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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: 2020 UTC Wed Dec 12, 2018
Valid: 12 UTC Dec 13, 2018 - 12 UTC Dec 14, 2018
 
Day 2 Excessive Rainfall Forecast
 
Forecast Discussion
 
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Wed Dec 12 2018
 
Day 2
Valid 12Z Thu Dec 13 2018 - 12Z Fri Dec 14 2018 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE 
OLYMPIC PENINSULA...

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS 
OF THE LOWER/MID MISSISSIPPI VALLEY AND THE CENTRAL GULF COAST 
REGION...

...Olympic Peninsula...
Models are still struggling with the timing of the next trough and 
shortwave activity into the Pacific Northwest thus resulting in a 
somewhat challenging forecast.  Regardless, a residual boundary 
from the previous system will continue to feed ample moisture into 
the Olympic Mountains through 14/00Z.  Model solutions continue to 
diverge after that however.  Based on discussions with the WPC 
model diagnostician and 00Z trends, put more weight on the slower 
ECMWF solution in part because of the ridging upstream in the wake 
of the southern stream system on Day 1 and Day 2 combined with the 
idea of the continued slowing trend shown by the GFS.

Given precipitable water values of near 1 inch will advect into 
northwest Washington from low level flow of 40-50 knots which will 
produce enhanced upslope across the windward side of the terrain. 
Rain rates are expected to be less than 0.5 inches per hour.  With 
days of an atmospheric river set up across the region, expect 
soils to be fairly saturated. Therefore, with areal averages of 
1.5 to 2.5 inches, expect Marginal Risk should cover the localized 
chance for flash flooding. 

...Lower/Middle Mississippi Valley and the Central Gulf Coast...
An upper level trough across the central CONUS will amplify as it 
moves east as the southern stream energy amplifies. In the 
mid-levels, strong vorticity rounding the trough will dive south 
and eventually result in a cut-off low across the Lower 
Mississippi Valley. As a result, a 40-50 knot jet will advect 
precipitable water values of 1.5-2 inches from the Gulf into the 
Lower MS Valley, steadily migrating into the Southeast by 14/12Z. 
Strengthening low level winds will draw deep moisture ahead of the 
system which results in an increasingly unstable atmosphere ahead 
of the front as shown by MUCAPE values at or above 2000 J/kg. As 
the system becomes more vertically stacked through the period, 
expect to see better convection develop across the central Gulf 
Coast states while the comma-head of the surface low will lead to 
training of light to moderate rain across the Lower/Middle MS 
Valley. 

Saw little reason to make more than minor adjustments to the 
on-going Marginal Risk areas at this point given reasonably 
consistent model QPF.  

Bann/Pagano

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

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