Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
225 AM EDT Thu May 10 2018
Valid 12Z Sun May 13 2018 - 12Z Thu May 17 2018
...Pattern overview...
Upper ridging will remain dominant along the North America west
coast through the medium range, with a persistent upper low just
northeast of Hudson Bay. In between, a broad area of modestly
cyclonic upper-level flow will remain in place from the Canadian
prairies to New England, with relatively active flow and frequent
passage of frontal boundaries. A cutoff upper-level low in place
across the Great Basin at the start of the forecast period (Sun)
is expected to meander across the Great Basin/Rockies into at
least Tue before drifting east into the plains. Farther east, the
west Atlantic subtropical ridge will exert its influence from the
lower Mississippi valley to the Mid-Atlantic. 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.
...Guidance/uncertainty assessment/preferences...
Models/ensembles continue to show above average consensus for days
3-5 (Sun-Tue), and consensus was sufficient to justify use of a
multi-model deterministic blend including the ECMWF/UKMET/CMC/GFS.
This blend represents a consensus solution well for the western
upper low, not transporting it east too quickly by Tue as has been
a bias among nearly all deterministic guidance except the CMC in
recent days. Additional features well resolved by a multi-model
blend during days 3-5 include a shortwave/upper low that develops
across the northeastern Gulf of Mexico, a trough/upper low
remaining off the U.S. West Coast (the 18Z GFS was much farther
south with the system than consensus, and was excluded in favor of
the 12Z run), and northern stream shortwave energy crossing south
of Hudson Bay, which will push a cold front into northern New
England by Tue.
During days 6-7 (Wed-Thu), majority weighting was given to the 12Z
ECENS/18Z GEFS ensemble means, but with continued use of the
ECMWF/CMC as well. The ECMWF/CMC showed decent consensus with the
large upper low approaching the West Coast by the middle of next
week (the GFS was likely too fast), and with additional northern
stream energy approaching the Great Lakes, and energizing a
surface frontal wave tracking from the Great Lakes to New England.
...Sensible weather/threats highlights...
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 Sun-Tue. 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.
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 north, and locally heavy rains will
be possible. Farther west, precipitation may begin to increase
across the West Coast states by the middle of next week as the
aforementioned trough/upper low approaches the region and a cold
front moves ashore.
Temperatures will be the other big story during the medium range,
with subtropical ridging leading to what may be perceived as the
first heat wave of the season for portions of the
southern/southeastern states. High temperatures Sun-Mon of 10 to
15 degrees above average are possible, and a sizable number of
record high temperatures (many in the low to mid 90s) look 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.
Ryan
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