Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Thu Sep 28 2023

Day 1
Valid 01Z Fri Sep 29 2023 - 12Z Fri Sep 29 2023

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS
OF THE MID-ATLANTIC AND NORTHEAST...

...01Z Update...
Updates made were based on recent observation trends and the most
recent hi-res guidance. 

In the Northeast, did not make any significant changes.  Recent
hi-res model guidance indicates heavier rain moving onshore
overnight with locally heavy accumulations becoming likely from
coastal New Jersey into the lower Hudson Valley and southwestern
Connecticut.  The 18Z HREF is showing high neighborhood
probabilities for accumulations of 2 inches or more centered
across this region overnight.  These amounts may prove
problematic, especially for areas with antecedent wet soils and
low FFGs across portions of northern New Jersey into southeastern
New York.

In the Ohio Valley, trimmed away the northeastern extent of the
previous outlook, but maintained a small area centered across
western Kentucky and Tennessee where isolated cells remain
ongoing, with some potential for additional development farther
west.

There remains a localized heavy rainfall threat across portions of
Florida this evening into the overnight, especially along the
Atlantic coast to the north of a slow-moving boundary, where
easterly flow and deep moisture will help to support additional
activity.

Pereira

...16Z Update...

In coordination with the local WFO offices in Mount Holly and
Upton, have upgraded the D1 ERO to a Slight Risk given the trends
in short term guidance in terms of placement and magnitude of
heavy rainfall this evening. General synoptic scale pattern
remains fairly similar to previous forecast package, but more
emphasis on heavy rain potential has arose given the signals on
the latest ensemble means and deterministic QPF distribution. 12z
HREF neighborhood probabilities of >1" and >2" totals across
northern and central NJ, as well as the NYC metro into the
southern Catskills was sufficient to upgrade the area to the SLGT
risk. There is still some discrepancy on the exact placement of
the heaviest QPF footprint, however the highly anomalous u-vector
wind fields across northern NJ and adjacent areas are all
prevalent across all guidance which would generate a robust
upslope component typically found in these types of heavy rain
events. Convective potential is still on the low side, but
non-zero given the theta-E advection regime likely overnight into
early tomorrow morning as indicated by all deterministic, global
or hi-res base. Total QPF between 1-2" with locally as high as 4"
are being depicted within the HREF blended mean QPF which only
lends credence to the higher potential with the SLGT. Main areas
of concern will be the higher terrain in NNJ and southern NY
state, as well as the urban areas in-of NYC/Newark/Jersey City.

Kleebauer

...Mid-Atlantic into Northeast...
We added a Marginal risk across portions of eastern PA into NJ and
southeast NY with this update. Lower confidence than normal for a
day 1 forecast as an area of low pressure develops offshore and
attempts to push an inverted trough/coastal front inland. The
extent of the flash flood risk really comes down to the
positioning of this front/convergence axis later tonight into
Friday morning. If this axis stays offshore then any instability
will also remain offshore, resulting in just some stratiform
rainfall moving inland. However if the axis is along the coast or
inland, then some weak instability should get ashore as well, and
set the stage for the potential of shallow efficient convection.
Most of the HREF members do bring some heavier rain onshore, with
the highest probabilities focused over NJ. However each HREF
member has a slightly different location, and most of the 00z
global models keep the bulk of the heaviest rain offshore through
12z Friday. Given this spread and uncertainty opted to stick with
a Marginal risk for now. The main threat for day 1 is the last 6
hours (06z-12z Fri), so will let the day shift take a look at the
12z HREF and make a call on whether to upgrade to a Slight or not
at that time.

...Ohio Valley...
A Marginal risk was maintained across portions of TN/KY into
southern OH. Convection will be ongoing at 12z this morning, with
some brief training/backbuilding remaining possible. We might end
up with a downward trend in convective activity by mid to late
morning...but with an elongated area of vorticity continuing to
traverse the area through the day good synoptic ascent will remain
in place. Thus once instability recovers from the morning activity
we could see some additional convective development across the
region this afternoon/evening. The magnitude/organization of this
additional activity is conditional on the amount of recovery we
get....but if we area able to ramp instability back up then some
additional brief training could occur given the persistent low
level inflow and convergence in place. Isolated flash flooding
will be possible, both this morning and potentially again this
afternoon/evening.

...Florida...
Only minimal changes to the inherited Marginal risk area over FL.
Anomalous moisture will remain in place, with a stationary front
and stronger upper level flow helping trigger above average
convective coverage again today. High rainfall rates will drive a
localized flood risk over any more susceptible urban locations.

Chenard


Day 2
Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN NEW JERSEY, FAR SOUTHEASTERN NEW YORK, AND THE NEW YORK
CITY METROPLEX...

...20Z Update...

Increasing consensus in a higher impact event across portions of
the Northeast U.S, including the NYC metro area has lead to an
upgrade to a Moderate Risk over the aforementioned area. After
coordination with the local WFOs over portions of the Mid-Atlantic
and Northeast, confidence was high enough to introduce the
Moderate given the signals across much of the 12z deterministic
suite and ensemble guidance. More on the setup below...

Only minor adjustments were made to the MRGL over the Upper
Midwest to account for convective signals within the means and 10
year ARI exceedance probabilities over the western Michigan U.P.
There were no changes to the MRGL in FL as the convective
environment remains favorable for locally heavy rainfall within an
elevated PWAT regime thanks to persistence in the southern stream
jet and location of a slow-moving front over the northern sections
of the FL Peninsula.

...New York/Northern New Jersey...

12z suite of guidance has narrowed the corridor of expected heavy
rainfall that will transpire thanks to a prominent easterly fetch
event expected on the northern periphery of an inverted trof in-of
the northern Mid-Atlantic coast. The signal for a prolonged heavy
rain event has increased markedly over the past 24 hrs with the
current deterministic and ensemble mean guidance now pinpointing a
swath of 2-4+" rainfall footprint across the NNJ and the adjacent
NYC metro and southern NY state towards Poughkeepsie. 12z HREF
mean is approaching 3-4" for just the NYC metro which would cause
considerable problems due to urban sprawl and inability for
rainfall to penetrate the impervious surface. 12z HREF
probabilities are also depicting an upper threshold event within
some of the probabilistic signatures. 3"/3 hr rainfall probability
is exceeding 70% for the NYC area up through the exo-burbs outside
the metroplex. 1-2"/hr rainfall rate probabilities are also into
the 70-80% territory which is typically found in the most robust
setups, usually within a tropical moisture field which is what is
expected given the enhanced IVT signatures on global deterministic
correlating well with the axis of heavy rainfall. GFS/ECMWF PWAT
anomalies are both approaching the 1.5-2 deviations above normal
range focused in the corridor of higher forecast QPF. Instability
will be lackluster for the most part, but some theta-E advection
into coastal NJ, NYC, and Long Island will pose a lower-end
convective threat across the region which would only exacerbate
flash flooding concerns.

The biggest change outside the magnitude of rainfall is the
propagation of the main QPF field further inland thanks to the
advection of low-mid level moisture being forecast due to very
anomalous U-vector wind fields generated from our inverted trof.
Enhancement on the NW side of the precip field will be possible
Friday morning and early afternoon as the shortwave trough over
the Great Lakes pivots into the northern Mid-Atlantic and closes
off across northern PA. This would allow for a more dynamic
signature within the northern and eastern fringes of the
upper-level circulation creating localized banding structures over
the terrain as depicted by a few hi-res deterministic. This is one
of the reasons we see secondary and tertiary maxes within the
general QPF forecast from deterministic as banding structures will
be found during the event evolution. Local maxes between 6-8" will
be plausible across the outlined Moderate area and even within the
confines of the SLGT risk. This has evolved into a higher impact
potential and will be monitored closely for future updates to the
orientation of the MOD risk with coordination from the local FO's
involved.

Kleebauer

...Upper Midwest...
Convection should be ongoing Friday morning across portions of
SD/ND/MN, although do think this activity will be pretty quick
moving. The better chance of isolated flash flood issues appears
to be Friday night as the front slows and low level moisture
transport sees a nocturnal uptick in intensity. This should be
enough to result in another convective round, with propagation
vectors supporting some backbuilding potential by this time. With
PWs running near to above the climatological 90th percentile any
training/backbuilding of cells could pose a localized flash flood
risk

...The Florida Peninsula...
Pretty much a persistence forecast across FL...with 2"+ PWs and a
favorable environment for good convective coverage in the area of
a slow moving frontal boundary. High rainfall rates over urban
locations may again result in some localized flood concerns.

Chenard

Day 3
Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND FLORIDA...

...2030Z Update...

No changes were necessary to the D3 outlook as the overall
synoptic environment remains on track from previous forecast
issuance. Digging upper trough across the west will aid in
advecting a deeper moisture presence across far west TX and the
eastern half of NM. 12z guidance is all-in on a prominent theta-E
advection regime correlating with above normal PWATs being
funneled up on the eastern flank of our upper trough. The
combination of terrain and instability will generate a period of
scattered convection during peak diurnal heating which will induce
a localized threat of flash flooding concerns within the confines
of the terrain. Areas of interest will be along the slopes of the
southern Rockies down into the Sacramento's, as well as any
urbanized towns/cities within the axis of convective development.
Added the El Paso area to the MRGL risk considering their recent
heavy rain episode that left some FFW issuances over parts of the
metro. Enough of a signal was present to also include the area
with some guidance pinpointing more thunderstorm activity
developing near El Paso and adjacent Juarez, MX.

Florida will remain in a persistent pattern of elevated
instability and deep, tropical moisture bisecting the FL Peninsula
thanks to a sub-tropical jet plume lying south of the mean eastern
trough. Localized flash flooding concerns over urbanized corridors
will continue to be the primary focus.

Kleebauer

...Southwest...
A deep and anomalous closed low will move into the Southwest this
weekend...with 500mb heights below the 5th percentile for the time
of year. To the east of this deep low there should be an overlap
of favorable PWs and instability across portions of far west TX
into eastern NM. This corridor will also be within an axis of deep
and fairly unidirectional southerly flow, which should favor some
repeat convective activity. Thus overall the inherited Marginal
risk looks in good shape, as would expect to see pockets of heavy
and locally excessive rainfall. The ECMWF seems a bit dry given
the ingredients in place...so prefer a wetter outcome, closer to
the GFS. Of course as we get closer we'll have more high res
guidance to look at and hopefully get a better idea of convective
details.

...Florida...
Pretty much a persistence forecast across FL...with 2"+ PWs and a
favorable environment for good convective coverage in the area of
a slow moving frontal boundary. Global deterministic and ensemble
guidance appears a bit wetter Saturday compared to
Friday...probably due to slightly better positioning of the upper
jet and mid level trough. So possible we will have a bit better
convective coverage, but still generally only looking at a
localized urban flash flood threat driven by high rainfall rates.

Chenard


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