Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Wed Jan 06 2021
Valid 12Z Sat Jan 09 2021 - 12Z Wed Jan 13 2021
...Overview...
Latest guidance continues to show establishment of an amplified
mean ridge aloft over western North America and downstream mean
trough extending from eastern Canada through the southern
Plains/northern Mexico. There is reasonable consensus that the
core of the ridge over the Pacific should strengthen by next
Tue-Wed, raising heights over the West at that time. With decent
confidence expect Pacific shortwaves/fronts to support periods of
precipitation focused over coastal Washington and Oregon with a
tendency for heavier activity early next week. However within the
downstream mean trough, individual model runs show a seemingly
infinite array of possibilities for individual shortwaves/possible
upper lows. Thus guidance clustering rapidly diverges after being
more agreeable for Gulf Coast rainfall and associated northern
Mexico/Gulf low pressure from the weekend into Mon, with
confidence decreasing accordingly next week. This system will
support a period of cool daytime temperatures across the southern
tier while the large scale pattern should promote persistent
warmth over the northern Plains and vicinity.
...Guidance/Predictability Assessment...
Given the below average predictability of some of the fairly small
features that could play a significant role in the forecast, the
updated blend employed an operational model composite early
followed by a transition toward relatively higher emphasis on the
00Z ECMWF mean and WPC continuity.
Into about day 4 Sun there is decent clustering for the shortwave
that drops into the southern Plains and northern Mexico. Then what
becomes of this feature and Gulf low pressure depends on the
evolution of various features within what could be at least two
separate streams--one covering the northeastern quadrant of the
lower 48 and another bringing energy south from Canada into the
Plains and vicinity. Over the past couple cycles there have been
increasing hints of an upper low dropping into the Plains (GFS
being the last to trend this way, in its 12Z run) but there is
still disagreement over which piece of energy creates this
feature. Plus the new 12Z ECMWF has decided to divert this upper
low energy eastward across southern Canada instead. Meanwhile
Great Lakes/Northeast upper flow may also play a significant role
in the eventual track of the wave crossing the Gulf along with the
westward extent of precipitation during the first half of next
week. By early next Tue the one aspect of the forecast maintaining
some consistency from yesterday is a GEFS/ECMWF ensemble signal (a
little better defined in the 00Z ECMWF mean versus the 06Z GEFS)
for weak low pressure to be just off the East Coast. Then some
combination of this wave and/or development associated with
northern stream flow reaches south of the Canadian Maritimes by
early Wed. The updated blend yielded a slightly slower trend
versus continuity from the Gulf into the Atlantic but maintained a
similar evolution in principle. Also this provided intermediate
timing between the faster GFS/GEFS and slower 12Z CMC/00Z ECMWF.
The main forecast consideration along the West Coast would be to
downplay whichever solutions are strongest with individual
shortwaves heading into the mean ridge. Notable examples include
the new 12Z UKMET around Sun and early Tue as well as the 00Z
ECMWF near the end of the period (approaching British Columbia).
...Weather/Threats Highlights...
The Pacific Northwest will continue to see periods of
precipitation with few breaks during the period. Low confidence in
details of shortwaves carried by fast flow aloft will temper
confidence for precise intensity and southward extent at any
particular time. Still the overall guidance signal is for heaviest
rain/high elevation snow to be during the first part of next week
and especially over Washington. The system emerging from the
southern Rockies on Sat may access enough Gulf moisture to produce
some locally enhanced rainfall along and just offshore from the
Texas coast, while recent southward trends suggest that areas
farther east along the Gulf Coast will see only light to moderate
rainfall. It would not take too great of a trend reversal to
change that though. Some snow will be possible from the
Colorado/New Mexico Rockies into parts of the southern Plains.
Confidence remains low for precipitation coverage and type over
the East during the early/middle part of next week. The fairly
broad coverage of light amounts in the manual forecast reflects
the high uncertainty and an ECMWF mean tilt while awaiting better
clustering.
Expect locations from Montana into Minnesota to be in the core of
warmest temperature anomalies through the period, with some
coverage of plus 10F or greater anomalies each day. Morning lows
should generally be farther above normal than the daytime highs
with plus 20F or greater anomalies for lows most likely on Sat and
next Wed. Strengthening upper ridge should lead to more widespread
plus 5-10F or so anomalies over the West Coast states into Nevada
heading into midweek. Within a cool pattern for the southern tier,
the best potential for highs more than 10F below normal will
extend between the southern Plains and Lower Mississippi Valley
from the weekend into the start of next week. Readings across the
South should moderate gradually by midweek.
Rausch/Fracasso
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Pacific Northwest and
the Northern Great Basin,
Mon-Tue, Jan 11-Jan 12.
- Heavy rain across portions of the Southern Plains, Sun, Jan 10.
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