Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Sat Sep 19 2020
Valid 12Z Sat Sep 19 2020 - 12Z Sun Sep 20 2020
...A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS
OF THE NORTHERN ROCKIES AND THE NORTHEASTERN FLORIDA PENINSULA...
...Northeastern Florida Peninsula...
Strengthening low level flow from the northeast in the wake of a
slowly sagging cold front through the Florida Peninsula will
support locally heavy/excessive rainfall along portions of the
coast between to a point just south of Melbourne. From just prior
to 18Z to at least 06Z, 850 mb winds are forecast to increase into
the 20-30 kt range, roughly perpendicular to the coast.
Precipitable water values above 2 inches and forecast CAPE in
excess of 1000 J/kg just inland of the coast will be in place
during the afternoon/evening hours along with forecast cell
motions of less than 10 kt. Frictional convergence near the coast
are expected to develop slow moving cores of heavy rain with
potential rates of 2-3 in/hr, sagging slowly southward during the
day as high pressure builds south, pushing the better moisture
axis southward as well. The hi-res models from 00Z showed very
heavy rain with 24 hour totals of 3-6 inches, and maximum totals
near 10 inches in the 00Z NSSL. 10 inches is probably overdone,
but the HREF probability of 5+ inches in 24 hours is 80-90 percent
over Volusia County. After coordination with the JAX and MLB WFOs,
a Marginal RIsk was maintained across this region of Florida.
As a seasonably strong mid-level trough tracks across the interior
northwestern U.S. today, anomalous moisture will move across ID,
MT and northwestern WY. 00Z GFS forecast precipitable water values
approach 1 inch across MT by 00Z/20, which translates into +1.5 to
+2.5 standardized anomalies. The orientation of the upper trough
should become increasingly negative tilt through 00Z with
dynamically induced upper level divergence and diffluence
overspreading the northern Rockies. Instability is forecast by a
majority of the 00Z models to remain the 500-1000 J/kg range, but
will locally be a bit higher just ahead of the associated cold
front and beneath the core of the upper trough, which will support
thunderstorms with heavy rainfall rates. Storms will be
progressive for the most part with the frontal progression, but
short term training with rainfall rates of 0.5 up to 1 in/hr will
be possible where instability maximizes. These values would be in
excess of area flash flood guidance values, therefore, WPC
maintained the Marginal Risk across portions of the northern
Rockies through 12Z Sunday.
The Day 2 outlook will be updated by 0830Z.
The Day 3 outlook will be updated by 0830Z.
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