Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1140 AM EDT Thu May 10 2018
Valid 12Z Sun May 13 2018 - 12Z Thu May 17 2018
...Locally heavy rain possible for much of Florida next week...
...Pattern overview...
Upper ridging initially off the North America west coast on Sunday
will slowly nudge eastward atop an upper low over the Great Basin.
A persistent upper low just northeast of Hudson Bay will favor
broad cyclonic flow that will skirt across the US/Canadian border
from the Upper Midwest eastward to New England, with relatively
active flow and frequent passages of frontal boundaries. The
cutoff upper low in place across the Great Basin at the start of
the forecast period is expected to meander across the Great
Basin/Rockies into at least Tue or Wed 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 to the Mid-Atlantic with another upper low
trapped in between just west of Florida. That will slowly lift
northward into the Southeast midweek and dissipate, but will bring
a widespread modest rainfall to much of the Sunshine state. 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 continued
use of a multi-model deterministic blend including the 00Z
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 the upper
low that develops across the northeastern Gulf of Mexico (06Z GFS
appeared too far west), a trough/upper low remaining off the U.S.
West Coast, 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), nearly equal weighting was given to the
00Z ECENS/GEFS ensemble means and the 00Z ECMWF/CMC as the Pacific
upper low approaches California. There remains north-south spread
in the location of the frontal boundary east of the Mississippi
due in part to the strength of high pressure over northwestern
Ontario next Thursday and the evolution of both the exiting Great
Basin/Rockies upper low and the southeastern weakening upper low.
Ensemble consensus provided a best position for now. given the
lead time.
...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 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 be possible into Georgia.
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 be the other 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 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) may be 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 midweek.
Much of the Great Basin will moderate from below average to above
average temperatures through the period as the upper low exits the
region.
Fracasso/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