Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1043 AM EDT Sat May 28 2022
 
Day 1
Valid 16Z Sat May 28 2022 - 12Z Sun May 29 2022 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF 
EASTERN PENNSYLVANIA AND AROUND THE TRI-STATE AREA OF NEW JERSEY 
AND NEW YORK...


...Northeast...
15Z Update...

A targeted slight risk has been added to far eastern PA, northern 
NJ, and southern Upstate NY after coordination with the affected 
WFOs. Rapidly developing thunderstorms ahead of a trough of low 
pressure will continue to expand into the aftn as MLCape rises to 
1000 J/kg and PWs remain around 1.2", above the 75th percentile 
for the date. Mid-level divergence downstream of an upper low 
combined with the RRQ of a departing upper jet streak will drive 
deep layer ascent into these favorable thermodynamics, and the 
available 12Z high-res guidance suggests convection will become 
scattered to widespread from PA into New England. Most of this 
area has been dry recently, but MRMS observations indicate 1-2.5" 
of rain has fallen across parts of eastern PA, NJ, and the Hudson 
Valley of NY in the past 24 hours leading to saturated soils and 
compromised FFG as low as 1"/3hrs. With weak mid-level winds 
driving slow storm motions, this FFG could be exceeded in a few 
areas as noted by the 06Z HREF 3-hr FFG exceedance probabilities. 
While the coverage of flash flooding instances will likely be 
limited, the slow storm motions and favorable antecedent 
hydrologic conditions support an upgrade to a SLGT risk for this 
targeted area.

Otherwise, the inherited MRGL risk was expanded only slightly to 
the north to account for coverage of thunderstorms this aftn/eve 
with 1-2"/hr rain rates. Rainfall outside of the SLGT risk area 
has been much less, so soils should mostly be able to absorb these 
rain rates. Additionally, storm motions will likely be faster as 
the upper trough and associated deep layer ascent begin to weaken 
and merge into the westerlies. This is likely to limit the 
duration of any heavy rainfall across New England, leading to only 
an isolated flash flood risk, so the MRGL risk was maintained.


...South Florida...
A washed out frontal boundary draped across the Peninsula 
interacting with daytime heating sea breeze convection should 
allow for the development of a few clusters of strong 
thunderstorms this afternoon. These cells will be capable of 
producing intense rain rates over the highly urbanized south 
Florida corridor. HREF probabilities indicate that some 2" an hour 
rains are probable, with daily totals locally exceeding 3" in 
spots. If these higher totals end up over an urban location then 
some flash flood issues could arise. Thus we will maintain the 
Marginal risk.


...Northern Plains...
A mid level impulse out ahead of the large scale troughing over 
the western U.S. will lead to convective development over the High 
Plains this afternoon. There is still a signal in most guidance 
for an axis of 1-2" of rain from eastern MT into western ND in the 
afternoon/evening hours. The lack of deeper instability and drier 
soils/streamflows should keep any flash flood risk fairly isolated 
at best. However there is some signal for amounts locally as high 
as 3" near the comma head of the developing low where increased 
convergence should support a longer rainfall duration. If amounts 
of 3" were to occur then some runoff issues seem possible, thus we 
will maintain the Marginal risk over the region.


Chenard/Weiss
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt