Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1144 AM EDT Sat May 11 2019
Valid 12Z Tue May 14 2019 - 12Z Sat May 18 2019
15 UTC Update...
A blend of the 00Z ECMWF/CMC and 06Z GFS was used as basis during
days 3-5 (Tue-Thu) for this forecast update, with particular
weight placed toward the ECMWF/CMC solutions. Models continue to
slowly converge on the eventual track of a surface low along/off
the Northeast U.S. coastline Tue-Wed, which shows some potential
to produce accumulating snows across the higher elevations of
interior northern New England. The ECMWF/CMC were more closely
clustered with consensus on the track of this system while the GFS
was a bit farther south. Solutions handle weakening southern
stream shortwave energy crossing the Southern Plains Tue-Wed
similarly, with differences mainly confined to the amplitude of
the feature. Models show reasonably good consensus on an amplified
upper trough reaching the West Coast Wed-Thu (a leading shortwave
Wed followed by stronger height falls Thu). Solutions differ on
the specifics of embedded smaller-scale features within the larger
trough, favoring gradually increasing ensemble weighting. Stronger
height falls crossing the Rockies by late next week should induce
surface cyclogenesis across the High Plains by Fri night-Sat.
Larger differences begin to emerge from Thu onward with the
evolution/timing of a few northern stream shortwaves. The chaotic
and somewhat blocked upper-level flow across Canada reduces
predictability for smaller scale features. Thus, trended the
forecast toward ECENS/NAEFS ensemble means during days 6-7
(Fri-Sat).
Ryan
Previous Discussion (issued at 0525 UTC)...
Tonight's forecasts were derived from a blend of the latest
deterministic 12z GFS /12z ECMWF blended with the 12z ECMWF/18z
GEFS Ensemble Means days 3-7.
Models are slowly converging on the latitude of the low developing
off the New England coast Mon night-Tue. The 12z ECMWF/ECMWF
Ensemble Mean/12z UKMET remain farther north than the GFS/18z GEFS
Mean/12z Canadian. The forecasts used an intermediate solution to
provide continuity and a consensus based solution.
The models show an upper level low near the AZ/NM border with
Mexico 12z Mon that slowly moves east and shears into an upper
trough as it progresses across the Southern Plains/Lower
Mississippi Valley through early next week. Well clustered
forecasts lend the situation to a consensus-based forecast.
Elsewhere, models have typical timing/amplitude differences with
northern stream energy diving from the upper MS Valley into the
Midwest/Great Lakes Tue night to Wed and crossing the central
Appalachians/mid Atlantic/coastal waters Thu, with potential low
pressure development off the coast Fri 17 May. There remains a
large latitudinal spread as to where low pressure develops on the
surface front. A blend of the 12z GFS/ECMWF models and their means
was used until better agreement develops.
Agreement amongst the models/ensembles supports an amplified upper
trough reaching the West Coast by Wed 15 May, with a secondary
trough reaching the west coast Thu 16 May, which then moves inland
into the Great Basin and towards the Pacific northwest coast Fri
17 May. Models differ on the exact timing and phasing of the
trough, with the ECMWF/ECMWF ensemble mean faster taking the upper
trough across the Rockies on to the high Plains. Blending with the
slower timing of the GEFS was used following historically slower
motion of higher amplitude upper troughs.
...Weather Highlights/Threats...
Expect showers over the New England early in the week as
consolidation of low pressure off the northeastern coast occurs,
with the system moving slowly. Some of the precipitation could
fall in the form of snow over highest elevations of northern New
Hampshire and western Maine as the core of the upper trough passes
through.
A cold front and frontal wave moving east across the Upper MS
Valley, Upper Midwest/Great Lakes/Ohio Valley should produce
showers during the early to middle portions of next week before
the system moves into the northeast later in the week.
The upper low tracking from the western U.S.-Mexico border into
the southern Plains will be responsible for locally enhanced areas
of showers/storms over the southern Plains and western Gulf coast
in the early to middle portions of next week. Showers/storm also
develop in the vicinity of a stationary front lingering over the
Fl peninsula in the middle to latter portions of next week.
The upper trough nearing the West Coast should bring increased
clouds/precipitation to the Pacific Northwest and northern CA Wed
15 May, continuing inland into the Great Basin and northern
Rockies Thu 16 May, and then the Plains on Fri 17 May and Sat 18
May.
Short-range warmth over the Northwest will continue into the first
half of next week,
with highs of 10-15F above normal over the Interior Northwest and
northern Rockies Tue, continuing in MT/WY Wed. The warm core
moves east into the northern to central Plains Thu with cooling
over the northwest. With the approaching trough, CA/OR/WA will
see a reversal to cooler than normal temperatures in the middle to
latter portions of next week.
The system affecting the Mid Atlantic, Great Lakes, and northeast
Tue will keep highs 10-15F or so below normal, continuing on Wed
in northern Maine. Gradual moderation is expected as the trough
moves offshore Thu 16 May.
Petersen
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml