Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EST Fri Dec 04 2020
Day 1
Valid 01Z Sat Dec 05 2020 - 12Z Sat Dec 05 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-ATLANTIC...
01Z Update: Did expand the Marginal Risk area westward in
response to the persistence of rainfall close to the terrain of
the Appalachians...and concern about the warm frontal convection
from the Atlantic stream becoming/remaining anchored along the
intersectin of the comma head. In addition, some convection had
formed over southwest North Carolina and western South Carolina.
However, this convection appeared to be progressive enough to
preclude more than isolated run-off problems.
Generally light rain...with some embedded moderate rainfall
rates...was growing in coverage early this evening across portions
of the southern and mid-Atlantic region as low pressure over the
Southeast U.S. becomes better organized as it moves into the
Mid-Atlantic region by Saturday morning. Through 05/00Z, hourly
rainfall rates were peaking ~0.35 inches based on radar and METAR
reports near the North Carolina/South Carolina state line. This
area of rain will gradually shift eastward overnight...and 04/12Z
and 04/18Z model guidance continues to be in good agreement that a
stripe of 1 to 2 inches of rain will develop over parts of the
mid-Atlantic region in response to good upper level jet support
and the strengthening of frontogenesis in the low- and mid-levels.
Within that area, the development of instability in proximity to
the fgen forcing/deformation zone may be enough to push hourly
rainfall rates to an inch per hour and result in 2 and 4 inch
amounts in a fairly narrow corridor from southeast Virginia to the
Delmarva peninsula overnight. While FFG will probably not be
exceeded...stream flows are running high across this region...and
this additional rain should be enough to at least result in some
urban, small stream and river flooding over portions of the
Marginal risk area overnight. We did consider the need for a
Slight risk...however heaviest rainfall amounts/rates were still
aligned with the highest FFG in the region...so think a Marginal
risk should suffice.
Bann
...Previous Discussion...
Strong height falls pushing into the Mid Mississippi/Tennessee
Valley region on Friday will become increasingly negatively tilted
Friday night as they push out of the Southern Appalachians and
into the Mid-Atlantic. Rapid deepening of a surface low expected
to begin across the southern Mid-Atlantic Friday night and move
off shore of the Delmarva by 1200 UTC Saturday. Through the day 1
time period, the 0000 UTC NAM is an outlier in being weaker and
faster with this low given weaker upper level dynamics and was not
favored. The 0600 UTC NAM has trended stronger with the upper
vort and slower with the surface low. UVVs should be
strengthening quickly in association with the deepening surface
low, supporting an axis of heavy precipitation from the Southern
Appalachians, northeastward into the Mid-Atlantic with model
consensus for the heaviest amounts in an axis from south central
to eastern Virginia into central to southern Maryland and across
the southern Delmarva peninsula where a well defined comma
head/deformation precipitation band is expected to set up. There
were only some minor changes made to the previous marginal risk
outlook across this region to fit the latest model consensus. Max
hourly rates in the expected well defined comma head/deformation
band may reach .50-.75"...supporting isolated runoff issues,
especially in urbanized regions.
Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Fri Dec 04 2020
Day 2
Valid 12Z Sat Dec 05 2020 - 12Z Sun Dec 06 2020
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ALONG COASTAL
SECTIONS OF THE NORTHEAST...
2030Z Update...
The afternoon update maintains the Marginal Risk area from
southern/southeastern New England through coastal/Downeast Maine,
with forecasts consistent in principle for the intensifying storm
expected to track from near the Mid-Atlantic coast early Sat to
near/just east of Downeast Maine early Sun. The average of 12Z
models has adjusted back to the west a bit (ECMWF fairly steady
though) which at least maintains confidence in the axis of
heaviest rain falling within the depicted area. Recent model
behavior reflects the complexity of evolution aloft, keeping the
door open to further minor adjustments one way or the other. This
favors minimal continuity change while awaiting improved stability
or consensus trends for exact details. Expect rainfall rates to
remain lower than FFG values in general but areas that had
significant rainfall early this week will likely have added
sensitivity to this event.
Rausch
Previous discussion...
A surface low pressure system moving through the Mid-Atlantic will
phase with a trough axis diving through the Eastern Great Lakes at
the start of the forecast period, Saturday morning. This will lead
to rapid cyclogenesis as the low moves offshore. It is the
progression of this surface feature and the approaching trough
axis that will be vital to the evolution and track of this
developing nor'easter. Models are coming into better agreement at
this update, with a bulk of them trending a bit farther
offshore/east than the 12Z model suite. Regardless of the exact
track, this system will be rather complex with dynamic cooling on
the back side resulting in the transition to snow across portions
of New England. So not only is the track of this system
important, but the thermals as well. Luckily, the winter weather
forecaster is handling the latter (as can be seen in the Heavy
Snow Discussion) with the focus within this discussion on the Day
2 QPF and track of the surface low as it emerges off the
Mid-Atlantic coast.
It appears based on model consensus, continuity and latest
ensemble data, that the slightly faster progression is the way to
trend. The 00Z GFS is a bit too slow with the phase, meanwhile
the 12km NAM too fast. The best blend appears to be the 00Z
EC/UKMET/CMC with a touch of the GFS and the ECENS/GEFS means. In
addition, blended some of the higher resolution guidance, with the
best track favoring the NAMNest and ARW2.
The heaviest precipitation observed with this deepening storm
system will likely be along the NE coast as moisture wraps around
the surface low, with most training occurring on the north and
west side of the system. Precipitable water values of around 1.5
inches, which is 1.5 standard deviation above the mean, will nose
into the coast from off the Atlantic. With instability remaining
offshore, the bulk of the forcing will be aligned with the strong
cyclone, mid-level vorticity and jet dynamics aloft resulting a
broad area of heavy QPF. Generally rain rates will be less than
0.50 inches per hour, though there could be pockets of slightly
higher hourly totals along the immediate coast of MA and ME.
Areal average precipitation will range between 1-2.5+ inches with
a fairly tight precipitation gradient between the New England
coast and 100 miles inland. A little shift in the track will have
implications not only to the QPF amounts but the potential extent
of flash flooding. We will continue to monitor the model trends.
Based on the latest WPC QPF amounts and hourly rates, the WWD for
snowfall/ptype transition and antecedent conditions, decided to
trim the Marginal Risk to be a bit closer to the coast with the
slight eastward trend in the models.
Pagano
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
