consisting of 80
percent operational guidance for Tue-Thu quickly trended to 70 and
100 percent ensemble weight (00z NAEFS/ECMWF means) for Fri-Sat
respectively.
...Sensible weather/threats highlights...
Confidence is gradually increasing that the majority of rainfall
associated with the wavy front off the East Coast should remain
offshore. Some probability still exists that some moisture could
extend back to parts of the North Carolina/Delmarva/southeastern
New England coasts for a time. At least in principle the forecast
is fairly consistent for the system tracking eastward from the
Plains Tue onward to generate an area of rain/thunderstorms with
some locally heavy/strong activity possible between the Plains and
central Appalachians. The initial eastern Pacific system and
trailing energy will bring a period of rainfall to the Pacific
Northwest/northern Rockies mid-late week. As the energy aloft
continues along, expect another episode of rainfall/possible
convection over some locations from the northern and possibly
central Plains eastward. Uncertainty over the western U.S.
pattern by late week/weekend lowers confidence in the distribution
and intensity of any western U.S. precip at that time, as well as
details of where heaviest rainfall may occur farther eastward.
Expect very warm temperatures from the Interior West into High
Plains during the period, but with gradual erosion of the warmth
across northern areas in particular as upper ridging weakens.
Within a broad area of plus 10-20F (and perhaps locally higher)
anomalies, there will be some opportunities for daily record
highs/warm lows. Pattern uncertainty by the end of the period
decreases confidence in the temperature forecast. Readings over
the East will rise from within a few degrees on either side of
normal into solidly above normal territory with the gradual
advance of the Plains system/front. By the latter part of the
period the northern tier states are likely to be on the cool side
and the South warmer than climo, separated by what could be a
fairly sharp front extending from the Plains to East Coast. Thus
toward Fri-Sat there is fairly high ensemble spread for
temperatures over the middle latitudes of the central/eastern U.S.
Rausch
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