Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Fri Sep 29 2023

Day 1
Valid 16Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023

...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER
PORTIONS OF THE NORTHEAST...

...16Z Update...

Only minor adjustments were made to the D1 package across the
CONUS. Tightened up the eastern extent of the Moderate to align
with current obs and short term trends where heaviest rainfall has
occurred and continued higher end impacts are forecast. Also
brought in the southern extent of the MRGL-SLGT to adjust for the
trends in guidance. Main axis of each outlook was aligned with
HREF blended mean and ensemble-bias corrected QPF footprint.

No change was initiated over the MRGLs in the Midwest and Florida
as the overall synoptic pattern is still on track. Flash flood
threat will remain isolated within the respective convective
environments this afternoon and evening within areas of anomalous
PWATs. 

...Mid-Atlantic into Northeast...

Heavy rainfall has impacted the western Long Island area into the
NYC metro with area totals on the order of 4-6" with locally
higher amounts located within the Brooklyn and Manhattan boroughs.
1-3" is very common in the surrounding boroughs and encompassing
areas within the Lower Hudson, CT, and the northern third of NJ.
Strong low-level frontogenesis located parallel to the NYC/NNJ/LI
coast is analyzed on mesoanalysis and very well depicted within
the banding structures persistent over the above locations.
Cross-section analysis of both the 12z HRRR and 12z NAM nest show
a rich moisture presence deep into the profile over northeast NJ
up into western CT within a field of elevated Omega which aligns
perfectly with the forecasted 1000-925mb frontogenesis fields.
This is the main axis of concern through the rest of the D1 time
frame where a north-south precip maximum will likely develop later
this afternoon and evening. 12z HREF mean still shows a widespread
area of 3-5" totals as far south as southern Monmouth county in NJ
up through the Lower Hudson where terrain influences will aid in
local heavy precip banding signatures as the strong U-vector wind
anomalies provide upslope enhancement within the hills north of
NYC across Orange and Putnam counties. Local totals exceeding 8"
are increasingly likely within the Moderate coverage area,
especially those areas that are already over 5-5.5" at this hour.
12z HREF neighborhood probability of >8" is up to 20-25% within
NYC proper, a fairly robust signal in the means and the center of
the MOD risk bounds.

Our upper low to the west is slowly propagating through PA with
center of circulation over the Laurel Highlands as depicted on WV
satellite. The upper-level progression is important in the outcome
of the downstream heavy rain threat as increased dynamics and
textbook synoptic scale evolution will evolve over the course of
the afternoon and evening, finally ending within the back end of
the D1 period.

Kleebauer


...Upper Midwest/Great Lakes...

Shortwave energy propagating south-north into Canada will drive
showers and scattered to isolated thunderstorm activity over the
Upper Midwest today. Another inverted surface trough will direct
and enhance anomalous moisture plume with PWATs of 1-2" into much
of Minnesota and northwestern Wisconsin. Rainfall totals of 1-2
inches are possible over portions the Upper Midwest by tomorrow
morning.

...Florida...

The back end of a slow moving cold front will act as a focus for
scattered to isolated showers and thunderstorms today. A Marginal
Risk area was continued to account for any localized flash
flooding concerns that may arise.

Kebede


Day 2
Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT BASIN, NEW MEXICO AND FAR WEST TEXAS, UPPER MIDWEST, AND
FLORIDA...

...20Z Update...

There is little change to the previous MRGL issuances from the
previous forecast cycle. Upper trough will dig across the western
CONUS with downstream convective implications across the southern
high plains, southern Rockies, and Upper Midwest. The main
addition to the D2 ERO was the MRGL risk in place across the upper
Midwest with primary focus on the U.P of Michigan and northern
Wisconsin. More details on that threat below...

Elsewhere, locally heavy rainfall will be possible across the
Great Basin within a marginal convective environment. 12z forecast
bufr soundings across several locations in NV indicate steep lapse
rates and deep moisture presence that is typical under the center
of a deepening mid and upper trough axis. PWAT anomalies are
running between 2-2.5 devaitions above normal under the mean
trough which coincides with the higher QPF outputs given by
guidance. Both the HREF blended mean QPF and ensemble-bias
corrected precip field indicate a widespread area of 0.75-1.25" of
precip which would occur within a 3-6 hr time frame when
convection develops and traverses the area. The consistently low
FFG's in place over the interior west with the convective scheme
forecast would lead to isolated areas of flash flooding, which
would ultimately arise if the rates can exceed 0.5" or more per
hour. Considering the dynamic nature of the setup, this is well
within the cards, and accentuated within the core of higher PWAT
anomalies present.

West Texas and New Mexico will be under the influence of the
difluent pattern on the eastern side of the mean trough. Modest
instability over the terrain as indicated by HREF mean MUCAPE
500-1000 J/kg Saturday afternoon within a corridor of modest PWAT
indices between 1-1.5 devaitions above normal lends credence to
isolates flash flood concerns from diurnally driven convection.
Theta-E advection into eastern NM is fairly prolific for this time
of the year, putting the area within a more late summer-like
environment which is consistent with heavy rainfall potential.
Main areas of concern will continue to be terrain favored regions
and local urban corridors.

Florida will remain locked into a persistent pattern of elevated
PWATs and focused ascent along and south of the frontal boundary
drapped over central FL. Primary focus will be within the coastal
urban corridors.

...Upper Midwest...

Energy ejecting to the northeast out of the central plains will
move overhead of the northern fringes of the Midwest by Saturday
evening. At the surface, a frontal boundary will lift north in the
form of a warm front with positioning to the south of Superior
after 00z Sunday. Modest instability across northern WI into the
Michigan U.P will create an environment favorable for convective
development within a core of elevated PWATs approaching 2-2.5
deviations above normal. The combo of ascent from weak vorticity
advection and surface convergence along the warm front will be
plentiful for convective initation along the southern lake shore.
12z HREF neighborhood probability is fairly robust for local heavy
rain signatures with 10 ARI exceedance probability running between
20-30% over northern WI and even a max of 45% showing up within
the means between Sault Ste. Marie and Marquette. Considering the
signal for locally heavy rainfall within a convective scheme
rooted in a very anomalous, moist environment, went ahead and
added a Marginal Risk encompassing higher QPF areas forecast
within the HREF blended mean and WPC forecast QPF. 

Kleebauer

...Great Basin/Nevada...

A deep upper-trough will transport a modest plume of moisture into
the West on Saturday. A cold front associated with a deepening low
pressure system will act as a focus for convection over the Great
Basin in particular. Some parts of central Nevada, southeastern
Oregon and southwestern Idaho could see rainfall rates in excess
of 0.25"/hr. Ample forcing over elevated terrain could lead to
more efficient/enhanced rainfall. Between 0.5-1" total areal
rainfall is forecast over 3-4 hour period therefore, localized
flash flooding is our primary concern.

...New Mexico/west Texas...

Convection supported by an upper trough to the West and shortwave
energy downstream from it will support another localized flash
flood threat over parts of eastern New Mexico and into west Texas.
Decent instability, modest moisture flux and low level jet may
produce bursts of heavy rainfall over our marginal risk area.
Urban areas and dry washes are most vulnerable of flash flooding.

...Florida...

A slow moving shortwave trough will allow for the continuation of
scattered to isolated thunderstorms across the central/southern
peninsula on Saturday. Some cells may produce localized heavy
rainfall within the marginal risk area. Antecedent conditions will
also contribute to the sensitivity to any isolated heavy rainfall
threats.


Kebede

Day 3

The Day 3 outlook will be updated by 2030Z.


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