Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
126 AM EST Mon Dec 23 2019
Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019
...Overview and Guidance/Predictability Assessment...
An amplified flow pattern is expected across the CONUS during the
medium range forecast period, with a progressive northern stream
and a less progressive southern stream with a slow-moving
upper-level low. A consolidated vortex across the Arctic Ocean and
an active Pacific wave pattern will keep any significant arctic
air outbreaks away from the CONUS, with many locations, especially
across the central and eastern U.S. experiencing unusually mild
conditions for late December.
A shortwave and associated frontal system are forecast to cross
the Midwest/Great Lakes/Northeast Thu-Fri night. Models were
relatively agreeable here, with differences primarily confined to
the strength of the shortwave and the depth of the surface low as
it moves into east central Canada. Farther west, an upper-level
low is forecast to be initially near or just off the southern
California coast early on Thu (day 3). Models have struggled to
resolve the timing of this system as it moves east across the
Southwest and eventually into the central U.S., and guidance is
not much closer to a resolution than this time last night. The
ECMWF continues to be on the slow side of the spread, but with a
substantial degree of support from a seemingly underdisbursed
ECENS. Meanwhile, the GFS remains on the fast side of the spread
(albeit at least a bit faster than the GEFS ensemble mean). These
described differences worsened through time during the forecast
period, ultimately resulting in cascading spread downstream by the
weekend as the system approaches/reaches the central U.S., and
potentially interacts with progressive northern stream waves. The
preference in this forecast was to lean at least a bit more toward
the faster timing, using blend of roughly 1/3 each of the 12Z
ECMWF/UKMET and 18Z GFS for days 3-4, with increased weight placed
on ECENS/NAEFS ensemble means by day 5 due to quickly increasing
guidance spread. This solution still supports the idea of low
pressure developing along a front across the Southern Plains Fri
night/Sat, quickly moving northeast into the Great Lakes by Sun
morning, and the cold front sweeping off the Eastern Seaboard on
Mon. Forecast confidence with this system by Sun-Mon remained
rather low.
Models show general agreement that another trough and potential
upper-level low should dig along the West Coast by Sun-Mon (days
6-7). Run-to-run differences among guidance were significant
enough with this feature to go heavily toward ECENS/NAEFS ensemble
means by that time period.
...Weather Highlights/Threats...
Rain and mountain snow are expected Thu across the Southwest as
the upper low moves onshore. Progressive upper-level flow will
keep rain/mountain snow a possibility across the Olympics and the
Cascades late this week. Moisture return ahead of the southern
stream system will create an overrunning setup north of a frontal
boundary across the Southern/Central Plains Thu-Fri, with
relatively widespread (and at least locally heavy) rain expected
to develop. By next weekend, with potential development of a low
pressure system across the central U.S., models indicate the
potential for a band of moderate to possibly heavy precipitation
from the Southern Plains to the Ohio Valley/Great Lakes, although
details remain elusive for reasons described above. Nonetheless,
there appears at least some potential for an area of winter
weather on the north side of this system (despite the relative
lack of significant cold air). Once models get a better handle on
the specifics of next weekend's system, a clearer picture will
emerge as to what degree of heavy rain and/or winter weather
threat exists. At this time, the greatest probabilities associated
with this potential low pressure system are indicated across
portions of the southern Rockies and southern High Plains Fri/Fri
night. See the medium range Winter Weather Outlook products for
more details on the winter weather threat.
Temperatures are forecast to be well above average across much of
the eastern half of the CONUS through late this week. The largest
temperature anomalies are forecast from the Mid-Mississippi Valley
to the lower Great Lakes and Ohio Valley on Thu, where high
temperatures may approach or exceed 20 deg F above average in some
areas. A broad area will see highs 5 to 15 deg above average
through Saturday before the arrival of cooler temperatures behind
the cold front. Meanwhile, with persistent upper-level troughing
in place much of the Great Basin and Southwest will see high
temperatures 5 to 10 deg below average into the weekend.
Ryan
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