Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Mar 12 2024

Valid 12Z Fri Mar 15 2024 - 12Z Tue Mar 19 2024

...Heavy Snow possible across parts of the Southern Rockies into
Friday...

...Multi-day heavy rainfall event likely for parts of the South
late this week and weekend...


...Overview...

The general flow over the CONUS will be settling into a much more
amplified regime during the medium range period. By the start of
the period on Friday, a sharp/amplified upper ridge off the West
Coast will support a deep Southwest upper low that will likely
meander in place into early next week. This feature would promote
a multi-day period of heavy rainfall potential across parts of the
Southern U.S. late this week and into the weekend with possibly
significant snowfall ongoing through Friday across parts of the
Southern Rockies. A leading Plains through Northeast system may
also produce some enhanced rainfall along its track late this
week.  The East Pacific into western Canada upper ridge will help
to amplify downstream troughing into the eastern half to
two-thirds of the lower 48 by this coming weekend into early next
week, with a colder trend behind a northern tier to East Coast
front.


...Guidance/Predictability Assessment...

Models appear to be in reasonable alignment regarding the overall
pattern evolution across the lower 48 during the medium range
period. The main uncertainty arises toward the end of the period
when the upper low over the Southwest is expected to dislodge from
its holding pattern and move into the Plains. There's additional
uncertainty around the amplitude of an upper trough dropping out
of Alaska and into the Northwest/western Canada early next week.
On day 3, a general model blend consisting of equal parts 00z
ECMWF and 06z GFS with some 00z UKMET/CMC was utilized due to
general agreement on the broad scale setup across the country.
There still remains some inconsistencies between the different
deterministic runs regarding a bit of shortwave energy moving
through the Midwest. The day 3 blend is continued into day 4 due
to consistent agreement between the models. By day 5 the EC/ECE
try to weaken the Western ridge and trough pattern which is
inconsistent with almost all of the other guidance so the 06z GEFS
was introduced to the blend to account for that. This EC trend
continues into the end of the period so the 00z CMCE is introduced
to the blend on day 6, replacing the UKMET, and is continued
through day 7.


...Weather/Hazards Highlights...

A long-duration winter storm will be coming to an end across parts
of the southern Rockies on Friday, with the some possibly
significant storm total accumulations across the higher terrain. 
Farther east, a Midwest through Northeast wave and frontal system
will support some enhanced precipitation near its path into
Friday. On Day 4/Friday, favorable moisture/instability in the
warm sector, a subset of operational models with locally
significant QPF, and fairly wet antecedent conditions/FFGs, seem
to favor holding on to the marginal risk into parts of the
Northeast. Some moisture reaching far northern New England could
also produce a little snow.

Meanwhile the moisture streaming northward ahead of the stagnant
upper low over the Southwest and interacting with the front
trailing from the aforementioned northern wave will support what
should be a multi-day heavy rainfall threat across the South,
which will start towards the end of the short range period. The
cold front may get hung up for a period of time supporting
repeat/training thunderstorms across parts of the Lower
Mississippi Valley into the central Gulf Coast states, but there
is plenty of uncertainty in the exact placement of the highest QPF
still. The days 4 and 5 Excessive Rainfall Outlooks both depict
slight risk areas across this region, and there is potential for
an upgrade as the event gets closer in time especially considering
possible overlap in the heaviest QPF. Some heavy rainfall
potential may extend northward along the boundary into the
southern/central Appalachians as well. The heavy rainfall threat
may linger across the Southeast through the weekend. The system
tracking through southeastern Canada Saturday-Monday and trailing
front may produce areas of rain and perhaps cold sector snow over
parts of the Great Lakes/Northeast.

Mean ridging aloft over the Eastern U.S. will continue to support
an expansive area of above normal temperatures, with the core of
the warmest anomalies over the Mid-Atlantic and Ohio Valley on
Friday where high temperatures could reach 15-20 degrees above
normal. Temperatures across the East will trend cooler with time
after Friday, but should generally remain above normal through the
weekend. Over the West, moderately below average highs and
near/above average lows will linger over the Great Basin/Southwest
Friday-Saturday, before shifting slightly east into the Plains on
Sunday. Strong surface high pressure will allow for below normal
temperatures to expand southward with time early next week into
the Tennessee Valley and parts of the Southeast, with some
possible Frost/Freeze concerns in the Tennessee Valley for any
areas which may be starting to see their spring green-up already.
Out West, temperatures should increase by Friday and through the
weekend as strong ridging builds in aloft, with the Northwest
seeing the best potential for some highs reaching 10-15 degrees
above normal.

Kebede/Santorelli