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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 1416Z Apr 12, 2024)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1015 AM EDT Fri Apr 12 2024

Day 1
Valid 16Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024


Mid-morning Update: Two main areas of precipitation ongoing this
morning, one on the leading edge of the warm front advancing
northward across New England. This activity will continue to stream
northward through New Hampshire and Maine, bringing widespread
moderate to locally heavy rainfall. The combination of higher rain
rates over sensitive ground conditions (melting snowpack, higher
soil moisture) will lead to some instances of rapid runoff and
flash flooding, particularly over the higher terrain areas. The
Slight Risk area was largely unchanged from the previous cycle and
looks to line up well with current radar and early 12Z guidance.

Another area of precipitation is advancing east/northeast along
the sweeping cold front across eastern PA and central NY. Based on
the recent HRRR runs and other early 12Z guidance, this activity
should lift east/northeast into western New England (western
MA/VT/NH) over the course of the morning/afternoon hours. While
more progressive, there are indications of localized higher rain
rates due to a ribbon of instability developing just ahead of it
(upwards of 500 J/kg MUCAPE). While the ground conditions are not
as sensitive as areas to the east, the soil moisture in the 0-40cm
layer is still fairly high and this additional rainfall could
result in some flooding concerns. The Marginal Risk was adjusted to
cover more of VT for this threat.


---previous discussion---

Strong southerly low level flow in an axis of much above average
PW values...3+ standard deviations above the mean...expected along
and ahead of the lead frontal boundary pushing across eastern New
England early Friday. A period of heavy rains likely to push fairly
quickly eastward Friday morning across New Hampshire, eastern
Massachusetts and Maine. There is pretty good agreement on a stripe
of heavy precip in the area of strong upslope flow from northern
New Hampshire into central Maine. Not a lot of changes made to the
previous slight risk area, which continues to correspond well where
the HREF neighborhood probabilities are high for 1 and 2"+ amounts
and where the HREF EAS for 1"+ amounts are also high. This heavy
rain, along with snow melt will continue the risk of flash
flooding, especially during the morning hours of Friday.

A second line of convection may form along the secondary frontal
boundary pushing eastward into far eastern Pennsylvania, northern
New Jersey and eastern New York. The previous marginal risk area
across northern NY State was extended southward to cover locally
heavy rains with this second band of convection moving across this
area Friday morning into early afternoon. The marginal risk fits in
well where the HREF neighborhood probabilities are high for 1"+


Day 2
Valid 12Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024


There is good model agreement of the southeastward movement of the
strong closed low off the Pac NW coast to a position off the
northern California coast by the end of the day 1 period and then
an eastward inland movement day 2 across central California. A
well defined area of precip expected along the slowly southeastward
moving frontal boundary. PW values along and ahead of this slow
moving front will rise to 1 to 2 standard deviations above the mean
along with 850-700 mb moisture flux anomalies of 2 to 4+ standard
deviations above the mean. The anomalous moisture flux values will
impact the central California coast range from Santa Cruz southward
to the western portions of the Transverse Range and the upslope
regions of the northern to central Sierra. Across these areas max
rainfall totals of 1 to 1.5"+ are possible. No significant changes
made to the marginal risk along the central California coast, with
a marginal risk introduced for the upslope area of the northern to
central Sierra.


Day 3
Valid 12Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: