Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 27 2023 00:36:25 FOUS30 KWBC 270036 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 735 PM EST Sun Nov 26 2023 Day 1 Valid 01Z Mon Nov 27 2023 - 12Z Mon Nov 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR NORTHEAST NEW JERSEY, FAR SOUTHWEST CONNECTICUT AND SOUTHEAST NEW YORK, INCLUDING NEW YORK CITY... ....01Z Update... The Marginal Risk area added at the midday update was maintained with this update. An area of rain is beginning to encroach into the NYC metro area, which is well forecast nearly unanimously in the CAMs. Where agreement is not so good is where an area of heavier rain embedded within the broader rain shield will form. Most of the CAMs from a few hours ago put that area of heaviest rain squarely over the NYC metro...and hence the introduction of the Marginal. More recent runs have unfortunately diverged a bit, with the HRRR suggesting the heavier rain forms further west and largely misses NYC. Thus far that westward solution is panning out, as the greatest increases in rainfall rates based on trends in the reflectivities from the area radars has been over eastern PA and far western NJ, which is actually even further west than even the HRRR solution, which focused on the area from north-central NJ into the Catskills. Heavier shower activity has been forming near Philly and moving northeast, so the development of this heavier rain could still eventually impact the NYC metro, justifying the retained Marginal risk area for now. As far as hydrology goes, NASA Sport data suggests that much of the northern mid-Atlantic is still "recovering" from last Tuesday's few inches of rain with respect to soil moisture levels. Much of the area is right around normal for this time of year, which would generally be the most favorable status of the soils to diminish the flash flooding threat. Thus, the area of the Marginal remains over mostly urban areas of NY/NJ/CT, where urbanization lowers FFG values. Thus, should the heaviest rainfall remain west of the NYC metro, it's unlikely anywhere will see flash flooding materialize, as those areas along the Delaware River are far less urbanized and should be much better able to handle the extra rainfall. Wegman ....Previous Discussion... Added a small marginal risk area across the urban areas just west and north of NYC, from far northeast NJ, far southeast NY State, including NYC, and far southwest Connecticut. The 12Z hi res runs all show increasing rainfall along the Northern Mid-Atlantic Coast this evening into Sunday night associated with surface low pressure deepening and moving northward just off the NJ coast and across Long Island into southern New England. The simulated hi res radars do show intensifying radar echoes in the 0300 to 0770 UTC time frame moving across the urban corridor from far northeast NJ into far southeast NY State, including NYC and far southwest CT in a region of strengthening boundary layer convergence along an inverted trof extending west northwestward from this low. Hourly amounts of .50-1"+ possible during this period, with isolated urban runoff issues the primary concern. HREF neighborhood probabilities for hourly amounts of 1"+ peak at 15-25% at 0500 UTC from far northeast NJ into NYC. .50"+ hourly rates are higher, 50-90%, in the 0400-0700 utc time period and across a broader region closer to the marginal risk area. Oravec Day 2 Valid 12Z Mon Nov 27 2023 - 12Z Tue Nov 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. 2100 UTC update No changes made to previous outlook for day 2. Oravec Previous discussion. Locally heavy rain is expected to be ongoing within the Gulf of Maine and near the coast of New England at the start of the period (Monday morning). A deepening surface low is forecast to quickly track NNE from eastern MA at 12Z into eastern Maine through 18Z Monday with peak rainfall rates of 0.5 to near 1.0 in/hr to the immediate northwest of the surface low track. The low track and axis of heavy rain should be fast moving within a region lacking appreciable instability, limiting potential for higher rainfall totals along the coast. Therefore, while some of the more aggressive members of the 00Z HREF show 3-hr QPF near or locally in excess of 3-hr FFG for coastal portions of Maine, the probability of exceedance was considered too low for a risk area at this time. In addition, the speed of the surface low should limit maximum rainfall totals into the 1.5 to 2.5 inch range, not considered significant for flooding impacts given recent dry weather across the region and after coordination with the affected WFOs. However, newer model guidance for coastal Maine will have to be monitored for increases and possible changes to the Monday to Tuesday morning Excessive Rainfall Outlook. Otto Day 3 Valid 12Z Tue Nov 28 2023 - 12Z Wed Nov 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8KrK8A5F1MifTkZ7kQo4GY3uVyItdN7rNah_tWTJpNvy= ymiTQTonCq-zCfbabPGoy9iYWRmlT--b1pTmb89-gn7fSSY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8KrK8A5F1MifTkZ7kQo4GY3uVyItdN7rNah_tWTJpNvy= ymiTQTonCq-zCfbabPGoy9iYWRmlT--b1pTmb89-nRxPjVY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8KrK8A5F1MifTkZ7kQo4GY3uVyItdN7rNah_tWTJpNvy= ymiTQTonCq-zCfbabPGoy9iYWRmlT--b1pTmb89-Wcl2m_4$=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) .