Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 07 2023 07:40:33 FOUS30 KWBC 070740 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 AM EDT Thu Sep 07 2023 Day 1 Valid 12Z Thu Sep 07 2023 - 12Z Fri Sep 08 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND MID-ATLANTIC... A mid-level shortwave over Michigan this morning will lift into Ontario and weaken as its surfaced cold front continues eastward through the Upper Ohio Valley and toward the Appalachians. In addition, a separate minor vort max over WV this morning is progged to move northeastward along the Appalachian crest this afternoon. Deep SSW flow will continue to supply sufficient moisture to the region, with precipitable water values 1.50-1.75 inches (+1 to +2.5 sigma from south to north). Increased instability in afternoon heating will support scattered showers/storms over the region as MLCAPE values rise to near 2000 J/kg. FFG values are relatively lower across parts of VT, the Catskills/Southern Tier in NY and into NEPA (as well as along the Appalachians), though the entire region has seen little rainfall in the past week so much of the rain will be beneficial. 00Z HREF guidance shows neighborhood probabilities for >1"/hr rainfall up to ~50% and 2"/hr 10-25% which is close to or above the lower FFG values between NY/PA/VT and along the WV/VA border, which stands the greater risk within the Marginal area especially given the more hillier terrain. Constrained the area toward I-95 where the rain chances are lower but the flash flood risk remains non-zero. Fracasso Day 2 Valid 12Z Fri Sep 08 2023 - 12Z Sat Sep 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND MID-ATLANTIC... Surface frontal boundary will slow/stall in the I-81 to I-95 corridor on Saturday with continued S to SW flow through much of the column. Upper level divergence maximizes over the central Appalachians during the afternoon when daytime heating will support another round of showers/storms. Precipitable water values will stay sufficient (1.50-1.75") but the vertical column shows a fair amount of dry air aloft. Still, 00Z HREF neighborhood probabilities for at least 1 and 2"/hr amounts are near 50% and 30%, respectively, which may cause a flash flood threat for more sensitive areas (as PA can be). Model QPF has been relatively stable for a few days with only isolated chances of heavier rates/amounts, so the Marginal Risk area should be sufficient. The caveat will be how much rainfall falls on day 1 (Thursday) which may lower FFG enough to warrant changes to the day 2 (Friday) area. Non-zero threat extends into eastern VA/MD and DC but the consensus is clearly closer to the Appalachians/Blue Ridge with the 00Z guidance. Future CAM runs may not be so consistent. Fracasso Day 3 Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE APPALACHIANS... Maintained the inherited Marginal Risk area from upstate South Carolina north-northeastward along the Appalachians to the Catskills in NY. Stalled front will remain over the region for another day with elevated precipitable water values still in place (1.50-1.75" or so) as a mid-level trough out of the TN Valley may close off as it slowly moves toward northern GA. In addition, a shortwave over Indiana this morning (Thu) will take a long arc through the Southeast and eventually lift back through the Carolinas on Saturday afternoon, which may help to act as a focus for increased coverage of showers/storms especially over western portions of NC/SC, thus the southward extension compared to the days 1-2 EROs. Afternoon heating will mean another round of showers/storms for much of the region with the focus again on the terrain initially but all areas to the coast have a non-zero threat of localized heavy rain that may exceed FFG values. Those values may vary quite a bit by then compared to current values based on rainfall Thu-Fri. Again, model QPF is somewhat lackluster but CAM guidance will likely show more potential for heavier rates/amounts as the ingredients remain in place overall.=20 Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4RIFQGE3FTftqZnWsAb_Q21cosFVPRSFp04DWICXyUZW= BfCfwZ5ot9XtojN6nYZ8VsRV-PxIcaJzQ0yu5eXY6536aFc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4RIFQGE3FTftqZnWsAb_Q21cosFVPRSFp04DWICXyUZW= BfCfwZ5ot9XtojN6nYZ8VsRV-PxIcaJzQ0yu5eXY9vvnhxo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4RIFQGE3FTftqZnWsAb_Q21cosFVPRSFp04DWICXyUZW= BfCfwZ5ot9XtojN6nYZ8VsRV-PxIcaJzQ0yu5eXYm6FRaJk$=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) .