Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Mar 30 2025

Valid 12Z Wed Apr 02 2025 - 12Z Sun Apr 06 2025


...Multi-day heavy rain/flooding and severe threat for
Lower/Middle Mississippi, Tennessee, and Ohio Valleys this week...


...Overview...

The medium range upper pattern will be amplified, blocky, and
active through the period. A surface low emerging into the Plains
by Wednesday will track towards the Great Lakes as a building upper
ridge over the western Atlantic into the Gulf should keep the
associated frontal boundary from making much progress on the
southern side, especially later in the week. This sets the stage
for a multi-day heavy rain and severe weather event from the Lower
Mississippi into the Ohio Valley, and significant flooding is
likely. Late-season snow is likely north of this system from the
northern Plains to Great Lakes and Interior Northeast for midweek,
and the Colorado Rockies are likely to see snow by late week. The
West should stay relatively cool underneath persistent troughing,
while above normal temperatures progress from the central U.S. and
Midwest, eventually settling late week across the Southeast and
into the Mid-Atlantic.


...Guidance/Predictability Assessment...

Model guidance is relatively agreeable on the large scale pattern
midweek into next weekend, but still uncertain with some of the
details, which could have sensible weather impacts. A building and
blocky ridge over the Atlantic to Gulf should help amplify
troughing over the West through the period. Shortwaves within that
trough show relatively more spread, but the first shortwave/compact
upper low and its associated surface low tracking from the Central
U.S. to Great Lakes into Thursday continues to show agreement. By
Friday and into the weekend, there is somewhat more uncertainty in
the details of another shortwave digging through the western side
of the trough. ECMWF runs in particular have been on the western
side with this energy compared to the larger consensus of other
models and even EC-based AI/ML models. The new 12Z ECMWF seems to
be coming into better alignment with the majority in the energy and
trough placement. Regardless there is still some spread in what
becomes of this energy next weekend, perhaps producing a closed
upper low within the trough becoming positively tilted while models
are converging on showing an upper ridge axis over the Northwest.

At the surface, a stationary frontal boundary will set up between
the Southeast ridge and Western trough that persists for several
days. There remain subtle differences still in the exact placement
of this front, which has notable implications on the exact
locations of heavy rainfall expected to accompany the front. Expect
these to vary a bit in future forecasts, though the general idea
of heavy rain leading to possibly significant flooding in parts of
the east-central U.S. is pretty locked in. A northern stream trough
may help to slowly nudge the stagnant east-central U.S. boundary
eastward a bit next weekend, though some typical spread remains
with the timing and depth of that trough.

The WPC forecast was able to utilize a deterministic model blend
through Day 4. By Day 5, began incorporating some ensemble mean
guidance to help temper the detail differences, while reducing the
proportion of operational models (especially the 00Z ECMWF) in
favor of over half means for Days 6-7. This approach maintained
relatively good agreement with the previous WPC forecast.


...Weather/Hazards Highlights...

Overall pattern amplification and a leading compact closed low
will support a surface low pressure system forming in the central
U.S. by Wednesday with widespread thunderstorms and heavy rainfall
across the east-central U.S. in its warm sector. Moist and unstable
inflow into the associated surface fronts along with the strong
upper level support will lead to widespread severe weather and
flash flooding concerns. SPC shows severe probabilities from the
south-central U.S. into the Midwest on Wednesday and Thursday. The
greatest concern for flash flooding on Wednesday and Thursday will
stretch across similar areas from the Ark-La-Tex northeast into the
Ohio Valley, as the front anchors storms for significant training
concerns. On Wednesday, precipitable water anomalies above the 95th
percentile with ample instability will be in place. Models are
indicating embedded rainfall totals of 5 to 8 inches during the 12Z
Wednesday to 12Z Thursday period alone, with the GFS already
explicitly forecasting hourly rain rates to 2", and certainly
expect the CAMs to show higher rates than that once we get into
their timeframe. Thus plan to upgrade the Day 4/Wednesday ERO to a
Moderate Risk from northeast Arkansas across the Ohio River Valley
to far southern Indiana. Much the same environmental pattern sets
up on Day 5/Thursday, with the added concern of very wet antecedent
conditions by then. The previously issued Moderate Risk for Day
5/Thursday was maintained, with a southwestward expansion per
coordination with the Little Rock local forecast office and
considering the higher instability there. Heavy rainfall is likely
to continue into late week, with perhaps a bit of a westward shift
into the southern Plains at times as the West trough pivots.
Through midweek into the weekend, significant flood impacts are
likely with the multiple rounds of heavy rain. High Risks are not
out of the question in future EROs as the event draws nearer and it
becomes clearer how the details could evolve, especially into the
timeframe of the CAMs.

Through Wednesday, some April snow is likely on the backside of
the initial low track, stretching across the Dakotas to Upper
Midwest and Interior Northeast. Snow amounts have typical
uncertainty, and an axis of freezing rain and/or sleet is possible
as well. Meanwhile as troughing sets up over the West with multiple
shortwaves moving through, rounds of precipitation are possible
from the West Coast into the Intermountain West and Rockies.
Generally lower elevation rain and higher elevation snow are
expected, with mainly light to modest amounts. Additionally, high
winds are possible for the central Rockies on Wednesday. Into late
week, a possible upper low in the Four Corners region could lead to
enhanced precipitation totals across the southern and central
Rockies and Plains. Snow is likely in higher elevations of the
Rockies in Colorado and northern New Mexico, with uncertainty at
this point in if/how much snow could spill into lower elevations of
the High Plains.

Above normal temperatures will be located across the south-central
U.S. to Midwest on Wednesday, and settle across much of the
southern and eastern U.S. Thursday and beyond. Daytime highs 10 to
20 degrees above normal are possible. Meanwhile, the West should
trend and stay cooler underneath upper troughing through Friday. By
next weekend, temperatures may moderate somewhat along the West
Coast while eastern Pacific upper ridging moves in. This shifts
cooler temperatures slightly east, and daytime highs across parts
of the southern Rockies could be 10 to 20 degrees below normal next
weekend.


Tate/Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw