Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 PM EDT Sun Sep 27 2020
Day 1
Valid 01Z Mon Sep 28 2020 - 12Z Mon Sep 28 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE SOUTHERN PLAINS AND ACROSS PARTS OF THE SOUTHEAST U.S....
...Southern Plains into the Lower Missouri Valley...
While a progressive cold front and upper shortwave have limited
the amount of time for moisture to return northward, showers and
thunderstorms are forming in its vicinity as CIN develops which is
allowing for them to hang up in the vicinity of northwest AR at
the present time. The Marginal Risk area is confined to a small
area of the Southern Plains where the models focus the best
instability. Storms should be organized considering the degree of
effective bulk shear in the region -- embedded mesocyclones cannot
be ruled out which would ramp up totals in their vicinity.
Thunderstorms with heavy rain could hold up for 2-3 hours before
marching/redeveloping south and southwest with time across
northwest AR and eastern OK. While rainfall has been limited as
of late, the Ozarks/Boston Mountains could lead to local issues
where hourly rain totals get close to 2". The best chance of this
is in the very short term as the storms begin to utilize
instability aloft due to developing CIN. The mesoscale guidance
continues to focus the heaviest rain in the vicinity of northwest
AR, which makes sense given where the surface wave resides and how
MUCAPE has been trending the past several hours. Adjusted the
Marginal Risk area to best fit where such issues are most possible
between now and 12z per the 18z HREF guidance.
...Central Florida to southern Alabama/Georgia...
A Marginal Risk of excessive rainfall maintained for central
Florida where convection is attempting to organize as CIN sets in.
The airmass should be characterized by precipitable water values
at or above 2 inches...so cells have been efficient rainfall
producers. Storms with heavy rainfall could hang on another few
hours before dissipating.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
533 AM EDT Mon Sep 28 2020
Day 1
Valid 12Z Mon Sep 28 2020 - 12Z Tue Sep 29 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE FLORIDA PENINSULA...
...Florida Peninsula and Adjoining Areas of the Coastal Southeast
U.S....
Introduced a small Slight Risk area across the east coast of the
Florida peninsula given the overlap of localized 4 to 7 inch
rainfall amounts on Sunday and the potential for additional heavy
rainfall today. The high resolution CAM guidance had a signal for
scattered convection capable of producing heavy rainfall this
afternoon and early evening, with the 28/00Z HREF 40 km
neighborhood probabilities for 3 inch per hour rainfall rates
peaking in the 10 to 20 percent range north of Cape Canaveral as
early as 21Z. Overall, instability should be in place ahead of an
upstream front with a better chance for convection in the axis of
high precipitable water values...1.75 to 2.25 inches.
Farther north, the Marginal Risk continued northward covering
portion of coastal Georgia and South Carolina where on-shore flow
interacting with the instability ahead of the front could result
in brief downpours that produce 2 or 3 inches in a short duration.
The SREF had several ARW and NMM members which produced 2 inch
per 3 hours this afternoon while the GEFS predominantly kept the
heaviest rainfall amounts/rates immediately offshore. Rainfall
across much of the area...especially along the coast...has had
below normal rainfall in the past week so the Marginal Risk area
was maintained with only minor adjustments from the previous Day 2
outlook.
..Upper Tennessee Valley into the Southern Appalachians...
By later this afternoon/tonight...we should see a pretty good
overlap of stronger low level convergence along a eastward moving
front and upper level divergence in the right entrance of the
250mb jet sweeping into the region. This should result in
expanding convection from northeast AL into northern GA and
eastern Tennessee. While the activity should be progressive...some
brief south to north training along the front is possible as the
overall trough sharpens. That, combined with lingering post
frontal rain given the approaching mid/upper tough, still supports
a localized flash flood risk with isolated 2" rains possible.
Portions of this area have elevated streamflows for late
September, suggesting a few runoff issues could evolve despite the
modest QPF values forecast.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Mon Sep 28 2020
Day 2
Valid 12Z Tue Sep 29 2020 - 12Z Wed Sep 30 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CAROLINAS
INTO THE COASTAL MID-ATLANTIC...
There continues to be a strong model signal for a widespread
moderate to heavy precipitation event day 2 along the east coast
ahead of the closing off upper low across the Southeast. The
surface frontal boundary moving east from the Appalachians Tuesday
morning should slow in response to the closing off Southeast upper
low. Strong upper difluence and surface frontogenesis in an axis
of pw values 1,5 to 2+ standard deviations above the mean will
support potential for a narrow area of heavier rains along the
front Tuesday into early Wednesday. Mu-cape values of 500+ j/kg
forecast along this front day 2, supporting the potential for
convection in what may be a narrow area of more intense rains
right along the front. With the slow movement of the front,
training of precip is possible with isolated runoff issues,
especially in the east coast urban corridor, despite recent dry
conditions,
Oravec
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Mon Sep 28 2020
Day 3
Valid 12Z Wed Sep 30 2020 - 12Z Thu Oct 01 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM COASTAL
MID-ATLANTIC INTO NEW ENGLAND...
Heavy rains possible day 3 from the coastal Mid-Atlantic into New
England. This heavy precip axis will be from two precip events
during day 3. The continuation of heavy rains late day 2
spreading northeast from the coastal Mid-Atlantic into New England
along the slow moving frontal boundary stretching northeast to
southwest across this area, followed by a second surge of
potentially heavy rains as surface low pressure deepens along the
Mid-Atlantic coast in response to the northeast movement of the
closed low from the Southeast into the Mid-Atlantic. While there
is expected to be a narrow axis of instability along the front day
2 across the Mid-Atlantic, the instability is expected to be
offshore day 3. This should support rainfall rates not as intense
as they may be day 2 along the front, with a general overrunning
precip pattern expected to the north and northwest of the surface
low pushing up the Mid-Atlantic coast. For these reasons, the
threat level was kept at marginal for day 3.
Oravec
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
