Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 01 2024 20:13:48 FOUS30 KWBC 012013 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 413 PM EDT Thu Aug 1 2024 Day 1 Valid 16Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST AND OHIO VALLEY... ....16Z Update... Only minor changes were necessary to the previous SLGT risk forecast and surrounding MRGL risk. The MRGL was pulled south across all of Eastern TN to match the Flood Watch issuance from the Morristown WFO which maintains a consistent message. Scattered thunderstorms are forecast within the zone south of Knoxville with lower FFG indices overall thanks to a barrage of convective impacts the past 48 hours. Further north, the primary zones for flash flooding will encompass both the Ohio Valley across IN/KY/Western OH thanks to the expected development of an MCS originating over IL with propagation vectors expected to be east to southeast through the complex life cycle. Progressive nature of the complex will aid in limiting the magnitude of flash flooding, however some heavier cores in-of the complex and upwind redevelopment on the southwest flank of the MCS could offer a better chance at significant flash flooding, especially if occurring over an urban corridor with higher propensity for run off. One of the stronger signatures for the potential is the 3hr FFG exceedance probability between 50-70% extending across Eastern IL through all of Central IN. This correlates well with the HREF blended mean QPF running above 2" along the same line where the MCS is anticipated on the northern flank of the complex. This is the area with the highest probability for flash flood impacts. Second area of interest is further north across WI as an upper low centered over Southern MN will move southeast with scattered to widespread convection expected this afternoon across Central and Southern WI. The expected environment has deviated very little from previous forecast with an overall slower storm motion and narrow band(s) of heavy rain under and just to the east of the upper low center. The SLGT risk across the area was more than justified as neighborhood probs around 60-70% for at least 3" are documented over the aforementioned area in WI from the latest 12z HREF. This is consistent with the signal from yesterday and overnight leading to higher confidence of the threat with locally significant impacts plausible in any urbanized settings. Out west, the MRGL risk area was relatively unchanged from the previous forecast, but a general expansion across the OK Panhandle and along the KS/OK state lines was initiated given the increased signal of a narrow corridor of thunderstorms expected overnight within the bounds of a quasi-stationary front draped over the region. A tongue of enhanced theta-E's aligned within the confines of the front provides an axis of stronger instability on the order of 3000-4000 J/kg MUCAPE, aligning with an areal PWAT anomaly bordering 1-1.5 deviations above normal. Scattered convection across Southeast CO into Northeast NM will develop later this afternoon with outflow generation and associated mid-level energy projecting eastward after full maturation. The combination of relevant cold pool propagation and mid-level energy perturbation will intersect the thermal gradient along the quasi-stationary front creating an environment favorable for rapid cell initiation from west to east over the course of the evening. Some cells will be capable of wet microburst potential leading to isolated instances of flash flooding if storms meander over any given area for too long. The signal was enough to warrant the additional MRGL expansion. Kleebauer ....Previous Discussion... ....Midwest/Great Lakes/Southern Appalachians... The suite of numerical guidance continued to support a Slight Risk area over portions of the Ohio Valley from Thursday into Thursday night as fairly impressive forcing approaches the region. Low-level moisture should be place (precipitable water values up to 2") as the closed mid- level low drops in from the north...helping to focus moisture flux convergence along a quasi- stationary front in the area at the same time that divergence aloft increases in response to the presence of an upper level speed maximum. The low-level inflow and effective bulk shear should be high enough in magnitude (25-30 kts) to lead to convective organization. Hourly rain totals to 2.5", and local amounts to 5", are possible. The ingredients still suggest a Slight risk area is warranted. Changes to continuity were minimal. ....Southwest into southern California... The upper ridge over the Southern Plains is forecast to build to the west and northwest across the Rockies, Great Basin and Southwest. An easterly wave under its base moves south of CA, helping push the axis of above average precipitable water values (as high as 1.75-2") farther west into southern to central California and southern Nevada. This will foster an environment in which scattered diurnal convection will be expanding into portions of central and southern California/southern Nevada. Hourly rain totals to 2.5" are possible, given the above. This would be problematic in area burn scars and dry washes/arroyos. Roth/Bann Day 2 Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY, SOUTHEAST MICHIGAN, AND THE NORTHERN MID ATLANTIC... ....20Z Update... Biggest changes for the period were focused around the inherited SLGT risk across the Ohio and Tennessee Valleys, as well as an additional SLGT risk situated across portions of the Northern Mid Atlantic encompassing areas of Eastern PA, Northwestern NJ, and Southeast NY state. More on the additional SLGT risk in the "Mid Atlantic" sub-heading below... Elsewhere, the signal for heavier convective precip across Northwestern OH and Southeast MI was enough to consider and follow through on a general SLGT risk expansion further north to encompass all of Western OH and the area surrounding Lake Erie. The premise for the upgrade had a lot to do with the forecasted propagation of the closed upper reflection positioning itself across the Northern Ohio Valley. HREF blended mean QPF footprint was well aligned with the ensemble bias corrected QPF signal of scattered heavier totals represented by stronger cores situated under the primary circulation. Somewhat similar to today's risk over WI, the colder core under the 5H trough axis will lead to steeper lapse rates and stronger downburst potential under the mean trough. Area PWATs are still forecast to be well-above normal within the confines of Lake Erie (+1-1.5 deviations), and this will generally be in a developing axis of deformation when looking at the 7H RH field alignment of higher moisture returns. This is the location of the higher HREF neighborhood probabilities for at least 3", upwards of 50-70%, well within the definition of flash flood concerns when situated over the Northern Ohio Valley corridor. Some general priming is forecast out of D1 which will also lower area FFGs prior to any event, so the threat will be "maturing" in the lead up and eventual impact. A SLGT risk was extended north from the previous forecast to cover the threat and add a southern buffer where some guidance has returns again over west-central OH.=20 The MRGL risk out west was extended northeastward into portions of the WY/CO/NE Front Range as latest deterministic output indicates the approach of a mid-level shortwave dropping southeast around the northern extension of the western ridge. The prospects for a well- organized area of convection is low, but the instability involved=20 with ample large scale ascent could offer some stronger convective cores that could offer some isolated flash flood concerns, mainly over that tri-state corridor to the northeast of I-25. Rainfall rates between 1-2"/hr are most likely within the stronger thunderstorms, the main factor for any flash flooding threat. ....Mid Atlantic... Favorable large scale pattern for scattered to widespread thunderstorms will materialize as we step through the period with a heightened chance for flash flood potential late-afternoon Friday through potentially the overnight period Friday into early Saturday morning. Closed 5H trough will slowly open across the Ohio Valley by later Friday, but a well-defined axis of diffluence will be=20 present extending through much of the Central and Northern Mid=20 Atlantic during the upper level evolution. Southwesterly flow aloft with southerly winds within the boundary layer will aid in a deep layer moisture advection regime through the morning and afternoon allowing area PWATs north of I-64 to ramp up to around +2 deviations above normal according to the latest ECMWF ENS and NAEFS output. Scattered thunderstorms will first develop upstream over the higher terrain in Western PA into Northern WV with energy quickly propagating northeast with the flow. More storms will fire across East-Central PA and points Northeast with a much more widespread coverage of convection by the early evening as a surface trough bisecting the the area from NoVA up through the Southern NY Catskills lights up with the increased forcing presence approaching from upstream.=20 12z HREF blended mean QPF really ramped up from the previous=20 forecast iteration with the signal for 1-2" of rain now=20 encompassing much of Eastern PA from I-78 and points north and=20 east. Multiple large bullseyes within the 1" HREF EAS probability=20 fields exist across the aforementioned corridor, as well as into=20 Northwest NJ and Southeast NY with a particularly bullish 70-80%=20 probability located over NJ. This would bisect the western extent=20 of the population centers based in Northern NJ, but just to the=20 west of the major NJ metros surrounding NYC. Regardless, the=20 pattern is very much favorable for a heightened threat of flash=20 flood concerns, thus leading to a targeted SLGT risk positioned=20 over much of Eastern PA, Northwest NJ, and Southeastern NY state. This matches a new area of emphasis within the UFVS First Guess Fields which escalated from a widespread MRGL risk to now a widespread SLGT across PA and points eastward.=20=20 Kleebauer ....Previous Discussion... .....Ohio & Tennessee Valleys... The upper pattern agrees on the approaching trough and focused ascent over the Ohio Valley into the Mid Atlantic. Precipitable water values up to 2" are advertised with this system. The low- level inflow and effective bulk shear should be high enough in magnitude (25-30 kts) to lead to convective organization. The available ingredients suggest hourly rain totals to 2.5" and local amounts to 5" would be possible. However, there is much spread in scattered convective signals underneath the mean trough. The environment is conducive for locally heavy rains anywhere from the Central Midwest through the Central and Northern Mid Atlantic with the primary likely falling under the center of the upper circulation and mean trough. This pinpoints areas like Ohio and Western PA down into WV/KY as the best chance for convective coverage capable of more flash flooding capabilities. Much is contingent on the pattern evolution and with time leading into the setup, but the mass fields along with the GFS/NAM suggest that portions of central and eastern KY should be most favored; added a Slight Risk in this area to address that concern which was coordinated with the JKL/Jackson KY and LMK/Louisville KY forecast offices. The previous Marginal Risk east of the Mississippi was expanded slightly, but remains fairly close to continuity. ....West... With the upper level ridge continuing to build across the Intermountain region and monsoonal moisture lingering near the International Border (precipitable water values up to 1.75")...another round of late day/evening showers and thunderstorms is expected. Continuity was generally maintained. Hourly rain totals to 2" should be possible. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. Roth/Kleebauer/Bann Day 3 Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE SOUTHWEST, OREGON, FLORIDA, UPPER MIDWEST, NORTHEAST, MID=20 ATLANTIC, AND SOUTHEAST U.S... ....20Z Update... Minor adjustments to the bounds of the MRGL risk areas across the Upper Midwest and Oregon were made to match the recent ensemble bias corrected QPF footprint and core of instability within each area in question. The overall synoptic pattern for all the MRGL risk threats have changed very little from run to run across most of the CONUS, however some smaller scale pattern changes have lead to more emphasis within the Eastern U.S broad MRGL risk in place from New England through the Mid Atlantic. Approaching cold front and mean trough across the east will be the primary forcing mechanisms for convective development and subsequent heavy rain potential. There was a greater signal for the interior portions of the Carolinas to see some heavier precipitation which would fall over an area that has seen quite a bit of precipitation the past few weeks. Surface forcing within a very moist, unstable environment will create a focal point for flash flood potential within the Carolina Piedmont with emphasis over the I-95 corridor and points west where increasing low-level convergence within the confines of the front will be greatest during the D3 time frame. This allowed for an expansion of the previous MRGL risk to the=20 north to cover much of the western two-thirds of the Carolinas=20 outside the Escarpment between the NC/SC/TN border. Pending the=20 convective evolution on D2, an upgrade is plausible across parts of the Mid Atlantic and Northeast as defined by the latest UFVS First Guess Field, but will allow more assessment of the setup as the=20 pattern evolves over the next few days.=20 The pattern across Florida is trending increasingly active with the National Hurricane Center now upping the potential for the tropical wave pattern over the Florida Straits to become something organized within the next 5-days. The threat for locally heavy rainfall of tropical origin will need to be monitored closely as the path of the wave and associated moist advection pattern could be enough to garner a targeted SLGT risk across parts of South FL as we move closer to the period. Primary axis of heavy precip remains focused offshore, so we're still within the bounds of the MRGL risk, but it would not take much for coastal areas of South FL to see an upgrade if the disturbance becomes more defined. Stay tuned for updates in the coming periods.=20 Kleebauer=20 ....Previous Discussion... ....Eastern U.S... A generally wet and unsettled pattern is likely with ample=20 moisture (precipitable water values up to 2") and instability in=20 place as a slow moving upper trough and surface/frontal systems all support areas of showers and thunderstorms. Inflow at 850 hPa of=20 25 kts implies some level of convective organization is expected.=20 Hourly rain totals up to 2.5" should be possible here, and local=20 amounts in the 2.5-5" range are advertised by the guidance, which=20 could easily fall within an hour or two. As there remains=20 uncertainty in placement of the higher amounts, and a decent=20 section of the Richmond/Washington D.C./Philadelphia megalopolis=20 has descended into drought, raising flash flood guidance values=20 fairly highly, believe the Marginal Risk continues best for the=20 time being. Urban areas are at the highest risk for flash flooding=20 in this region. ....Southwest... Monsoonal convection over the Southwest (precipitable water values up to 1.75" near the International Border) continues to support a Marginal Risk area. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. Expanded the area westward to account for the signal seen in the 00z Canadian Regional which implies a risk as far west of the Peninsular Ranges of CA. ....Florida Peninsula... Moisture and instability increase as a tropical disturbance moves in the vicinity of the FL Peninsula. Precipitable water values rise to 2.25"+, and plenty of instability resides offshore over the warm waters. There has been a westward shift in the guidance over the past 24 hours more into the Gulf of Mexico due to its slower than anticipated (in model land) development, and the best indication as to the center of any low- to mid-level vorticity has been near Puerto Rico. With the potential for 3" an hour totals and a possibly better organized system to enhance rainfall potential, went ahead and broadened the existing Marginal Risk area to encompass the FL Peninsula for the time being to account for the uncertainty. ....Upper Midwest... The guidance is generally showing a modest signal for heavy=20 rainfall in and near Minnesota this period. Precipitable water=20 values rise about 1.25" with a warm advection pattern. This led to=20 an introduction of a Marginal Risk here, as flash flood guidance=20 values are modest after recent heavy rainfall. ....Oregon... Moisture advecting northward through the West gets drawn into the leading edge of an upper level trough between southwest OR and northeast WA. Both the GFS and NAM get the precipitable water values up to 1-1.5", which would be most anomalous in terrain. There is some reflection of the trough in the 850-700 hPa wind pattern (the NAM appears more bullish on the low-level convergence prospects). Considering the weakness of the upper level feature, it shouldn't preclude daytime heating/diurnal insolation, so there should be decent instability as well. The concern is highest in the terrain, particularly near any burn scars. This led to a new Marginal Risk area across portions of OR, which have the best QPF signal, albeit not that high. Hourly rain totals to 2" are considered possible. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-G75Uy6xSpK3Ki77n_8-49h2aLhNpvNUUS53XpM-Pefn= A31kUa6NlHz0CNwANj1IGyPH63ao6ERfHNmBvVLrxuWKDoE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-G75Uy6xSpK3Ki77n_8-49h2aLhNpvNUUS53XpM-Pefn= A31kUa6NlHz0CNwANj1IGyPH63ao6ERfHNmBvVLrvHNEfUo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-G75Uy6xSpK3Ki77n_8-49h2aLhNpvNUUS53XpM-Pefn= A31kUa6NlHz0CNwANj1IGyPH63ao6ERfHNmBvVLrXSdagM4$=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) .