Quantitative Precipitation Forecast
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000
FOUS30 KWBC 221542
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1142 AM EDT Fri Mar 22 2024

Day 1
Valid 16Z 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...

...16Z Update...

Limited changes necessary across the Marginal risk areas focused
over the Southeast/Mid Atlantic, as well as the northern CA coast.
Guidance is consistent in the placement and magnitude of the
expected rainfall from each disturbance with QPF maxima generally
positioned in similar locations when analyzing run-to-run trends.
Northern CA will see its heaviest rainfall from now until early
this evening before the primary moisture plume shoves inland in
wake an advancing cold front. This will likely end the necessity of
the risk area as the focus shifts inland with winter weather
becoming the primary concern.

The heaviest rainfall across the Southeast and Mid Atlantic will
occur along the NC coast by the end of the period as a maturing
surface low will aid in projecting a strengthening 85H LLJ off the
adjacent Atlantic with modest theta-E returns centered over the
coastal Carolinas. Latest HREF probability fields indicate a nose
of elevated 1-2"/hr probability signals within the aforementioned
corridor which could lead to localized flooding within any training
cells. Storm totals of 1-3" with locally up to 4" along the NC
coastal plain are plausible in the setup, but much of the rainfall
occurring over an area of higher FFG thresholds given sandier soils
limiting runoff.

Across Florida, the trends have been consistent within the latest
hi-res guidance on the southern FL Peninsula being the primary
beneficiary of multiple waves of heavy rainfall that present
totals breaching 5" over the course of the D1 time frame. The
Slight risk was expanded to the north over the southeastern FL
coast to align with the latest HREF blended mean QPF forecast and
accompanying probability fields signaling a threat for 2-3"/hr
rates (25-45%) affecting the population centers from West Palm
Beach down to Miami through all the of the FL Keys. The lower FFGs
centered over the I-95 corridor will be the focus for the greatest
flash flood risk, but the FL Keys will have the greatest threat for
the higher totals, so the entire area is well-within the criteria
for the SLGT risk being maintained from the previous forecast
cycle.

...Previous Discussion...

...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 Sun Mar 24 2024 - 12Z Mon Mar 25 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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