Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Mon May 21 2018
Valid 12Z Thu May 24 2018 - 12Z Mon May 28 2018
...Guidance and Predictability Assessment...
Over the eastern Pacific and western U.S., recent ECMWF/ECMWF mean
runs along with the 18z GFS/GEFS mean provide a decent cluster for
the handling of the eastern Pacific upper low that will likely
track inland by next weekend. Over recent days typically slower
GFS runs have been steadily catching up to the ECMWF scenario
which has tended to include the CMC as well. Once again today the
18z GFS has advanced relative to the 12z version to yield the
improved consensus. There is still a significant question of
exact track once the feature moves inland with recent CMC runs
tending to take a farther southeast track than the ECMWF. UKMET
runs and the new 00z GFS are also farther southeast. For now
prefer to lean somewhat more in the ECMWF direction as the fairly
strong downstream ridge may lower potential for an upper low track
on the southeast side of the envelope. By days 6-7 Sun-Mon
upstream Pacific flow may complicate the forecast as well. Models
and individual ensemble members are quite diverse for specifics
with the most common signal being some degree of cyclonic flow
centered farther offshore than depicted in the 12z GFS by the end
of next Mon. Confidence is not especially great in 18z/00z GFS
runs either, with a rather sharp/amplified trough versus broader
cyclonic flow in most other models/means.
From the general consensus perspective the forecast is fairly
stable for the shortwave forecast to track eastward from the
northern High Plains and corresponding low pressure that may reach
the Great Lakes/Northeast by the latter half of the period.
Primary detail uncertainties by later in the period involve the
extent to which the northern tier energy could try to feed into
the upper trough to the south by later in the period (6-hourly GFS
runs steadily backing off) and interaction of potentially
amplifying southern Canada flow (trends toward more in the ECMWF
and less in the GFS). Ongoing adjustments in the guidance favor
an intermediate solution that yields fairly good overall
continuity.
The forecast over lower latitudes continues to provide significant
uncertainty, with surface forecasts over the Gulf of Mexico
ranging widely for strength and track of any potential low
pressure. CMC runs have been characteristically well-developed
while UKMET runs thus far have showed no low pressure at all. For
track recent ECMWF runs have been on the western side of the
spread and GFS/GEFS on the east. 18z and new 00z GEFS runs have
adjusted noticeably westward with their overall area of low
pressure versus earlier runs, which provides more support for the
idea of a central/west-central Gulf track. By day 7 Mon there is
actually decent agreement from the means toward low pressure
gravitating to the central Gulf Coast region under the core of the
upper trough that deepens over 90W longitude. Consensus also
sides with the ECMWF scenario of the upper trough being somewhat
sharper versus recent GFS/GEFS runs that have showed more
broadness on the eastern side.
Forecast considerations continued to favor a blend that tilted
60-70 percent toward ECMWF/12z ECMWF mean guidance relative to the
18z GFS/GEFS mean. By mid-late period the forecast incorporated
both the 12z and old 00z runs for the operational ECMWF component
of the blend to downplay lower confidence detail differences.
...Weather/Threats Highlights...
Areas from the northern Plains eastward will see
rainfall/convection in association with the shortwave energy and
surface low pressure/frontal system initially emerging over the
northern Plains on Thu. Some activity may be locally moderate to
heavy. Areas from the lower MS Valley through the Southeast
including Florida will likely see a fairly wet period with the
upper trough deepening along 90W longitude promoting a northward
flow of moisture aloft and surface low pressure possibly tracking
toward/settling over or near the central Gulf Coast. Later in the
period there will be a question of how much of the moisture over
the Southeast spreads northward ahead of the front approaching the
eastern states. Heavy rainfall potential will require close
monitoring given the significant totals already observed over some
locations in the past week. Areas from the central/northern West
Coast into the Rockies will likely see another episode of rainfall
of varying intensity with the upper low forecast to come inland
from the eastern Pacific.
Much of the lower 48 will see above normal temperatures during the
period. Exceptions will be over the Southeast/Florida where highs
will be lower due to clouds/rainfall and California into the Great
Basin under the eastern Pacific upper low that tracks inland. The
most prominent warm anomalies should exist ahead of the system
emerging from the northern Plains, and then from the Northwest
through most of the Rockies and eventually into the Plains in
association with upper ridging. Within both of these areas
scattered locations may see daily record values for highs/warm
lows.
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
Rausch