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WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 1957 UTC Sat Oct 23, 2021
Valid: 12 UTC Oct 24, 2021 - 12 UTC Oct 25, 2021
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Sat Oct 23 2021
Valid 12Z Sun Oct 24 2021 - 12Z Mon Oct 25 2021
...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL IN THE UPSLOPE
REGION OF THE SIERRA NEVADA RANGE IN NORTHERN CALIFORNIA...
...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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt