Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Wed Mar 13 2024

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

...Multi-day heavy rainfall event continuing across parts of the
South into 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
Midwest/South/East Coast. These two features combined will promote
a multi-day period of heavy rainfall potential into this weekend
across parts of the Southern U.S., continuing from the short range
into 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 a stubborn Southwest closed low.

...Guidance/Predictability Assessment...

Recent models remain in good agreement with the flow pattern
through at least the weekend, with some uncertainty in the details
and timing of individual systems. A GFS/ECMWF/UKMET/Canadian
composite seems to provide a good forecast basis and detail
consistent within a pattern with seemingly above normal
predictability. By early next week, there is more uncertainty with
the influence of complex systems into/offshore the Northwest/West
Coast and any eventual interaction with a lead closed low set to
anchor/meander over the Southwest that may slowly lose influence
over time. Given uncertainties and continuity, a composite blend
of the GEFS/ECMWF ensemble means, WPC continuity and closest
matching ECMWF model input offers a consistent solution in line
with trends.

...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
slow moving cold front across The South to support what will be a
multi-day heavy rainfall threat from Texas to the Southeast,
starting during the short range period. Cold front may get hung up
for a period of time supporting repeat/training thunderstorms, but
plenty of uncertainty in the exact placement of the highest QPF.
Given latest model trends, the Day 4/Saturday Excessive Rainfall
Outlook still offers Slight and Marginal Risks. Flash flood
guidance values are high and antecedent conditions are dry, but
the favorable moisture and instability are favorable. On Day
5/Sunday, there is a Marginal Risk area centered along the central
Gulf Coast near the front. Elsewhere, light showers may accompany
a secondary cold front as it sinks through the Midwest/East
underneath a deep surface low working across southern Canada
underneath ample upper troughing. 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 per the WPC Winter Weather Outlook (WWO) probabilities.
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 dry 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 by next Tuesday-Wednesday.

Santorelli/Schichtel