Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 16 2023 08:08:44 AWUS01 KWNH 160808 FFGMPD MEZ000-MAZ000-NHZ000-RIZ000-VTZ000-CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-MDZ00= 0-161400- Mesoscale Precipitation Discussion 0744 NWS Weather Prediction Center College Park MD 407 AM EDT Sun Jul 16 2023 Areas affected...Portions of the northern Mid-Atlantic into the Northeast and southern New England Concerning...Heavy rainfall...Flash flooding likely Valid 160800Z - 161400Z Summary...Scattered instances of flash flooding early this morning will likely become widespread across portions of southern New England by mid-morning. Localized 3-5"+ totals are possible, and some significant flash flooding is expected to develop across more vulnerable terrain. Discussion...Showers and thunderstorms have been organizing over the past couple of hours across portions of the northern Mid-Atlantic, within a pre-frontal surface trough and broad cyclonic flow aloft. The mesoscale environment in the vicinity and downstream of this convection is characterized by ML CAPE of 500-2000 J/kg, precipitable water values of 1.6-2.1 inches (between the 90th percentile and max moving average, per IAD/ALY sounding climatology), and effective bulk shear of 30-50 kts. This parameter space should continue to support organized convection, which has been orienting itself linearly from SSW-NNE (parallel with the mean flow) and allowing for localized training of 1-2"/hr rainfall rates. This is very concerning, as most of the region has already seen anywhere from 300-600% of normal rainfall (ranging from 3-12 inches) over the past week (through Saturday morning). This has resulted in rather low Flash Flood Guidance (FFGs), generally ranging from 1-2" (with the exception of portions of the Delmarva and southern NJ, where guidance is as high as 3-4" for a 3-hr period). Flash flooding is considered likely across the region, given the very favorable environment and very wet antecedent conditions. Scattered coverage is generally expected through much of the morning, and mainly occurring where the aforementioned training bands ultimately track. Scattered convective activity is also beginning to proliferate across portions of southern New England, and this shallower activity may also be capable of 1-2" short-term totals with highly efficient warm rain processes dominating. As the low-level flow continues to back towards the south across this area, the approaching deeper convection from the southwest will start to merge with and overcome this convection, where the 00z HREF indicates alarmingly high 40-km neighborhood exceedance probabilities for 3" and 5" thresholds through 15z (30-60% and 10-30%, respectively). This suggests that flooding will become more widespread across portions of southern NY/NH and western CT/MA by mid-morning, and some of this flooding will likely become significant (i.e. life threatening) as well (particularly where 3-5"+ totals coincide with more vulnerable terrain, including flood-prone low-lying and urbanized areas). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7aMzVTzbQetKb8bJX4XnCjWaLsRmE8EbhjKiUx8LVZAqLcICmIz1Oyr1wKun-rNPjukH= 38BKGxtMnvM0HAvXu8dXGE4$ for graphic product... ATTN...WFO...AKQ...ALY...BGM...BOX...BTV...CTP...GYX...LWX... OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 45387068 44227065 43317075 42407152 41617203=20 40957290 40117413 39357488 38577557 39067657=20 39887617 40747585 41217603 41957546 42527509=20 43237415 43687394 44537403 44957356 45197271=20 45247188=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .