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000
FOUS30 KWBC 142258
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
658 PM EDT Sun Apr 14 2024

Day 1
Valid 01Z Mon Apr 15 2024 - 12Z Mon Apr 15 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL NEAR THE OHIO-
WEST VIRGINIA-PENNSYLVANIA BORDER JUNCTION...

A line of convection across PA has been generally progressive and
has shown signs of organization due to the degree of effective bulk
shear present and upstream instability across OH. It`s the upstream
instability that`s the possible issue as there has been recent
signs of some backbuilding showers. While precipitable water values
are fairly low -- 1-1.25" at best -- where backbuilding and
resultant cell training can manage to occur, 1.5" an hour totals
would be possible which would be problematic in rugged terrain and
urban areas. Have opted to maintain a portion of the Marginal Risk
near the tristate border of WV-PA-OH due to possibility of
backbuilding.  From an excessive rainfall/flash flood threat,
expect the risk to be on the lower end of the marginal risk scale
-- i.e. closer to 5%.  Due to the modest precipitable water values
 present, expect CIN to develop after sunset and cause the
 activity to wane in intensity and coverage, with the expectation
 that it won`t last too far into the very early morning hours on
 Monday, perhaps 0700 UTC.

Roth

Day 2
Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS...

...2000 UTC Update...
Minor changes were made to the Day 2 ERO, mainly to nudge the
southern flank of the Marginal Risk area a bit farther east across
the eastern Plains and Lower MO Valley based on the latest guidance
trends. Deepening low slowly pivoting into the Upper Midwest will
begin to draw moisture from the western GOMEX later Monday
afternoon and overnight, with 850 mb southerly flow and moisture
flux anomalies reaching 4 standard deviations above normal per the
12Z GEFS. Pre-frontal quasi-linear convection across the eastern
Plains will likely be fairly transient per the latest simulated
reflectivity and 1-3 hour QPFs per the guidance. However, with
mixed-layer CAPEs of at least 1000-2000 J/Kg over the southern
portion of the Marginal Risk area, there will be a higher
probability of more intense 1-3hr rainfall rates compared to
areas farther north. Higher FFGs (lower soil moisture percentiles
lower) over the southern portion of the outlook area will maintain
a Marginal or localized flash flood risk.

Meanwhile, lower FFGs/wetter antecedent soils over the northern
portion of the outlook area, along with a better chance for more
prolonged activity along/north of the surface warm front, will be
offset by decided lower deep-layer instability (even elevated). So
even here, again the flash flood risk remains marginal.

...Previous Discussion...
The latest model guidance is in good agreement on the eastward
push of the strong closed low that is expected to move inland from
central California into the Great Basin day 1, across the Central
Rockies into the Central High Plains day 2. A broad Marginal Risk
area maintained through portions of the Plains in a region of
strengthening low level southerly flow and increasing PW values. PW
values expected to increase to 2 to 3 standard deviations above
the mean along and ahead of the strengthening north to south
oriented frontal boundary forecast to push eastward into the Plains
on Monday. This and and a broad region of instability along and
ahead of this front will support increasing convection, especially
late Monday afternoon into the early hours of Tuesday. There
continues to be a large spread with qpf details among the models
for the upcoming day 2 period. We kept a fairly broad Marginal Risk
area to cover this spread. Two areas of concern continue in this
broad Marginal Risk area. 1: For a convective line forming late
Monday afternoon into the early hours of Tuesday along and ahead of
the front over the Southern to Central High Plains. This line will
likely be fairly progressive to the east, limiting flash flood
potential. The other area of concern would be with a slow moving
comma head/deformation precip area farther to the north to the
north of the closed low track. The 850-700 mb moisture flux to the
north of this closed low becomes very anomalous Monday afternoon
into Monday night/early Tuesday with values 2 to 5 standard
deviations above the mean in the strengthening southeasterly flow
level flow. There is general consensus for a comma head/deformation
max from far southeast MT/far northeast WY into SD. With precip
totals generally below average over the past few weeks across large
portions of the Plains, and current model qpf spread, the risk
level was kept at marginal.

Hurley/Oravec


Day 3
Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN
PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE
MISSISSIPPI VALLEY...

...2000 UTC Update...
Very few changes made to the Day 3 Marginal Risk area based on the
latest guidance trends. Similar to the day 2 period, we continue
to depict a broad marginal risk area ahead of the strong closed low
forecast to push from the Central High Plains into the Upper
Mississippi Valley during day 3. An axis of anomalous PW values, 1
to 2.5+ standard deviations above the mean, will persist day 3
along ahead of the well defined front forecast to push east through
the Central and Southern Plains and Lower Missouri Valley. There
continues to be potential for organized convection in this
anomalous PW axis ahead of the front. However, this convection will
likely be fairly progressive, again limiting the heavy precip
potential. This will keep the threat level at marginal for the day
3 period along and ahead of this front.

Farther to the north...model consensus is for a greater chance of
organized precip along and to the north of the west to east
oriented frontal boundary stretching from the Northern Plains
eastward into the Upper Mississippi Valley. 850-700 mb moisture
flux anomalies rise to 4 to 5 standard deviations above the mean
into this boundary where strengthening southerly to southeasterly
flow level flow will impact the front. An axis of heavy rains
likely to the north of the front, with model consensus for areal
average 1-2" amounts. This region has been relatively dry over the
past few weeks, resulting in dry soils and below average stream
flows. At the moment, we have kept the threat level also at
marginal across this area.

Hurley/Oravec

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