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< Day 2 Outlook
WPC Day 3 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2022 UTC Wed Dec 12, 2018
Valid: 12 UTC Dec 14, 2018 - 12 UTC Dec 15, 2018
Day 3 Excessive Rainfall Forecast
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Wed Dec 12 2018
Day 3
Valid 12Z Fri Dec 14 2018 - 12Z Sat Dec 15 2018 


...21Z Update on Tuesday...
Made only a few minor adjustments to the periphery of the on-going 
Slight Risk over the Southeast U.S..  The models largely 
maintained consistency with the depiction of the deep layer 
moisture and the synoptic scale dynamics...although there were 
some differences in terms of where convection forms over the Gulf 
States and how it progresses into the Southeast.

The model spread with respect to precipitation associated with the 
closed low really limits confidence...but certainly think 
localized heavy rainfall with an associated risk of excessive 
rainfall remains.  Made few changes here since the risk still 
appears to be covered by the on-going Marginal risk areas. 


...Previous Discussion...
A high amplitude trough will dive into the central CONUS on Day 2 
before transitioning into a cut-off low across the Lower MS Valley 
that will eventually shift eastward. At the surface cyclogenesis 
will take shape across the Southern Plains eventually moving into 
the Southeast as it becomes vertically stacked.  This will lead to 
strong warm air advection and rich moisture from the Gulf and 
Atlantic. Typically with this set up, anonymously high QPF events 
occur. Models are certainly in good agreement with this sentiment 
as there is an enhanced risk of excessive rainfall leading to 
flash flooding across a portion of the Southeast. 

At the surface a cold front will cross the northeast Gulf Coast 
through GA/FL and eventually work through the Carolinas/VA.  Ahead 
of this boundary there will be a focus for rich moisture as seen 
by precipitable waters of 1.5 to 2 inches surging north thanks to 
a 45-55 knot jet.  Instability will also be surging north, most 
notably across north Florida with elevated instability observed 
from model soundings farther inland into the Carolinas.  This will 
lead to higher rain rates approaching 1 inch per hour (mainly 
across north Florida and coastal Carolina). This may even give 
Wilmington, NC (ILM) a chance to exceed the 100 inch mark for 
their annual precipitation observed thus far; approaching twice 
the yearly average. Also, in looking at the corfidi vectors and 
the 850-300 mb flow, they become aligned across north Florida as 
the surface boundary tries to edge east. This will lead to even 
higher QPF amounts across this region.  

Given the aforementioned ingredients and the dynamical forcing 
within this system as seen from the strong divergence aloft, mid 
level shortwave energy and surface convergence, expect areal QPF 
output to average 3-4.5 inches across north Florida and 2-3.5 
inches across the Carolinas/southern VA and points south.  Based 
on the aforementioned QPF, the current FFG and expected snow melt 
in the Carolinas/VA, placed portions of north FL, southeast GA, 
the Carolinas and south VA into a Slight Risk for flash flooding.  
A Marginal Risk was crafted to incorporate areas bounded by the 
Slight Risk while also encompassing the QPF from the comma-head 
slowly working across the Lower MS Valley northeastward into the 
Kentucky River Valley.

As models come into even better agreement on placement and 
amounts, anticipate the Marginal and Slight Risks will be adjusted 
with the potential for a Moderate Risk if conditions warrant it. 


Day 3 threat area:

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