Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Fri Feb 22 2019
Valid 12Z Mon Feb 25 2019 - 12Z Fri Mar 01 2019
...Pattern Overview...
Latest guidance maintains and to some degree strengthens the Rex
block pattern that has generally been in place over the
east-central Pacific for the past couple weeks. A significant
aspect of the forecast evolution during next week will be the
expected northward drift of a strong upper high likely to start
the week near the southeastern coast of Mainland Alaska. As this
occurs, the core of strongest positive height anomalies should
lift into the Arctic. Teleconnections relative to such a
northward anomaly center favor a mean ridge close to the West
Coast and troughing over eastern North America. This trough
position is an eastward adjustment from the recent pattern but the
end result keeps a high likelihood of below normal temperatures
anchored over the northern Plains and vicinity. Meanwhile
progressive mid-latitude Pacific flow will favor potential for
significant focused precipitation over parts of the West Coast and
some inland terrain. Embedded energy in low amplitude flow
continuing eastward may produce one or more areas of precipitation
over the South and East--the latter in possible combination with
northern stream flow.
...Guidance/Predictability Assessment and Preferences...
Multi-day means into the D+8 time frame agree quite well with each
other for the overall pattern, and aforementioned teleconnections
with the positive height anomaly center lifting into the Arctic
yield a favored pattern consistent with what these means are
showing. Thus there is decent confidence that the general pattern
should gravitate toward a solution close to what the means show by
the end of next week.
However confidence is significantly lower for day-to-day details.
Upper highs over the northeastern Pacific/northwestern North
America can cause havoc with forecast details over the eastern
Pacific and the Lower 48, due to complex/high-sensitivity
evolution for shortwave energy on the southern/southeastern side
of these highs. Specifically for today's forecast, the
complexities involve a shortwave just off the Pacific Northwest as
of early Mon and an upper low approaching from the west along with
their possible interaction and subsequent movement. Details of
the upper ridge will also affect the ultimate shape of Canadian
flow that will eventually affect the Lower 48. Currently it
appears that the upper ridge will reach far enough north to have
less influence on a possible Pacific system nearing the West Coast
by next Fri.
The updated forecast blend starts with a combination of 18Z GFS
and 12Z ECMWF/CMC/UKMET model runs from day 3 Mon into day 5 Wed.
As of early Mon there is good overall clustering for the strong
storm lifting away from New England and low pressure nearing the
north-central West Coast. There is a general theme in the
guidance that features off the Pacific Northwest should evolve
into an upper low but with a moderate amount of spread for
position by Wed. Also by Wed there are hints in some guidance
that northern stream shortwave energy may produce a weak surface
wave reaching the east-central U.S. Latest GFS runs and 12Z ECMWF
are on opposite sides of the spectrum while a blend yields a
modest reflection similar to continuity. As operational runs
further diverge detail-wise, the forecast rapidly
incorporates/increases ensemble mean weighting (18Z GEFS/12Z ECMWF
mean) so that the means comprise 2/3 of the blend on day 6 Thu and
100 percent on day 7 Fri. On a relative basis the most agreeable
features by the end of the week appear to be the system coming
into the picture off the West Coast and southern Canada low
pressure whose trailing front would bring a reinforcing push of
cold air to the northern states.
...Weather Highlights/Hazards...
The pattern over the West will favor an axis of significant
precipitation (rain and higher elevation snow) from the
central/northern West Coast into the northern half of the Rockies.
The best probability for highest 5-day totals currently covers
northern California/Sierra Nevada range and western Oregon with at
least 5-10 inches liquid possible at some locations. The
precipitation axis should extend into favored terrain of
Idaho/Wyoming with somewhat less extreme totals.
Farther eastward expect strong winds to persist over the northeast
quadrant of the country into Mon as the deep weekend storm tracks
away from New England. Also anticipate some lake
effect/terrain-enhanced snowfall early in the week. At least one
area of moisture may spread mostly light-moderate snow from the
northern Plains into the Northeast during the week, but with low
confidence in specifics due to uncertainty over dynamics aloft and
associated surface reflection. Meanwhile progressive mid-lower
latitude shortwave energy may periodically enhance rainfall along
and north of the Gulf Coast region but guidance spread/continuity
changes decrease confidence in details. Depending on stream
interaction there is the possibility that some of this moisture
could reach farther north over parts of the East but the signal is
not very pronounced at this time.
Expect very cold conditions to continue over and near the northern
half of the Plains with temperatures at least 15-20F below normal
most days. Some areas in the extreme northern Plains/eastern
slopes of the Rockies may see highs 30-40F below normal early in
the week. A late week cold front could bring highs back down to
25-30F below normal on Fri. To a less extreme degree some of this
chilly air should extend across the Great Lakes/Northeast.
Pacific flow will tend to bring above normal mins to about the
southern 3/4 of the West while daytime highs should be within a
few degrees on either side of normal. The forecast looks similar
over the southeastern states.
Rausch
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found 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