Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
119 AM EST Thu Feb 14 2019
Valid 12Z Sun Feb 17 2019 - 12Z Thu Feb 21 2019
...Multi-day heavy rain event possible for portions of the
Southeast, Tennessee Valley, and Southern Appalachians next week...
...Overview...
Upper-level ridge across the North Pacific shows no signs in the
guidance of breaking down during the medium range, and thus will
continue to influence the large scale flow across the CONUS. This
will reinforce a pattern with a trough in the West and ridging off
the Southeast U.S. coast through the next week. A rapid succession
of Pacific shortwave energy will traverse the flow, amplifying the
mean trough over the West before ejecting east and interacting
with a lingering polar front along the Gulf Coast to generate
frequent frontal waves which will affect the Southeast and
Mid-Atlantic regions during the forecast period.
...Guidance/Predictability Assessment and Preferences...
Model consensus was sufficient during days 3-4 (Sun-Mon) to use a
multi-model deterministic blend (12Z ECMWF/UKMET/CMC and 18Z GFS)
as a basis for the forecast. Models showed some variability on the
specifics of a frontal wave crossing the eastern U.S. during that
time frame, with variations as to how quickly energy transfers
from a low over the southern/central Appalachians to a second low
off the Mid-Atlantic coast, but a multi-model consensus was
preferred. Models showed relatively good consensus with amplified
shortwave energy digging into the Southwest Sun-Mon as well.
From day 5 (Tue) onward, models continue to show relatively good
agreement when it comes to the big picture, but larger differences
begin to emerge on the specifics. Confidence is relatively high
that energetic shortwave energy should move from the Four Corners
region Tue morning into the central U.S., weakening to some degree
as it begins to encounter anticyclonic flow on the western
periphery of the subtropical ridge. Differences among the guidance
as to how quickly this wave lifts out and the degree of
interaction with the northern stream result in varying surface
evolutions. There is at least general consensus that another
frontal wave should develop in the northwestern Gulf of Mexico Tue
and then lift northeastward across the Southeast Tue night/Wed.
Run-to-run variability among deterministic solutions but a
moderate degree of ensemble clustering lent to gradually heavier
use of ECENS/GEFS mean solutions. Meanwhile, as additional
shortwave energy dives south along the West Coast and reinforces
the mean trough, solutions vary widely as to the specific nature
of the next wave, and its effect on the larger mean trough.
Teleconnections relative to the strong North Pacific ridge support
the idea of keeping the western U.S. mean trough from progressing
east too quickly, and the ECENS mean has more consistently held
this feature farther west relative to the GEFS mean by the middle
of next week. Thus, the forecast during days 5-7 was based more
heavily on the ensemble means, with more weight placed on the
ECENS relative to the GEFS.
...Weather Highlights/Hazards...
The polar front will linger across the Southeast/Gulf Coast
through the forecast period, with multiple waves/surface lows
traversing the front bringing several rounds of widespread and
potentially heavy rain. Model/ensemble guidance show the strongest
signal for heavy rains across portions of central/northern
Mississippi, Alabama, and Georgia, as well as Tennessee and
western portions of the Carolinas. Multi-inch rainfall totals are
possible across these areas from Sun onward into the middle of
next week. Colder air in place on the northern periphery of these
low pressure systems may result in areas of wintry precipitation
for portions of the central Appalachians and Mid-Atlantic.
Meanwhile, conditions along the West Coast will be at least a bit
quieter than seen recently, as the main upper trough axis moves
inland. This will spread snows into the southern Rockies Sun-Mon.
More widespread rain/snow may return to the Pacific Northwest by
next Tue-Wed as a more significant shortwave approaches.
Well-below average temperatures will persist across much of the
western/central U.S. during the next week as the upper trough axis
moves inland. Temperatures as cold as 15 to 30 deg F below average
will initially be concentrated across the northern/central High
Plains Sun, but will become more widespread from Mon onward, with
much of the western and central U.S. expected to see temperatures
ranging from 10 to as much as 30 deg below average.
Ryan
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