281
FOUS30 KWBC 062004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Thu Jun 6 2024

Day 1
Valid 16Z Thu Jun 06 2024 - 12Z Fri Jun 07 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID ATLANTIC AND NORTHEAST...

...16Z Update...

Little change was made to the inherited risk area as the prospects
for flooding remain similar to the previous forecast(s) as backed
by hi-res probability fields and associated numerical output. The
latest HREF signals maintain relative continuity with the area
across New England and over the southern Mid Atlantic as the
primary focal points for convective based flash flooding this
afternoon and evening. A small shift in the MRGL was necessitated
over portions of coastal southern New England due to a small, but
formidable axis of convergence along a warm front bisecting the
coasts of RI over into southeastern MA. Rainfall rates have reached
upwards of 2-2.5"/hr at times within the band of heavy rainfall.
While not overly convective in nature, anomalous low to mid-level
moisture with a zone of enhanced 925-850mb frontogenesis aligned
parallel to the coastal plain has provided a tongue of precip
totals breaching 1.5-2" over the aforementioned areas. The setup
will eventually vacate the area and end the threat, but current
radar presentations indicate at least another hour or two of
potential before the threat subsides.

This was the greatest change from the previous forecast.
Otherwise, general continuity with the threat across the Mid
Atlantic evolving from convection along and just ahead of an
approaching cold front. Heavy rain over northern New England is due
to primed upper forcing within a broad axis of diffluence present
downstream of a potent shortwave trough swinging through the Great
Lakes and adjacent Ontario Province.

Kleebauer


...Previous Discussion...

...Mid Atlantic to Northeast...

A slow moving cold front, currently analyzed across portions of the
Ohio Valley and Appalachians, will cross through the Mid Atlantic
and Northeast today into tonight. Meanwhile, a warm front will
continue lifting northward through the Northeast this morning
before stalling or slowing across southern New England this
afternoon. The airmass will be quite moist, with the latest
guidance pointing toward PWs well above normal (exceeding 95th
percentile) with values over 1.75" from the Mid-Atlantic into
southern New England and near 1.5" across northern NY and portions
of VT/NH. This morning there should be an initial round of locally
heavy downpours with the warm front passage across southern NY and
southern New England, which could bring localized flooding concerns
with rain rates exceeding 1"/hr. A second round is then expected
for the afternoon/evening with the approaching cold front but also
could train/repeat with the warm front stalling nearby. Convective
initiation is likely between 16-18Z across western NY and PA with a
line of showers and thunderstorms moving east/northeast through
the remainder of the day.

The latest HREF probabilities for 24-hr totals of at least 2" are
spotty but show some signal exceeding 40-50 percent in locations
form eastern/northern NY and southern New England). Probabilities
for hourly totals of 1-2" are slight (under 30 percent).

Further south into the Mid-Atlantic, higher moisture ahead of the
cold front and sufficiently warm temperatures will support a line
or two of stronger showers and thunderstorms during the
afternoon/evening hours. Some of these will be capable of producing
heavy downpours, particularly from the I-95 corridor from
Baltimore to Richmond and eastward including southeast VA where the
2" HREF neighborhood probabilities are locally higher near 50
percent.

Taylor

Day 2
Valid 12Z Fri Jun 07 2024 - 12Z Sat Jun 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN KANSAS AND WESTERN MISSOURI...

...20Z Update...

Minor adjustments were made to the inherited SLGT risk across the
central plains. Recent trends favor an axis of heavy rain a tad
further north within the confines of an increasingly favorable
theta-E surge on the western fringe of a surface ridge positioned
over the Tennessee Valley. HREF probabilities for totals >2"
signal a greater likelihood of the heaviest totals bisecting the
KS/NE/MO/IA border, in-line with the recent ML output for location
of the heaviest corridor of precip. The upper quartile of outcomes
is generally within the 3-5" range thanks to the progressive nature
of the expected complex as it initiates over northern NE by the
late afternoon, driving southeast into MO by the end of the period.
Expected rates are forecast to top out between 2-3"/hr at peak
intensity, although the probabilistic output of generally 20-30%
for at least 2"/hr on the latest HREF neighborhood probability
intervals remains highly scattered in nature signifying less of
widespread higher rates in the complex and more of a few embedded
heavy cells intertwined within the main core. In either case, the
prospects for localized flash flooding are high enough to warrant
the continuation of the SLGT risk with only some minor shifts on
the northern periphery of the risk areas based off current ML and
ensemble output of the heaviest precip core.

Kleebauer


...Previous Discussion...

The setup continues to look favorable for a heavy rainfall event
across portions of the Southern/Central Plains Friday/Friday night.
A strong ridge of high pressure will remain anchored over
Mexico/Texas and along the periphery of this ridge, shortwave
energy is expected to ripple through out of the Rockies into the
Plains Friday afternoon/night. Meanwhile, a cold front passing
through today (Thursday) is expected to return/lift back through as
a warm front Friday/Friday night, bringing a surge of higher
moisture with it as low level flow increases. The combination of
this moisture transport, favorable dynamics (including right
entrance lift) will likely initiate a complex of thunderstorms that
then dive south/southeast along the instability gradient into
KS/MO. For this forecast cycle, the guidance remains in pretty good
agreement overall, with perhaps a nudge to the south compared to
the past 24 hours though the relative magnitude/amounts of QPF have
trended down somewhat and the latest ensemble guidance supports
lower QPF in general. There appears to be enough support to
maintain the Slight based on the environmental ingredients and
synoptic setup, but the risk probably leans toward the lower end
of the probability range for now.

Taylor


Day 3
Valid 12Z Sat Jun 08 2024 - 12Z Sun Jun 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS TO MID MISSISSIPPI VALLEY...

...20Z Update...

Recent consensus amongst global deterministic and associated
ensembles continues to pin a corridor of heavy rainfall potential
across eastern KS extending into central and southern MO. This
signal aligns perfectly within a forecasted theta-E maximum that
advects poleward Saturday afternoon and evening with an approaching
cold front to the north providing ample surface convergence as we
enter peak diurnal instability. As mentioned in the previous
forecast, environment will be ripe for heavy rain thanks to an
anomalous low to mid-level moisture flux that will jettison PWATs
to well-above normal levels. The moist environment coupled with
favorable low to mid-level dynamics will be plentiful for incite a
swath of heavy convection along and ahead of the surface front that
will be moving into the area. Current ensemble blends, including
the NBM and bias corrected ensemble pinpoint a mean of 2-2.5" of
rainfall across eastern KS into south-central MO with the areal
extent of at least 1" encompassing the area 75-100 miles
surrounding.

The biggest change to the previous forecast was mainly
the positioning of the SLGT being a bit further south from current
consensus, so the shift north was in response to the recent
northward deviation of previous forecasts with ample support from
pretty much all global deterministic and ensembles. A lot of the
pattern is contingent on the meridional push of the advancing cold
front, so we`ll be keeping a close eye on the general model output
for the boundary in question as that will dictate eventual SLGT
risk location and latitudinal expanse of the risk area(s) across
the plains.

There was little change in numerical output for the secondary area
of interest across the Front Range where general consensus points
to isolated heavy thunderstorms capable of 1-3" of rain falling in
any given storm that materializes along the terrain in CO/WY.

Kleebauer


...Previous Discussion...

A wavy frontal boundary is expected to settle across portions of
the Southern Plains eastward into the Mid Mississippi Valley while
aloft the quasi zonal/northeast flow continues with periodic
shortwave energies moving through. A pool of higher moisture will
remain along/ahead of the frontal boundaries and is likely to be
characterized by PWs well above 1.5" and approaching 2",
particularly across portions of MO/AR. This higher moisture
combined with the lift/convergence and instability due to peak
heating should spark a few rounds of strong thunderstorm complexes
that track east/southeast across the risk area. There remains model
spread in placement (north vs. south) and magnitudes as well, and
some of the uncertainty will be tied to how the previous day
(Friday/Friday night) plays out but the consensus remains favorable
for an area of heavy rainfall that could lead to scattered
instances of flash flooding, especially as some of these areas are
still running above normal for soil moisture anomalies.

Further west, moisture banking against the terrain of the Front
Range combined with daytime peak heating and shortwave trough
energy moving through will lead to isolated/scattered showers and
thunderstorms, some of which will be slower moving and produce
intense rain rates and isolated/localized flash flooding.

Taylor



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