Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
152 AM EST Thu Dec 19 2019
Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019
...Heavy rain over parts of the Southeast exiting Monday...
...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 potent southern stream system
will track across Florida late this weekend and then push slowly
out to sea. Locally heavy rain will be possible around its
circulation between northern Florida and coastal North Carolina.
...Guidance/Predictability Assessment...
Across the West, the general troughing pattern seems well-resolved
in the longwave pattern, but there remained timing differences
through about Tuesday and then both feature and timing differences
thereafter. A consensus position was favored, somewhat near the
12Z ECMWF/Canadian until next Wed/Thu when they may become too
deep/slow (near the slowest 10% of ensembles) but not unreasonable
given the pattern. The GFS/GEFS may be too quick overall by later
in the period which can be their bias.
In the Southeast, models were still struggling with the forward
speed and track of the closed low and surface low but forecast
convergence continues. The GFS continues to be on the quicker side
while the 12Z UKMET lagged the more centered 12Z ECMWF/Canadian,
which were close to the ensemble consensus and was the preferred
solution. Did not rule out the quicker GFS given the
quasi-progressive nature of the pattern--western trough would not
allow too much of a slower solution though it could trend slower
in tandem. Multi-day ensemble trend has mostly stabilized so will
continue to favor the best-clustered models.
...Weather Highlights/Threats...
Western troughing will support fairly widespread but light to
modest precipitation next week, spreading inland as height lower
to the east. There remains considerable uncertainty as to how much
precipitation moves into the Plains around Christmas Day into the
26th, owing to how much Gulf moisture may be tapped. For now,
favored the slower/drier ECMWF/Canadian ensembles vs the
quicker/wetter GEFS. Temperatures will trend cooler in the West
behind the front next week.
The Gulf system will provide at least modest rainfall for much of
the Southeast with locally heavy amounts continuing out of the
short range Saturday. Several additional inches of rain is quite
possible Sun-Mon with sufficient moisture in the lower atmosphere
and good inflow off the Atlantic. 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 nearly all rain.
Warmer than average temperatures are forecast to spread from the
Northern Rockies and Plains eastward across much of the rest 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.
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