Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Mon Sep 16 2019
 
Day 2
Valid 12Z Tue Sep 17 2019 - 12Z Wed Sep 18 2019 

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

...Texas Coast...
A disturbance in the northern Gulf of Mexico will attempt to 
organize, at least subtly, as it moves westward onto the upper 
Texas Coast. There remains considerable disagreement into how 
organized this system will become, and how far south it will 
advect, but the general trend has been for an uptick in model QPF, 
especially beyond this day 2 period. On Tuesday and Tuesday night, 
the 500mb trough and associated vort lobe will move westward to be 
along the TX coast late in the period, while the 850-700mb low 
lifts slowly northward. The decreasing spatial distance between 
these features should lead to some strengthening, while the 
subsequent intensifying inflow will drive higher PWAT air onshore 
in conjunction with WAA/MLCape development. PWATs nearing 2.25" 
and MLCape rising to above 1000 J/kg will support increasing 
coverage of convective rain bands, some of which may train onto 
the central and upper Texas Coast. The most clustered model signal 
is for the heaviest rainfall in this area, and while total QPF on 
day 2 should be generally 1-3" along the immediate coast where 
frictional convergence will enhance ascent, rain rates may at 
times touch 2"/hr which could lead to isolated flash flooding. The 
SLGT risk has been expanded very slightly northeast to encompass 
the Houston area for its more sensitive FFG, with a MRGL risk 
extending inland and enclosing the SLGT risk area.


...Pacific Northwest...
A short duration atmospheric river will move onshore the Pacific 
Northwest 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. PWATs may briefly exceed 1.25", especially along the Oregon 
Coast, which is as much as 2 standard deviations above the 
climatological mean for mid-September. Despite the expected lack 
of instability, robust moisture transport on an 850mb jet 
approaching 50 kts will impinge orthogonally into the Coastal 
Ranges, and ECENS rainfall probabilities show a high risk for more 
than 1", and even low-end probabilities for 3" in the terrain, 
where isolated higher amounts are likely in the favored W/SW 
upslope regions. In general, this rainfall could be more 
beneficial than hazardous due to antecedent dry conditions, but 
rain rates potentially exceeding 0.5"/hr could produce isolated 
flash flooding, especially in any sensitive terrain or along burn 
scars, and the MRGL risk was left unchanged from previous.


...Minnesota...
Convection expected to blossom within a narrow warm sector 
associated with a frontal system late Tuesday night into 
Wednesday. This system will advect E/NE beneath a potent shortwave 
ejecting from the Northwest, with deep SW flow ahead of this 
feature drawing moisture and instability northward. PWATs are 
forecast to climb to/above 1.75 inches, 2-3 standard deviations 
above the climo mean, and near daily highs for September 18th. 
Although most of the activity is expected well after diurnal 
heating, a robust LLJ approaching 50 kts will drive strong WAA and 
increasing MUCape to fuel thunderstorms after 00Z/18 across the 
Upper Midwest. In addition to the synoptic ascent through height 
falls ahead of the shortwave, the 850mb LLJ is progged to be more 
than twice the cloud layer mean wind, suggesting intense low-level 
omega to further drive lift. While storm motions are likely to be 
rapid to the northeast noted by 0-6km mean wind forecasts of 20-30 
kts, deep layer unidirectional shear along the boundary supports 
training potential with rain rates becoming more efficient as warm 
cloud depths rise towards 13kft. Across this region, 3-hr FFG is 
generally 1.5-2.5", so where training of the heaviest rain rates 
can occur, flash flooding will be possible, and have added a MRGL 
risk after coordination with DLH/MPX focused in the 18/00Z-12Z 
timeframe before the LLJ veers to reduce the moisture confluence.

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