Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 07 2023 20:27:42 FOUS30 KWBC 072027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Thu Sep 07 2023 Day 1 Valid 16Z Thu Sep 07 2023 - 12Z Fri Sep 08 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE NORTHEAST AND MID-ATLANTIC... ....16Z Update... Only minor adjustments made this cycle to the D1 outlook, more in line with the latest 12z HREF blended mean QPF and exceedance probabilities within the hi-res ensemble suite. Main adjustment was to northern MD and SEPA where some guidance indicated a round of heavy rainfall from convection firing near the Mason Dixon, moving NE with the mean flow. Given the convective premise of today with a Slight Risk via SPC and the enhanced moisture field located over the Northeast U.S, felt that there was merit in a slight southern and eastward expansion into the corridors above. Current deterministic keeps the heaviest rain to the northwest of the I-95 corridor, so kept the MRGL contour outside of the metro areas. The rest of the forecast remains on track with convective initiation over the southern Appalachians this afternoon, as well as more widespread convection over central PA up into NY state thanks to our lead shortwave pivoting NE'ward out of WV this morning. Kleebauer ....Previous Discussion... ....Northeast U.S 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 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE NORTHEAST AND MID-ATLANTIC, AS WELL AS EAST-CENTRAL FLORIDA... ....2030z Update... Only minor changes to the previous D2 ERO package. Totals have been bumped up a tad from previous runs, but core synoptic pattern remains unchanged. Antecedent conditions from today's precipitation could have an impact on future updates with regards to any area needing an upgrade. 12z HREF still indicating a large areal extent of 1"+ across south-central PA with other local maximums within the Capital District of NY state, as well as along the Blue Ridge down into WV/VA. Neighborhood probabilities for 1-2"/hr rates are still within that zone of 30-50% with localized signals around 65% now showing up between the I-81/83 corridor in south-central PA. As of now, did not feel it was necessary for any upgrades to the forecast and maintained the MRGL to the west of the Mid-Atlantic metros. MRGL threat for Florida can be found below... Kleebauer ....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 ....East-Central Florida... Sea breeze convergence will initiate a round of heavy thunderstorms within a narrow corridor near the Melbourne area Friday afternoon. Hi-res deterministic has been consistent over the past succession of runs for a line of 2-3"/hr rainfall rates to run along the eastern FL coast within the more urbanized corridor extending from Melbourne to up near Daytona beach. There will be a little more enhancement in the upper-levels thanks to a digging shortwave trough forecast to the enter the area, providing a bit more ascent compared to the typical mesoscale boundary convergence. After some collaboration with the local WFO, have added a targeted MRGL for the aforementioned area. Kleebauer Day 3 Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE NORTHEAST DOWN THROUGH THE WESTERN CAROLINAS... ....2030Z Update... The main adjustments made were to extend the MRGL through the Hudson Valley into the Capital District given the latest global deterministic and ensemble trends with a core of heavy rainfall expected further north than recently forecast. The expected upper-level evolution remains the same with the primary greatest threat located under the large scale ascent provided by a difluent signature north of a closing 500mb disturbance dropping through the Southeast U.S. Terrain influences within the zone of highest expected precipitation would exacerbate the potential for localized flash flooding, especially within some deterministic solutions that drop 2-3" rain from convective enhancement over the NC/SC Piedmont. Kleebauer ....Previous Discussion... ....Northeast through the Western Carolinas... 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. Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ow02bthQXIeGyG6_6-i9psjXJ-qsd91qTlNAbC6v1uf= vsBAPPeJoG5_-uoHan-URW5HqUwGC7Ca58LEDnWLd2i-7kA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ow02bthQXIeGyG6_6-i9psjXJ-qsd91qTlNAbC6v1uf= vsBAPPeJoG5_-uoHan-URW5HqUwGC7Ca58LEDnWLb1ZGxTg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ow02bthQXIeGyG6_6-i9psjXJ-qsd91qTlNAbC6v1uf= vsBAPPeJoG5_-uoHan-URW5HqUwGC7Ca58LEDnWLvYRL598$=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) .