Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM 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 will eventually
break down/shift eastward early next week as shortwave energy
drops into the Northwest, and this may finally help to weaken and
push the stubborn Southwest low eastward.


...Guidance/Predictability Assessment...

Models are in good agreement with the large scale pattern through
at least the weekend, with some uncertainty in the details and
timing of individual systems. The greatest area of uncertainty
early next week is with the evolution of the upper low over the
Southwest. Models have certainly trended towards the initial
amplified trough into the East missing this system completely,
allowing it to lag longer over the Southwest, but there is a lot
of uncertainty with the influence of the next system into the
Northwest and how quickly this works to finally weaken/kick the
system eastward. There's a lot of run to run variability of this,
but the GFS is the most amplified with the shortwave and a little
quicker to move that system out. Yesterday's 12z run of the CMC
and ECMWF were much less amplified/slower with the Northwest
shortwave which kept the upper low to linger over the Southwest
longer and eventually just weaken. The new 00z run of the CMC
(available after forecast generation time) interestingly enough
pulls the energy back northward as upper troughing approaches from
the West next Wednesday. Even the ensembles and EC-initialized
machine learning models are very dispersed so confidence in the
forecast beyond day 5 is very low. Based on the ensemble means,
the preference seemed to lean more towards holding back the energy
so the WPC forecast also stayed the course as previous continuity
using a blend of the 12z CMC/ECMWF with the ensemble means.


...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 about Monday. 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 parts of the Southern Plains 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 new Day 4/Saturday
Excessive Rainfall Outlook trimmed back the slight and marginal
risks significantly from the previous issuance. The slight risk
now only stretches from southeast Texas into central Louisiana,
where although flash flood guidance values are high and antecedent
conditions are dry, the favorable moisture and instability warrant
the slight. On Day 5/Sunday, included just a marginal risk along
the central Gulf Coast into the Southeast along the front, since
some of the guidance shows the heaviest QPF actually offshore by
then. Elsewhere, light showers may accompany a secondary cold
front as it sinks through the Midwest/East while the late period
system into the Northwest may bring some light precip back into
that region.

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