Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 09 2023 01:06:02 FOUS30 KWBC 090105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 905 PM EDT Sat Jul 08 2023 Day 1 Valid 01Z Sun Jul 09 2023 - 12Z Sun Jul 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTH-CENTRAL PLAINS AND MID-MISSISSIPPI VALLEY... ....Central High Plains/Oklahoma and Mid-South... Retrograding ridge into the Southwest continues to slightly amplify tonight 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. The surface boundary is currently along the MO/AR border up along the OH River and will merely sag south overnight. A distinct area of heavy rain risk is over the Mid-South east to the south of the Ohio River where a Slight Risk is maintained. Upstream, the increasing ridge and veering southeasterly flow allows expanding organized thunderstorm activity over the High Plains of eastern CO into KS. The strength of return moisture over the Plains is a bit limited given the stronger mid-level shortwave is now farther east, but the westerly flow over the Rockies remains potent enough for thunderstorms to once again grow upscale into an MCS crossing SW KS into NW OK through the central and into eastern portions of the state, likely to overlap areas recently saturated by prior few days of MCSs. A Slight Risk remains in place from southwest KS/southeast CO through OK. ....NY and New England... Southwesterly flow ahead of a mid-level wave rounding a low centered west of Hudson Bay that will shift northeast from NY/PA this evening will provide lift to persistent instability and +1.5 sigma PWs in a corridor from the lower Hudson Valley through northern New England. Expect localized heavy rain and propagating cells with localized flash flooding into the overnight. The Marginal Risk is maintained. ....Upper Ohio Valley through Central Appalachians... Subtropical moisture advection ahead of a cold front continues from the Mid-South up the eastern/Upper Ohio Valley with persistent instability overnight over the TN Valley/Upper OH Valley and central Appalachians. Localized heavy rain and generally lower FFG could lead to isolated instances of flash flooding overnight, so a Marginal Risk was expanded a bit east through the central Appalachians. ....Eastern North Carolina... Marginal Risk is maintained over eastern/northeastern NC into Southeast VA where 2" PW and 1000-2000 J/kg MLCAPE will persist this evening in light westerly mean layer flow allowing ongoing heavy activity to persist/advect east. ....Southern Florida... Diurnal convection is waning, so the Marginal Risk has been removed. Jackson 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 Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF INTERIOR NEW ENGLAND... ....20Z Update... In light of the forecast rainfall expected on Day 2/Sunday and the Moderate Risk, portions of New England were also upgraded to Moderate Risk for the rainfall expected to continue into Day 3/Monday and Monday night in coordination with all of the impacted forecast offices. A slow moving surface low tracking up the New England coast will continue to advect abundant moisture and rainfall from the Atlantic into New England, where the terrain will support upsloping, as well as embedded instability. Thus, the periods of heavy rain will continue for this area from Sunday night into as late as Monday night for some eastern areas of ME. Assuming ample rainfall across VT/NH from Sunday will further worsen antecedent conditions, the addition of yet another 2-4 inches of rain with locally higher amounts in these same areas again on Monday increased the confidence, despite much of the short-term guidance that Moderate Risk level flooding will continue, particularly across VT. The Slight Risk was also expanded to include much of central and western Maine. Conversely, along the Gulf Coast, the signal for heavy rain remains very low, especially when considering the very high FFGs in place along the central Gulf Coast. While it's possible some of these areas may see heavy rain from Day 2/Sunday, most of that convection is likely to stay north/inland from where the heaviest rains may fall on Monday along the immediate coast. In coordination with MOB/Mobile, AL forecast office, the Slight Risk was downgraded to a Marginal for this forecast update. Wegman ....Previous Discussion... ....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!-sflSJ29olWdx84j-TIoMpmHBjPWK3xZ3j8OP_wXNwSY= wiBmtN08hw-hVWVeJwKhgjoJtgpPSdFSqObnVVQl8dN-Zgs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-sflSJ29olWdx84j-TIoMpmHBjPWK3xZ3j8OP_wXNwSY= wiBmtN08hw-hVWVeJwKhgjoJtgpPSdFSqObnVVQl4qDn4bQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-sflSJ29olWdx84j-TIoMpmHBjPWK3xZ3j8OP_wXNwSY= wiBmtN08hw-hVWVeJwKhgjoJtgpPSdFSqObnVVQlTlWH76s$=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) .