Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
151 PM EST Wed Dec 18 2019
Valid 12Z Sat Dec 21 2019 - 12Z Wed Dec 25 2019
...Heavy rain possible over parts of the Southeast this weekend...
...Overview...
Upper troughing off the West Coast will be reinforced from the
northwest and slowly move onshore next week, maintaining an
unsettled pattern. Concurrently, a somewhat unusual southern
stream system will track through the Gulf of Mexico and across
Florida late this weekend. Locally heavy rain will be possible
around its circulation.
...Guidance/Predictability Assessment...
Across the West, the general troughing pattern seems well
resolved, but there are some differences in the wavelength of the
trough and the amount of energy that makes its way into the West
Coast. A model blend of the 06Z GEFS and 00Z EC means was used
along with some incorporation of the operational models.
In the Southeast, a potent upper low likely cut off from the
northern stream is expected to move eastward from the Southern
Plains on day 3/Sat across the Gulf Coast through day 4/Sun, while
a surface low tracks eastward with it. Model spread is mainly
related to timing--GFS and GEFS mean runs have been consistently
faster than the ECMWF and ECENS runs on Sun/Mon. The 00Z UKMET was
an outlier even Sat, because it separated the 500 mb vorticity
into pieces instead of wrapping up one upper low, but it appears
the new 12Z UKMET is in better alignment with other guidance. The
00Z CMC seemed to be a good compromise between the GFS and EC
solutions with the low position, so incorporated that into the
blend, as remaining in between the faster and slower solutions
seems prudent. Overall this forecast issuance did not change
significantly from continuity.
...Weather Highlights/Threats...
The moisture plume associated with the atmospheric river into the
Pacific Northwest during the short range period should continue to
sink southward through coastal OR/CA as the cold front moves
onshore Saturday. An additional few inches of precipitation are
possible over northern Califonia before the focused plume
diminishes and precipitation becomes lighter through central and
then southern coastal California. As the upper trough and surface
front move inland next week, precipitation will spread through the
southern Great Basin and Four Corners region. Then there is
uncertainty as to how much precipitation moves into the Plains
around Christmas Day, with considerable run-to-run variability.
Temperatures will trend cooler behind the front.
The Gulf system will provide at least modest rainfall for much of
the Southeast with possible locally heavy amounts. Models have
been showing over five inches over several days with sufficient
moisture in the lower atmosphere. Some light snow is possible on
its northern fringe in the marginally colder air of the southern
Appalachians, but the event is expected to be mainly rain. Warmer
than average temperatures are forecast to spread from the Northern
Rockies and Plains eastward across much of the U.S., with high
temperatures up to 20 degrees above average. Dry conditions should
persist across much of the Mississippi Valley to Ohio Valley and
Mid-Atlantic/Northeast.
Tate/Fracasso
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