Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Mon Dec 09 2019
Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019
...Heavy Southeast Rains to lift northward over a Wintry/Cold Air
...Wet flow from the Pacific Northwest to the Snowy North-Central
...Pattern Overview and Guidance/Predictability Assessment...
The cold Hudson Bay mean upper vortex will linger this week. Flow
underneath over the lower 48 will become less amplified, but
remain quite active and unsettled/stormy this week as a long
series of northern and southern stream systems traverse overall
mean cyclonic flow.
The WPC medium range product suite was mainly derived from a blend
of reasonably compatable GEFS/ECMWF ensemble means. This is amid
rapidly growing forecast spread that includes significant system
and stream phasing uncertainties despite a similar larger scale
pattern evolution. This maintains good WPC continuity vs more
run-run inconsistent model solutions.
Models still offer varied low scenarios from the Gulf of Mexico to
the Southeast then up the Eastern Seaboard into next weekend.
Forecast spread has been considerable, especially with recent
outlier GFS runs, but seems to trending a bit better with the 00
UTC guidance with respect to the timing/stream phasing,
cyclogensis potential and eventual system tracks. This pattern
seems likley to focus moisture and lift to produce a risk of heavy
rainfall across the Southeast. 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.
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 heavier snows will develop with system
progressions inland, especially for favored terrain of the
north-central Great Basin/Rockies later week through the weekend.
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: