Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 29 2024 00:59:08 FOUS30 KWBC 290058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Wed Aug 28 2024 Day 1 Valid 01Z Thu Aug 29 2024 - 12Z Thu Aug 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, NORTHERN PLAINS TO THE UPPER MISSISSIPPI VALLEY,=20 AND PARTS OF THE MID MISSISSIPPI/OHIO VALLEYS AND MID-ATLANTIC... ....01Z Update... Based on current radar/satellite trends and recent hi-res guidance - the biggest adjustment was to reduce the extent of the Marginal=20 Risk area previously extending from the mid Mississippi Valley to=20 the Mid Atlantic. In the northern Plains and the Southwest, made=20 only minor adjustments. Pereira ....16Z Update Summary... Minor adjustments were made to the MRGL risk area across the Ohio Valley/Midwest/Mid-Atlantic with a general expansion on the northern end into Southeast MI to account for a low-end urban flash flood risk over the Detroit Metro. Little to no deviation in the previous risk areas over the plains and Southwestern U.S were made this update. ....Southwest... 16Z Update: Changes were very minor to the MRGL risk across the Southwest with the main focus continuing to lie over the NM terrain, mainly the burn scar locations within the Rockies and Sacramento's. PWAT anomalies continue to meander around 1-1.5 deviations above normal with sufficient low-level buoyancy progged through afternoon heating. Some remnant cloud cover this morning will inhibit some of the convective potential early on, however the focal points within the terrain will still manage to muster enough destabilization to initiate a period of thunderstorms capable of localized flooding, especially within those sensitive burn scar areas. The most notable areas within the MRGL are covered by a Flash Flood Watch to account for the potential. Will make a small mention of a non-zero flash flood threat within the Lower Trans Pecos and Concho Valley in West TX. Antecedent soil moisture is down <10% in the area leading to more beneficial rainfall within cell development this afternoon. That said, the rates within any stronger storms will be capable of exceeding 2"/hr based off the latest HREF probability fields. This could allow for an isolated flash flood risk within any urbanized areas and/or low water crossings. Kleebauer ...Previous Discussion.. Guidance mid-level vorticity progs continue to support a more north-south outlook area compared to Day 1, though nonetheless still quite similar. Instability and moisture parameters (afternoon and evening CAPE and PW profiles) remain similar, and thus the anticipation of more isolated/localized instances of flash flooding than otherwise, particularly after 18Z coinciding with peak diurnal heating. Hurley ....Eastern Dakotas-Western Minnesota... 16Z Update: Little to no change was needed for the MRGL risk across the Dakotas into Northwest MN as guidance remains consistent within their presentations of a quick-moving complex of thunderstorms capable of locally heavy rainfall. Some discrepancy in exactly where the heaviest rain will occur with signals mixed between the SD/ND border up to the Canadian border. The environment is primed for impact somewhere within the confines of the Central and Eastern Dakotas, so didn't feel it was necessary to change the previous forecast with some split in the specific locations of the convection. The MRGL risk was maintained and falls within the low to middle end of the risk spectrum. Kleebauer ...Previous Discussion.. Strong, deep-layer warm and moisture advection ahead of the next vigorous mid-upper level trough (closed 500-300 mb low) and associated surface fronts will favor more organized/widespread elevated convection by Wed night ahead of the advancing low-level warm front. After 00Z, 850 mb southerly flow increases to 40-50 kts from west to east across the outlook area, which is around 4 standard deviations above normal per the SREF and GEFS. Not surprisingly, 850-700 mb moisture flux anomalies also climb to +3 to +4. Given the strength of the low-mid layer moisture transport and uptick in IVT, isolated QPF totals of 3-5" per some of the 00Z CAM runs should not come as a surprise. However, at least for now, reasons for keeping the ERO below the Slight threshold over this region Wed night include (a) the continued high spread in guidance QPFs (including CAMs), along with (b) progressive southeast motion of the convection growing upscale (MCS) per the forecast Corfidi Vectors, given the shear between the low-level flow and mean 850-300 mb wind. Hurley ....Eastern Portions of the Mid Mississippi Valley-Ohio Valley- Mid Atlantic... 16Z Update: The biggest change this period involved a minor expansion of the MRGL risk to the north to encompass the Detroit Metro. Current visible satellite across Southern MI depicts a small circulation migrating eastward off Lake Michigan with some CAMs now picking up on this feature leading to convection downstream later in the afternoon. Frontal boundary is just north of the line from Grand Rapids over into Detroit with a southern motion anticipated through the day time hours. Thin axis of instability located across Southeastern MI coupled with the approaching energy to the west will allow for a pocket of enhanced low-level convergence capable of a few thunderstorms initiating just to the west of Detroit, eventually moving overhead during the peak instability. The threat is low-end within the flash flood potential, but the proxy of Detroit with lower FFGs due to urbanization factors corresponding to rates that could breach 1"/hr allowed for the previous MRGL risk to be expanded northward to account for the threat. Further south, the front to the north will make progress into the Ohio Valley with an axis of thunderstorms developing along and out ahead of the front. A broad axis of instability is noted within the current theta-E analysis extending from Southern IL across into IN/OH with a secondary instability max located within the Central Mid-Atlantic. Front over the Northern Mid-Atlantic will migrate south with a convergent pattern developing in-of the front as it approaches Southern PA into Northern MD. Guidance isn't as keen on the potential, but there's enough of a signal remaining to maintain the previous MRGL risk given the focal point of the front, as well as the strong instability presence located over the above area. Adjustments to the northern fringe of the risk area were made in conjunction with the HREF blended mean QPF footprint and neighborhood probability for >2". Kleebauer ...Previous Discussion.. W-E frontal boundary will sink southward on Wednesday, then remain quasi-stationary Wed night as the mid-upper trough traverses the central-northern Rockies and High Plains. Deep-layer moisture and instability will be sufficient for areas of heavy rainfall along this boundary, while the easterly storm motions (parallel to the low-mid level fronts) would support the risk of cell training, especially when the Corfidi Vectors drop to 5 kt or less). WPC QPF and smart blends (NBM and HREF) have come up from the previous forecast, with the D1 period now fully within the high-res CAM window. Uptick in right-entrance region forcing (upper divergence... low-level FGEN) from the upper level jet streak over northern NY- New England will favor more widespread convection, especially when coinciding with peak diurnal heating. However, lack of robust low- level moist inflow/transport will likely not sustain/prolong strong updrafts and intense hourly/sub- hourly rainfall rates. As a result, have maintained the Marginal Risk from yesterday's Day 2 ERO, albeit with a few tweaks based on the latest guidance trends. Hurley Day 2 Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST, CENTRAL MID ATLANTIC, AND ALONG THE SOUTHEAST TEXAS COAST... ....20Z Update Summary... The SLGT risk across the Midwest was generally maintained with only minor adjustments necessary. Additional SLGT risks were added across the Piedmont of VA into the WV Panhandle, as well as along the Southeast TX coast. The MRGL risk was expanded further southwest into NM. ....Lower MO Valley and Corn Belt into the Upper MS Valley... 20Z Update: The previous forecast remains steadfast with little variance from run-to-run creating a general continuity in the MRGL and SLGT risk proposals across the Midwest and Central Plains. Increasing ascent from a vigorous upper low over the Northern Plains will generate a period of scattered to widespread convection with embedded heavy rainfall within the strongest cell cores. HREF neighborhood probabilities for >2" and >3" are both fairly aggressive (40-70%) within the outlined SLGT risk area with the core of the potential lying across the Corn Belt of IA into Southern MN. This area has been dry as of late, so some of the precip will be beneficial, but anomalous moisture and strong mid- level ascent could generate rainfall rates capable of flooding, especially within any urban footprint. Kleebauer ...Previous Discussion.. Aforementioned upper trough and associated surface cold front will continue eastward Thu-Thu night as the mid-upper low lifting north into western Ontario by 12Z Fri. Given the progression of synoptic features, the bulk of the deep-layer forcing and more anomalous low-level inflow/moisture flux will pivot across the Upper MS Valley and southern Canada. Compared to the southern portions of the Slight Risk area, generally speaking instability will not be as high over the northern portion (i.e. along the path of the triple point). However, owing largely to the SSW LLJ, especially as it becomes more aligned with the mean 850-300 mb flow, the eastward progression of the quasi-linear convective segments will slow during the period as per the weakening Corfidi Vectors (decreasing forward propagation). Therefore the risk for cell training will increase along and ahead of the front. Farther south (IA and eastern NE), the pre-frontal activity should be more forward-propagating, however much higher deep-layer instability south of the triple point, especially by late Thu-Thu night, will maintain the risk of more intense hourly rainfall rates (1.75-2.00+ inches) a little while longer (beyond 00Z Fri) per the 00Z HREF exceedance probabilities. ....Mid Atlantic Region... 20Z Update: A SLGT risk has been introduced across the VA Piedmont down into portions of Southeast VA and as far north as the WV/MD/PA line, including the WV Panhandle and Allegheny Front. Backdoor front will be co-located within the confines of the above area with a fairly well-depicted axis of low-level instability and anticipated surface convergence. A core of elevated theta-E will align with the front from northwest to southeast during the afternoon hours while a jet streak to the northeast catches the Central Mid Atlantic just within the right-entrance region of the jet leading to appreciable upper forcing during peak destabilization. Widespread convective activity will initiate across the Blue Ridge and adjacent Appalachian Front, drifting eastward under the guide of a very slow forecast storm motion as indicated from the latest forecast soundings off various CAMs and global guidance. MLCAPE between 2500-3500 J/kg will be common within the vicinity of the front, so ample surface based instability will be present to maximize potential of any thunderstorm development. Local amounts of 2-4" will be plausible based on the recent HREF blended QPF output and enhanced probabilistic signals based off the 12z CAMs. In coordination with the local WFOs in the region, a SLGT risk was added with emphasis on areas west of the Potomac in WV/VA. Kleebauer ....Louisiana and Middle-Upper Texas Coast... 20Z Update: The premise noted in the previous forecast still maintains merit with little change in the overall pattern. The most notable difference is the uptick in the QPF based within the ensemble mean(s), as well as a strong consensus within the CAMs that elevated the probabilities for totals exceeding 5" with some lower end potential for upwards of 8" along the TX coast. HREF EAS signal for 2" is also relatively robust with a solid 40-60% marker along the coastal plain extending from Eastern Matagorda up through Galveston into the Upper TX coast into the Lower Sabine. EAS for 3" is also within the 15-20% range, which considering the conservative nature of the parameter is a pretty notable output over the areas mentioned above. The current evolution maintains the heaviest rainfall and accompanying rates away from the urban center of Houston, but sits just south of the city. Considering the recent trends and the anomalous moisture source off the Gulf helping aid the pattern, a SLGT risk was added along the TX coast between Matagorda up through Port Arthur and the far Southwest corner of LA. Kleebauer ...Previous Discussion.. Underneath the broad upper ridge, models show a subtle mid-upper level shortwave lifting gradually northward toward the central- western LA Coast on Thu. Subtle upper-level forcing (DPVA/upper divergence) along with persistent (albeit not overly robust) low- level moisture transport from the GOMEX will make for a localized flash food risk, especially coinciding with peak diurnal heating Thu afternoon. PW values increasing to 2.3-2.4 inches along with MLCAPEs between 1000-2000 J/Kg (tall/skinny distribution) and WBZ heights climbing to aoa 15,000 feet will establish an optimal thermodynamic profile for heavy rainfall. Sub-hourly rainfall rates of 2-3" will be possible underneath the strongest cores. By the same token, 0-6km bulk shear generally 20kts or less will lead to more pulse-type convection and thus more short-lived intense rainfall rates. Still enough of a flash flood risk to support a Marginal Outlook area in the ERO, especially given the uptick in southeasterly low-level flow from the Gulf on Thursday, which would be of equal or greater magnitude compared to the 850-300 mb wind. Corfidi Vectors aligning in the opposite direction than the inflow would support increased upwind propagation and cell training. Hurley Day 3 Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA COASTAL PLAIN... ....Southeast Texas and Southwest Louisiana Coasts... A continuation from D2 will spill over into Friday as a weak disturbance from the Western Gulf will pivot northward into Eastern TX with a prevailing south to southeast flow funneling tropical moisture into the coastal plain of Southeast TX and Southwest LA. Still some discrepancy within the recent global deterministic on the depth of the protrusion of the elevated moisture field and accompanying heavy rain footprint across the region. Despite the differences, the environment is very much favorable with the recently ML and ensemble mean QPF output indicating another round of heavy precip impacting areas that will have seen a considerable amount of rain the prior 24 hrs. The trajectory of the mid-level disturbance off the Gulf will make the difference in exactly where the best heavy rain axis will occur as ascent provided under the guide of the complex will determine where the more appreciable totals would occur. As of now, a general 2-4" with locally higher is expected, leading to a chance for 2-day totals to exceed 8" in areas that see repeated action in both D2 and D3 time frames. A SLGT risk was added to very similar locations to D2 with a small extension eastward into more of Southwest LA as the southerly fetch off the Gulf is forecast to bisect that region when analyzing ensemble outputs. Kleebauer ....Southern Rockies-Plains into the Mid MS Valley, Northern OH Valley, and Lower Great Lakes... 20Z Update: There were very little changes necessary to the expansive MRGL extending from the Southwest through the Great Lakes. Convective pattern will be driven by relevant moisture anomalies aligned along the cold front as it slowly migrates to the south and east with high pressure pressing down from the north. Currently, there is no one favored area of convective organization, however the zone within the Mid- Mississippi Valley and down across West TX is the most likely candidate for any potential upgrade. Both areas have been dry recently, so the antecedent conditions work against the potential, so maintained continuity given the pattern remaining the same from the previous issuance. Kleebauer ...Previous Discussion.. Elongated Marginal Risk inherited from yesterday's Day 4 ERO continues to look good; made a minor adjustment by extending the southern periphery farther south across TX-AR-MO-KY given the latest guidance trends. QG forcing, kinematic, and forcing along the expansive frontal boundary will doesn't appear to be robust given the upper level pattern; however, pooling of above normal PWs ahead of the front along with the diurnal instability will support the potential for locally excessive rainfall and thus an isolated flash flood threat. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgbh_dC06c$ Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgb0A_wYck$ Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgblHgtigE$=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) .