Quantitative Precipitation Forecast
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000
FOUS30 KWBC 220801
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
401 AM EDT Fri Mar 22 2024

Day 1
Valid 12Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
GOLD COAST AND THE FLORIDA KEYS...

...Southern Florida...

Convection ongoing across the central and southeastern Gulf is
tracking southeast towards southern Florida and the Keys this
morning. The storms have formed along a robust moisture gradient,
south of which PWATs are forecast to exceed 2.25 inches, which is
impressive even by summertime standards. The storms are training
across the Gulf, so the real question is where the line of storms
sets up once they reach south Florida and the Keys. CAMs guidance
continues to sag southward with where the heaviest storms will
track, some even as far south as the north coast of Cuba. Despite
this, the incredibly moisture-rich environment the storms have to
work with, even by summer standards, should easily result in
multiple inches of rain for the Keys especially, and depending on
where the storms track could impact the Miami metro. Storm total
rainfall amounts to 5 inches are forecast around Key West, closer to
3 inches around Key Largo, and 1-2 for Miami. Again these amounts
are quite uncertain as any small northward deviation in the storm
track could bring all of these numbers higher, while continued
southward shifts would lower them.

The inherited Slight was trimmed out of all of southwest Florida, as
the heaviest convection is most likely to miss that area to the
south and west, and the swampy nature of the area should be able to
handle the 1-3 inches of rain forecast since this will fall over a
longer time frame as outflow from the storms to the south and west.
The surrounding Marginal was left unchanged given the inherent
uncertainty as to where isolated flooding may develop.

...The Southeast...

Cyclogenesis occurring ahead of a slow-moving but intensifying
shortwave trough located in the "sweet spot" in between the left
exit region of a 150 kt jet over coastal Texas and the right
entrance region of a 140 kt jet along the US/Canada border from NY
through ME will be the catalyst for heavy rain across the area. With
the storm only just forming through 12Z Saturday, most areas should
see a long-duration light rain, though with potential embedded
convective elements. The most likely areas of isolated flash
flooding will be into the southeast facing slopes of the southern
Appalachians through the Great Smokies, where upslope will locally
enhance rainfall rates, as well as along the Carolina coast where a
plume of moisture to 1.75 inches PWAT will stream northward ahead of
the main low. The extra moisture in this plume will locally enhance
rainfall rates from Charleston, SC through the southern Outer Banks.
Elsewhere in between, expect a rapidly expanding precipitation
shield that will move up the coast well into New England by 12Z, as
increasingly strong frontogenesis ahead/northeast of the low focuses
the precipitation shield all up the East Coast.

...Northern California Coast...

A strong vertically stacked will will approach the Washington/Oregon
coast through today and into tonight. Already moving slowly, the low
will turn northward up the coast tonight. The slowing eastward
motion of the parent low will translate to a slowing eastward
translation of the trailing cold front, the latter of which will be
the focus for the heaviest rainfall leading to potential isolated
flash flooding. The area where the cold front will be slowest moving
is along the northern CA coast and far southwestern OR, where
coastal terrain will further enhance rainfall amounts. The inherited
Marginal Risk for this area was unchanged with this update as the
terrain-focus of the heaviest rainfall remains in excellent
agreement.

Wegman

Day 2
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...

Few changes were needed to the inherited Slight Risk across portions
of the Northeast. Rainfall amounts are nominally higher from
previous forecast, but there remains very good agreement that a
coastal front will be enhanced by a strengthening southerly jet
streak in the right entrance region along the coast. This will
contribute to the already impressive frontogenesis across the
northeast with 850 temperatures to -12C over the upper Lakes
contrasting with +6 along the Eastern Seaboard at 00Z Sunday. This
extreme temperature gradient will enhance the southerly flow ahead
of it, which will force abundant moisture into the coastal front,
extending the precipitation shield northward well away from the
surface low, which will increasingly become secondary to the coastal
front as far as the most impactful feature. Given the very cold air
north of the front, there will be a stark rain-snow line across New
England, with plowable snows expected north of the front, and up to
3 inches of rain to the south.

As usual the bulk of the moisture will be on the warm side, which
will align with the I-95 corridor from DC into southern Maine. Thus,
expect a period where rainfall rates could approach 1 inch per hour
along the urban corridor. Fortunately, HRRR guidance suggests that
the period of time where rainfall rates could be that high will only
be for a 1-3 hour time frame in any one location, as the heaviest
rainfall quickly races off to the north and east. This will be the
main limiting factor preventing more widespread flash flooding.
However, given soils that are only beginning to green up across the
area, much of the soils will be unable to handle this amount of rain
falling even this quickly, so widely scattered instances of flash
flooding are likely through the Slight Risk area. A higher-end
Slight is forecast from Wilmington, DE through Boston. This area is
where the strongest frontogenesis will align with the leading edge
of the moisture plume for the aforementioned 1-3 hours at the ending
of the event, and the corridor where any potential upgrades to a
Moderate are likely to be focused. Despite the long-duration rain
throughout the Slight Risk, the relatively brief period of heaviest
rains should preclude the need to upgrade at this time, but future
shifts will evaluate.

The primary changes from inherited were to include more of the
northern Virginia suburbs of DC in the Slight due to lower FFGs in
this area, as well as a significant northward expansion of the
Slight from near Boston north through Portland, ME. In this area the
rain-snow line will be the dominant factor as to which areas get
potentially flooding rain versus plowable snow.

...South Florida...

Lingering rainfall from the Day 1 period will be ongoing Saturday
morning. The front causing the rain will shift eastward during the
morning, ending the flooding threat rapidly from west to east. The
Marginal risk was adjusted to closely match the Day 1 Slight area.
Since the vast majority of the rainfall with this event will be over
before the start of this period, the Marginal is mainly
acknowledging continuing ongoing flooding, rather than any new
excessive rainfall. Please see the Day 1 discussion for the fuller
meteorological setup for this area.

...Northern California Coast...

As the surface low off the Oregon coast moves inland and rapidly
weakens, the overlying upper level low will slowly shift southward
down the coast through Sunday morning. In advance of the low,
rainfall will continue into the northern California coast, with
another 1-3 inches of rain anticipated. These amounts generally
remain below concerning levels, but given the similar amounts of
rain expected in the Day 1 period, a doubling of that rain through
Day 2 could again lead to isolated flash flooding concerns. The
inherited area was nudged southward a row of counties on both ends,
but is otherwise largely unchanged.

Wegman

Day 3
Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024

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

...Southern California...

Onshore flow ahead of a strengthening upper level shortwave diving
southward parallel to the coast will keep a steady light to locally
moderate rain ongoing across southern California, largely south of
Los Angeles through the Day 3/Sunday period. Light rain will begin
in this area in Day 2, but the bulk of the rain is expected on Day
3. Given the sensitivity of the soils and urbanization, a Marginal
Risk area was introduced in response to increasing forecasted
rainfall through the day Sunday. Rainfall amounts of around three-
quarters of an inch may cause isolated flash flooding in the terrain
east of San Diego, along with any poor drainage areas in the city
and surrounding suburbs. The overall low-level windflow pattern from
the northwest will prevent any tap of subtropical moisture, so the
maximum amount of rainfall will be quite limited, so further
upgrades in this area are not anticipated.

...Nebraska/Iowa...

The inherited Marginal Risk area for eastern Nebraska and western
Iowa was downgraded with this update. While moisture amounts with
the developing storm are well above climatology, most of the
guidance suggests the heaviest precipitation will be realized in the
cold conveyor belt of the storm, and any locally heavy rainfall
associated with embedded convection will be progressive. Thus, the
winter storm aspect of this low will be the far more impactful.
Further, antecedent conditions have been very dry over this area, so
the limited amounts of rain that are largely expected to remain
under an inch should be well handled by the soils of this area,
precluding even isolated flash flooding.

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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