Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Thu Sep 28 2023

Day 1
Valid 16Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023

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


...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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE THE MID-ATLANTIC REGION INTO SOUTHERN NEW ENGLAND...

...Mid-Atlantic and Northeast...
The risk of flash flooding appears to be increasing across
portions of the Northeast Friday into Friday night, although there
remains quite a bit of uncertainty with the magnitude and location
of the highest threat. The main feature to focus on will be an
inverted trough/coastal front which will help focus a persistent
area of convergence. We are also likely going to have enough
instability advect in off the Atlantic along this convergence axis
to support at least shallow convective elements and primarily
efficient warm rain processes. The 00z HREF focuses the highest
probabilities across NJ into southeast NY...with 5" exceedance
probabilities ~50% and 8" over 20% from 00z Fri-00z Sat. The HREF
also shows some low 100 year ARI exceedance probabilities over
this same area. The HREF is likely depicting a plausible worst
case scenario...where the inverted trough and persistent
convergence focuses over land resulting in this excessive
rainfall. In this scenario a significant flash flood risk may
evolve. However I do think the uncertainty is probably greater
than what the HREF is depicting. For one, the location is not
locked in...with the 00z ECMWF further north and focusing the
heaviest rainfall more over southern New England. On top of that
it is not a given that the inverted trough sets up onshore...there
is some chance this axis stays just offshore. If this were to
happen then instability would stay offshore as well...and rainfall
totals/rates would not be as high over land.

For now we will carry a Slight risk, as the conditional flash
flood risk is pretty significant, and higher end flash flooding is
a possibility with this setup (although not a given at this
point). We will focus the risk from northern NJ into southeast NY
and into CT, which is where the best consensus amongst the high
res and global models currently resides. However we will need to
watch areas further northeast into southern new England...as while
it appears to be a lower probability outcome at the moment, we can
not rule out the further northeast 00z ECMWF solution either. 

...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...

...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: 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