Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 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 nrn stream flow dug to the U.S./Canadian border from the
Upper Midwest to the Northeast will support and periodically
reinforce a persistent and wavy frontal zone underneath that will
tends to shift slowly southward across the nrn third of the
nation. Hot and increasingly moist warm sector conditions to the
south will offer stark contrast. Mid-latittude impulses running
over this frontal zone and a more mid-latitude stalled front will
further act to focus instability and moisture pooling on each to
support multiple rounds of heavy convection. It remains unclear
the effect of widespread heavy convection within the warm sector
will effect the overall distribution to the north, but the
gradually weakening but quite lingering effects from a Gulf of
Mexico upper low and anomalously deep moisture meandering up into
the srn/southeastern U.S. will in itself be quite the focus for
widespresd heavy convection over the region and with time up the
Appalachians to the Mid-Atlantic as enhanced from terrain and
slowed cell translations. In this pattern, record high temps may
persist into mid-later this week over the mid-lower MS valley
where there is a relative break from heat breaking thunderstorms.
Record minimum temps will be more widespread during that period,
northeastward across the Mid-South to the Mid-Atlantic.
Meanwhile...the West will also be quite active as an amplified
mid-upper level trough and associated surface frontal system
slowly works inland from CA midweek to the Great Basin/Southwest
later week and into the Rockies by next weekend. The greatest
precipitation threat will be along and north of system track as
enhanced by terrain/deformation. Ample but uncertain
energies/height falls also ejecting out across the Rockies into
the plains next weekend also offers a threat for
afternoon/nocturnal convective development/clusters along/north of
the lingering fronts, with warm sector meso-boundaries, and a
dryline.
...Guidance and Predictability Assessment...
Model and ensemble mass field guidance offers average clustering
days 3-5 (Wed-Fri), with forecast spread/uncertainty increasing
significantly next weekend on days 6/7. Accordingly, a composite
model and ensemble mass field 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 compatable
12 UTC NAEFS/ECMWF ensemble means days 6/7. WPC continuity was
mixed due to effects from significant differences with the local
focus of heavy rainfall between guidance in a widespread
convective pattern.
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
Schichtel