Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Feb 23 2019
Valid 12Z Tue Feb 26 2019 - 12Z Sat Mar 02 2019
...Pattern Overview...
The expected mean pattern and embedded uncertainties look fairly
similar to 24 hours ago. A Rex block configuration should persist
over the east-central Pacific with a strong ridge/closed high
meandering over eastern Alaska and northwestern Canada while
features dropping into the mid-low latitude mean trough progress
toward the West Coast. In the multi-day mean perspective
teleconnections relative to the ridge's positive height anomaly
continue to favor the consensus idea of a modest mean ridge close
to the West Coast by late period and troughing over eastern North
America. There are multiple significant uncertainties with
embedded details though. The most common themes in guidance
continue to be the potential for significant precipitation from
the central and northern West Coast into the northern Rockies and
persistence of very cold temperatures over and near the northern
half of the Plains. There will likely be multiple precipitation
areas over the eastern half of the country during the Tue-Sat
period but specifics will be sensitive to the uncertain details at
the surface and aloft.
...Guidance/Predictability Assessment and Preferences...
The difficulty that an upper high over the northeastern
Pacific/northwestern North America tends to create for the
forecast is evident again today, with significant spread quickly
developing for features to the south and southeast of the high.
Already from very early in the period there are meaningful
differences for flow details over and offshore the Pacific
Northwest as well as for downstream flow over the northern
U.S./southern Canada. Regarding the former, there is agreement in
principle that initial energy should close an upper low while
mid-latitude energy approaching from the west is incorporated into
the southern side of the circulation by day 4 Wed. However there
are significant position differences for this upper low as of
early Wed with further divergence thereafter. Latest GFS runs
including the 00Z version are particularly fast with this system
versus most other guidance, ultimately leading to another very
deep Great Lakes storm by Sat. The 18Z FV3 GFS clusters toward
the other slower solutions. Overall preference leans increasingly
away from the GFS by mid-late period. Meanwhile out-of-sync
shortwave details from the northern Plains eastward (with
sufficiently small scale leading to fairly low predictability)
affect possible surface waviness and snowfall amounts/coverage.
The GFS continues to be on the strong side of the full spread with
any low pressure while ECMWF/ECMWF mean runs persist with no
reflection at all. Taken as a whole there seems to be more
potential for modest waviness than indicated by the ECMWF scenario
but it should be weaker than in the GFS--favoring an intermediate
solution.
Later in the period there is a reasonable degree of
persistence--though with a somewhat slower trend--for the system
likely to track toward the West Coast. This slower trend argues
for at least intermediate timing between the faster 12Z
ECMWF-ECMWF mean/18Z FV3 GFS and other somewhat slower solutions.
By Sat ECMWF/ECMWF mean scenario of keeping the early period upper
low offshore Vancouver Island would lead to some influence on the
system while other guidance shows inland ejection albeit slower
than in the GFS. Farther east one notable divergence is for the
path of a southern Canada upper low, for which GFS runs lean to
the southern part of the spread. Even if the sequence of events
leading to the deep eastern storm by next Sat currently looks
questionable, there is a signal for some amount of surface
development (such as in the past two ECMWF runs and 12Z/18Z GEFS
means. Albeit with very uncertain latitude there is somewhat more
agreement toward the idea of low pressure just off the East Coast
during the day Sat as height falls aloft approach.
The updated forecast starts with a multi-model (18Z GFS/FV3 GFS
and 12Z ECMWF/UKMET/CMC) blend during the first half of the period
to reflect a consensus or intermediate solution as appropriate for
individual features early in the period. By mid-late period the
forecast includes 30-60 percent total input of the 18Z GEFS/12Z
ECMWF means, with some ideas from the 12Z ECMWF/CMC and 18Z FV3
GFS for localized detail.
...Weather Highlights/Hazards...
Over the West some of the day-to-day details have varied but
guidance is consistent in principle with the potential for
significant precipitation--in the form of rain and higher
elevation snow--from the central/northern West Coast into the
northern Rockies. There may be a brief break in activity around
late week between initial moisture flow and the arrival of another
system by Sat. Best probability for highest 5-day totals still
covers northern California/Sierra Nevada range and southwestern
Oregon with at least 5-10 inches liquid possible at some
locations. Expect meaningful but somewhat lower max totals over
the northern Rockies.
Precipitation coverage and intensity from the Plains to East Coast
continue to be difficult to pinpoint due to significant
differences in shortwave energy and surface evolution. From Tue
into Wed night an area of mostly light snow may progress eastward
from the Plains. Meanwhile southern stream impulses may promote
periods of rainfall across the Gulf Coast states over the course
of the period. Toward Fri-Sat there may be an expansion of
precipitation coverage over the eastern states (snow north and
rain south) in association with some degree of surface development
over the East and/or extreme western Atlantic.
Within the area of much below normal temperatures centered over
the northern half of the Plains, coldest anomalies (at least 30F
below normal at some locations) should exist early and late in the
period. Some of the cold air will extend eastward early-mid
period followed by a moderating trend. Flow from the Pacific will
keep morning lows above normal over all but the extreme northern
parts of the West, as well as over the southern tier, while clouds
and precipitation will tend to keep highs closer to normal.
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