Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Fri May 04 2018
Valid 12Z Mon May 07 2018 - 12Z Fri May 11 2018
...Pattern overview...
The large scale flow pattern across North America during the
medium range looks to return to a ridge in the west/trough in the
east pattern which has been typical for much of the past several
months. The amplitude of the pattern will be less than in recent
months, however, more typical of mid to late spring.
Characteristics of this pattern will include weakening Pacific
frontal boundaries moving into the western U.S. as the associated
upper-level shortwaves encounter a broad area of anticyclonic
upper flow. Meanwhile, the central U.S. to the Great
Lakes/Northeast will be the preferred storm track east of the
Rockies. Additionally, a lingering surface front off the Eastern
Seaboard shows at least some potential among the guidance for
surface wave development.
...Guidance and uncertainty assessment...
Models show widely varying solutions for an upper shortwave
initially across the Ohio/Tennessee valleys, with the 12Z ECMWF
the most aggressive with amplifying this feature and keeping an
upper low in place across the Southeast even into Day 5 (Wed). The
ECMWF was supported to some degree by the 12Z CMC. Meanwhile, the
12Z/18Z GFS along with the UKMET were much weaker. Ensemble means
seemed to support a middle ground solution, with the GEFS/NAEFS
means both deeper than the GFS/UKMET. The evolution of this
features will also significantly influence the development of a
potential wave of low pressure along a lingering surface front off
the Eastern Seaboard. The WPC forecast took a compromise approach,
not as deep as the 12Z ECMWF, but more so than the weaker
solutions.
Elsewhere, models showed typical timing/amplitude differences with
additional shortwave energy crossing the Midwest Tue-Wed, as well
as with an upper low approaching the Pacific Northwest during the
same time frame. The 18Z GFS was likely the strongest outlier with
respect to the latter, seeming to weaken the system and move it
inland too quickly. Ensemble means tended to favor a slower/deeper
solution as shown by the ECMWF/CMC/UKMET. By Day 7 (Thu),
solutions differ as the energy cuts into the upper ridge across
western North America. The 12Z ECMWF suggested some potential for
interaction/phasing with higher latitude energy rotating around a
broad area of negative height anomalies center over Hudson Bay,
while the GFS/CMC keep a stronger ridge in place, keeping the
Pacific energy more separated from the northern stream. Given
significant run-to-run variability by late in the period,
confidence is very low in the details, and the forecast was
trended heavily toward the ECENS/NAEFS means.
...Sensible weather/threats highlights...
The aforementioned variations among the guidance with respect to
the shortwave across the eastern U.S. on Days Mon-Tue have
significant implications for if/where precipitation will occur. A
stronger ECMWF-like solution would result in the potential for
heavy rainfall across portions of the Southeast. The compromise
approach favored for this system would temper the heavier precip
shown by the ECMWF somewhat, although the potential for at least
locally heavy rainfall is there. Meanwhile, showers and
thunderstorms will be possible from the central/northern High
Plains to the Midwest Tue-Thu as a surface frontal wave develops
east of the Rockies.
The building ridge across the West will most of the region dry,
with above average temperatures (high temps 10-15 deg F above
average). The Pacific Northwest will be the only area to see
potentially wet conditions, showers increasing by mid-week as a
Pacific front/upper low move onshore.
Ryan
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