Quantitative Precipitation Forecast
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FOUS30 KWBC 240055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sat Mar 23 2024

Day 1
Valid 01Z Sun Mar 24 2024 - 12Z Sun Mar 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN MASSACHUSETTS...

Heavy rainfall was ongoing at 00Z across eastern Massachusetts into
coastal New Hampshire and Rhode Island in association with a
strengthening surface low located near TAN in southeastern
Massachusetts. Observed rainfall rates since 23Z have peaked in
the 1.0 to 1.5 in/hr range between PVD and BOS despite little to no
instability. The lack of instability has been made up for by
strong dynamics with low level convergence (70-80 kt low level jet)
and frontogenetic forcing beneath the right entrance region of a
150+ kt upper jet over northern New England.

The 12Z hires models have had a relatively good handle on
precipitation amounts but have been a bit slow to translate the
heavy rain axis across the northeastern U.S. Even recent runs of
the HRRR have been about an hour slower when compared to the
observed radar.

The strong axis of low level moisture transport will soon shift
east of Massachusetts and focus into Maine where temperatures have
been at or below freezing supporting snow or freezing rain. Strong
low level warm advection is expected to push surface temperatures
above freezing from the Mid-Coast to Downeast Maine through 06Z as
the surface low tracks very near the eastern tip of Maine. Hourly
rainfall along the immediate coast of Maine is expected to reach
into the 0.5 to 1.0 inch range for a brief time, shifting fairly
progressively toward the east overnight, limiting additional
rainfall to 1-2 inches for the region. Beyond ~06Z, the axis of
heavy rain should have exited or nearly exited Maine into New
Brunswick and Nova Scotia. Localized flooding/flash flooding may
result across these regions of New England prior to the exiting of
precipitation.

...South Florida...

The Marginal Risk was removed for the 01Z update as deep moisture
and associated heavy rainfall has exited off to the east.

Otto


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...

...20Z Update...

Little change in the overall forecast for Sunday as increased
onshore flow across southern CA will allow for a regional maximum
in precip across the Peninsular Range down towards the San Diego
metro area. Guidance is fairly steady within the 1-1.5" range
across the interior with HREF EAS probabilities over 80% for at
least 1"/24-hrs in a small zone in-of of the Peninsular Range.
Lower probabilities are located within the San Diego metro itself,
but still non-zero around 10-20%. Primary concern will be terrain
influences within the zone of heaviest rainfall, as well as poor
drainage areas within the risk area that could be more prone to
flooding. The MRGL risk was maintained.

Kleebauer

...Previous Discussion...


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...

...20Z Update...

The Slight Risk was maintained for the Lower Mississippi Valley for
Monday as the overall synoptic pattern supports heavy rainfall
thanks to improving dynamics and increased buoyancy ahead of a
sharp cold front advancing east out of the plains. No major changes
were made to the overall risk alignment, but a few places were
added to the threat areas based off the latest ensemble trends and
ML based guidance. The MRGL risk was expanded to the southwest to
include the eastern section of the hill country to the east of
I-35 as the latest ensemble trends show the tail end of the cold
front tilting back to the southwest across south-central TX which
would allow for trailing thunderstorms to propagate over the more
flood-prone areas just to the east of the I-35 corridor. This also
matches the GFS and ECMWF Graphcast ML models with the trailing
convection idea that could spur trouble for localized flooding in
that particular zone. The other area of interest was the expansion
of the Slight Risk to the south over northern LA where deep-moist
convergence along the advancing cold front into a well-defined
instability axis will project a threat of heavy rainfall as ML and
deterministic guidance paints upwards of 2" or more of precip
within the corridor between I-10/20. Considering the stout
meridional pattern expected over the central US, the trends for a
slower moving convective scheme is favored which would allow for a
greater threat of training. This is also agreed upon by the latest
ML total precip depiction, as well as the ensemble mean forecast
from the NBM and associated global ensembles.

Kleebauer


...Previous Discussion...


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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