Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Fri May 11 2018
Valid 12Z Mon May 14 2018 - 12Z Fri May 18 2018
...Pattern overview and guidance/uncertainty assessment...
A mean upper vortex centered just to the northeast of Hudson Bay
favors broad cyclonic flow underneath that will skirt the
US/Canadian border from the Upper Midwest eastward to New England,
with relatively active flow and frequent passages of frontal
boundaries. Upper ridging initially along the North America west
coast Monday will slowly nudge eastward atop an active upper low
over the Great Basin. 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 deepened moisture will be trapped in
between just west of Florida. This low will slowly lift northward
into the Southeast into 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.
Later week upstream closed trough development and gradual track
from the eastern Pacific inland into CA then south-central Great
Basin offers potential for widespread precipitation in a pattern
with increasing model spread, but generally good overall
support/continuity.
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 18 UTC GFS/GEFS, 12 UTC NAEFS/ECMWF
ensembles and ECMWF.
...Sensible weather/threats highlights...
It remains evidemt 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 (+2 to +3
standard deviations), 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 over the Southeast. Farther west, precipitation may
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 fronts move ashore.
Temperatures will also remain a big story during the medium range,
with subtropical ridging leading to what may be perceived as the
first significant heat wave of the season 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) in jeopardy.
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 after Monday will be in the west (Oregon through
California) as the upper low moves ashore mid-later 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.
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