Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
122 AM EDT Sun May 06 2018
Valid 12Z Wed May 09 2018 - 12Z Sun May 13 2018
...Pattern overview...
An upper ridge will initially be in place across western North
America, downstream from a mean upper low migrating from near the
Aleutians into the northern Pacific. A couple Pacific shortwaves
ejecting from the mean upper low will attempt to erode the ridge
through the forecast period, but by late in the period most
guidance show the ridge once again exerting itself along the West
Coast. Downstream of the broad area of anticyclonic upper flow,
the tendency will exist for shortwaves to amplify across the
central U.S. and then move east along the northern periphery of
the subtropical ridge, which will also attempt to raise heights
across southeastern states.
...Guidance and uncertainty assessment...
The WPC forecast was initially based on a blend of majority
deterministic guidance, including the 12Z ECMWF/UKMET along with
the 12Z/18Z runs of the GFS. Most emphasis was placed on the ECMWF
and GFS. The forecast was transitioned to a majority ensemble
means (12Z ECENS/NAEFS) during days 6-7 to account for
substantially decreased confidence during that time frame.
Model solutions have come into somewhat better agreement with
respect to the trough initially along the Eastern Seaboard on Wed,
and the blended approach described represents the current
consensus solution. The same holds true for additional shortwave
energy crossing the Midwest/Great Lakes Wed-Thu. Timing
differences shown by the guidance at this time last night have
decreased, and the ECMWF/GFS solutions were well clustered within
the consensus with respect to a surface low tracking across the
Midwest/Great Lakes Wed night/Thu, and the cold front reaching the
East Coast by Fri.
The most uncertain aspects of the forecast involve energy
originating in the northern Pacific which moves into the Pacific
Northwest/western Canada and attempts to weaken the upper ridge.
The first of these systems should reach the Northwest on Wed.
There has been somewhat of a trend over the past day of weakening
this shortwave/upper low more quickly (the GFS was the model to
lead this trend), but a number of ensemble members as well as the
12Z UKMET continued to suggest keeping the system closed for
longer, which results in a slower inland progression and somewhat
less weakening of the ridge. The WPC forecast was adjusted in the
direction these latest trends, and a somewhat quicker progression
of the shortwave and associated surface front inland Wed-Thu. A
second shortwave reaches the coast Wed night-Thu, and the
evolution of this features as it moves inland is the area of
greatest uncertainty this forecast period. The ECMWF continues to
quickly amplify this second wave across the northern Great
Basin/Intermountain Region Fri-Sat. The GFS has trended a bit
toward the ECMWF, but remains less amplified. At the same time
both solutions show the upper ridge becoming reestablished
along/just off the West Coast, with perhaps a highly amplified
flow pattern developing from the north central Pacific into the
western half of North America. ECENS/NAEFS means represented a
good compromise between the ECMWF/GFS solutions, and a blend
weighted toward these two means was favored by late in the week.
The 18Z GEFS was a significant outlier in terms of the ensemble
means, quickly bringing the mean upper low east across the north
Pacific, which displaces the re-amplifying ridge farther east
(across the Rockies) relative to the ECENS/NAEFS which keep it
more along the West Coast. Based on the broad agreement shown
between the latter to means (and a large amount of spread among
GEFS members), the 18Z GEFS solution was discarded.
...Sensible weather/threats highlights...
The shortwave/cold front crossing the Midwest/Great Lakes/Ohio
valley Wed-Thu is expected to produce scattered to numerous
showers and thunderstorms. Widespread heavy rainfall is not
expected, but locally heavy rain will be possible due to the
convective nature of much of the precipitation. Showers and storms
may reach the Northeast by Thu night. Meanwhile, the Northwest and
northern Rockies will see scattered precipitation Wed-Thu as the
aforementioned Pacific shortwaves move inland. As the second wave
amplifies across the Rockies by late in the week, the impacts
associated with the system could increase. While confidence is low
in any specifics for reasons described above, a surface low
pressure area forecast to develop along a stalled frontal boundary
across the central plains Fri-Sat has the potential to produce
widespread convection and heavy rainfall for portions of the
central/northern High Plains and Midwest into the weekend. The
orientation of the upper flow by that time would be supportive of
efficient transport of Gulf of Mexico moisture northward to the
front, and a number of deterministic solutions show the potential
for heavy rainfall across differing areas.
Temperatures will start off the period well above average across
much of the interior western U.S. beneath the upper ridge. Highs
temperature anomalies of +10 to +20 deg F are possible Wed and
Thu, before moderating slightly (but remaining above average)
Thu-Sat as the ridge flattens a bit. Much of the central/eastern
U.S. will begin the forecast period near or above average on Wed.
The northern tier, from the northern plains to the Northeast will
see cooling temperatures Thu-Sat in the wake of a cold front, and
highs of 5 to 10 deg F below average are possible. By late in the
week, a gradually expanding subtropical ridge will support
increasing temperatures across the Southeast. High temperatures
anomalies Fri-Sun of +10 to +15 deg F are possible.
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