Mesoscale Precipitation Discussion 1206
NWS Weather Prediction Center College Park MD
210 AM EST Mon Dec 18 2023

Areas affected...Mid-Atlantic into southern New England

Concerning...Heavy rainfall...Flash flooding likely

Valid 180709Z - 181300Z

Summary...Deep convection continues to train along an axis from
Norfolk, VA to Salisbury, MD.  2 inch rainfall totals have been
noted in a few spots along this axis over the past three hours. 
These heavier rain rates should continue to spread northeastward
toward southern New York, New Jersey, and southern New England
through 13Z.  Flash flooding is likely.

Discussion...Recent observations indicate a heavy rainfall
scenario that is generally overperforming model guidance over the
past couple hours.  Axes of heavy rainfall have materialized from
northeastern North Carolina to southeastern Maryland, and spots of
local axes of 2 inch/3-hr rain rates have developed especially
across urban areas of southeastern Virginia, resulting in flash
flooding.  The axes are tied to 1) strong convergence along a warm
front extending north from a surface low near Wilmington, NC and
2) increasing instability, with SPC Mesoanalyses now indicating
1000-1500 J/kg MUCAPE along an axis located very near the front. 
Deep southerly flow aloft was also allowing for training of deep
convection, further enabling the heavier rain rates despite
relatively quick cell movement.

Concern has increased that this focused axis of convergence and
instability will gradually extend northward through portions of
Delaware, New Jersey, and southern New York through 13Z this
morning, prompting heavy rainfall and flash flooding.  These areas
are already experiencing rainfall now, which will only prime soils
and lead to minor flooding in low spots until heavier
precipitation moves in from the southwest.   Again, areas of 2
inch/3-hr rain rates should exceed 3-hour FFG thresholds
(generally in the 1.5-2.5 inch range - lower in urban areas) and
result in potentially widespread flash flood potential across the
discussion area.  Given current trends, flash flooding is
considered likely through 13Z this morning.

Cook

ATTN...WFO...AKQ...ALY...BGM...BOX...CTP...LWX...OKX...PHI...

ATTN...RFC...MARFC...NERFC...SERFC...NWC...

LAT...LON   42097373 41947276 41377258 40667290 39637391
            37797510 36837595 36717675 37157713 38267703
            39687656 41477485