Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 AM EDT Fri Apr 03 2020
 
Day 2
Valid 12Z Sat Apr 04 2020 - 12Z Sun Apr 05 2020 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS 
OF CENTRAL AND EASTERN TEXAS...

...Texas and nearby Louisiana...
Short waves in the quasi zonal mid level flow over TX during Day 2 
interact with deep moisture and instability to produce convection 
capable of heavy to locally excessive rainfall over portions of 
central and east TX (as well as far southwest LA). The main threat 
appears to be along the mid and upper TX coast, where the best 
instability is expected to lie (with MLCAPE values between 
1000/1500 J/KG). There is some spread in the placement of the 
surface front, which should be the line of delineation for higher 
instability. 

Along the coast, deep moisture (with precipitable water values 
near 1.75 inches, which is between two and three standard 
deviations above the mean) is focused by the low level convergence 
of the dropping front, which is expected to result in the highest 
rainfall amounts (possibly 2.50+ inches) over the mid TX coast. 
While moisture in place is expected to be deep, the low level 
inflow should be less than 15 knots, which would suggest that the 
convection may remain near or just off the TX coast (as indicated 
by the 00z ECMWF). The moisture plume could support rainfall rates 
between 1.50/2.00 inches (as indicated by the 00z NAM CONUS Nest), 
though the placement of the highest rainfall rates is not yet 
clear. Based on this, a Marginal Risk was placed over much of the 
TX coast. Should there be better model agreement concerning the 
placement of the highest rainfall near the TX coast, a Slight Risk 
could be needed here in later forecasts.

Further inland across central TX, there has been a general 
increase in QPF (led by the 00z ECMWF/NAM), as the short wave 
energy interacts with the deep moisture. Instability here is not 
quite as robust as it is closer to the surface boundary dropping 
south. Elevated instability in the low level flow could allow for 
higher rainfall amounts across the Hill Country and southern 
Edwards Plateau (where hourly rainfall amounts could exceed an 
inch), Seven day rainfall amounts have been below normal across 
this area and three hour flash flood guidance values are generally 
2.50 inches or higher. However, longer term wetness could result 
in a flash flood threat, especially where training or cell mergers 
occur. Because of this, the Marginal Risk was extended back 
westward from the coast to over the threat.

...Northern California Coast,,,
Two short waves associated with a developing closed low 
approaching the northern CA coast interact with moisture in an 
increasing low level upslope flow, resulting in locally heavy 
rainfall during the first half of Day 2. A 25 knot low level 
southwest flow ahead of the initial short wave transports 0.75 
inch precipitable water air (which is about two standard 
deviations above the mean) to the upslope areas of Del Norte and 
northern Humboldt counties, mainly before 05/00z. The combination 
of upslope flow and moisture could support hourly rainfall rates 
approaching 0.50 inches in these locations in this time period.

There has been an overall increase in QPF in this area, led by 
some of the 00z high resolution guidance, mainly before 05/00z. 
This is plausible given the depth of the moisture in the flow, but 
as heights fall in the wake of the first short wave, it is not 
clear how much of the QPF falls as snow. Conditions have been 
relatively dry here recently, and after collaborating with WFO 
EKA, we decided to forgo issuing a Marginal Risk with this 
forecast. Should the threat become clearer before any 
precipitation phase change, a Marginal Risk could be needed in 
later forecasts.

Hayes


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