< 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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Wed Dec 12 2018
Valid 12Z Fri Dec 14 2018 - 12Z Sat Dec 15 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST INTO THE MID-ATLANTIC...
...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.
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: www.wpc.ncep.noaa.gov/qpf/99epoints.txt