Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
228 PM EST Mon Dec 09 2019
Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019
...Developing low pressure system could bring areas of heavy rain
up the Appalachians Friday into the Saturday with wintry
precipitation at first across the interior Northeast and central
...Moist and fast flow across the West could lead to next low
pressure system developing over the Deep South early next week...
...Pattern Overview and Guidance/Predictability Assessment...
Model guidance this morning continues to indicate that a cold core
upper vortex will meander in the vicinity of Hudson Bay through
this weekend. Across the U.S. mainland, the pattern will become
more amplified this weekend as a shortwave exiting the southern
Rockies/northern Mexico is forecast lift toward the northeast and
interact with the northern stream flow over the eastern U.S. The
GFS continues to show a faster motion of this shortwave across the
northern Gulf of Mexico Thursday into Friday compared with the
ECMWF, resulting in a much faster progression of the wave through
the Eastern Seaboard during the weekend. A 30% blend of the 06Z
GEFS with 70% the 00Z EC mean (together with some of their
deterministic solution) was used to handle this system. This
yielded a slower forward motion of this system with the associated
moisture pushing further into the interior sections of the entire
East Coast this weekend. A similar blend of the 06Z GEFS with 70%
the 00Z EC mean was used for Days 6 and 7 to handle the
uncertainty on the back side of the low over the Northeast.
Models still offer varied low scenarios from the Gulf of Mexico to
the Southeast then up the Eastern Seaboard into next weekend.
Forecast spread and run-to-run variability remain substantial.
But there appears to be a trend for the associated moisture
pushing further into the interior sections of the entire East
Coast this weekend. This could bring areas of heavy rain up the
Appalachians Friday into the Saturday Subsequent system lift up
through the East Coast and interaction with a slow to recede lower
atmospheric cold air damming offers an interior snow/ice threat to
monitor. As is often the case, the degree of phasing/interaction
between the two initially separate streams and baroclinic
zone/coastal setup will be determining factors of eventual
evolution and any potential for any deeper coastal storm
developments and threats. Nevertheless, wintry precipitation can
be expected at first across the interior Northeast and central
Appalachians Friday night into early on Saturday.
Out West, the pattern will become wet across the Pacific Northwest
into later this week as Pacific shortwave energies/height falls
punch onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored terrain of the
north-central Great Basin/Rockies later week through the weekend.
By next Monday, the exiting wave will possibly inject and then
interact with Gulf moisture to spwan a developing low pressure
system over the Deep South.
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: