Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
216 AM EST Wed Feb 27 2019
Valid 12Z Sat Mar 02 2019 - 12Z Wed Mar 06 2019
...Pattern Overview...
Broadly cyclonic mean flow across the Lower 48 at the start of the
weekend should transition toward increasingly amplified troughing
over the East by the middle of next week as an upper ridge reaches
the Rockies and a trailing trough approaches the West Coast. Mean
ridging aloft over northwestern North America (specifically an
initial upper high dropping south over western Canada followed by
another ridge building over eastern Alaska and the Yukon) will
support a core of extremely low temperatures over the central U.S.
with anomalies up to 30-45F below normal during Sat-Mon and likely
widespread new daily records. Some of this cold air will extend
into northern parts of the West and through much of the East. Two
southern stream Pacific systems will bring more significant
precipitation to parts of the West with heaviest focus over
California, while there is still a signal for heavy rainfall
potential over parts of the Southeast. Farther north over the
eastern states there will be one or more possibilities for snow
but with a fair degree of uncertainty over the specifics.
...Guidance Evaluation/Preferences...
Early in the period a blend of operational model guidance through
the 12-18Z cycles provided a good starting point for yielding the
desired solution for systems of interest. There have been
continued differences for the overall area of low pressure
expected over the northeastern U.S./southeastern Canada and off
the New England coast during the weekend. In the 12-18Z array of
guidance there were essentially two clusters, the GFS/CMC and
their means that were better defined along/inland from the New
England coast and the ECMWF/ECMWF mean/UKMET that placed more
emphasis on offshore low pressure. Dependence on low
predictability small scale features (originating in part from the
upper low now near the Pacific Northwest) favored a blended
approach that kept the forecast close to continuity. There have
been some trends toward a more fragmented ejection of that upper
low energy so the weaker adjustment in the 00Z GFS is plausible.
Meanwhile trailing waviness affecting the Southeast/Mid-Atlantic
will be influenced by energy from the system initially nearing
California (consensus mean still looking good for this system) and
possibly interaction with northern stream flow. The multi-model
blend yielded a forecast close to the 12Z ECMWF mean, a reasonable
intermediate solution between the weaker GFS/GEFS mean runs and
stronger/northward 12Z ECMWF/UKMET/CMC. A majority of new 00Z
runs (minus the GFS) maintain support for significant waviness.
During the latter half of the period, model/ensemble clustering
has shown a pronounced deeper trend with the upper trough axis
that crosses the Midwest/Great Lakes and vicinity. However latest
operational runs have shown less evidence of a closed low than 24
hours ago. Recent GFS runs strayed a little faster than consensus
with this trough over the East by Tue-Wed. Meanwhile by day 7 Wed
the next system to affect the West Coast shows expected
track/timing differences but in general the clustering is not bad
for that time frame and an intermediate solution appears
reasonable at this time. One complexity that will have to be
monitored is potential for elongating southwestern Canada energy
to close off an upper low off Vancouver Island and have some
influence on the incoming system. Overall both the models and
means appeared to provide some useful information to contribute to
the days 6-7 forecast so the blend followed an even weight between
the 18Z GEFS/12Z ECMWF means and 12Z ECMWF-CMC/12-18Z GFS.
...Weather Highlights/Hazards...
Across the West the primary axis for heaviest rain and higher
elevation snow will extend from California eastward through the
central/south-central Rockies with the Sierra Nevada likely seeing
the highest 5-day totals. Two systems will affect the region
during the period, one arriving early in the weekend and spreading
moisture rapidly eastward then followed by a second arriving
Tue-Wed. As moisture from the second system spreads northward
there may be potential for wintry precipitation at low elevations
over interior portions of the Northwest given the cold air likely
to be in place.
The system affecting the Northeast during the weekend should bring
some snowfall to the region but weakening and possibly offshore
trends for low pressure as well as fast progression may limit
accumulations. Southern stream energy interacting with the
trailing front may produce a period of heavy rainfall over parts
of the Southeast with the weekend into early Mon time frame the
most likely period for this activity. Guidance has not yet fully
converged for details so confidence is only moderate thus far.
Snow will be possible in the northern part of the moisture
shield--from the south-central Mississippi Valley into the
Mid-Atlantic. Lighter rainfall could linger over parts of the
extreme Southeast/Florida into next week.
Within the widespread area of extremely cold air over the central
U.S., expect the most extreme anomalies to be over the
northern-central Plains during Sat-Mon with anomalies of 30-45F
below normal. Especially on Sun-Mon many places over the Plains
and Midwest should challenge or exceed daily record
values--especially for cold highs but also for morning lows at
some locations. Less extensive coverage of daily records may
continue into Tue. The eastern U.S. will see a modified form of
this cold air by the first half of next week with readings "only"
10-20F below normal. Localized parts of the interior Northwest
may see temperatures 10-30F below normal. Relative warmth will be
confined to the southern half of the West and more so for morning
lows than afternoon highs.
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