Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Tue Feb 27 2024
Valid 12Z Fri Mar 01 2024 - 12Z Tue Mar 05 2024
...Large system to spread a Heavy Rain/Snow/Wind threat across the
West this weekend and the North-Central to Southern U.S. through
early next week...
Guidance forecast spread has recently decreased through medium
range time scales in active and amplifying upper flow to bolster
forecast confidence. Favored a 18 UTC GFS and 12 UTC
ECMWF/Canadian/UKMET composite for Friday into Sunday.
Predictability decreases into early next week and a composite of
best compatible 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF
ensemble means seems to provide a good forecast basis along with
the National Blend of Models through these longer time frames. WPC
product continuity is well maintained with this plan. However, it
is a bit concerning that while latest machine learning models and
the bulk of latest 00 UTC guidance remains in line, the 00 UTC
ECMWF has trended toward less amplified flow into/out of the West.
...Overview and Weather/Hazards Highlights...
A leading southern stream upper trough with origins over the East
Pacific will eject from northern Mexico and work across The South
Friday before exiting the East Saturday, and may prompt a
modest/uncertain coastal low to lift focus up/off the East Coast.
In this pattern a surface low/frontal system also lags back over
the Gulf of Mexico. Moisture return the Gulf of Mexico and onshore
fetch from the Atlantic will fuel an organized area with enhanced
rain/convective chances. Locally heavy downpours may focus over
the Central Gulf Coast late week, with potential for a secondary
maxima over the Southeast/southern Appalachians. These areas will
be monitored for runoff issues, but no WPC Excessive Rainfall
Outlook (ERO) threat area has been issued at this time given
relatively high Flash Flood Guidance values. Some rain/showers
will linger over the Southeast into early next week with weaker
impulses, lingering boundaries and inflow.
Upstream, there continues to be a very strong guidance signal that
an amplified upper trough digging into the eastern Pacific later
this week will shift over the West this weekend. This will support
chances for a moderate to heavier rain focus into the Pacific
Northwest mid-later week that along with a moderate long fetch
moisture plume/Atmospheric River will shift steadily down the
coast through southern California later week into the weekend
before easing. The WPC Day 4/Friday and Day 5/Saturday EROs shift
Marginal Risk areas southward from SW Oregon through southern
California. There is also a significant threat for high winds and
very heavy snow to spread inland across the Intermountain West and
Rockies, especially for favored terrain/mountains as highlighted
in the WPC Winter Weather Outlook (WWO). Confidence continues to
increase in the risk for moderate to extremely disruptive high
snowfall rates/blowing snow, particularly for the Sierra Nevada.
Additionally, valley snowfalls may develop more over time as snow
levels decrease with upper system and wavy frontal push. This
translation should also favor development of an emerging
precipitation shield over the cooling Rockies/Plains by
Sunday/Monday with Plains cyclo/fronto genesis. Widespread
showers/thunderstorms look to develop across the
Plains/Mississippi Valley then east-central U.S. with focus over
the Gulf Coast States into Tuesday. This may include some severe
weather potential, as well as the chance for heavy snows in quite
windy flow on the northwest periphery of activity across the
Northern Plains to Upper Midwest with low deepening and the
digging of cold Canadian air southward in the wake of this main
system, as per the WWO.
Pacific upper trough amplification and inland progression this
weekend will cool temperatures over much of the West, leading to
the spreading of values as cold as 15 degrees below normal.
Downstream, expect much above normal pre-frontal warmth later week
over the north-central U.S. that will spread to much of the
central and eastern U.S. this weekend/early next week downstream
of the main upper trough/lead frontal system approaching from the
West. Springlike minimum and maximum temperatures ranging upwards
to 20-30+ degrees above normal over the Midwest will overspread
the Northeast and offer widespread record values.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at: