Excessive Rainfall Discussion

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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Sat Oct 23 2021

Day 1
Valid 01Z Sun Oct 24 2021 - 12Z Sun Oct 24 2021


A strong atmospheric river is on track to impact CA later tonight
into Sunday. Heavy rains should move into northwest CA between
06-12z tonight...with as much as 2-4" falling by 12z. Rainfall
impacts through 12z should end up limited in scope...thus the
Marginal risk should be okay...although any more sensitive burn
scar areas could experience some runoff issues before 12z. The
bigger and more widespread impacts still look to be after
12z...where we have Moderate and High risks of excessive
rainfall...with significant impacts expected.

...Central Plains into mid MS Valley...
Convection should expand in coverage later this evening from
northeast KS/southeast NE...east into portions of southern IA,
central/northern MO into IL. Strong southerly flow will increase
ahead of the approaching trough, resulting in a strengthening
corridor of low level moisture convergence. Flow in the upper
levels will also become gradually more divergent...although the
real uptick in this looks to hold off until after 12z.
Nonetheless, the low level convergence, modest upper level
divergence, and plentiful upstream instability should support an
expanding region of convection. Corfidi vectors may become quite
weak overnight pointed towards the south...which should support
some backbuilding/training of convection given the persistent low
level convergence and increasing mid/upper level support with
time. Hourly rainfall approaching 2", and localized swaths of over
3" in a couple hours seems probable with this setup. It will
likely be a pretty narrow swath and some lingering uncertainty on
the exact latitude...with the most probable location somewhere
from northern MO into far southern IA into west central IL.

We will go ahead and upgrade this corridor to a Slight risk for
the remainder of the overnight hours...as some exceedance of the
1hr and/or 3hr FFG seems probable on an isolated to scattered
basis. While 2-4" amounts are most likely within the swath, there
is at least some risk of localized 4-7" totals if training ends up
a bit more robust/persistent.


Day 2
Valid 12Z Sun Oct 24 2021 - 12Z Mon Oct 25 2021


...California and far southwest Oregon...

21Z update... The latest WPC forecast keeps 8 to 10+ inches over
parts of the Sierra Nevada Range with a minor broadening of the
footprint on the western side. As such, the High and Moderate Risk
areas required very small adjustments. The amounts within the
Moderate Risk area that is closer to the coast are in the 4 to 8
inch range. Should trends increase in the coming update cycles
over this particular area there may be the need to upgrade to a
High Risk over a very focused area. 


There is a High Risk of Excessive Rainfall over portions of
northern California...especially along the west face of the Sierra
Nevada range where a pronounced atmospheric river is directed
normal to the mountain range by low level flow approaching 50 kts.
 This results in maximum rainfall amounts of 8 to 10 inches and a
significant risk of life threatening flash floods...especially in
regions of large burn scars left by recent wildfires.  Also
modulating the threat will be enough instability to support
enhanced rainfall rates.

A well defined atmospheric river takes aim at the west coast of
North America as low pressure deepens rapidly at about
45N/135W-130W. The axis of deepest moisture, with precipitable
water values approaching 1.75 inches in the GFS, gets directed
on-shore by 850 mb winds 30 to 45 kts and into the Sierra Nevada
as the entire axis of deep moisture and strongest low level flow
shifts south and east.  GFS-based Integrated Water Vapor Transport
values remain on the order of 1250 to 1500 kg/m/sec are directed
along the northern California coast from roughly SFO eastward at
the start of the Day 2 period at 24/12Z before making its way
inland. This focuses the one area of maximum rainfall in and near
the northern California ranges and farther east in the Sierra
Nevada.  Snow levels initially begin quite high but gradually sink
as heights aloft/temperatures decrease.  As a result, there was
some overlap between some Marginal and Slight Risk areas and snow
areas.  Even so, kept the highest risks of excessive rainfall
separated from the highest elevations where precipitation type was
likely to primarily be snow.

...Mid-Mississippi Valley toward the Southern Great Lakes...

21Z update... There was a trend amongst the guidance that there
would be increased convection across central Iowa and thus WPC QPF
increased over this area. The Marginal Risk was extended further
north and west into Iowa to account for this change.


Moisture from the Southern Plains gets drawn northward as
cyclogenesis continues over portions of Kansas and Nebraska on
Sunday morning in response to the approach of a mid-level wave
dropping out of the Dakotas. Models now show a stripe of 2 to 4
inches extending from the Mid-Mississippi Valley eastward toward
the southern end of the Great Lakes with isolated spot maximum
amounts of 5 or 6 inches.  Only change needed was to expand the
Slight and Marginal risk areas.  The Slight Risk area remains
confined to the region of best CAPE/LI and upper level
divergence...while the Marginal Risk area eastward somewhat to
account for the possibility that the more easterly solutions


Day 3
Valid 12Z Mon Oct 25 2021 - 12Z Tue Oct 26 2021


21Z update... During this period the scattered to widespread
precipitation will shift south, focusing more on central and
southern parts of the state. Western portions of the Sierra Nevada
will have had wetting rains on the day prior and will have lowered
FFG for this period. With additional heavy rain expected there
will be an elevated risk for excessive rain, with flash flooding
and debris flow possible near recent burn scars. The Slight Risk
area was broadened slightly as well as the Marginal Risk bounds.


Introduced a small Slight Risk over portions of the west aspect of
the Sierra Nevada as the core of heaviest precipitation associated
with an atmospheric river moves deeper inland.  QPF from the model
guidance and WPC focuses heaviest amounts well in the mountains
during the Day 3...with snow being the predominant precipitation
type.  However, there will still be enough precipitation at lower
elevation to warrant a risk area given antecedent conditions. 
Felt a Marginal should suffice in most areas...although introduced
an area in the mountains southeast of Fresno which has been
rendered more sensitive to flooding by a recent large wildfire. 
Highlighted this particular area in a Slight Risk.


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

Last Updated: 858 PM EDT SAT OCT 23 2021