Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 29 2024 20:17:28 FOUS30 KWBC 292017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Mon Jul 29 2024 Day 1 Valid 16Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS... ....1745Z Update... The Marginal Risk in the Southwest was expanded to include more portions of southern AZ, central NM, and far western TX. For additional details on the flash flood potential in the Desert Southwest, please read WPC's MPD #0755 which is in effect until 2330Z. ....16Z Update... Added a Marginal Risk for portions of the northern Mid-Atlantic coast and as far north as Long Island. According ot RAP mesoanalysis, a narrow band of moisture convergence over Long Island is generating a ribbon of heavy thunderstorms that MRMS QPE shows is producing as much as 1.25-1.75"/hr rainfall rates over eastern Long Island. RAP guidance suggests this moisture convergence zone may advance northeast up along the southern New England coast this afternoon where as much as 500-1000 J/kg is present. This same instability axis will be present on south and west towards the Delaware Valley and southern NJ where there is also a pivoting 500mb vort max on the backside of the anomalous 500mb low that will traverse the region this afternoon. 12Z surface analysis also depicts a surface trough along the I-95 corridor that could assist as a trigger for storms this afternoon. There is the potential for backbuilding convection along the surface trough that may generate hourly rates as high as 2"/hr over more urbanized environments. Given the potential for max hourly rainfall rates as high as 2"/hr and the areas from northern DE to Long Island sporting some higher concentration of hydrophobic surfaces, a Marginal Risk was introduced this forecast cycle. Elsewhere, there is the potential for considerable flash flooding in parts of the Ohio Valley tonight where the potential for training heavy thunderstorms tonight. The latest 12Z HREF does depict low chances (20-30%) for 6-hr QPF > 100-yr ARI exceedance in south central IN between 00-06Z tonight, as well 30-40% probabilities for >5" of rainfall for that same time period. The area has been dry of late, but even despite the drier soils, excessive rainfall rates between 2-2.5"/hr are likely within the intense clusters of thunderstorms tonight that would stretch from south-central IL and southern IN to central KY. Minor tweaks to the Marginal in the Southwest were made. The Marginal Risk in the Lower Great Lakes was expanded to cover more of central MI. This is due to a remnant MCV approaching from southern WI that will act as a trigger for storms that would have tap MLCAPE >1,000 J/kg and PWs >1.8" at their disposal this evening. Lastly, moved the Slight Risk in northern GA more to the east and trimmed off the far southern extent of the Slight Risk given latest 12Z HREF probabilities moving the higher axis of >3" rainfall amounts north and east. Mullinax --Previous Discussion--- ....Ohio/Tennessee Valleys & Southern Appalachians... West northwesterly mid to upper level flow will persist during the upcoming day 1 period from the Northern Plains, east southeastward through much of the Mississippi Valley, Ohio Valley, Southern to Central Appalachians, Mid-Atlantic and Southeast. Embedded shortwaves in this flow will support another day of active convection across much of these areas where PW values will be mostly above seasonal averages. A slight risk area was drawn where the latest HREF neighborhood probabilities were the highest for 1, 2 and 3"+ totals and overlapped the lower FFG values and observed precip axis from recent rains. This results in a slight risk from southern Illinois, east-southeast through the Ohio Valley, Upper Tennessee Valley into the Southern Appalachians. Changes from the previous issuance were to elongate the previous slight risk farther westward through the Lower Ohio Valley and southward into the Southern Appalachians where heavy rains fell over the past 24 hours. The eastern portion of the slight risk over west central NC was trimmed westward given relatively high FFG values and no precip overlap potential from past 24 hours and the upcoming day 1 period. ....Florida... Concentrated the marginal risk area across the southern portion of the peninsula where model consensus is for heavy rainfall in the high PW axis along and south of the stationary frontal boundary/trof currently analyzed off of the FL east coast. HREF neighborhood probabilities are high for 1, 2 and 3"+ amounts across the southern half of the Peninsula, supporting isolated runoff issues, especially in the more urbanized regions. ....Southeast Arizona into southwest New Mexico... No significant changes made to the marginal risk area across southeast Arizona into southwest New Mexico. Scattered convection likely again associated with shortwave energy rotating northward on the western side of the Southern Plains upper ridge and across portions of the Southwest. There is the usual low confidence in qpf details, but potential for isolated heavy totals and localized runoff issues. The marginal risk area fits well with the axis of the 1"+ HREF neighborhood probabilities. Oravec Day 2 Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN MID-ATLANTIC INTO THE CENTRAL APPALACHIANS... ....2030Z Update... ....Northern PA & Southern NY... An approaching 500mb vort max that is also associated with a remnant MCV moving through the Great Lakes on Monday will make its way into the northern Mid-Atlantic on Tuesday. The upper trough will generate ascent atop the atmosphere while the environment sports MLCAPE up to 1,000 J/kg and PWs maxing between 1.8-2.0". While storm motions should generally range between 15-20 kts, ECMWF area averaged soundings in the Slight Risk area do show upshear Corfidi vectors <10 kts, suggesting the potential for back-building thunderstorms. This is particularly the case along mountain ranges that are oriented favorably to the 850-300mb mean flow out of the SW. 1-hr FFGs are lowest over the southern tier of NY where values are <1"/hr in some spots. For these reasons, a Slight Risk was introduced for parts of Northeast PA and the southern tier of NY given their more sensitive soils. ....Eastern IA & Northern IL... A 500mb disturbance tracking across the Northern Plains will track towards the western Corn Belt while a 50kt 500mb jet over the Upper MS Valley helps to provide favorable synoptic-scale lift over the region. At the same time, 850mb ridging over the Southeast and troughing over the northern Plains will cause an acceleration in the LLJ (925mb winds 2-3 standardized anomalies above normal in=20 the Central Plains) to deliver a surge of 925-850mb moisture over=20 the Midwest. This LLJ will also favorably intersect a stationary=20 front draped WNW to ESE, which along with veering winds as air=20 parcels rise will support organized thunderstorm activity. PWs up=20 to 1.8", MUCAPE >2,000 J/kg, and hodographs that provide sufficient shear to sustain long duration thunderstorm activity could result=20 in back building and training thunderstorms either along the=20 stationary front and/or along outflow boundaries that are supported by deep cold pools. Latest 12Z HREF's 3-hr QPF > 3-hr FFGs product depict up to moderate chance probabilities (40-50%) for 3-hr FFG=20 exceedance while low-to-moderate probabilities (20-40%) for 1-hr=20 QPF > 1-hr FFGs were present over eastern IA. There remains some=20 notable spread in guidance with regards to the position and=20 orientation of where the heaviest QPF area will set up. That said,=20 the region highlighted is also currently sporting 1-hr FFGs that in most areas are <2"/hr. Given the optimal atmospheric setup for=20 storms generating >2"/hr rates and sensitive soils, a Slight Risk=20 was introduced for portions of eastern IA and northern IL.=20 ....Northern Rockies... A fast moving -1.5 to -2.0 standardized anomalies 500mb trough=20 tracking through the Northern Rockies will also be accompanied by=20 PWs that are hovering around 1.0", which are as much as +2 standardized anomalies over the region. In addition, MUCAPE up to=20 500 J/kg will support rainfall rates as high as 1.5"/hr in the most intense areas of convection. Storms should be progressive so=20 that will help keep the flash flood threat to a very localized=20 scale, but the available atmospheric moisture and instability could produce intense enough rainfall rates to cause rapid runoff along=20 complex terrain and burn scars. A Marginal Risk was introduced to=20 account for the very localized flash flood potential Tuesday=20 afternoon and evening. Outside of minor tweaks to the Southwest's Marginal Risk, the remainder of the previous shift's forecast discussion is on track. Mullinax ---Previous Discussion--- ....Southwest Florida... The previous marginal risk area was decreased in size and concentrated over Southwest Florida. This corresponds to where the HREF neighborhood probabilities for the 12 hour period from 12Z Tue to 00Z Wed are highest for 2 and 3"+ amounts. Isolated runoff issues possible, especially over urbanized regions. ....Southwest... No changes expected to the large scale flow with the upper ridge remaining centered over the Southern Plains. This will again support shortwaves rotating northward on the western side of this ridge into the Southwest, supporting scattered convection, localized heavy rains and isolated runoff issues. Oravec Day 3 Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST... ....2030Z Update... Minor adjustments were made to the Slight Risk in the Midwest as=20 there was not enough consensus on guidance to make significant=20 adjustments at this moment. The only tweaks made were to take out the Marginal Risk in western NC and southern VA where QPF has dropped compared to the previous cycle and to expand the Marginal in the Northeast southward to include more of the southern tier of NY where soils are more sensitive. Otherwise, the previous forecast discussion remains on track. Mullinax --Previous Discussion-- ....Upper Mississippi Valley into the Midwest... The progressive northern stream flow will continue into day 3 with the next strong area of height falls expected to push eastward across the Northern Plains and into the Upper Mississippi Valley. There is potential for additional organized convection/heavy rainfall to push eastward with these height falls across eastern portions of the Northern Plains and into the Upper Mississippi Valley. There are some typical latitude differences with the max qpf axis. The day 3 slight risk area was drawn to favor the more southern solutions of the EC, NAM and CMC. The previous slight risk area over the Upper Mississippi Valley was suppressed more to the south and southwest to better match the more southern solutions with the next round of potentially organized convection. ....Eastern New York State into western New England... The strong height falls moving out of the Central Appalachians into the Northern Mid-Atlantic day 2, will progress across eastern NY State into New England day 3. There is a typical amount of qpf spread for the day 3 period, but general consensus for potential of moderate to locally heavy rainfall amounts in a region of much above average PW values ahead of these height falls. A broad marginal risk area was maintained from the previous issuance to capture the heavy rainfall spread in the models across this area day 3. ....Southwest... Similar to the days 1 and 2 period, the mid to upper level ridge will remain centered over the Southern Plains. Scattered convection again likely over portions of the Southwest as shortwave energy again rotates northward on the western side of this ridge and into the Southwest. Localized heavy rains and isolated runoff issues again possible for southeast Arizona into southwest to central New Mexico. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!45aBUEZskfnQKKvGsAYqhtxD45t_NBXVadiHmKXEzVxP= 7SLtAiUIDsN_j3wafuVMediipoi6TReQvH1BpeXqMOG9sDk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!45aBUEZskfnQKKvGsAYqhtxD45t_NBXVadiHmKXEzVxP= 7SLtAiUIDsN_j3wafuVMediipoi6TReQvH1BpeXqpScdAAk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!45aBUEZskfnQKKvGsAYqhtxD45t_NBXVadiHmKXEzVxP= 7SLtAiUIDsN_j3wafuVMediipoi6TReQvH1BpeXqo6Cb9rA$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .