Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Sat Mar 25 2023
Day 1
Valid 01Z Sun Mar 26 2023 - 12Z Sun Mar 26 2023
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST...
A Marginal Risk for excessive rainfall has been introduced across
portions of the Southeast, mainly encompassing south-central
Mississippi and Alabama (largely overlapping the Marginal Risk
going into Day 2, though displaced a bit towards the southwest).
CAMs have been very consistent in depicting convection imitation
after 06z tonight along a stalled outflow boundary/frontal zone. A
weak perturbation in the mid-levels looks to be responsible for
this expected initiation, and little to no large scale ascent/lift
looks to be present. The environment, however, will be very
supportive of convective organization and sustainment as ML CAPE
should range from 1500-2500 J/kg, precipitable water values look
to range between 1.2-1.5 inches (between the 90th percentile and
max moving average, per BMX sounding climatology), and deep layer
(0-6 km) shear is on the order of 50-70 kts. Given the ample deep
shear and somewhat curved hodographs, storm mode of deep
convective activity should favor supercell structures (especially
initially) with both splitting storms and localized training
segments being possible. Supercell storms will likely be capable
of 1-2"/hr rates, and 18z HREF neighborhood probabilities for 2"
exceedance (through 12z) are as high as 15-30%, and 3" exceedance
probabilities as high as 10%. These probabilities may be a bit
underdone as well, considering that more recent HRRR runs (through
22z) depict the potential for localized 3" totals (and supercells
will often exceed what the HRRR depicts as well, especially when
associated hail production is at a minimum). Even so, localized
totals of this magnitude will be fairly close to associated 3-hr
FFG.. therefore the threat for any instances of flash flooding
through 12z should remain rather isolated at best (and may not
even occur if deep convection fails to initiate from the lack of
large scale lift).
Churchill
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sun Mar 26 2023
Day 1
Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL ALABAMA AND GEORGIA...
There has been quite a notable flip-flop in all of the guidance as
pertains to the weather expected to unfold across portions of the
Southeast today. A stalled out front remains in place across the
Slight and Marginal Risk areas from Louisiana to southern North
Carolina. This front is already serving as a focus for convection
from central MS through far western GA. The convective activity is
only expected to increase for the rest of the predawn hours and
into the morning. 30-40 kts of southwesterly 850 mb flow is
drawing plentiful Gulf moisture northward into the front.
Meanwhile, a pair of subtle disturbances are expected to track
eastward along the front, causing local increases in both
convective coverage and severity. This is probably the largest
change from 24 hours ago, when we had eliminated even a Marginal
risk of flash flooding! These disturbances acting to organize the
convection along the front now will be quite effective at taking
advantage of as much as 2,500 J/kg of MUCAPE and somewhat unusual
levels of atmospheric moisture at 2 sigma above normal and PWATs
exceeding 1.5 inches.
Uncertainty remains above average given the rainfall event is
already beginning, as to where the heaviest rain is expected to
fall. While most of the CAMs have highlighted the Slight Risk
area, the primary area of uncertainty is how a subtle upper level
disturbance interacts with the ongoing convection and associated
cold pools this evening. The wave will track well north and west
of the area, but nonetheless will have enough impact on the
ongoing thunderstorms to result in a local increase in storm
coverage and intensity. Some of the CAMs suggest this will result
in the heaviest convection displaced well to the north of the
surface front, highlighting Birmingham and Atlanta, whereas others
think the primary forcing will stay with the front further south,
resulting simply in an area of strong thunderstorms as the latest
salvo in the wave train. The impacts from this difference would be
how severe the likely ongoing flooding can get if the stronger
storms track over the same areas, or if they're sufficiently
displaced to the north to result in fewer impacts (despite the
lower FFGs due to yesterday's rain across northern AL). If the
former scenario pans out where the storms stay with the main
surface front, then there's some potential for extreme rainfall
totals exceeding 6 inches (as depicted in the 00Z HRRR primarily),
and the potential need for an additional upgrade to a Moderate
Risk.
As regards training storms, the potential is very high, and in
fact is already happening in the Slight risk area of eastern
AL/western GA along the front. These storms will continue training
at least through the morning hours, at which point the overall
convective activity will wane a bit with the typical weakening of
the LLJ. Convection then increases again with the approach of the
second wave in the late afternoon and evening hours. For all
convection today, the motion of the storms will be
east-northeastward, which is parallel to the surface front that is
not expected to move much during the day. Thus, the flow orienting
parallel to the front supports continued training through the day.
Already, currently ongoing severe thunderstorm activity is
producing local rainfall rates up to 1.5 inches per hour, and that
intensity of convection is expected to continue over the next
several hours, and again late this afternoon and evening.
For the surrounding Marginal Risk, expansions were made in all
directions with this morning's update. Some convection has already
developed in northwestern Alabama and central and southern
Mississippi as of this writing, and continued scattered convective
development is likely through the day in both areas. The increased
coverage and intensity required an eastward expansion of the
Marginal Risk to the Carolina coast, as all of the convection will
track along this corridor (albeit likely in a somewhat weaker
state than areas further west), and therefore training convection
once again will be the dominant factor favoring development of
flash flooding, particularly if those stronger storms move over
urbanized areas.
...Potential for Future Risk Changes...
Depending on storm coverage through the morning, and if guidance
remains consistent on continued training storms in much of the
Slight Risk area late this afternoon and this evening, a Moderate
Risk may be needed.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sun Mar 26 2023
Day 2
Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
By Monday morning, most of the convective activity associated with
the front in the Southeast will have drifted south of the main
areas impacted by the strong storms today, i.e. closer to the Gulf
Coast. With little in the way of overlap from Day 1 and good
agreement overall that convective coverage and intensity will be
greatly reduced as compared with Sunday's storms, a somewhat low
confidence forecast of no flash flooding remains for this forecast
update. However, should there be any northward trend with the
forecast convection on Monday, then a Marginal Risk will be needed.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Sun Mar 26 2023
Day 3
Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
CENTRAL CALIFORNIA COAST AND ADJACENT MOUNTAINS...
...Central California Coast...
In coordination with the MTR/Monterey, CA and LOX/Los Angeles, CA
forecast offices, the inherited Slight Risk area was expanded both
north and south with this morning's forecast update. An upper
level polar cutoff low begins the day Tuesday morning centered off
the Oregon coast, and tracks southeastward to be centered over the
northern California coast by Wednesday morning. A 100 kt westerly
jet streak wrapping around the low will direct a plume of Pacific
moisture and precipitation into California as it tracks south.
PWATs will approach 3/4 inch, which is about 2 sigma above the
climatological normal. This will bring with it a brief increase in
snow levels as high as 7,000 ft during the day Tuesday. The
attendant surface low will follow a similar track as the upper
level cutoff low through this period, making it a vertically
stacked low, that will result in largely unidirectional flow
throughout the column south of the low. Since the vertically
stacked low has polar origins, it will have plenty of cold air and
a lack of plentiful Pacific moisture with it. Thus, this rainfall
event is not expected to be anywhere close to as intense as the
atmospheric rivers impacting the state in recent weeks.
Nevertheless, the residual impacts from the atmospheric rivers
remain across coastal central California, and the injection of up
to 3 inches of rain into the coastal mountains are still expected
to cause or worsen flooding impacts as that rainwater flows down
the mountains and swells already high rivers. Thus, the Slight
Risk was expanded due to increasing forecasted rainfall through
the Bay area and especially the mountains north of there, and
expanded south to include the westernmost Transverse Ranges of
Santa Barbara County. Further inland, slightly lower snow levels,
much higher mountains, and more recovery time from the last round
of atmospheric rivers were factors that led to the continuance of
the Marginal Risk, as much of the heaviest precipitation is likely
to fall as snow. The highly favorable antecedent conditions for
flash flooding remain a significant factor in the Slight Risk area.
...Potential for Future Risk Changes...
Despite the expansion of the Slight Risk area, at this point it
appears further upgrades to the Slight Risk area are unlikely, as
precipitation totals have remained fairly steady, and the lack of
tropical moisture will preclude rainfall rates more typical of
true atmospheric rivers.
...Gulf Coast through Coastal Georgia...
The Marginal Risk area along the Gulf Coast is largely unchanged
from the Day 4 update yesterday. There has been a small southward
shift in most of the guidance, so the immediate Gulf Coast and
points up to 100 miles inland remain in the Marginal Risk area for
this update. The front will be in roughly in the same place
Tuesday as it was on Day 2/Monday. Thus, expect Monday's rainfall
to have lowered FFGs a bit in the Marginal Risk area by the time
we get to Day 3/Tuesday. Forecasted rainfall is a little higher in
this area than on Day 2/Monday, particularly around southern
Mississippi. Thus, think continued convection in the same areas on
Tuesday as on Monday warrants the continued Marginal Risk for the
same stalled out front.
...Potential for Future Risk Changes...
As with the current Day 1 update, if future model trends move
higher for forecasted rainfall, it's very possible a Slight Risk
may be needed for portions of the Marginal Risk area on Tuesday.
Since a large component to any upgrades will be the rainfall
distribution in this area on Day 2/Monday, confidence and current
forecasted rainfall are low as to where any upgrades would be.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
