Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 08 2023 08:36:19 FOUS30 KWBC 080836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 435 AM EDT Sat Jul 08 2023 Day 1 Valid 12Z 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... ....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"/24r 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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN OKLAHOMA TO THE WESTERN FLORIDA PANHANDLE AS WELL AS THE MID-ATLANTIC INTO NEW ENGLAND... ....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 (ie, 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 (ie GFS). 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 Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND AND THE CENTRAL GULF COAST... ....Northeast... Strong deep layer cyclone closes off from the main body of the large scale trough across the Northeast on Monday into Tuesday. Nose of 250mb Upper-level jet provides favorable weak diffluent and right exit divergence for solid broad scale ascent across the region. Deep layer warm conveyor will tap highly anomalous moisture over the Northwest Atlantic with 1.7-1.9" Total PWats along/ahead of the slow cold front, nearly 2.5 standard deviation from normal. However, strength of low level flow does not appear to be highly anomalous in the day 3 period having shifted into Canada. Still, ample moisture convergence and deep layer southerly steering flow should allow for short-term south to north training of thunderstorms along/ahead of the cold front has it propagates eastward. Additional 2-4" totals are expected across northern New England with some hints at the upwind edge as far south as W CT. Compounding from the day 2 period, flash flooding is growing more likely across the terrain in northern New England. A higher-end Slight Risk of Excessive Rainfall remains in place from the prior cycle, with a Marginal Risk extending into the Pocono Mtns and NYC. ....Gulf States into South Carolina... Deep Layer trough as pressed the cold front and pooled moisture/instability along the Central Gulf Coast. Northwesterly flow with modest south-southwest flow intersecting the front across N LA will provide continued opportunity for training thunderstorms along the front toward NOLA before rounding the base. Upper-level diffluence may allow for some clusters to grow upscale and train with best convergence of heavy rainfall signature in proximity to SE LA and coastal MS. A subtle shortwave/MCV may enhanced strengthening low-level flow/convergence downstream of the mean trough axis into S AL/GA, but there remains sizable uncertainty to the strength of the flow and heating to produce larger organized areas of thunderstorms/clusters through the day 3 period. As such, a Slight Risk of Excessive Rainfall remains in place across the Central Gulf states were the potential for repeating thunderstorms is greatest, with a broader Marginal Risk up and down stream cross S AR to SC. ....Central to Southern Plains... Stronger closed ridge over the Southwest and reduced activity overnight late Monday across Central High Plains, should have allowed for weaker moisture return to build up higher values, increased instability for potential activity into Tuesday evening. 00z guidance along with prior cycles continues to hint at an organized cluster or small complex to form at the upwind side of the cold front in the vicinity of NE NM into the Texas Panhandle/W OK. Storm mode will likely determine localized very intense rainfall rates capable of inducing an isolated few instances of flash flooding, in proximity to saturated grounds across central Oklahoma, a Marginal Risk is extended from the area in the Lower MS Valley back across and north of the Red River.=20=20 Additionally, northern stream cold front as leading edge of larger scale height-falls will be pressing southward across the central Plains and intersecting some of the untapped moisture/instability that rolled back through eastern Colorado on Sunday and Monday.=20 Instability of 2000-3000 J/kg and 1.5"+ total PWats pooled along strengthening (20-25kt) LLJ is likely to converge in early Tuesday for thunderstorm development across Nebraska, eventually propagating into KS. As such an extension of the Marginal Risk is brought northward into southern Nebraska extending toward the Iowa boarder.=20 ....Upper Midwest... There is uncertainty to the level of moisture flux return along/ahead of the cold front dropping out of Canada crossing Lake Superior to N Nebraska in the day 3 period. Stronger storms are likely to dot the front, but cell motions should mitigate the highest totals across portions of Iowa into MN, particularly given recent dry soil conditions. At this time, it does not rise to the level to be delineated within a risk area. However, nearer the surface low, timing of the cold front may be ideal in timing with Lake Breeze boundaries for thunderstorm collisions across the U.P. of MI. A small Marginal Risk was considered for this potential, but may be still too early for confidence in such a mesoscale interaction and sufficient moisture availability for such a scenario unfolding. Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!514eDm01nsTRQ5QPkN8FORE11z-nx3iaYeWWjmQ_4uMb= LKL3uu31PON6HxS6MphqWZ0yIqGBKHycAZ7SlNKNKNYSIYk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!514eDm01nsTRQ5QPkN8FORE11z-nx3iaYeWWjmQ_4uMb= LKL3uu31PON6HxS6MphqWZ0yIqGBKHycAZ7SlNKNMGN8h7Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!514eDm01nsTRQ5QPkN8FORE11z-nx3iaYeWWjmQ_4uMb= LKL3uu31PON6HxS6MphqWZ0yIqGBKHycAZ7SlNKNWqTw6fs$=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) .