Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Fri May 04 2018
Valid 12Z Mon May 07 2018 - 12Z Fri May 11 2018
...Pattern overview...
Latest guidance continues to show a less amplified mid-late spring
variation of the western ridge/eastern trough mean pattern that
has tended to be present across North America in recent months.
Within this pattern expect Pacific systems to weaken as upper
support heads into the mean ridge aloft, while downstream low
pressure will likely track from the northern-central Plains into
Great Lakes/Northeast. Shortwave energy within the eastern mean
trough may encourage wave development along a front lingering off
the East Coast.
...Guidance and uncertainty assessment...
Confidence for details of the upper trough initially over the East
is still fairly low. Among current guidance, energy within
stronger northern stream flow crossing New England is
slower/sharper in the 00z/06z GFS. The 06z GEFS mean hints at the
slower timing but is less amplified as of early day 3 Mon and then
compares well to the non-GFS consensus for the overall pattern by
early Tue. Meanwhile the diffuse nature of the portion of the
trough over central/southern portions of the East lowers
predictability for important details. The guidance average has
trended a bit deeper with the core of the feature aloft, resulting
in a somewhat better defined wave along the leading surface front.
From midweek onward guidance diverges significantly as 00z
ECMWF/CMC runs offer fairly rapid northeastward progression while
00z/06z GFS runs are quite slow while wrapping the surface wave
back to the East Coast. Overall prefer an intermediate solution
that is closer to the ensemble means given the lack of confidence
in details aloft and potential for the general evolution of flow
aloft to support somewhat faster progression than seen in the GFS.
There is generally better agreement/stability in principle for the
shortwave energy initially crossing the West and heading into the
eastern U.S. mean trough. Aside from a slightly faster trend over
the past couple days there has been reasonable model/ensemble
clustering for associated low pressure to track from the
northern/central Plains into or near the Great Lakes from Mon into
early Thu. After that time solutions show more divergence with
increasing influence from Canadian/northern U.S. troughing aloft
and leading cold front.
Over the past day guidance has greatly improved its clustering for
the path of the upper low initially over the eastern Pacific, with
the guidance majority now showing a track across Washington state
in contrast to some ECMWF runs that were farther southeast and GFS
runs that were farther northwest. Timing still becomes a problem
by the latter half of the period as the 00z/06z GFS stray faster
than other operational models and to some degree the GEFS mean as
well. This is at least partially due to faster arrival and/or
greater southeastward amplitude of upstream energy in the GFS.
Toward the end of the period the 00z GFS becomes particularly
pronounced with height falls reaching just off the West Coast. A
number of 00z GEFS members lean in that direction but the 06z
GFS/GEFS have backed off some. Remaining guidance keeps higher
heights/possible shortwave ridge over the extreme eastern Pacific.
By late in the period the combined teleconnections relative to
negative height anomalies south of the Alaska Peninsula and near
southern Greenland in multi-day D+8 mean charts offer better
support for some variation of the ECMWF/CMC and their means which
bring modest troughing into the West--though perhaps with a weaker
anchoring upper low than depicted in the operational runs.
Based on guidance comparison/considerations, an operational model
blend represented preferences well while downplaying any less
confident details of an individual run. Preferences over both the
West and East after Wed led to trending the days 6-7 forecast
toward more 00z ECMWF mean relative to the 06z GEFS mean along
with a lingering minority 00z ECMWF/CMC component. Any
adjustments from continuity were generally in the realm of typical
run-run variability.
...Sensible weather/threats highlights...
Detail uncertainty within the upper trough initially over the East
keeps confidence low regarding coverage/intensity of any potential
rainfall along the East Coast. The highest probability scenario
is for most significant activity to remain just offshore, but
there is still minority potential for moisture to wrap back as far
as the coast or slightly inland. This evolution will require
monitoring over the coming days as specifics gradually become more
clear. Farther west expect moisture initially over the northern
Rockies and vicinity to progress along with surface low
pressure/frontal system into and through the northern/central
Plains and eventually reach the East. Some Plains into Great
Lakes/Ohio Valley convection could be locally heavy and/or strong.
Meanwhile the system arriving from the eastern Pacific will bring
a period of enhanced rainfall to the Pacific Northwest/northern
Rockies around midweek.
The Interior West and portions of the High Plains should see the
warmest temperature anomalies during the period--plus 10-20F and
locally higher. There may be some daily records for highs/warm
lows. The Northwest will see a cooling trend Wed-Fri with the
arrival/passage of the system tracking out of the East Pacific.
Eastern states will see near to slightly below readings Mon-Wed
followed by a modest rebound to warmer conditions.
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