Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Fri May 18 2018
Valid 12Z Mon May 21 2018 - 12Z Fri May 25 2018
...Guidance and Predictability Assessment...
During the Mon-Wed period there is reasonable agreement in the
large scale evolution. Expect gradual weakening of western U.S.
troughing, with the most prominent embedded upper low center
likely to drift from California into the Great Basin. At the same
time latest guidance indicates that another piece of energy may
drop southward from the Pacific Northwest. A persistently strong
western Canada ridge will promote cyclonic flow over eastern
Canada, perhaps amplifying enough to interact with initial
Plains/MS Valley shortwave energy. Farther southeast an upper
ridge northeast/east of the Bahamas will steadily expand southward.
Within this evolution the greatest overall uncertainty seems to be
with the Plains/MS valley shortwave and associated surface low
pressure. Through the 12z/18z cycles there was a fair degree of
operational model spread for timing and the ensemble envelope was
even broader--ECMWF leaning on the fast side and the GFS a bit on
the slower side. If anything model spread is greater with 00z
runs thus far as the GFS has become weak/slow and the UKMET/CMC
are slower than prior runs--at the very least supporting slower
timing than the 12z ECMWF. Beyond the timing differences is the
question of northern stream interaction that will influence the
strength and track of low pressure. Within the western trough
aloft the 18z GFS started out slower than consensus with the CA
upper low as of early Mon before gradually catching up.
Confidence is low with trailing energy, with a consensus blend
downplaying its specifics until better clustering/continuity
emerge. Based on these considerations the Mon-Wed portion of the
forecast included various components of the 18z-12z GFS and 12z
ECMWF/UKMET/CMC to reflect the most common ideas of guidance or
intermediate solution as appropriate.
Guidance spread for eastern Pacific flow that develops by Wed
ultimately leads to increasing uncertainty for larger scale
aspects of the eastern Pacific/North America forecast for late
next week. In varying ways recent GFS runs have tended to split
Pacific trough energy such that the northern energy breaks down
the western Canada ridge while leaving a lower latitude upper low
back toward 135-150W longitude. On the other hand the 12z ECMWF
and recent CMC runs (including the new 00z version) maintain the
western Canada ridge while bringing the core of Pacific trough
energy closer to the West Coast. Highlighting the delicate nature
of the forecast evolution, the 12z UKMET through the end of its
run was close to the ECMWF cluster but the new 00z UKMET shows a
GFS-type solution. Confidence is not particularly high in such a
deep upper low approaching the West Coast as depicted in the 12z
ECMWF and recent CMC runs. However the GEFS/ECMWF ensemble means
lean more toward the overall ECMWF/CMC scenario of maintaining the
western Canada ridge while nudging the Pacific trough toward the
West Coast--which is closer to the established long-term mean
pattern. The occasional bias of the GFS to break down ridges too
easily further recommends leaning away from the GFS scenario at
this time.
Farther east the primary forecast issues will be details of
northern stream cyclonic flow aloft and effect on surface
fronts/waves, and at lower latitudes guidance signals for a
possible tropical feature to emerge from the northwestern
Caribbean. Both have fairly low confidence. Regarding the latter
there is a dramatic amount of spread regarding
timing/track/strength. GFS runs/GEFS members have tended to be on
the faster side of the full envelope with the the 00z GFS slowing
down to near the 12z ECMWF as of early day 7 Fri. The 00z CMC has
strayed faster though, and with a track farther west. It may take
a while for guidance clustering to improve significantly for this
feature.
For the days 6-7 time frame the forecast transitioned to half
weight of the 12z ECMWF mean with remaining input spread among the
18z GEFS mean and 12z ECMWF/CMC. The 18z GEFS mean compared
favorably to the other solutions over the western states and
eastern Pacific but was likely too quick to lower pressures over
the eastern Gulf/FL Peninsula.
...Weather/Threats Highlights...
The early-mid week upper trough with embedded low(s) over the West
will support areas of precip that should be best organized along
and north of the best-defined circulations aloft--with highest
totals likely to extend from favored terrain in northern-central
CA through the northern/north-central Rockies. Locations
downstream will see periods of enhanced rainfall as energy ejects
eastward. A leading system tracking out of the Plains/MS valley
will likely generate episodes of convection over the eastern half
of the country during the first half of the week. The front
trailing from this system should extend back to low pressure that
becomes established over the central High Plains. This front
combined with energy ejecting out of the West may promote one or
more convective events that should produce highest rainfall totals
over/near the north-central Plains. Meanwhile confidence is quite
low in specifics of any defined feature that may emerge from the
northwestern Caribbean and affect the Florida Peninsula, but there
is at least potential for some enhanced moisture to reach the area
given the flow around ridging at the surface and aloft to the east.
The greater proportion of temperature anomalies will be on the
warm side of the spectrum. Highest anomalies should be for min
temps over the East Mon-Wed and for max/min temps from the Pacific
Northwest eastward along the Canadian border Tue onward. Most of
Florida will tend to see modestly below normal highs most of next
week while upper troughing will keep the Southwest/Great Basin on
the cool side for daytime highs Mon-Wed.
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