Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun May 13 2018
Valid 12Z Wed May 16 2018 - 12Z Sun May 20 2018
...A convectively active and quite wet pattern for much of the
country...
...Weather/Threats Highlights...
Energetic northern stream flow swinging across/through the
U.S./Canadian border from the Upper Midwest to the Northeast will
support and periodically reinforce a persistent and wavy frontal
zone from the southern Plains through the Ohio Valley to the
Northeast. Hot and juicy warm sector conditions to the south will
offer stark contrast to the cool/continental air mass to the
north. Additional mid-latitude impulses running over this frontal
zone will further act to focus instability and moisture pooling
which will support multiple rounds of heavy rain and embedded
quasi-disorganized convection. It remains unclear how the
convection within the warm sector will affect the overall rainfall
distribution to the north, but the gradually weakening (and
lingering) effects from a Gulf of Mexico upper low and anomalously
deep moisture on its east side will likely lift northward through
the southeastern U.S. into the mid-Atlantic which will spread at
least modest to potentially locally heavy rain up the Appalachians
to the Mid-Atlantic later this week into this weekend. In this
pattern, record high temps may persist into mid-later this week
over the mid-lower MS valley to the west of the Gulf upper low
with possible record warm mins farther east in a rather humid air
mass.
Meanwhile...the West will also be quite active as an amplified
mid-upper level trough and associated surface frontal system
slowly work inland from CA midweek to the Great Basin/Southwest
Thu-Fri and into the Rockies by the weekend. The greatest
precipitation threat will be along and north of the upper system
track as enhanced by terrain/deformation. Weaker ejecting
shortwaves out of the Rockies into the plains will help initiate
scattered showers/storms ahead of this weakening upper low, which
will likely exit onto the Plains in weakened state late Sat into
next Sun. SPC does not show a risk area for severe weather through
the period but enough instability/moisture should meander around
the sfc boundary and dry line to fire off at least some storms
through the period.
...Guidance and Predictability Assessment...
Upper highs over British Columbia and near Bermuda will mostly
hold in place leaving the lower 48 wavering between zonal and
quasi-blocked flow. Model and ensemble mass field guidance offers
good clustering days 3-5 (Wed-Fri), with forecast
spread/uncertainty increasing significantly by the weekend on days
6/7. Accordingly, a mostly deterministic model blend was used to
derive the WPC medium range surface fronts/pressures and 500mb
progs days 3-5 before turning to a more simple blend of the
reasonably compatible 06 UTC GEFS / 00Z UTC ECMWF ensemble means
days 6/7 due to northern stream uncertainties across Canada and a
breakdown of Pacific flow agreement.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation forecasts, and winter
weather outlook snow/sleet probabilities are 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
Fracasso/Schichtel