Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sat May 19 2018
Valid 12Z Tue May 22 2018 - 12Z Sat May 26 2018
...Guidance and Predictability Assessment...
Overall, there is not large differences in the overall synoptic
pattern with models. On Day 3/4 (Tuesday/Wednesday), a large
closed 500 mb low will move over the Great Basin/Desert Southwest
and weaken while a shortwave skirts across the Great Lakes. The
upper ridge still stays in tact across the central U.S. with very
strong ridging in western Canada. By Day 5 (Thursday), mid-level
energy will move from the Great Basin to the northern Plains as
the upper ridge breaks down and the upper trough over eastern
Canada will also dip into the Northeast. By late week and into the
weekend (Friday/Saturday), another trough will meander toward the
West Coast as a low from the Caribbean moves into the Gulf of
Mexico.
There are not significant differences in the 06Z GFS/00Z ECMWF/00Z
UKMET/00Z CMC on Days 3 and 4 at the 500 mb level. All agree on
the strength and timing of the closed low over the Great Basin
evolving into an open trough by Wednesday. Meanwhile, a shortwave
will quickly slide across the Great Lakes. WPC blended all of
these with a little bit of the 06Z GEFS/00Z ECENS.
By Thursday, models begin to differ on the timing of the upper
trough approaching the West Coast. The 00Z ECMWF has consistently
been the faster solution and the 00Z CMC along with the means also
back up this faster solution. The GFS has been much slower with
the GEFS leaning toward the faster solution of the ECMWF and its
mean. the 06Z GFS/00Z ECWMF and their means are in relative
agreement with the shortwave energy over the northern Plains. By
Friday and into Saturday, the GFS/GEFS has consistently been on
the eastern side of the spread with respect to the tropical low in
the Caribbean moving northward into the Gulf of Mexico. The CMC
runs also continue to be very aggressive with this low and being
on the western side of the spread. The ECMWF had been in agreement
with the GFS/GEFS but the 00Z run is faster and on the western
side of the spread. For now, confidence remains low with the
details of this system but trended toward slightly west of the
GFS/GEFS solution. WPC took a blend of the 00Z ECMWF/ECENS and 06Z
GFS/GEFS from Days 5-7 but put much more weight on the ECMWF and
its mean due to its treatment of the evolution for the Pacific
trough late in the week.
...Weather/Threats Highlights...
With the upper trough over the western U.S. and the shortwave
ejecting toward the northern Plains, expect an increase in
precipitation from the Great Basin to the northern Plains through
Thursday. By Friday and into Saturday, precipitation will move
eastward into the MS and TN Valley/Great Lakes as the shortwave
continues moving eastward. As the second Pacific trough moves
toward the West Coast, California and perhaps parts of the Pacific
Northwest can expect rainfall through the end of the week.
Rainfall will likely increase by the weekend as the trough
approaches closer to the coast. Though confidence of the timing
and position of the low in the Gulf of Mexico, the Gulf Coast
region can expect more coverage in convection as there will be an
abundant amount of moisture in place.
Most of the country will experience above average temperatures
throughout the medium range period. The biggest exception is
Florida which will have continued precipitation throughout the
week; thus, maximum temperatures will be nearly 5 degrees below
normal. The California coast will also be below average for much
of the medium range period. As the upper trough dips into the
Northeast from eastern Canada, cooler conditions will follow which
will be felt on Thursday. Temperatures are expected to recover
back to near normal by Friday.
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
Reinhart