Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 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 that should migrate from
near the Aleutians into the northern Pacific before possibly
wobbling back to the north somewhat. A couple Pacific shortwaves
ejecting from the mean upper low will erode the western ridge
during the forecast period, but by next weekend most guidance
shows a building eastern Pacific ridge reaching at least as far as
the West Coast. Downstream from the mean ridge expect shortwave
energy to amplify over central U.S. and then move east along the
northern periphery of a strengthening subtropical ridge that will
serve to raise heights across southeastern states through the end
of the week.
...Guidance and uncertainty assessment...
There is still significant model/ensemble spread over the eastern
Pacific/western U.S. from late week into the weekend but trends
over the past 12-24 hours have at least led to better guidance
agreement in principle for the time being. This is a result of
ECMWF/CMC runs toning down the depth and closed nature of upper
energy dropping into the West from Fri onward, ensemble means
trending somewhat deeper/sharper with the upper trough, and
GFS/GEFS guidance becoming less aggressive in trying to push
upstream Pacific flow toward the West Coast--thus allowing for
higher East Pacific heights and more western troughing as have
been favored by teleconnection relationships relative to the
negative height anomaly center between Hudson Bay and southern
Greenland. Among latest runs the 00z GFS represents a scenario
more similar to older ECMWF/CMC runs that were deeper and more
closed over the West (but similar to consensus in concept) while
after Fri the 06z GFS appears to stray to the fast side with the
trough and upstream Pacific flow. The 06z GEFS mean also shows a
somewhat faster trend versus the 00z cycle, so the 00z cycle would
be preferred for any GFS/GEFS input.
Farther east, a consensus forecast maintains continuity well for
the system tracking out of the Plains into the Great Lakes early
in the period. Toward Fri-Sat a multi-solution blend helps to
temper the 00z ECMWF that is on the stronger side (albeit within
the ensemble envelope) with the overall upper troughing that
crosses eastern North America. By the latter half of the period
the greatest uncertainty will likely be the position of a fairly
strong and wavy front extending between Plains low pressure and
the East Coast. The stronger dynamics aloft in the 00z ECMWF lead
to a farther south backdoor front along the coast while wave
details farther westward will be sensitive to shortwave specifics
(impulses possibly ejecting from the developing western trough as
well as the southern fringe of northern stream flow) that have low
predictability several days out in time. Prefer to maintain an
intermediate scenario with below average confidence.
The updated forecast maintained continuity in principle while
refining some details per the majority guidance cluster. Primary
emphasis days 3-5 Wed-Fri was on the 00z ECMWF/00z-06z GFS with
lesser input from the 00z UKMET/CMC and 00z NAEFS/ECMWF means.
Ensemble mean weight increased for days 6-7 Sat-Sun with minority
components of the 00z ECMWF/GFS/CMC retained through the end of
the period.
...Sensible weather/threats highlights...
The system crossing the Midwest/Great Lakes/Ohio valley Wed-Thu
will likely produce scattered to numerous showers and
thunderstorms. Some locally heavy/strong activity is possible but
the probability of widespread heavy rainfall appears fairly low.
Showers and storms may reach the Northeast by Thu night. Arriving
Pacific energy will support a period of precip over the Northwest
and northern Rockies around midweek followed by areas of
rain/thunderstorms progressing across the northern half of the
Plains and beyond. There is still uncertainty in the evolving
pattern but some improvement in confidence toward the threat of
significant rainfall over and downstream from the north-central
High Plains. There is increasing potential for slow moving low
pressure over the High Plains with the wavy front extending
east-northeast serving as a focus for rain/storms as flow to the
south of the front transports Gulf of Mexico moisture northward.
For the overall period the full array of model/ensemble guidance
highlights an area from the north-central Plains into Great Lakes
as having the greatest probability of receiving the highest
rainfall. Meanwhile the Florida Peninsula should see a steady
increase in rainfall, some potentially becoming heavy, as flow
brings moisture in from the east/southeast.
Over the West expect well above normal temperatures to become more
suppressed relative to Wed as shortwave energy erodes the western
ridge aloft. Best potential for plus 10-20F or slightly greater
anomalies during Wed-Fri, and scattered daily records, will extend
from the Great Basin into southern High Plains. Cooler air will
spread from northern areas into the Interior West/Rockies while
the building ridge nearing the West Coast will promote a
significant warming trend over WA/OR/CA next weekend with some
highs 10-20F above normal. Farther east expect cool air to push
southward from the northern Plains with some highs at least 10-15F
below normal from the northern Plains/upper MS Valley into
northern-central High Plains around Fri-Sat. Warmth moving into
the East by Thu will persist through the rest of the period to the
south of a sharp front setting up over north-central latitudes,
with readings 5-15F above normal.
Rausch
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