Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Fri Feb 15 2019
Valid 12Z Mon Feb 18 2019 - 12Z Fri Feb 22 2019
...Multi-day Heavy Rain Threat for the Tennessee Valley and
South-Central Appalachians/Mid-Atlantic and Heavy Snow/Ice Threats
Overtop into the Ohio Valley, Central Appalachians, North-Central
Mid-Atlantic and Southern New England...
...Wintry Western-Central U.S Weather focuses over
Southwest/Southern Rockies...
...Overview...
It remains the case that upper-level ridging across the North
Pacific and near the Bahamas shows minimal signs in guidance of
breaking down during the medium range, and thus will continue to
influence the large scale flow across the CONUS. This reinforces a
pattern with a cold/unsettling trough in the West/Southwest,
arctic air intrusions into the n-central U.S. and wavy southwest
flow over the eastern half of the CONUS through the next week. A
rapid succession of Pacific shortwave energies will traverse the
flow, amplifying the mean trough over the West before ejecting and
interacting with a lingering polar front along the Gulf Coast to
generate frequent frontal waves which affect the east-central and
eastern U.S.
...Guidance/Predictability Assessment and Preferences...
A composite of decently clustered guidance of the 12 UTC
ECMWF/ECMWF ensembles and favorably trending of the 18 UTC
GFS/GEFS seems reasonable as a basis for the forecast Mon-Wed.
Models show 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 consensus was preferred. There is also relatively
good consensus with amplified shortwave energy digging into the
Southwest Mon-Wed.
Strating Day 6/Thu and especially by day 7/next Fri, guidance
continues to show relatively good agreement with the big picture,
but larger differences begin to emerge on the specifics. Opted to
derive this forecast period primarily from the 12 UTC ECMWF
ensemble mean and to a lesser extent the 18 UTC GEFS mean to
address growing uncertainty and maintain as much WPC continuity as
feasible. The ECMWF ensemble mean offers more amplified/less
progressive flow than the GEFS mean.
...Weather Highlights/Hazards...
A polar front lingers over 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 over the
Tennessee Valley and South-Central Appalachians/Mid-Atlantic.
There is a threat of multi-inch rainfall totals 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 settled in place
on the northern periphery of these low pressure systems offers
threats of heavy snow/ice for the Ohio Valley, central
Appalachians and from the north-central Mid-Atlantic into southern
New England.
Meanwhile, conditions next week along the West Coast will be
quieter than seen recently as the main upper trough axis moves
inland. This will spread snows into Southwest/southern Rockies
terrain. Some widespread rain/snow may return to the Pacific
Northwest by midweek as a more significant shortwave approaches
and dives south along the coast into the wintry Great Basin then
Rockies/n-central U.S.
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.
Schichtel
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