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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0059Z Dec 13, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
758 PM EST Wed Dec 12 2018

Day 1
Valid 01Z Thu Dec 13 2018 - 12Z Thu Dec 13 2018

...A MARGINAL RISK OF EXCESSIVE RAINFALL CONTINUES OVER WESTERN
WASHINGTON...

...Western Washington, including western portions of the Olympic
Peninsula...
A strong compact shortwave will enter the Gulf of Alaska with an
enhanced plume of increased moisture between 00z and 12z Thursday.
Strong southwesterly LLJ with 45-55kts of 850 hPa flow will direct
this TPW plume (~1.0") toward far northwest Washington supports
IVT values around 500 kg/ms.  Despite virtually no instability,
hourly totals towards 0.5" are possible in this regime, as
supported by the latest (18Z) HREF 40 km neighborhood
probabilities. Snow levels of 3000-4000 ft early will climb to
5000-6000 ft by 12Z Thu with the strong, deep-layer warm/moist
advection. As such, WPC is maintaining a Marginal Risk of
Excessive Rainfall over portions of western WA, at elevations
generally below 4500 ft where the pcpn is expected to change over
to mod-heavy rainfall later tonight. See the Probabilistic Heavy
Snow/Icing for more details on the winter weather impact.

Hurley/Roth


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 3
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.

Bann

...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.


Pagano/Bann



Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt