Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 21 2023 08:34:52 FOUS30 KWBC 210834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 433 AM EDT Fri Jul 21 2023 Day 1 Valid 12Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO, SOUTHEAST/MID-SOUTH, AND NORTHEAST/MID-ATLANTIC... ....Northeast and Mid-Atlantic... Convection is ongoing as of 08z across portions of eastern PA/NY and should continue to push eastward early this morning. It should gradually weaken with eastward extent, although the southern portion of the line is maintaining longer than most guidance indicated. This does raise some question with regards to how quickly instability will recover this morning behind this line from DE/NJ north towards NYC. Not expecting much of a flash flood risk with this initial line, with the threat primarily expected with the redevelopment later this morning into the afternoon/evening hours. A slowing surface front and persistent upper level divergence will support the potential of multiple convective rounds today across portions of the Northeast. Mean flow is out of the west or west southwest at about 20-30 kts...which would support rather swift eastward movement of cells. However with the front slowing, lower level convergence will be rather persistent, which will support a few rounds of storms, and may also support some brief anchoring of cells along the convergent axis. Both of these would locally extend rainfall duration and increase the flash flood risk. PWs are only modestly above average, so not expecting the most efficient rainfall producers. But we are in mid summer, and so PWs are still high enough for heavy rainfall rates within any convection...and the potential brief training should be enough to at least partially overcome the modest PW values. HREF data supports decent coverage of 1" in an hour rainfall, with more isolated areas around 2". Storm total rain should approach 3" in spots, and can not rule out an isolated 5" amount. Do expect that we will see some flash flooding today across portions of NJ northward into eastern NY, VT and western MA/CT. If antecedent conditions were neutral this would be a solid Slight risk, but the fact that soil and streamflow conditions are above average does begin to push this event towards the higher end of the Slight risk probability range. Gave some consideration to a MDT risk upgrade, but the coverage and magnitude of impacts will probably stay just below MDT risk levels. Will consider this a higher end Slight risk though, and can not rule out localized higher end impacts if the wrong basin or urban area gets impacted by a quick 2-3" of rain. ....Mid-South/Southeast... An MCS will likely be ongoing across portions of OK at 12z this morning. The complex should remain rather progressive as it moves across OK and AR, however heavy short duration rainfall rates are likely, with a quick 1-3" of rain expected. The forward speed of the complex will probably keep the flash flood risk isolated in nature for the most part. While some weakening is possible by later this morning, do expect the complex to maintain to some extent as it moves into southern TN and northern MS/AL. As daytime heating increases, we should see increasing instability out ahead of the feature, along with enhanced moisture convergence along its track, especially where interaction with the southward dropping cold front occurs. Thus additional development along and ahead of the complex appears likely by this afternoon. This should allow for some cell merging and/or training along the track of the complex, and supports an isolated to scattered flash flood risk extending east southeast across southern TN, northern MS/AL and eventually northwest GA. Additional development is also likely Friday night as convergence along the southward dropping front is progged to increase. Some west to east training is initially possible with this development across portions of AR/MS. Activity may then grow upscale into another MCS that would track southeast across portions of MS/AL. While this would probably be progressive...this may be round two for some areas after what transpires this morning and afternoon. Add all this up, and despite the high FFG, think a broad Slight risk is warranted from OK into western GA...accounting for convection this morning/afternoon and for what develops later tonight into early Saturday. ....New Mexico... Weak shortwave impulses embedded within an amplifying ridge over the Desert Southwest will produce ascent within an environment of increasing heat and instability. HREF guidance suggests MUCAPE will rise to around 2000 J/kg during peak heating...with strengthening low level upslope flow and convergence along/behind a southward dropping front helping initiate convective development. The region will also be clipped by the right entrance region of an upper jet to the northeast, enhancing divergence aloft and supporting a maintenance of deep convection. HREF data supports a high likelihood of 1" an hour rainfall, with isolated/localized 2" within an hour possible over northeast NM. Convection will initially be slow moving near the terrain, but should eventually build upscale enough to propagate southeast along the instability axis into the western TX Panhandle. Expect that some flash flooding will occur over northeast NM where convection initiates and is initially slow moving, and then a lower but non-zero risk will exist as activity begins pushing off to the southeast towards the TX border. ....Southeast Florida... Not much change to the inherited Marginal risk across southeast FL. Another day of scattered afternoon air-mass type thunderstorms is likely across the FL peninsula, but some slightly greater organization and coverage is possible compared to the past few days. PWs around 2" will combine with MUCAPE reaching 3000 J/kg to provide extreme thermodynamics to support rainfall rates of 2-3"/hr within convection. Although most of the development will occur along the typical sea breeze/gulf breeze with additional storms occurring along outflows, a weak boundary dropping south across the peninsula may help slightly increase convective organization. The 00z HREF guidance still supports localized 3"+ rainfall amounts, which if occur over a sensitive urban area, would cause some flooding concerns. With generally weak low level flow, the latest HREF does focus the highest rainfall over inland areas, with a bit less of a threat along the urban coastline. So the risk of flash flooding is probably a bit lower, but still can not rule out some heavy amounts along the coast and will thus maintain the Marginal. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7OPz8P2oqqxkQvSWY7lAiqbNtgZwJh8xwVJKBh3vI46p= cf6LqvTuxIp1jmSLZTENKwlD8zasH6RW3CajSn4_rxL2TuY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7OPz8P2oqqxkQvSWY7lAiqbNtgZwJh8xwVJKBh3vI46p= cf6LqvTuxIp1jmSLZTENKwlD8zasH6RW3CajSn4_M9gRrGw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7OPz8P2oqqxkQvSWY7lAiqbNtgZwJh8xwVJKBh3vI46p= cf6LqvTuxIp1jmSLZTENKwlD8zasH6RW3CajSn4_ZY_ghHs$=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) .