Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Mon Oct 25 2021

Day 1
Valid 12Z Mon Oct 25 2021 - 12Z Tue Oct 26 2021

...THERE IS A SLIGHT RISK IN PORTIONS OF CENTRAL CALIFORNIA... 
 
...California... 
The Marginal and Slight Risk areas in California appear ideal
given the most recent WPC QPF and 00z HREF probabilities of 0.5"
an hour and 3"+ in 24 hours.  Scattered to widespread coverage of
precipitation will shift south, focusing more on central and
southern parts of the state. There has been a gradually shift in
model guidance to delay the end of the precipitation.  There will
be some overlap with heavy/excessive precipitation which occurred
Sunday and Sunday night and additional rainfall on Monday.  Burn
scars still present the greatest risk of excessive rainfall. 


...Olympic Peninsula of Washington...
Enough moisture, inflow from the Pacific, and instability leads to
a modest chance of 0.5" an hour totals.  The HREF supports a high
chance of 3"+ totals in the Olympic Peninsula, but snow levels
appear low, which limits the threat across its higher elevations.

 
...Northeast Mid-Atlantic States and Southern New England... 
Slowing trends have led to a southwest shift in the existing
Marginal Risk area.  Rainfall associated with the comma head of a
low pressure system off the East coast moves northward and starts
to draw moisture on-shore from the Atlantic Ocean, with
precipitable water values around 1.25".  Instability should be
greatest across the central Mid-Atlantic states, which along with
frontogenesis could aid hourly rain totals.  There is some spread
in terms of placement and associated rainfall amounts. As a
result, confidence is not high...but there is a signal for the
potential of locally moderate to heavy rainfall in the area.  Two
week precipitation has been minimal, but recent leaf fall could
clog area drainage and lead to standing water with rainfall below
flash flood guidance, so the Marginal Risk remains in place as a
precaution.
 
Roth/Campbell/Bann 


Day 2
Valid 12Z Tue Oct 26 2021 - 12Z Wed Oct 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND... 
 
There is agreement that low pressure off the East Coast will be
the major player in producing precipitation across the
region...but the degree of development and some model mass field
differences result in uncertainty regarding maximum values and
placement.  There remains some overlapping of solutions over
portions of southern New England and the eastern portions of the
Northern Mid-Atlantic states get 1.5-3" areal average rainfall,
with enough frontogenesis and instability to expect the
possibility of 1" an hour rain totals, which would be most
problematic in New York City and nearby northeast NJ.  Adding to
the concern is ongoing leaf fall across the area, which could
cause drainage issues.  Areas depicted from continuity remain
relatively similar, with some southwest adjustment made into
eastern PA and NJ to match model trends.
 
Roth/Campbell/Bann

Day 3
Valid 12Z Wed Oct 27 2021 - 12Z Thu Oct 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE ARKLATEX, CENTRAL GULF COAST, & PACIFIC NORTHWEST COAST...

ArkLaTex/Central Gulf coast...
A couple batches of convection are expected in the vicinity of
Louisiana near and ahead of a cold front.  Inflow at 850
hPa/effective bulk shear of 30-40 kts is expected to organize
convection, most likely into lines within the unidirectional
southwest flow.  Instability should be sufficient from eastern TX
down to the central Gulf coast for potential heavy rain issues. 
Overall, the threat should be minimal as much of the region has
had below average rainfall over the past couple of weeks.  Should
any mesocyclones form in this environment, they could hold up the
line for an hour or two and briefly pose a flash flood threat as
hourly rain totals to 2.5" appear possible in the expected
environment.

Coastal Pacific Northwest...
A returning warm front ahead of a surface low brings precipitable
water values of 1-1.25" on the heels of 850 hPa inflow of 35-50
kts.  The 21z SREF indicates that there could be a smidge of
instability, which when combined with the above factors could lead
to some chance of 0.5" an hour totals, particularly late in the
period (Wednesday night).  While two week rainfall through Sunday
morning was modest, ongoing rains and what is expected on Monday
across the Olympic Peninsula should lead to some degree of
saturation.  A Marginal Risk was raised as a precaution.

Roth



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