Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1059 AM EST Thu Feb 14 2019
Valid 12Z Sun Feb 17 2019 - 12Z Thu Feb 21 2019
...Multi-day heavy rain event quite possible for portions of the
Southeast, Tennessee Valley, and Southern Appalachians next week...
...Overview...
Upper-level ridging across 1) the North Pacific and 2) near the
Bahamas 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 wavy southwest flow in the eastern half of the
CONUS 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 (00Z ECMWF/CMC/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 TN Valley/Southeast Tue
night/Wed. Run-to-run variability among deterministic solutions
remains but a moderate degree of ensemble clustering lent to
gradually heavier use of especially the 00Z ECMWF ensemble mean
(and some of the 00Z GEFS mean) solutions. Meanwhile, as
additional shortwave energy dives south along the West Coast and
reinforces the mean trough, solutions vary as to the specific
nature of the next wave and its affect 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
eastward too quickly, and the recent ECENS mean runs have more
consistently held this feature farther west relative to the recent
GEFS mean runs by the middle of next week, which have been slowly
trending westward. Thus, the forecast during days 5-7 was based
more heavily on the ensemble means with more weight placed on the
00Z ECMWF ensemble mean relative to the 06Z 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 on the northwest side of the upper high
circulation 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 as the
blocking ridging channels moisture along the same axis. Ensemble
situational awareness table highlighted this area with "MAX"
values per the GEFS QPF relative to the reforecast database --
signaling the rarity of the GEFS ensembles to be this wet this far
out this time of year (Jan/Feb/Mar). 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 by Tue. 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 and dives south along the coast into the Great Basin.
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 40 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. Record cold
maximum temperature are quite possible over portions of the West
including southern California through the period. Farther east
near the upper ridge, record high temperatures will be possible
along the Gulf Coast into Florida.
Fracasso/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