Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1145 AM EST Sun Dec 01 2019
Valid 12Z Wed Dec 04 2019 - 12Z Sun Dec 08 2019
...Southern tier system to affect the Southwest
midweek/Mississippi Valley and Southeast late week...
...More moisture to spread into the West late week/next weekend...
...Pattern Overview and Guidance/Predictability Assessment...
The large scale pattern that has tended to feature mean troughs
over the eastern Pacific and eastern U.S. with a mean ridge over
the west-central U.S. will begin to progress eastward by next
weekend as the upper trough amplifying over the eastern Pacific
during the latter half of the week moves into the lower 48--albeit
with some timing uncertainty. Latest D+8 multi-day means have
been showing a prominent negative height anomaly center over the
North Pacific and/or Aleutians, with teleconnections supporting
some eastward progression of the positively tilted trough beyond
the end of the medium range period to an axis from near the Great
Lakes to the southern Rockies (with a modest hint of stream
separation). At the same time a ridge should begin to build into
western North America with best positive height anomalies over
western Canada.
There should be two dominant weather producers this period: 1) a
leading southern tier system that should be just offshore southern
California as of early Wed and then become more suppressed with
time from the southern Plains into the Southeast Thu-Fri as the
energy aloft becomes incorporated into progressive flow, and 2)
the amplifying eastern Pacific trough which by next weekend should
progress into the western and possibly parts of the central U.S.
Meanwhile a couple northern stream shortwaves may achieve a
negative tilt by the time they reach the Northeast (one Wed-Thu
and another around Sat). Most precipitation associated with these
shortwaves will likely be on the lighter side of the spectrum
though locally enhanced activity could be possible.
For the southern tier system a 00Z/06Z consensus blend position
(though the 00Z UKMET has strayed slower) maintains good
continuity as the upper low opens up over the Four Corners region.
The most significant difference for the larger-scale eastern
Pacific into CONUS trough is timing, with the GFS and ECMWF still
exhibiting their fast/slow tendencies respectively. By next
weekend the GEFS/ECMWF means ultimately support a slower timing
than the GFS for the upper trough, with a positive tilt that
appears to lead well into the teleconnection-favored pattern
expected to prevail soon after next Sun. Therefore the updated
forecast trended about 50 percent to the ensemble means late in
the period (with modest input of the 00Z ECMWF and continuity).
Potential for stream interaction within the overall trough
contributes to ongoing uncertainty mid-late period. The preferred
blend helped to downplay lower confidence detail uncertainties
with northern stream shortwaves affecting the northern
tier/Northeast. Consensus continues to show that low pressure
with the first shortwave will track off the Northeast coast
(though perhaps close enough to far eastern Maine) with the
trailing shortwave having minimal affect.
...Weather Highlights/Threats...
The system tracking across the southern tier will first bring a
brief period of locally heavy rain (and some snow at highest
elevations) over favored southern California terrain around
midweek with a lesser relative maximum likely over central
Arizona. Also expect higher elevation snow from the Sierra Nevada
into the central Rockies. Farther east guidance is starting to
show a somewhat more coherent signal for locally moderate
(possibly briefly heavy) rainfall over and just east of the lower
half of the Mississippi Valley late in the week before a cold
front approaching from the north suppresses the system. With
continued uncertainty in the exact onset, expect moisture ahead of
the amplified Pacific upper trough to reach the West Coast states
late in the week and push eastward with time during the weekend.
Thus far guidance has been consistent in highlighting northern
California and Sierra Nevada for highest precipitation totals.
Lower but still meaningful activity is possible over the Pacific
Northwest. Lake effect snow aided by the first shortwave crossing
the Northeast early in the period will tend to favor locations to
the lee of Lake Erie/Lake Ontario with lighter activity to the
northwest. The next shortwave aloft and surface wave/frontal
system may produce some relatively light amounts across the
upper/eastern Great Lakes and New England.
The greatest temperature anomalies (up to 10-20F) during the
period will be of the warm variety and for morning lows in
particular--in association with the mid-late week southern tier
system, over the northern tier Wed-Thu ahead of a cold front
drifting south from Canada, and from the West into the Plains late
week/next weekend ahead of the large scale upper trough moving
from the Pacific into the West. Meanwhile the Great Basin into
the Southwest should continue to see highs around 5-10F below
normal while the East Coast states will see temperatures vary with
frontal progression but on average tend to be somewhat below
normal.
Fracasso/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
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