Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 08 2023 19:54:27 FOUS30 KWBC 081954 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 353 PM EDT Sat Jul 08 2023 Day 1 Valid 1800Z Sat Jul 08 2023 - 12Z Sun Jul 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS INTO THE SOUTHERN MID-MISSISSIPPI VALLEY... ....18Z Update... Expanded the Marginal Risk farther west over NC and southwest VA per regional radar activity and areas covered by MPD 664 which can be consulted for further information on heavy rain in this region. Jackson ....16Z Update... In collaboration with ALY/Albany, NY; BTV/Burlington, VT, BOX/Norton, MA; and GYX/Gray, ME forecast offices, have opted to retain the inherited Marginal Risk area for portions of New England for today's storms that are starting to bubble up as per latest visible satellite imagery. Antecedent conditions are very favorable for flash flooding as AHPS output shows 2-week rainfall amounts exceeding 300% of normal for a large swath of the Northeast from the eastern Poconos of PA up to far southern ME. However, today convective coverage will be somewhat limited, and largely tied to the terrain, which should limit the impacts of any such resultant flash flooding. Things also remain steady state across the Plains, with excellent agreement among the 12Z guidance that a corridor of heavier rain associated with training convection possibly growing upscale into an MCS will occur in a northwest to southeast oriented corridor in Oklahoma from the Panhandle across the heart of the state, only a small northward expansion into extreme southwestern Kansas was included for this update. Finally, a Slight was also considered for central NC with this update. In coordination with RAH/Raleigh, NC office the decision was made to keep it at a Marginal for this update. While convective coverage should be greater due to enhanced forcing than surrounding states/areas, dry antecedent conditions and widely scattered coverage should keep the flash flooding threat isolated, consistent with the Marginal. For more on the meteorological setup, please see the previous discussion below for more details. Wegman ....Central High Plains/Oklahoma into Mid-MS Valley... Retrograding ridge into the Southwest continues to slightly amplify across the Rockies in tandem with the base of the larger scale trough over the Northern Plains/Upper MS Valley continuing to carve out cyclonic flow into the Ozarks and Ohio River Valleys. At the start of the forecast period, a mature to weakening MCS will be fading through the Red River Valley, but starting to round through the base of the aforementioned trof into increasingly diffluent flow aloft. The surface boundary will continue to sag southward across the southern Central Plains into S MO and become stationary. Insolation will allow for increased instability above 2000 J/kg by mid-afternoon and utilizing older outflow boundaries, weak convergence should allow for scattered thunderstorm activity to develop/expand though the Delta Region of the Mid-MS Valley with cells utilizing pooled deep layer moisture (in excess of 2" Total PWat) for efficient rainfall production. Loosely organized, training elements across AR/W TN/MS slowly moving into W AL with 00z Hi-Res CAMs and HREF PMM supporting 2-4" totals resulting in possible flash flooding conditions. 00z HREF 2-3"/3hr probabilities range from 40-50% and 25-40%, respectively and the inherited Slight Risk was adjusted slightly to encapsulate those signals. Upstream, as the ridge increases, the southeasterly flow veers a bit more across the upslope region expanding organized thunderstorm activity into E WY; however, best focus again remains along/south of the Lake/Palmer divide in SE Colorado. The strength of return moisture is a bit more limited given the stronger mid-level shortwave is now a bit further east, but the westerly flow over the Rockies remains potent enough for thunderstorms to once again grow upscale into an MCS crossing into NW OK through the central portions of the state, likely to overlap areas recently saturated by prior few days of MCSs. So while the overall rainfall intensity/totals is diminished versus prior waves, a Slight Risk remains in place (a tad south and west of prior tracks) connecting to the broader area in the Mid-MS Valley. ....Central Great Lakes and Ohio Valley... Large scale pattern remains on track with exiting shortwave across the Great Lakes into SE Canada to be replaced by deepening wave that will further carve out broader cyclonic flow further south into the Ohio and Mid-MS Valleys A surface low along the draped central CONUS frontal zone will ride northward across IL/IND today, with an attendant warm front across Ohio moving toward the Lower Great Lakes though the evening. This will increase overall deep layer moisture (1.5" toward pockets of 1.75" TPW) and broader pockets of typical summer instability values (1000 to 2000 J/kg, from Lower MI to SW KY/W TN). Proximity to the frontal zones may allow for increased clustering of stronger, but generally progressive thunderstorms capable of 1.5-2"/hr rates, localized cell training/repeating, with localized pockets of 2-3" totals, inducing isolated to widely scattered flash flooding incidents that seems appropriate for a Marginal Risk category. ....Carolinas, Mid-Atlantic into Northeast... Stubborn ridge over the NW Atlantic and main forcing wave lifted well into Eastern Canada, has left a residual sheared out higher Theta-E/moisture ribbon across central New England. Insolation will support a similar narrow instability axis of 1000-2000 J/kg by mid-afternoon to again initiate thunderstorms through the axis, but today forcing will be weaker and more isolated than prior days; however very weak steering will allow for pulse thunderstorms with high efficient rainfall production for isolated flash flooding risk. Further south, the deeper moisture is broader across Eastern Carolinas, a little more cyclonic curvature and low level convergence should allow for increased converge of thunderstorms with expected scattered shot QPF pattern with scattered to numerous spots of 2"+ with broad area of 3"/24hr probabilities of 50-80% in E North Carolina, though over sandy soils and higher FFG a broad Marginal Risk seems appropriate for level of widely scattered flash flooding potential here as well. ....Southern Florida... Flat/broad deep layer westerly flow remains across Peninsular Florida today with anomalous deep layer moisture in the 2-2.25" range which is generally 2-2.5 standard deviations from normal.=20 Near surface flow is backed across Southeast Florida, to increase moisture convergence/anchor expected sea-breeze a bit further south and east than prior days. This results in continued above average QPF signal and 00z HREF probability highlighted greatest near Martin/Palm Beach county, but extending west a bit into the Glades/Lake Okeechobee. As such, maintained the Marginal Risk of Excessive Rainfall across this area again today. Gallina Day 2 Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO WESTERN NEW ENGLAND... ....20Z Update... ....Mid-Atlantic through New England... In coordination with all the forecast offices from DC/Baltimore north through New England, a Moderate Risk area was introduced with this afternoon's update for portions of the Mid-Atlantic into western New England. All guidance continues to hone in on a corridor of heavy rainfall caused by numerous training showers and thunderstorms developing across the Moderate Risk area Sunday afternoon for southern and western areas, then progressing through New England Sunday Night. Expect a widespread 2-4 inch event especially from eastern PA north through VT. However local amounts are likely to exceed 6 inches of rain, with the most likely areas for the higher rainfall totals through the Catskills/Poconos of NY/PA and also through the Green Mountains of VT. Much of this area has been very hard hit in recent days with heavy rainfall, with a large area over 300% of climatological normal rains per 2-week AHPS data. Adding a stalling front into the mix which will allow for training of abnormally high moisture air on the order of PWATs to 1.75 inches makes for a likely scenario for flash flooding, especially when adding terrain influences into the mix. With the full day's worth of HREF data first available at 12Z, it's notable that portions of east central VT have a 30% chance of exceeding 100-year annual return intervals for the amount of rain expected, with the Catskills/Poconos and Green Mountains all over 70% chance of exceeding 3 and 6 hour FFGs Sunday afternoon and evening. The steady signal of widespread rain and good model agreement on both timing and areal amounts added enough confidence to the forecast to upgrade to the Moderate Risk for this update. ....Plains to the Gulf Coast... Very few changes were needed with this afternoon's update, as the model consensus continues to point to another MCS and/or training convection unfolding from Oklahoma to the Gulf Coast on Sunday. Only very small nudges were needed to the previous Slight as everything still looks on track. ....Eastern North Carolina... In collaboration with the MHX/Morehead City, NC and RAH/Raleigh, NC forecast offices, the Slight Risk was extended south to include portions of eastern NC, though largely away from the immediate coast. Forward propagating convection early will slow down and may train for areas east of I-95 Sunday afternoon. With favorable antecedent conditions in portions of this area as well due to previous days' convection, and a favorable signal in the 12Z HREF guidance, the upgrade was issued for this forecast update. Wegman ....Previous Discussion... ....Mid-Atlantic to New England... At the start of the forecast period 09.12z, a shortwave that carved out the base of the large scale trough extending into Canada, continues to amplify/strengthen through the Upper Ohio River Valley providing strong height-falls and large scale forcing across the eastern third of the CONUS. There still remains some modest spread in the latitude of the core of the wave and therefore timing of progression, but the low level response continues to support a surface low through the Lower Great Lakes driving deeper moisture flux across the Mid-Atlantic into the Northeast. Total PWats will be 1.7 increasing to over 2" through the Hudson Valley and with ample heating ahead of the cold front, will support thunderstorm activity from NY to the Carolinas.=20 Cells further south will be progressive but have stronger moisture available supporting streaks of enhanced rainfall across the Carolinas into Virginia, but should remain scattered enough to keep within the Marginal Risk of Excessive Rainfall. A secondary surface wave will form lifting northward across the mid-Atlantic, as such, deep layer steering along and north of the wave will be more southerly with slowing forward progression of the front.=20 This will allow for a favorable training profile for thunderstorms along and ahead of the front (i.e., perhaps two rounds of stronger thunderstorms throughout the day) particularly across the Delaware and Hudson River Valleys. Nearly all larger scale guidance members support 2-4" rainfall across the area, though longitude remains fairly uncertain due to the remaining timing spread.=20 Current consensus would place the axis across E PA, E NY into SW VT, though may be as far east as NH/Central MA, W CT (i.e. GFS).=20 A broad Slight Risk remains fairly similar to prior day 3 forecast with slight adjustment westward given increased clustering westward. This is crossing areas of increased soil saturation given AHPS 14-day precip anomalies of 200-400% across much of central MD, central & eastern PA into SE Upstate NY and Western New England, there remains a potential for further upgrade to a Moderate Risk of Excessive Rainfall though guidance needs better timing agreement for peak thunderstorm intensity to align with this precip anomaly axis. Additionally, lowered FFG from recent heavy rainfall, ample moisture and thunderstorms along/ahead of the front, as well as coordination with local forecast offices in Sterling and Wakefield VA; the Slight Risk extends south to central to Southeast VA including Charlottesville, Richmond and Norfolk.=20=20=20=20 ....Southern Plains to Central Gulf Coast... Like prior days, there is likely to be an ongoing MCS across Oklahoma at the start of the forecast period from prior night's upslope flow. Guidance shows some increase in elongation of the wave from west to east with weaker isentropic ascent to spur stronger thunderstorms, though the length should maintain duration for modest rainfall crossing saturated grounds from multiple prior days' complexes. By mid-day, insolation and convergence in proximity to the MCV across LA/S AR should spur an additional round of stronger, moderately organized thunderstorms capable of 2-4". Trends in the guidance continue to broaden the overall totals and reduce the concentration of rainfall magnitude to more scattered clusters/instances, while also shifting south and west.=20 Return moisture for upslope will further weaken for last Sunday into early Monday across the Southern Rockies, but given stalled frontal boundary and isentropic ascent from the LLJ overnight, a few scattered thunderstorms are probable as far west as W OK.=20 Given expected lower FFG from the multiple rounds over the last and next few days, WPC will maintain but narrow a Slight Risk of Excessive Rainfall from Central OK to Southern AL, encompassed by a Marginal Risk across the Tennessee Valley into the Central Appalachians. Gallina=20 Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_o5SQUFyl1f1ont61NCubQU2VlDd6qr9PQuYov7-FUUT= WipaVCBpkMUB2MHjlVF2iM-eRQMHYaPSDR-HcEhVMmnJSCc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_o5SQUFyl1f1ont61NCubQU2VlDd6qr9PQuYov7-FUUT= WipaVCBpkMUB2MHjlVF2iM-eRQMHYaPSDR-HcEhVaEZJOVE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_o5SQUFyl1f1ont61NCubQU2VlDd6qr9PQuYov7-FUUT= WipaVCBpkMUB2MHjlVF2iM-eRQMHYaPSDR-HcEhV5BDVZ54$=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) .