Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 AM EDT Wed May 09 2018
Valid 12Z Sat May 12 2018 - 12Z Wed May 16 2018
...Pattern overview...
An upper low is expected to meander over the Great Basin this
weekend into early next week with zonal flow across the
central/eastern states. An upper low just north of Hudson Bay will
promote broad cyclonic flow over southeastern Canada and along our
shared border into/through the Great Lakes and Northeast. At the
surface, a front associated with the upper low in the west will
slowly push out of the Rockies by next week as a Pacific front
finally approaches the Pacific Northwest coast around next
Tuesday. The eastern extent of the front will ebb and flow
northward and southward in the east as successive waves exit off
the east coast.
...Guidance/uncertainty assessment/preferences...
Models/ensemble have quickly to come a consensus over the past 24
hours, keeping an upper low back across the Great Basin for much
longer than most guidance previously indicated. Model/ensemble
spread during days 3-5 (Sat-Mon) was sufficiently low to justify a
multi-model deterministic blend (including the 12Z ECMWF/UKMET/CMC
and 18Z GFS) as a forecast starting point. This blend also
resolves the stationary front (and a few waves along it) well,
extending from the Great Basin/Rockies east across the plains/Ohio
valley to the Mid-Atlantic. One wave of low pressure develops
across the Ohio valley on Sat and then quickly crosses New England
by Sun morning. A couple additional waves of low pressure will
traverse the front across the central U.S. Sat-Sun, the strongest
of these likely crossing the central plains on Sat.
Spread does increase during the days 6-7 (Tue-Wed), with some
continued disagreement on how quickly the upper low moves east
into the plains. The ECMWF remains on the fast side of the spread,
outpacing the GFS and CMC, and even (slightly) the 12Z ECENS mean.
The 12Z ECMWF was excluded from the blend for days 6-7 given its
acceleration of shortwave energy into the Upper Midwest while the
consensus (and the trend) has been toward a slower progression.
Additional shortwave energy and associated surface front are also
expected to reach the Pacific Northwest by Tue, and models show
some timing differences, with the 12Z ECMWF on the fast side and
the GFS slower (also showing a closed upper low). Favored a shift
to an ensemble approach during this time frame, with increased
weight placed toward the 12Z ECENS/NAEFS means.
...Sensible weather/threats highlights...
The wavy/stationary frontal boundary extending from the central
Rockies and adjacent plains to the Great Lakes/Northeast from Sat
onward will serve as a focus for scattered to numerous showers and
thunderstorms over a large area. Models continue suggest the
potential for areas of heavy rain (and high elevations snows)
across portions of the central Rockies Sat-Sun (especially
northeastern Nevada eastward into Wyoming) as the vigorous upper
trough amplifies to the west, and a surface low develops along the
front across the central plains, enhancing low-level easterly
upslope flow. A number of deterministic solutions show the
potential for localized multi-day totals of at least a few inches.
A the front exits toward the plains next week, precipitation will
expand from northern Texas northward to Kansas and eastward along
the frontal boundary. By next Tue-Wed, convective activity may
once again expand north to the northern plains/Upper Midwest as
upper energy finally begins to exit the Rockies, enhancing
upper-level support for precip, and the central plains surface
front lifts north as a warm front.
Farther east, broad southerly flow across the plains will
transport moisture north and lift it across the frontal boundary,
and models show potential for multiple rounds of convection along
the front Sat-Sun across portions of the Midwest/Great Lakes and
perhaps even into portions of the Northeast or Mid-Atlantic, with
the potential for areas of locally heavy rainfall. Confidence in
the specific locations of any heavy rain is relatively low at this
time, and would likely be determined by mesoscale/convective
processes. Farther south, a dissipating frontal boundary will
focus increasingly widespread shower and thunderstorm activity
across the southern/central Florida Peninsula from late this week
into early next week. Areas of heavy rainfall will be possible
given the multi-day nature of the event and high Pwat values (+2
to +3 standard deviations).
High temperatures are expected to be 10 to 20 deg F above average
across much of the southern tier on Sat, with the center of
greatest anomalies shifting east to the lower Mississippi valley,
Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F
anomalies). Record high temperatures are quite possible from the
Mississippi valley to portions of the Southeast Saturday-Monday
and maybe into Tuesday. Temperatures to the north near the
Canadian border will be below average on the cool side of the
front.
Ryan/Fracasso
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