Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sat May 12 2018
Valid 12Z Tue May 15 2018 - 12Z Sat May 19 2018
...Rather wet pattern with scattered convection over much of the
lower 48...
...Pattern overview...
An upper low initially over Hudson Bay Tuesday will slide eastward
toward southern Greenland by the start of next weekend which
favors broad cyclonic flow to its south along the US/Canadian
border from the Upper Midwest eastward to New England. A weakening
upper low over the Great Basin on Tuesday will dissipate around
Thursday while another upper low west of California will move into
norCal and through NV into UT next Fri/Sat. Yet another upper low
will slowly move into the Southeast Tue/Wed and weaken as it lifts
into the TN valley/southern Appalachians late Thu into Fri. In
between these features--central/southern Plains into the
mid-Mississippi valley--will be a wavy surface boundary that
promote scattered showers/storms through the period.
...Guidance/uncertainty assessment...
Models/ensembles continue to show better than average agreement
despite the active pattern and a blend of the deterministic models
(00Z GFS/ECMWF/Canadian/UKMET and 06Z GFS) offered a good starting
point for the Tue-Thu period. After that, the model/ensembles
diverge in their handling of the northern stream flow across
southern Canada along/south of 50N with rising heights behind the
departing Hudson Bay low and lowering heights downstream of
building ridging over western Canada. With additional ridging
nosing westward from the western Atlantic near Bermuda, leftover
troughing along the Mississippi should squeeze northeastward
through the Great Lakes, lifting a sfc low into the St. Lawrence
valley. Trend has been toward increased Atlantic ridging which may
help guide this system more northeastward rather than eastward
even as a Canadian cold front pushes southeastward Thu-Fri into
Ontario. Trend toward increased ensemble weight helped temper any
changes from the previous cycle. To the west, ensembles favor
taking the upper low into Utah by next Saturday but may get stuck
beyond that given the possible Rex Block pattern formation in the
west and building heights to its east.
Fracasso
...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. Several areas of potentially heavy
rainfall are possible, with the potential for multiple rounds of
convection (though SPC does not indicate the potential for
organized severe weather). 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 as we near the event in the likelihood for heavy rainfall
across the Florida peninsula owing to very high PWs across the
Florida (about +2 standard deviations or around the 98th
percentile), and multi-inch rainfall totals are possible across
several days with the potential for especially urban flooding.
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 spread through the Southeast and perhaps into the southern
Mid-Atlantic by the end of the week. As a Canadian cold front
sinks southward to the Mason-Dixon line, precipitation may expand
along this boundary as well, setting up a potential modest/heavy
rain event for the mid-Atlantic to southern New England. Farther
west, precipitation will begin to increase across the West Coast
states by midweek as the aforementioned upper low approaches the
region and its attendant cold front moves ashore, focused mostly
over northern California/Nevada/Utah and southern Oregon/Idaho.
Schichtel
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