FOUS30 KWBC 192004

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Tue Jan 19 2021

Day 1
Valid 16Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021

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


Day 2
Valid 12Z Wed Jan 20 2021 - 12Z Thu Jan 21 2021


...Southern California/Arizona...
A cut off low off the Southwest coast will slowly migrate inland
over southern California and the northern Baja region through the
forecast period (Wednesday into early Thursday).  In response,
deep southerly flow will transport the moisture rich environment
north just ahead of the aforementioned low into southern CA/AZ. 
This is evident by the precipitable water values climbing to over
0.75 inches, possibly reaching 1 inch in far southern AZ, which is
2 to 3 standard deviation above the mean.  The moisture flux will
be aided by 25 knot low level south/easterly flow helping to
promote upslope along the terrain, most notable the Peninsular
Ranges.  As a result, the precipitation will be enhanced by the
upslope component.  In addition, with lapse rates are expected to
steepen as cooler air aloft moves in from the west allowing the
instability to increase (albeit weak).  Therefore, rain rates may
exceed 0.75 inches per hour at times. 

Areal average precipitation will range between 2-3+ inches along
the Peninsular Ranges and 1-2+ across the deserts and upslope
terrain in southern CA/AZ. This combined with the aforementioned
rain rates and QPF on Day 1 (through Wednesday morning), there is
the increased potential for flash flooding within this region. 
Given the orographic effects will be most robust along the
Peninsular Range with multiple rounds of showers and isolated
thunderstorms, a Slight Risk was issued for this location. The
rest of the region will see moderate to heavy rain, though the
impacts should be localized given the progressive nature of the
precipitation activity and less dynamic forcing and instability.


Day 3
Valid 12Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021


...Lower Mississippi Valley...
A shortwave over the Desert Southwest will transition quickly east
while a weak northern stream trough moves through the Great Lakes
and into the Northeast.  The southern and northern stream will not
phase resulting in this aforementioned shortwave energy shifting
east across the Lower MS Valley and into the Southeast through the
forecast period (Thursday into early Friday).  While the dynamics
associated with this system are not robust, there will be a period
of time across the region where training of moderate to heavy rain
could result in localized flash flooding.  Precipitable water
values will increase to around 1.5 to 1.75 inches, which is 2-3
standard deviations above the mean for this time of year.  This
will be aided by 30-40 knot low level southern flow with a
pronounced low level nocturnal jet developing Thursday night
helping to enhance rain rates along and south of the surface
front.  A key feature that may limit rain rates will be the weak
instability.  Despite this however, the propagation vectors should
align with the mean wind to result in training of precipitation as
activity moves from west to east along the front.  This will
increase hourly precipitation totals and thus whittle down the
high flash flood guidance that is currently in place. 

Areal average precipitation will range between 1-3+ inches, with a
narrow corridor of 2.5-3.5 inches likely.  The exact placement of
the heavier footprint of QPF will depend on the position of the
surface front and progression of the surface low along this
boundary.  While the antecedent conditions are dry, the training
as mentioned above could be enough to produce nuisance/localized
flash flooding issues. Therefore, a Marginal Risk was introduced. 


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