Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Thu Mar 21 2019
Valid 01Z Fri Mar 22 2019 - 12Z Fri Mar 22 2019
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-ATLANTIC STATES...
Mid level frontogenetic forcing along and ahead of a frontal
boundary extending from southeast VA into central and southeast PA
has resulted in a slow moving axis of heavier rain extending along
the front. For the most part, hourly rainfall rates have averaged
between 0.25 and 0.50 inches, with the best instability remaining
southeast of the banding. As a closed mid level low forms over the
Mid Atlantic states, the area of mid level lift moves slowly north
into central PA and northern NJ.
With instability remaining the limiting factor for higher hourly
rainfall rates, a general 1.00/1.50 inch rainfall is expected from
central PA into northern NJ before the frontogenetically forced
banding begins to weaken in place or drift slowly northeast with
time. Three hour flash flood guidance values are as low as
1.00/1.50 inches extending from western VA across western MD into
central PA, with another area of lower flash flood guidance
extending from near KPHL to the NYC metro areas. Since three hour
flash flood guidance values could be met or exceeded in these
locations, the Marginal Risk remains in place. The consensus of
high resolution guidance suggests that the highest rainfall rates
(generally between 0.25 and 0.50 inches) begin to wane after
Convection forming along a weak boundary (with several small waves
running along it) east of the southern DelMarVA coast has weakened
as it has moved north, away from the instability along the western
edge of the Gulf Stream. Its trajectory would place this activity
over portion of eastern NJ across portions of western Long Island
in the 20/02z through 22/06z time frame (which is supported by the
most recent runs of the HRRR). However, it is unclear where the
deep convection could survive the trip across the cooler waters
closer to the coast here (as it becomes elevated and weakens). If
it does, the higher rainfall rates could impact areas with lower
flash flood guidance, extending the flash flood threat into
western Long Island NY.
Based on this, the Marginal Risk was extended into this area, as
the convection is upstream of areas with lower flash flood
guidance. However, if the convection does not reach the NJ coast
intact, rainfall rates would be insufficient for flash flooding.
Valid 12Z Fri Mar 22 2019 - 12Z Sat Mar 23 2019
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Valid 12Z Sat Mar 23 2019 - 12Z Sun Mar 24 2019
...THERE IS A MARGINAL RISK OF FLASH FLOODING OVER PORTIONS OF THE
LOWER MISSOURI VALLEY...
...Eastern Nebraska, northeast Kansas, northwest Missouri,
A synoptic scale system coming across the Rockies will become
somewhat better organized as it moves over the Plains and spreads
precipitation over an area which is very sensitive to any
precipitation on Day 3.
Precipitation forecasts from the models are pretty modest and
would, under most instances, not warrant a risk area of excessive
rainfall. The modest model-QPF is consistent with limited
instability shown by the NCEP guidance. The global guidance
continues to show Lifted Index values of 0 to -2C and Best Cape
values of 250 to 500 J/kg nosing into the outlook area (especially
southern portions -- i.e. northeast KS and northern MO) Saturday
night into Sunday morning ahead of the next system as the
pre-frontal south-southwesterly flow increases to 30-40+ kts. The
magnitude of low-level frontogenesis and moisture flux east of the
system coincides with a modest spike in PW values (aoa 0.75"),
supportive of locally higher precipitation rates than shown by the
21/12Z models given the projected thermodynamic environment. These
higher rainfall rates would be occurring over an area where flash
flood guidance values are as low as 1 inch in 3 hours in spots.
Therefore, additional, more localized areas of excessive runoff
will be possible.
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