Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Feb 28 2019
Valid 12Z Sun Mar 03 2019 - 12Z Thu Mar 07 2019
...Pattern Overview...
The forecast of large scale flow has maintained good continuity
over the past day. Downstream from persistent mean
ridging/positive height anomalies over northwestern North America,
a trough will amplify into the eastern U.S. with greatest depth
around midweek before the trough axis continues into the western
Atlantic. At the same time a mean ridge will build into the
west-central U.S. as a mean trough settles near the West Coast.
There are embedded details that will take time to resolve though,
especially over the East early in the week and near the West Coast
from Tue onward. Accompanying this evolution will be an episode
of extremely cold temperatures for early March over the central
U.S. with some of this cold extending through almost all of the
eastern states as well as back into the interior Northwest. A
fast moving wave will spread a broad area of various precipitation
types across the East early in the week and another wet southern
stream Pacific system will bring more moisture into the West
starting Tue-Tue night.
...Guidance Evaluation/Preferences...
Over the past couple days there has been a pronounced
northwestward trend for a wave expected to track from the lower
half of the Mississippi Valley into the Canadian Maritimes during
Sun-Mon. This trend is due to a combination of faster adjustments
for shortwave energy streaming into California on Sat and changes
toward slower progression of northern tier U.S. height falls into
the Northeast--allowing for more interaction. The ECMWF has been
leading this trend over the past couple days while the GFS has
been slowest to trend to this scenario. The 12Z ECMWF was in the
northern minority among all ensemble members by late Sun-Mon so
preference was to follow a track slightly to its south (compromise
with latest UKMET/CMC runs), but trends do provide more confidence
in the northwest half of the spread versus the southeastern half.
New 00Z runs offer hope that the forecast could become a little
more stable soon.
The eastern Pacific/West Coast region will be a focus for
significant detail uncertainty by mid-late period. The full array
of model/ensemble guidance shows a variety of possible evolutions
involving three separate features: 1. sheared southwest British
Columbia energy that will likely form a closed low somewhere
between the Pacific Northwest and offshore Vancouver Island, 2.
Alaska/northeastern Pacific shortwave energy dropping practically
due southward from Sun onward, and 3. a closed southern stream
system reaching near 135W by Tue. Thus far recent CMC runs have
been farthest south/southeast with the northern upper low thus
suppressing surface low pressure nearing the West Coast by
midweek--a seemingly low probability solution at this time. In
spite of the potential complexity of evolution, the ensemble means
have been remarkably stable/agreeable thus far with low pressure
nearing extreme northern California by early Wed. The 12-18Z
GFS/12Z ECMWF runs were very close to their respective means so
those solutions provided the basis for the updated forecast with
good continuity. 12-hourly GFS runs have been fairly stable for
the past day or so while ECMWF runs have been oscillating between
12Z runs near the ensemble mean consensus and 00Z runs that were
12-24 hours slower.
Based on above considerations the updated forecast blend went 70
percent toward the 12Z ECMWF/UKMET/CMC cluster for days 3-4
Sun-Mon and then removed the CMC from consideration due to its
suspect solution along the West Coast. At the same time the
forecast started to incorporate the 18Z GEFS/12Z ECMWF means on
day 5 Tue with a gradual increase of the means to slightly more
weight than the 18Z GFS/12Z ECMWF by day 7 Thu.
...Weather Highlights/Hazards...
A broad area of precipitation will accompany a vigorous wave
tracking northeast over parts of the eastern states and into the
Canadian Maritimes Sun-Mon. Northwest trends in the past day have
pushed the most likely area for significant wintry weather to an
axis extending from the Mid-Mississippi Valley/Ohio Valley into
the Northeast. A more consistent theme from yesterday is the
potential for heavy rainfall over parts of the Lower Mississippi
Valley and Tennessee Valley/interior Southeast. Much of the East
will trend drier after the wave's passage though moisture may
linger for a time over the extreme Southeast/Florida. Expect
periods of lake effect snow downwind from ice-free areas of the
Great Lakes and one or two areas of light snow may pass through
the Plains/Midwest.
Rain and higher elevation snow in progress over the
central/south-central parts of the West late in the weekend will
taper off from west to east. Then from Tue onward the next system
in the series will spread a broad area of moisture across the
West, with greater northward extent than with the previous system.
Favored terrain over northern and central California should once
again see the highest precipitation totals with this system.
Moisture reaching northern portions of the West may bring wintry
weather to low elevations over some areas given the cold
temperatures in place from early in the week.
Continue to expect coldest temperatures relative to normal during
Sun-Mon with some central U.S. locations seeing anomalies of
30-45F below normal and a widespread area over the Plains/Midwest
with readings that will approach or break daily records--more
likely a greater number for cold highs. Interior Northwest
locations may also see some temperatures at least 20F below normal
with local challenges to daily records. The area of potential
daily records will likely shift more into the eastern half of the
country (Midwest/Great Lakes/Ohio Valley) by Tue. As of Thu
expect coverage of minus 20F or greater anomalies to decrease
considerably but the eastern half of the country will still be
chilly versus normal. The southern 2/3 of the West continues to
look warm for morning lows while highs should be within a few
degrees of normal.
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