Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu May 09 2019
Valid 12Z Sun May 12 2019 - 12Z Thu May 16 2019
...Overview...
Most guidance shows the large scale mean flow transitioning to a
trough-ridge-trough pattern aloft by the mid-late part of next
week. On the way to this configuration, a strong upper trough
will cross the eastern half of the country Sun-Wed with fine-scale
details likely playing a role in exact evolution of surface low
pressure and coverage/intensity of precipitation. Upstream energy
feeding into the mean trough should push a front into the northern
Plains early in the week and farther southeast thereafter. Over
the West the pattern will take on a Rex block appearance through
the first part of next week as an upper low tracks across extreme
southern areas while a ridge drifts over the north-central
latitudes. In principle the upper troughing that amplifies toward
the West Coast has fairly good confidence but there is fair degree
of detail uncertainty.
...Guidance Evaluation/Predictability Assessment...
Guidance is still in the process of trying to resolve specifics of
the upper trough crossing the East Sun onward, along with the
corresponding surface evolution. Consensus has the trough
core--most likely a closed low--progressing from the Upper Midwest
through the Great Lakes and Northeast. The 18Z GFS/FV3 GFS
strayed south of consensus with the upper low, ultimately leading
to an offshore surface low to the south of most other
models/ensemble members by Tue. The new 00Z GFS made a partial
northward adjustment while the 00Z UKMET has trended south, at
least maintaining support for a compromise relative to the farther
north 12Z ECMWF/ECMWF mean. Based on guidance through the 18Z
cycle the preferred multi-model/ensemble blend (using the 12Z run
for the GFS component) yielded a surface low track between the 18Z
GEFS/12Z ECMWF means by Tue. Current clustering has the upstream
Upper Midwest wave becoming fairly suppressed after early Sun--as
previously advertised by some ECMWF/UKMET/CMC runs.
The next frontal system dropping into the northern Plains by Mon
and then continuing southeast has good agreement on existence but
differences aloft have been resulting in meaningful timing spread
and run-to-run variability. This favors a model/mean blend as the
best deterministic starting point. The average of current
guidance yields a slower trend compared to the cycle 12 hours ago.
Note that the 00Z GFS has a low-confidence Northwest U.S.
shortwave around Mon which disrupts downstream flow in later days,
so consensus of other models/means would appear better.
Multi-day trends for the upper low forecast to be just southwest
of Arizona as of early Sun have generally been slower along with
maintaining somewhat better definition as the feature reaches the
Plains. This favors somewhat more account for operational model
solutions relative to the means late in the period versus
preference for other areas of the forecast domain.
For the amplifying upper trough approaching the West Coast,
model/ensemble behavior thus far favors solutions in the middle to
slower half of the spread. In contrast to GEFS mean runs that
have been comparable to the ECMWF mean aside from modest details,
the operational GFS has tended to be faster bringing in height
falls over at least a part of the West--northern areas in some
runs, farther south in others. This tilts preference toward the
CMC/ECMWF and GEFS/ECMWF means.
Based on above considerations the forecast started with varying
weight of the 12Z models and minority 18Z GEFS/12Z ECMWF mean
input for about the first half of the period and then rapidly
adjusted to 60-70 percent weight of the ensemble means while
keeping some ideas of the ECMWF/CMC.
...Weather Highlights/Threats...
Expect a broad shield of precipitation over the East as frontal
waviness lifts through the region on Sun followed by possible
consolidation of low pressure off the northeastern coast. Best
potential for highest totals with this system should extend from
the central Gulf Coast through the Mid-Atlantic but with lingering
potential for significant amounts a bit northwest of this axis.
Confidence in specifics remains lower than desired in light of
persistent model spread. At least some of the precipitation could
fall in the form of snow over highest elevations of New England as
the core of the upper trough passes through. Behind this system a
cold front dropping southeast from the northern Plains may spread
rainfall of various intensity through the northern Plains and
Midwest/Great Lakes/Ohio Valley.
The upper low tracking from the western U.S.-Mexico border into
the Plains will be responsible for locally enhanced areas of
precipitation, including some convection, over southern parts of
the West and then likely extending into parts of the southern
Plains and vicinity. Confidence is still modest regarding
specifics of southern Plains activity, which will be important to
monitor given recent and upcoming short-range heavy rainfall over
the region.
The upper trough nearing the West Coast should bring increased
clouds/precipitation to the region after Tue. However
distribution and intensity of precip are uncertain as guidance has
yet to agree on the finer details of the trough.
Short-range warmth over the Northwest will continue into the first
half of next week, though with a steady cooling trend over
Washington/Oregon as upper troughing approaches. Expect highs of
10-20F above normal over the Interior Northwest and northern
Rockies Sun-Tue with coverage of plus 10F anomalies likely
decreasing significantly by Wed-Thu. Cool highs of 10-15F below
normal over the Southwest into southern High Plains will moderate
closer to normal as well. The system affecting the East during
the first half of the period will keep highs 10-15F or so below
normal for one or more days from parts of the Ohio Valley/eastern
Great Lakes into the northern Mid-Atlantic/Northeast.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml