Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Thu Dec 19 2019
Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019
...Heavy rain over parts of the Southeast departing after Monday...
...Overview...
The large scale pattern will feature a mean trough near the West
Coast. One or more leading embedded shortwaves will eject inland
followed by amplifying upstream energy that reinforces the trough.
Expect this mean flow to produce a period of unsettled weather
over portions of the West. At the same time a potent southern
stream system will track across the extreme Southeast/Florida late
this weekend and possibly into Mon, then push slowly out to sea.
Locally heavy rain will be possible around its circulation between
northern Florida and parts of the Carolinas. There is reasonable
agreement with the mean pattern but much less confidence over the
specifics of individual shortwaves reaching the West along with
their influence on surface features/sensible weather east of the
Rockies. Uncertainty is also fairly high regarding the extent to
which a ridge crossing western-central parts of southern Canada
will affect flow over eastern North America mid-late week.
...Guidance/Predictability Assessment...
From the eastern Pacific into the western U.S. preferences
continue to lean more toward ECMWF/ECMWF mean in general, with
slower shortwave progression than recent GFS runs through nearly
the entire period. By late in the period the 00Z ECMWF adjusted a
bit faster than its prior run while the 00Z CMC seemed a bit
slow/cut-off with its upper system so the slowest side of the
spread may still be a little too slow. The new 12Z CMC is close
to the 00Z ECMWF. Latest GEFS runs have been faster than the
ECMWF cluster but slower than operational GFS runs.
Downstream there is a general consensus that ejecting western
energy will support surface waviness to the east of the Rockies by
Thu-Wed Christmas Eve/Christmas Day. By mid-late week guidance
becomes increasingly dispersed over the eastern half of the
country due to differences in relative strength of southeast
Canada/northeast U.S. upper troughing (strongest in the 00Z ECMWF)
versus energy that may be ejecting from the western trough. Given
preferences toward the slower half of the spread within the West
Coast mean trough and the ECMWF/CMC plus their means all showing
notable trends toward higher eastern U.S. surface pressures by day
7 Thu, the updated forecast trended somewhat toward the more
suppressed eastern scenario at that time. Confidence is fairly
low though.
The early-period Southeast system continues to have a fair amount
of spread in the guidance with the GFS/CMC on the faster side (CMC
farthest north) and ECMWF/UKMET slower. The 06Z GEFS mean was a
bit slower than the operational run. Overall favor a tilt more
toward the slower timing but the pattern may be progressive enough
to support slightly faster timing than the slowest edge of the
spread--and the new 12Z ECMWF has indeed nudged its system a bit
faster.
Combining the above considerations led to a forecast blend that
emphasized the 00Z ECMWF and UKMET/WPC continuity with modest
inclusion of the 06Z GFS/GEFS mean early in the period. Then the
forecast transitioned toward more ECMWF mean emphasis versus the
operational run while maintaining GEFS mean/continuity input and
removing the 06Z GFS.
...Weather Highlights/Threats...
The mean trough aloft near the West Coast will support fairly
widespread but generally light to moderate rain and higher
elevation snow next week, spreading inland as shortwaves eject
from the overall trough. Uncertainty over timing and amplitude of
individual shortwaves will lower confidence in precise
timing/coverage/intensity of precipitation at any particular time.
The same is true to the east of the Rockies where preferences
maintained a lean toward less precipitation than the GFS/GEFS but
with low confidence. For any precipitation that does occur, it
would most likely be snow from the northern Plains through the
Upper Great Lakes and northern New England. The eastern
Gulf/Southeast system will provide varying rainfall totals for
much of the Southeast into Sun and/or Mon with locally heavy
amounts continuing out of the short range time frame that goes
through Sat. A period of robust inflow off the Atlantic may
promote heaviest activity along the Southeast coast.
The overall pattern will support a trend to modestly below normal
highs over the West while the Southeast system should be
accompanied by slightly below normal highs early in the period.
Otherwise the central/eastern states will see above normal highs
with warmest readings versus climatology likely to be over the
central Plains on Sun-Mon. High pressure may bring a couple cool
mornings to southern Texas early in the period but otherwise
expect min temps to be warmer than normal over nearly the entire
lower 48. The northern Plains into Upper Midwest will likely see
the highest anomalies.
Rausch
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
Hazards:
- Heavy precipitation across portions of California, Thu, Dec 26.
- Heavy rain across portions of the Southeast and the
Mid-Atlantic, Sun-Tue, Dec 22-Dec 24.
- Heavy snow across portions of the Central Great Basin,
California, and the Southwest, Sun-Mon, Dec 22-Dec 23.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Tue-Wed, Dec 24-Dec 25.
- Flooding occurring or imminent across portions of the Ohio
Valley and the Northern Plains.
- Flooding likely across portions of the Pacific Northwest and the
Northern Plains.
- High winds across portions of the Southeast, Mon-Tue, Dec 23-Dec
24.
- High winds across portions of the Southeast, Sun-Mon, Dec 22-Dec
23.
- High significant wave heights for coastal portions of the
Southeast, Sun-Mon, Dec 22-Dec 23.
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