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000
FOUS30 KWBC 141557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sun Apr 14 2024

Day 1
Valid 16Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
COASTAL SOUTHERN CALIFORNIA AND FROM PORTIONS OF THE NORTHERN MID-
ATLANTIC INTO THE CENTRAL APPALACHIANS...

...Coastal Southern California...
Only cosmetic changes were made for the 16Z Day 1 ERO update to
the previous Marginal Risk area across the Transverse range and
the southern portions of the central California Coast Range. A
second area of showers is developing early Sunday morning along the
central California Coast Range ahead of the strong closed low
along the central California coast. This upper center is expected
to rotate southeastward along the central California coast and
through the Transverse Range day 1. An organized area of showers
will accompany this upper low, supporting additional moderate to
locally heavy precip totals day 1. Hourly rainfall totals from the
hi res guidance suggests .10-.25"+ amounts are possible. HREF
neighborhood probabilities are rather low for any hourly totals
greater than .50". Isolated runoff issues possible where additional
moderate to locally heavy rains fall over regions that received
rain over the past 24 hours.

...Northern Mid-Atlantic into the Central Appalachians...
For the 16Z update, we removed the northern portion of the Marginal
Risk area that had extended into southern NY. Despite the 850-700
mb kinematic and moisture transport anomalies and steepening
700-500 mb lapse rates, lower (sub 850 mb) tropospheric dry air,
which is not currently being handled well by the models (including
13Z NBM), is a big mitigating factor for the overall flash flood
threat (in all aspects...real extent, duration, and intensity).
Believe the drier ARW, HRRR, and RRFS are having a better handle on
the evolving situation, vs. the much wetter ARW2, FV3, and NAM
CONUS-Nest. Have opted to maintain a portion of the Marginal Risk
area (over PA, far eastern OH, and northern WV), however 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% vs. near
15%.

...Previous Discussion...

A surface frontal boundary expected to sink southward
late Sunday afternoon into Sunday night across the Northern Mid-
Atlantic. A narrow axis of instability, LIs 0 to -4, MUCAPE
200-500 j/kg, and increasing PW values .75-1"+, expected along and
ahead of this front. HI res model consensus is for a line of
showers/thunderstorms to form along and ahead of this front across
the Northern Mid- Atlantic Sunday afternoon and sink southward into
early Sunday evening. Recent heavy precipitation from the Central
Appalachians into the Northern Mid- Atlantic has resulted in
saturated soils and high stream flows. Hi res model consensus is
for potential of .25-50"+ hourly amounts along this front. With
soils saturated, much of this will runoff, possibly resulting in
isolated flash flooding. No significant changes made to the
previous Marginal Risk across this region.

Hurley/Oravec


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...

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.

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...

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.

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