Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Thu Mar 14 2024

Valid 12Z Sat Mar 16 2024 - 12Z Wed Mar 20 2024

...Multi-day heavy rainfall event across the South to come to an
end this weekend...


...Overview...

The general flow over the CONUS will remain quite amplified into
early next week as a deep upper low meanders over the Southwest
and an amplified trough dives south and eastward through the
East. These two features combined will continue to promote a
multi-day period of heavy rainfall potential into this weekend
across parts of the Southern U.S., but should come to an end by
Sunday/the start of the medium range period. A leading cold front
will also bring a much cooler trend to much of the eastern half of
the lower 48, with some frost freeze concerns across the mid-
South. Strong ridging off the West Coast and into the Northwest may
eventually break down next week as shortwave energy drops
into/offshore the Northwest/West Coast, and this may eventually
work to interact with the stubborn Southwest closed low.

...Guidance/Predictability Assessment...

Recent models remain in good agreement with the flow pattern
through at least the first half of the period, with some
uncertainty in the details and timing of individual systems,
especially shortwaves through the mean Eastern U.S. troughing. The
biggest uncertainties arise by about Tuesday with the evolution of
the upper low over the Southwest. Recent runs of the CMC are an
outlier in showing the low lifting northward in favor of another
cut off low moving in towards Southern California. There is no
support for this solution and the CMC was not used in tonight's
blend. There is better agreement in the guidance/ensembles for the
low to weaken and shift eastward as a shortwave into the Plains by
mid next week (differences on timing), and this is supported by the
EC- initialized machine learning models too.

The WPC forecast for tonight used a majority blend of the GFS and
ECMWF (with some UKMET) through Monday. After that, gradually
increased ensemble mean contribution to about 50 percent of the
blend by Day 7, along with the GFS and ECMWF which showed fairly
good agreement with each other through the period.

...Weather/Hazards Highlights...

The upper low over the Southwest should continue to support some
generally light to moderate precipitation across the Four Corners
region, with snow across the higher terrain of southwest Colorado
and New Mexico through early next week. Meanwhile, moisture
streaming northward ahead of the low will be interacting with a
cold front moving through the Southeast/Gulf Coast region on
Sunday, and Florida on Monday. For the Excessive Rainfall Outlooks,
continued a marginal risk on Day 4 along the central Gulf Coast as
anomalous moisture and instability pool along the boundary where
there some increased risk for training of storms. There may be a
localized risk on Day 5 as the front moves through Florida, but the
front should be progressive enough and the model QPFs do not
support even a marginal risk at this time. To the north, a
secondary cold front through the Great Lakes/Northeast should bring
some light precipitation across the region. Sufficient cold air
and protracted wrap-back moisture feed may combine to produce some
plowable snows in windy flow, with main focus over northern Maine
Sunday as a surface low exits New England. Well upstream, a
complex late period system working offshore/into the Northwest may
bring some light precip back into that region, albeit with
significant uncertainty with support/interactions to monitor in
upcoming guidance cycles in an overall drier pattern by then over
the lower 48 into next week.

Temperatures from the Midwest to the East will trend much cooler
by this weekend as strong surface high pressure shifts south and
eastward. Daytime highs from the Mid-South to Southeast may be
10-15 degrees below normal, with some frost/freeze concerns for
any areas where spring green-up has started as temperatures dip
well into the 30s to near freezing especially Tuesday morning.
Temperatures across the Southwest will trend warmer with time as
the upper low weakens and above normal temperatures expanding over
the Northwest this weekend will move into the northern/central
Plains and Upper Midwest by next Tuesday-Wednesday.

Santorelli