Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
141 PM EST Tue Dec 10 2019
Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019
...Eastern Seaboard heavy rain and interior wintry weather threat
Friday/Saturday and again Monday/Tuesday-...
...Heavy mountain snows possible Friday-Saturday from Northwest to
Rockies...
...Pattern Overview and Guidance/Predictability Assessment...
An active and complex pattern should persist through the medium
range period (Friday-Tuesday) as northern/southern stream
shortwaves interact. As one shortwave/trough exits the Northeast
this weekend, another dives south and eastward into the West,
moving into the Central U.S. by early next week. Through the
period, there is fairly good agreement in the large scale pattern,
though forecast confidence remains somewhat tempered due to run to
run inconsistencies with the more complex details. Notable
differences is with the initial northern stream shortwave moving
into the Great Lakes/Northeast on Saturday, the GFS remains a bit
more amplified and results in a deeper surface low, but a blend
with other solutions provides a reasonable forecast. Late in the
period, troughing moving into the Central U.S. shows some slight
timing differences with the CMC and its mean a bit slower/stronger
than the rest of the guidance.
The model choices for this cycle of the medium range progs
features a majority deterministic GFS/ECMWF blend days 3-5, with
increased weighting of the ECENS/GEFS means to come to a 50/50
blend between deterministic and ensembles by day 7. This maintains
very good continuity with previous WPC continuity.
...Weather Highlights/Threats...
Southern stream shortwave progression and subsequent northern
stream flow interaction should support low development and track
from the Gulf of Mexico/Gulf Coast to the Southeast then up the
East Coast Friday/Saturday. This will bring heavy rain up the East
Coast mainly from the Southeast to the Appalachians, Mid-Atlantic
then Northeast. System interaction with lower atmospheric cold air
damming offers an interior snow/ice threat to monitor. The degree
of phasing between two initially separate streams and baroclinic
zone/coastal details will be determining factors of eventual
evolution and potential coastal system threat.
The pattern will also become wet upstream over the Pacific
Northwest into late week as Pacific shortwave energies punch
onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting wave will eject and then interact with Gulf
moisture to spawn a rainfall focusing low pressure/frontal system
forecast to move over the Deep South, then potentially again up
the East Coast.
Santorelli/Schichtel
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