Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1128 AM EDT Fri May 11 2018
Valid 12Z Mon May 14 2018 - 12Z Fri May 18 2018
...Pattern overview and guidance/uncertainty assessment...
An upper low initially centered over of Hudson Bay will slide
eastward toward southern Greenland by the end of next week which
favors broad cyclonic flow to its south along the US/Canadian
border from the Upper Midwest eastward to New England. Upper
ridging initially along the west coast Monday will slowly nudge
eastward atop an active upper low over the Great Basin Mon-Wed.
This cutoff low is expected to meander across the Great
Basin/Rockies into midweek before drifting east into the plains
and weakening/dissipating. Farther east, the west Atlantic
subtropical ridge will exert its influence from the lower
Mississippi valley and mid/deep south to the Mid-Atlantic as an
active and wet upper low and tropical moisture plume will be
temporarily trapped in between just west of Florida. This low will
slowly lift northward into the Southeast midweek and be slow to
dissipate, but will bring widespread and locally heavy downpours.
In the region of relatively weak upper flow between the western
trough/upper low and the southeastern ridge, expect a wavy surface
front to linger from the Rockies to the Mid-Atlantic through much
of the period and remain a focus for pooling moisture and periodic
convection. Another closed low will track from the eastern Pacific
inland into northern CA and then the south-central Great Basin
next Thu-Fri, which will offer the potential for widespread
precipitation along and to the north of its path. Accordingly,
with models/ensembles continuing to show above average consensus
for much of days 3-7 (Mon-Fri), it seems sufficient to justify use
of a multi-model and ensemble blend as mainly derived from the 00Z
GFS/ECMWF/Canadian/UKMET to start with a trend toward the 06Z
GFS/GEFS and 00Z ECMWF/ECMWF ensemble mean for next Thu-Fri.
...Sensible weather/threats highlights...
It remains evident that scattered to numerous showers and
thunderstorms will be possible across a large area in the vicinity
of the wavy surface frontal boundary, from the Great Basin/central
Rockies to the Mid-Atlantic/Northeast. Several areas of
potentially heavy rainfall are possible, with the potential for
multiple rounds of convection. The specific areas most likely to
receive heavy rains will likely be somewhat difficult to pin down
until the short term when models begin to resolve
mesoscale/convective processes. Farther south, higher confidence
exists in the likelihood for heavy rainfall across the Florida
peninsula. An upper low developing across the northeastern Gulf of
Mexico will advect very high PWs across the Florida (about +2
standard deviations or around the 98th percentile), and multi-inch
rainfall totals are possible across several days with the
potential for especially urban flooding along the I-95 corridor
from Miami to Jacksonville. Current indications among the guidance
are that deeper moisture may gradually advect farther north into
much of the southeastern U.S. as the upper low drifts northward,
and locally heavy rains will spread through the Southeast and
perhaps into the southern Mid-Atlantic by the end of the week. As
a Canadian cold front sinks southward to the Mason-Dixon line,
precipitation may expand along this boundary as well, setting up a
potential modest/heavy rain event for the mid-Atlantic to southern
New England. Farther west, precipitation will begin to increase
across the West Coast states by the middle of next week as the
aforementioned upper low approaches the region and its attendant
cold front moves ashore.
Temperatures will also remain a big story during the medium range,
with subtropical ridging promoting well above average temperatures
for portions of the southern/southeastern states. High
temperatures Mon-Tue of 10 to 15 degrees above average are
possible, and a sizable number of record high temperatures (many
in the low to mid 90s and even some upper 90s) are in jeopardy of
being tied/broken. Some relief may arrive across the Southeast by
Tue-Wed as rainfall becomes more widespread, but the lower/mid
Mississippi valley will remain well above average. The only other
area for below average temperatures outside Florida after Monday
will be in the west (Oregon through California the into Nevada) as
the upper low moves ashore later in the week. Much of the Great
Basin will moderate from below average to above average
temperatures through the period as the upper low exits the region,
only to cool again next Friday.
Fracasso/Schichtel
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