Flash Flood Guidance
Issued by NWS
Issued by NWS
632 AWUS01 KWNH 171513 FFGMPD VAZ000-NCZ000-172030- Mesoscale Precipitation Discussion 1029 NWS Weather Prediction Center College Park MD 1112 AM EDT Tue Sep 17 2024 Areas affected...Southeast VA...Northeast NC... Concerning...Heavy rainfall...Flash flooding likely Valid 171510Z - 172030Z SUMMARY...South to north training and efficient rainfall rates up to 2-3"/hr approaching urban centers of SE VA pose possible localized flash flooding/rapid inundation flooding through early afternoon. DISCUSSION...Active convection this morning over the Pamlico River supported a tightening of the surface triple point as well as an associated boundary layer to 850mb low. While these two features have separated with the occluded low still remaining south toward the mouth of the Pamlico River; the elevated circulation has lifted northward over toward the Chowan/Albermarle Rivers continuing to lift north. The trailing tail has become highly convergent with easterly surface to 850mb both north of the warm front (near Roanoke Island) lifting north and veered southeasterly flow out of the warm sector over the central sound and Atlantic east of the Outer Banks. Tds in the mid to upper 70s with temps increasing to the mid-80s in the warm sector is supporting increasingly unstable air with MLCAPEs over 1500 J/kg fueling stronger updrafts within the strengthening deep layer convergence axis. Total PWats of 2-2.25" are more than sufficient (loaded with 1-1.2" in the lowest sfc-850mb per CIRA LPW) with the strength of flux to support 2-2.5"/hr perhaps occasionally ticking up to 3"+/hr for short-durations and highly localized. Deep layer steering will support a south to north training that may result in a few hours of training. This may result in 3-5" localized totals across far SE VA and NE NC; however, there is some uncertainty that propagation to the east due to the strength of inflow may reduce the training duration but also shift the focus along or just offshore, limiting the flash flooding/rapid inundation risk. While much of the area is sandy soils (FFG 3-4"/3hrs) and supports higher infiltration, rates over 3"/hr or even proximity to urban centers have a higher probability of inducing flash flooding. Hi-Res CAMs are more persistent in allowing for more westerly cell motions expanding the flooding threat westward; however, nearly all do not resolve this low level wave lifting north acting like a barrier as well as reducing insolation with increased cirrus canopy and therefore unstable downstream air. The 12z ARW2 seems to handle this wave best of the overall solutions but trends appear west compared to observations. Bottom line, these variations due reduce overall confidence in precise placement but the rates/potential totals should result in localized flash flooding. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...AKQ...MHX... ATTN...RFC...MARFC...SERFC...NWC... LAT...LON 37847641 37717571 37347562 36797581 36067559 35427534 35247557 35717611 36137646 36437669 36857687 37347693 37667677