Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Mon Dec 23 2019
Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019
...Overview and Guidance/Predictability Assessment...
The latest guidance has an initial synoptic pattern with an
amplified trough over the West and ridge over the Southeast that
tracks toward the eastern U.S. by the end of the weekend. The
northern stream will be fairly progressive while the southern
stream will be slower. A consolidated vortex across the Arctic
Ocean and an active Pacific wave pattern will keep any significant
arctic air outbreaks away from the CONUS, with many locations,
especially across the central and eastern U.S. experiencing
unusually mild conditions for late December.
The main weather feature for the extended period will be shortwave
energy ejecting over the Southwest/Southern Rockies that will have
an associated surface low pressure system track across the
Midwest, the Great Lakes region and into the Northeast. In
general, there is a moderate signal within the guidance for this
feature however the spread greatly increases for the days 6 and 7
timeframe. The GFS is very progressive which translates to lifting
the energy much further north than the rest of the guidance and
nearly 12 to 18 hours quicker as well. The ECWMF and the UKMET are
on the slower end of the spectrum, but reasonably clustered near
the CMC (which seems to be a compromise of the ECWMF and the GFS).
Following a similar approach to the previous forecast, at least
initially, the model preference incorporated a blend of the 00Z
ECMWF/06Z GFS and 00Z UKMET/CMC. Weighting of the ECENS/NAEFS
ensemble means by day 5 were increased while decreasing influence
of the GFS. This solution still supports the idea of low pressure
developing along a front across the Southern Plains Fri night/Sat,
quickly moving northeast into the Great Lakes by Sun morning, and
the cold front sweeping off the Eastern Seaboard on Mon. Forecast
confidence with this system by Sun-Mon remained rather low.
...Weather Highlights/Threats...
Within the northern stream, there will be progressive upper-level
flow which will maintain the possibility of coastal rain and
mountain snow for the Olympic and Cascade ranges by the end of the
extended period. Rain and mountain snow is expected for portions
of the Desert Southwest as an upper-level low in the southern
stream pushes onshore. As the surface low pressure system crosses
the Southwest and Southern Rockies rain will become fairly
widespread over the Southern and Central plains by the end of the
week as Gulf moisture overspreads the quasi-stationary frontal
zone. Local areas of moderate to heavy rain will be possible with
this system.
By next weekend, with potential development of a low pressure
system across the central U.S., models indicate the depict a band
of moderate to possibly heavy precipitation (possibly in a mixed
form, or snow) from the Southern Plains to the Ohio Valley/Great
Lakes; however, the specifics are tricky given the uncertainty of
evolution. See the medium range Winter Weather Outlook products
for more details on the winter weather threat.
Temperatures are forecast to be well above average across much of
the eastern half of the CONUS through late this week. The largest
temperature anomalies are forecast from the Mid-Mississippi Valley
to the lower Great Lakes and Ohio Valley on Thu, where high
temperatures may approach or exceed 20 deg F above average in some
areas. A broad area will see highs 5 to 15 deg above average
through Saturday before the arrival of cooler temperatures behind
the cold front. Meanwhile, with persistent upper-level troughing
in place much of the Great Basin and Southwest will see high
temperatures 5 to 10 deg below average into the weekend.
Campbell
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