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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Sun Dec 22 2024

Valid 12Z Wed Dec 25 2024 - 12Z Sun Dec 29 2024

...Unsettled weather for the West Coast with periods of heavy rain
and mountain snow...

...General Overview...

The upper level pattern will be fairly amplified with split flow
steering multiple energetic shortwaves across the nation. A mean
upper trough will develop over the central U.S. on Thursday and
remain there through the rest of the week. Several shortwaves will
move into the West Coast and travel east through the mean trough,
directing Pacific moisture into the Northwest and providing support
for multiple progressive frontal systems in the Plains. An upper
level ridge over the East Coast will prevent frontal systems from
moving due east, instead forcing them to move north-northeast
towards the Midwest and Great Lakes.


...Guidance/Predictability Assessment...

Guidance has come into better agreement on the overall pattern
since the overnight forecast, but there is still significant spread
among model solutions towards the end of the week when
interactions between the northern and southern stream energies
become more uncertain. The main outlier from the guidance suite
today was the 00Z ECMWF, which was too slow with upper level energy
moving through the East Coast trough late this week into the
weekend. All other deterministic, ensemble, and AI guidance favored
a faster solution more similar to yesterday's 12Z ECMWF. For this
reason, yesterday's 12Z ECMWF run was used in the forecast blend in
place of the 00Z run. This faster solution is consistent with
today's 12Z ECMWF as well.

WPC's forecast blend consisted of a near even blend of the 06Z GFS,
12Z ECMWF from yesterday, and 00Z UKMET and CMC for the first half
of the period. For the second half of the period, the GFS and CMC
were favored since they were near the model consensus and ensemble
means from the GEFS and ECENS were added (up to 40% by day 7) to
smooth out smaller model differences.


...Weather/Hazards Highlights...

Multiple rounds of moderate to heavy rainfall are expected to
impact the West Coast later this week as multiple frontal systems
approach the coast. Flooding will be possible, especially near the
steep terrain of the Olympics and coastal mountain ranges, and
potentially into the foothills of the Cascades as well. A Marginal
Risk of Excessive Rainfall is in effect for coastal Washington,
Oregon, and northern California on Wednesday/Christmas Day and
again on Thursday. Flooding will likely remain an issue for this
region beyond Thursday as more heavy rain moves into the coast
Friday into the weekend.

Moisture spreading inland in the West will likely produce heavy
snow over the higher terrain of the Cascades and Rocky mountains,
and even portions of the Sierra Nevada as well. Heavy snow will
focus over the Northwest on Christmas and expand through the
Intermountain West/Rockies later this week.

Downstream, multiple frontal systems will bring rain and
thunderstorm chances to the Plains, Mississippi Valley, and
Southeast. Southerly flow will bring moisture up from the Gulf of
Mexico into the Ark-La-Tex region where multiple rounds of rain are
expected. Given the potential for higher instability, locally
heavy rain will be possible in the Ark-La-Tex on Thursday, and
there will be a Marginal Risk of Excessive Rainfall.

Widespread above normal temperatures by late December standards are
expected across basically all of the Central U.S. for the entire
forecast period, ranging from 5 to 20 degrees above average. The
greatest positive anomalies are likely across portions of the Upper
Midwest and extending into the Ohio Valley by Saturday. Readings
closer to climatology are expected near both the West Coast and the
East Coast.


Dolan/Hamrick


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

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw