Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Sun Sep 15 2019
 
Day 3
Valid 12Z Tue Sep 17 2019 - 12Z Wed Sep 18 2019 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MIDDLE 
TEXAS COAST...

...Pacific Northwest...
A quick-moving atmospheric river will push onshore the Pacific 
Northwest by Tuesday, bringing a round of heavy rainfall from 
coastal areas of northwest California north through Vancouver 
Island. This will be associated with a deep northern stream upper 
trough digging southeast from the Gulf of Alaska and an attendant 
cold front. The moisture anomalies do reach up to as high as 2 
standard deviations above normal, with a focus on northwest 
California and southwest Oregon along and ahead of the front. This 
will be facilitated by 45 to 50 kt southwest low-level jet which 
should help briefly drive PWATs along the immediate coast to 1.0 
to 1.25 inches, with the greatest moisture again concentrated near 
the Oregon/California border. Despite the expected lack of 
instability, a combination of robust moisture transport, warm air 
advection, and orographic forcing across the coastal ranges should 
yield locally heavy rainfall totals, with some occasional rain 
rates approaching or briefly exceeding a 0.50 inch/hr. The latest 
suite of forecast guidance suggests as much as 2 to 3 inches of 
rain, and some isolated heavier totals will be possible across the 
more orographically favored west and southwest facing slopes. 
However, in general these amounts should be well tolerated given 
the time of year and dry antecedent conditions. Nevertheless, the 
Marginal Risk area was maintained for the coastal ranges where 
there is some concern locally around any burn scar area. The risk 
area that was over the Cascades was paired back since the rainfall 
rates here should be a bit more modest compared to the coastal 
ranges, and with somewhat lower storm totals.


...Texas Coast...
The same evolving low center from Monday will advance inland 
across southern Texas on Tuesday bringing higher PWATs, greater 
instability and a likelihood of heavier convective rainfall. The 
12Z GFS remains on the weaker side of the model guidance with 
respect to how well organized the system will be, and thus has 
lower QPF. However, the 12Z NAM and full suite of 12Z non-NCEP 
solutions all favor a moderately organized system at least from a 
convective perspective, with some potential for training 
convective bands impacting the immediate coastal plain. As a 
mid-level ridge of high pressure persists north of the system, the 
track should continue to be off to the west, albeit rather slow. 
The slow motion will also increase the threat of some of the 
system's convective banding features hanging up over the same 
area. Given this and the presence of deep tropical moisture 
through the period, some very heavy rainfall totals will be 
possible and especially with an efficient environment for very 
high rainfall rates. As such, a portion of the Marginal Risk area 
has been upgraded to a Slight Risk with a focus on the middle 
Texas coast where the model guidance is best clustered on heavier 
rainfall amounts. Locally a few inches of rain will be possible 
this period alone, and some flash flooding will be possible as a 
result.

Orrison
 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt