Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 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: 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 Southeast Thu-Fri as the energy
aloft becomes incorporated into progressive flow, and 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 recent slower trend in the GFS has
aided guidance clustering in the 12Z/18Z cycle. The new 00Z UKMET
has strayed slower than consensus but remaining solutions maintain
good continuity. The most significant difference for the
larger-scale eastern Pacific into CONUS trough is timing, with the
GFS and ECMWF exhibiting their fast/slow tendencies respectively.
The 12Z CMC was close to the GFS and adjusted a little slower in
the new 00Z run. By next weekend the GEFS/ECMWF means ultimately
support an intermediate timing 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 50-60 percent to the
ensemble means late in the period (with modest input of offsetting
operational runs) after reflecting the 12Z/18Z operational model
consensus early-mid period. 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 well
off the Northeast coast while there has been somewhat of a weaker
trend with the surface wave associated with the trailing shortwave.
...Weather Highlights/Threats...
The system tracking across the southern tier will first bring a
brief period of locally heavy rain (perhaps 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.
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