Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Sat Oct 23 2021
 
Day 2
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.  

Campbell

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.

Campbell

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 
verify.

Bann

 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt