Quantitative Precipitation Forecast
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FOUS30 KWBC 230705
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Sat Mar 23 2024

Day 1
Valid 12Z Sat Mar 23 2024 - 12Z Sun Mar 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID-ATLANTIC INTO NEW ENGLAND...

...Mid-Atlantic into New England...

A wound-up low, associated upper level trough and shortwave, and
150 kt northwesterly jet streak have all aligned to result in a
very active weather day along much of the East Coast this morning.
Impressively deep moisture is being drawn northward up the Eastern
Seaboard ahead of the well-defined low circulation. PWATs are above
2.25 inches around the Florida Keys, but for the Mid-Atlantic into
New England, they`ll exceed 1.5 inches. This moisture will provide
ample support for widespread rain that is already evident on
radar. The precipitation shield will continue pushing
northeastward across the Slight Risk area through the morning.

Across the lower Great Lakes, an impressive cold front supported
by an even more energetic negatively tilted shortwave trough in the
upper levels is already beginning to rapidly intensify a second
jet streak that will roughly follow the US/Canadian border from NY
to ME on the Canadian side. While sufficiently separate from the
subtropical jet across the Southeast, the right entrance region of
the jet and impressive baroclinic zone associated with the front
will draw the aforementioned abundant moisture northward, clashing
with the advancing cold air from Canada. This boundary will act as
a focal line along which heavy rain (and snow on the north side)
will develop and track along the boundary to the northeast. It`s
likely the heaviest rain will begin to form on the southwestern
side of the Slight Risk area over eastern MD and the Delmarva, then
progress up the coast. As it moves through the NYC area and into
New England, the increasing baroclinicity will further enhance
lift. Most of the CAMs suggest embedded convective segments and
cells aloft across MD, DE, southeast PA, and NJ will coalesce into
more of a line as they approach NYC and continue as a line of much
heavier rain across southern New England. This heaviest line of
rain will precede the back edge of the rainfall associated with the
low`s dry slot. Ahead of the line, convective cells aloft are
likely, which will locally enhance rainfall rates. Fortunately they
will be brief in any one area as any convection will be racing
northeastward embedded within the broader precipitation shield.

The combination of the off-and-on convective elements within the
broader rainfall shield, followed by the line of much heavier rain
will result in widespread storm total rainfall from MD through MA
of 2 to 3 inches, with locally higher amounts. Urban areas along
the I-95 corridor will be most susceptible to resultant flooding,
with the highest risk from roughly Wilmington, DE through Boston.
This area remains in a "higher-end Slight". The area of greatest
concern within that corridor extends from NYC northeast through CT
and RI, where a coastal front, upslope from the terrain of southern
New England, and a well defined strong line of heavier rain will
have already formed. Widespread urbanization in this area will
contribute to lower thresholds for flooding. Within the line, rates
to an inch per hour are possible, which following the entire
morning of steady light to moderate rain should result in scattered
instances of flash flooding, with potential for considerable
flooding in the most susceptible areas.

Little change in storm-total rainfall and the evolution of the
rainfall across this region led to relatively little change in the
forecast. Given somewhat dry antecedent conditions and widespread
rainfall amounts largely unchanged, a Moderate was considered,
particularly in the NYC to southern New England area, but the short
duration of the heaviest rates and little change to the forecast
precluded that. The guidance shifted a bit to the east across the
Mid-Atlantic, so the Slight was removed from VA, and the Marginal
was shifted a row of counties eastward. Otherwise there was very
little change to the risk areas from NY into New England.

The Marginal risk was expanded to include the entirety of coastal
Maine with this update. The aforementioned convective line and the
warm advection out ahead of it should allow for a transition of
precipitation type for all of the coast from snow to mixed
precipitation and over to heavy rain. Heavy rain falling on ice
should enhance runoff, which will be offset by a lack of inflow
from interior Maine due to all of the precipitation falling as
snow. Given the uncertainty, the Marginal was hoisted but should
conditions verify warmer, then Slight level impacts are possible
as far north as Portland.

...South Florida...

A Marginal Risk will remain in place for portions of southeast
Florida from Ft. Lauderdale south and the Florida Keys. This
Marginal is largely a precaution as the bulk of the rainfall has
already occurred, but there remains some uncertainty as to whether
the rain will be fully pushed off to the south and east by 12Z. The
00Z guidance has pushed south as compared with the 18Z guidance,
which resulted in the heaviest rain remaining just south of the
Keys, which should preclude any flooding. However there is not
unanimous agreement among the guidance as to this scenario. There`s
better agreement that the heaviest rains will be clear of south
Florida by the 16Z midday update.

...Northern California Coast...

A strong vertically stacked low off the Washington coast will track
southward down the coast today, while weakening. The westerly flow
ahead of the low will enhance local upslope into the northern
California coastal mountains as well as far southwestern Oregon.
Expect another 1 to 3 inches of rain into this area, which when
added to yesterday`s rain continues the Marginal risk in more
flash flood prone locations.

Wegman

Day 2
Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

The vertically stacked low currently off the Washington/Oregon
coast will have opened up into a vigorous positively tilted trough
at the start of the period Sunday morning. The trough will track
down the coast through Sunday evening as a northerly 150 kt jet
streak approaches. The left exit region of the jet will align over
southern California, while onshore west to west-northwesterly winds
pump Pacific moisture into the Peninsular Ranges. Forecast rainfall
Sunday afternoon and evening has been gradually increasing, with
favored upslope west-facing slopes now expected to pick up around
an inch of rain for the day. Thus, the Marginal remains in place
with increased confidence for isolated flash flooding in the flash
flood prone foothills of the Peninsular Ranges as well as poor-
drainage locations in the San Diego metro.

Wegman

Day 3
Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY...

A positively-tilted upper level longwave trough over the West will
eject into the southern Plains Monday, becoming more neutrally
tilted with time. Vigorous shortwaves will lead the reorientation
of the longwave trough, with the northernmost one energizing a
mature low and winter storm across the northern Plains. That low`s
trailing cold front will provide the forcing for convection across
the Marginal and Slight Risk areas. Due to a second shortwave in
the subtropical jet moving the base of the trough eastward, the
trailing cold front will tap into increasing Gulf moisture in the
lower levels, leading to the rapid intensification of a low level
jet up the lower Mississippi Valley. Moisture-rich showers and
thunderstorms will develop ahead of the cold front, resulting in a
wide swath of 2+ inch forecast rainfall totals in the Slight Risk
area from south-central Missouri across most of Arkansas and much
of the northern two-thirds of Mississippi. Since these rainfall
amounts will be associated with thunderstorms, it`s likely they
will fall in a short-time frame, which in turn will increase the
flash flooding risk.

The Slight Risk was nudged a bit towards the northwest and
southeast to cover more of AR and MS, while trimmed across
northern LA following small changes in the latest guidance from
inherited.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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