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< Day 1 Outlook Day 3 Outlook >
 
WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
 
Updated: 0807 UTC Fri Oct 19, 2018
Valid: 12 UTC Oct 20, 2018 - 12 UTC Oct 21, 2018
 
Day 2 Excessive Rainfall Forecast
 
Forecast Discussion
 
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Fri Oct 19 2018
 
Day 2
Valid 12Z Sat Oct 20 2018 - 12Z Sun Oct 21 2018 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
SOUTH TEXAS...

...Texas...
The deepening flow around surface high pressure over the mid MS 
Valley funnels tropical moisture across portions of central and 
south TX during Day 2. A low level easterly flow transports 2.25 
inch precipitable water air (which is between two and three 
standard deviations above the mean) over south TX, along and over 
a quasi stationary boundary extending over the nearby western Gulf 
of Mexico. Model soundings showed generally less than 1000 J/KG of 
MUCAPE, which might suggest that instability could be a limiting 
factor for convection. However, the soundings also showed a moist 
column through 300 mb, which promotes warm rain processes in the 
developing convection. This could offset the less robust 
instability profiles expected near the coast.

There is a multi model signal for 1.00 to 3.00 inches of rainfall 
over far south TX (with the 00z ECMWF showing rainfall maxima over 
4.00 inches). Mean winds in the 850-300 mb layer and propagation 
vectors are both expected to remain below 10 knots, which should 
lead to cell mergers, as well as short term training. Given the 
deep moisture in place, these rainfall amounts seem plausible. 
Even with three hour flash flood guidance values are generally 
above 2.50 inches across much of south TX, the potential for 
training in the deep tropical moisture support a Slight Risk here 
for Day 2.

Further north across portions of west and central TX, the 
influence from the frontal boundary is less than closer to the 
coast, but deep moisture and marginal instability could support 
low topped convection during Day 2. These areas received as much 
as 600 percent of normal rainfall in the past week (which is 
reflected in the three hour flash, which is as low as 1.00/1.50 
inches), and additional rainfall could result in a flash flood 
threat. There is not a strong model signal for rainfall amounts 
this high, but given the wet antecedent conditions, portions of 
west and central TX were placed in a Marginal Risk for Day 2. 

...Southwest/Great Basin...
Moisture funneled northward ahead of a closed mid level low off 
the CA coast, coupled with marginal instability due to steepening 
lapse rates, could produce convection capable of heavy to 
excessive rainfall across portions of far southern NV and 
northwest AZ during Day 2. Precipitable water values near 0.75 
inches (which is between two and three standard deviations above 
the mean) during the second half of Day 2 spread over these areas, 
especially after 20/00z. Model soundings showed MUCAPE values near 
500 J/KG, which should be sufficient to support low topped 
convection. 

The 850-300 mb mean wind and propagation vectors are expected to 
be less than 10 knots, and this should result in cell motions less 
than 10 knots as well. Short term training is possible in this 
environment, and local rainfall amounts of an inch are possible 
(though the 00z GFS rainfall amounts may be too high, given the 
moisture in the profile). Based on this, and after collaborating 
with WFO VEF, a Marginal Risk was placed over portions of far 
southern NV and northwest AZ for Day 2. 

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

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