Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 10 2024 15:49:40 FOUS30 KWBC 101549 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1149 AM EDT Sat Aug 10 2024 Day 1 Valid 16Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINAS INTO SOUTHEASTERN VIRGINIA... ....16Z Update Summary... Only minor changes were implemented across the risk areas forecast with no changes to the SLGT risk in the Carolinas. Full updates can be found in each respective sub-heading below.... Kleebauer ....Carolinas & Virginia... 16Z Update: Not seeing much of an agitated cumulus field across=20 Eastern NC as of recently, but all CAMs show a general initiation=20 of convection across the Carolinas after 18z with scattered to=20 widespread showers and storms anticipated through the first half of the evening. HREF blended mean QPF depicts a corridor of 1-2"=20 totals with some locally elevated signatures in the mean across=20 Southeast NC, just inland of Wilmington. HREF neighborhood=20 probabilities are relatively high within SC (20-50%) for at least=20 3" with a much more robust signal in-of Eastern NC with an=20 extensive areal coverage of 50-80% for the >3" threshold, as well=20 as some low-end 15-30% signals for >5" northwest of Morehead City.=20 This area is a prime target for flood concerns as the region=20 continues to deal with the aftermath of Debby's rainfall. Moderate=20 to major river flooding is ongoing for much of the area within the=20 Eastern Carolinas, so additional rainfall will be a great concern=20 to exacerbate the flood risks when convection arises. The SLGT risk forecast was maintained given the persistent signals and=20 antecedent conditions that could escalate flash flood concerns.=20 Kleebauer ...Previous Discussion.. A stalled out front over the western Carolinas and central Virginia will act as a focus for storms to develop this afternoon on the warm/moist southeastern side of the front. The atmosphere will be characterized with plentiful deep moisture with PWATs over 2 inches across much of the region. HRRR guidance shows several clusters of storms forming across southeast Virginia and the central Carolinas late this afternoon. The storms will then track southeastward towards the coast into an area where instability could exceed 4,000 J/kg along the coast. This level of instability and moisture will support potential for storms to produce rainfall rates to 3 inches per hour. Fortunately the guidance suggests the storms will keep moving into the evening, but those heavy rainfall rates would have the potential to cause flash flooding even if antecedent conditions weren't so favorable. Debby's remnants tracking across this region over the last few days has led to very saturated soils all across Virginia and the Carolinas. With numerous streams, creeks, and rivers already well into flood stage, additional rainfall from storms Saturday afternoon will struggle to drain, resulting in additional flooding and prolonging ongoing flooding. In coordination with all of the impacted offices, a Slight Risk area was introduced with this update. 00Z HREF guidance peaked at over 50% chance of exceeding 6 hour FFGs in the Slight Risk area, with an over 70% chance of exceeding 3 inches of rain across much of eastern North Carolina. These values added to the confidence to upgrade the ERO risk. Wegman ....Rockies into the Plains... 16Z Update: Across the Plains, an elongated complex of thunderstorms is=20 beginning to fade across the Northeast NM through the TX Panhandle=20 into OK, but a few flash flood warnings were issued due to training convection along a convergent zone that aligned with the I-40=20 corridor. Outflow generated from the complex on the eastern=20 periphery of the convection is migrating slowly eastward along the=20 Red River into the neighboring areas of OK and TX. Models are not=20 handling that area well with much of the CAMs void of precip in the region despite the radar saying otherwise. This was a trickier=20 portion of the setup given the lack of consistency with ongoing=20 convection, so decided to hedge a little further south with the=20 MRGL risk into TX as it follows the Red River to as far east as the DFW metro. Kept the risk area out of the metroplex given the=20 southern extent of the outflow lying north of the I-20 corridor,=20 but wanted to cover for an isolated threat of heavy rain as=20 boundary layer destabilization along the Red River this afternoon=20 could offer the threat of some overachieving cells that may not be=20 well forecast within the current CAMs suite.=20 Further north into OK, the expectation for another round of nocturnal convection has remained persistent within the current hi-res suite with general agreement within the global deterministic and ensemble blends with some heavier convective cores likely cause localized flood concerns across portions of Central and Northern OK after 03z. The prospects for significant flooding are low, but non-zero given the probabilistic output off the latest HREF with a >3" neighborhood probability around 20-40% bisecting portions of West-Central OK across areas north of I-40 and OKC. The progressive nature of the storms combined with very dry antecedent conditions will mitigate a greater threat of flash flooding leading to a maintenance of the MRGL risk with a lower chance of a targeted upgrade.=20 The Southwest remains active with the Monsoon with the elevated moist pattern nestled across the entire Desert Southwest up through the Inter Mountain West as far north as WY. The greatest threat will occur within the terrain with the highest flash flood risks likely within remnant burn scars, slot canyons, and any urbanized corridors where slow-moving convection lingers. Signals for 1 and 3 hour FFG exceedance are low and scattered among the outlined areas, so the threat remains isolated in nature. The MRGL was maintained given the sporadic depiction with the probabilities and mean QPF. Kleebauer ...Previous Discussion.. An approaching positively tilted shortwave trough moving into California will increase lift to its east across the Four Corners region. Storms are likely to be widely scattered across this area, which would support an isolated flash flooding instance or 2, currently covered with the large Marginal Risk. Additional moisture in southern Arizona and lift into the Peninsula Ranges of southern California may cause local flash flooding issues, but the guidance is inconclusive at best as to where any potential Slight risk area could be drawn. In coordination with the impacted offices, have opted to hold at a Marginal for now, but will reevaluate each day's chances for flooding based on the previous day's convective behavior. MCS development with potential repeating storms in the Plains will be a concern across Kansas through Oklahoma late Saturday night. Antecedent conditions are quite dry for soil moisture across this area, and the CAMs are not in great agreement on the location and behavior of the storms, despite ample moisture available. Here too a Slight Risk is close, and may need to be considered with future updates. Wegman Day 2 Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS AND FOR PORTIONS OF THE OZARKS AND CENTRAL PLAINS... ....Carolinas... The stalled out front over the Carolinas will continue to draw deep tropical moisture northward up the Southeast coast. The front will have made enough progress to likely keep the rainfall down into southeastern Virginia, but there will likely be more storms into South Carolina. Thus, the Slight risk is just a bit south of the Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from Debby, additional rainfall, even if more widely scattered, will have an outsized potential for impacts from additional flooding. 00Z HREF guidance through 00Z Monday suggests additional elevated potential for exceeding FFGs in NC, and plentiful moisture and instability with a stalled out front providing the forcing will likely help organize the storms, increasing their flooding potential. ....Central Plains/Ozarks... MCS development on the leading edge of a low level jet (LLJ) is expected from southeast Kansas through northwest Arkansas. Despite recent dry weather, training convection with PWATs increasing to around 2 inches will support storms capable of rates to 3 inches per hour at times. This very heavy rainfall combined with terrain interactions in the Ozarks will favor widely scattered instances of flash flooding. In coordination with the impacted offices, a Slight Risk upgrade was introduced with this update. Given CAMs frequent trouble with correctly simulating storms with MCSs, there are likely to be both temporal and intensity adjustments with the expected rainfall. Despite this potential, there was good enough agreement in the guidance to draw in the Slight risk, which for now remains low-end given the recent dry weather and dry soils. ....Southwest... More widespread convection is likely to impact the Four Corners region Sunday afternoon. The shortwave that will start the day in northern California will continue eastward across the Intermountain West. Thus, it appears the best potential for storms capable of flash flooding will be across central UT and western CO, generally north of the UT National Parks. Nonetheless with better moisture further south, AZ and northwestern NM will not be out of the woods, and isolated flash flooding will remain possible there. Storms are also likely along the Peninsular Ranges of southern California again Sunday afternoon, where a small Marginal risk was introduced. Wegman Day 3 Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS... Ongoing rainfall from Sunday night may continue into Monday morning across southern Missouri. With only a few hours of rain in the morning, the Marginal across Missouri was maintained. However, should rainfall from Day 2/Sunday overperform, then a future Slight may be needed. Once again another round of storms Monday afternoon across the Four Corners region is likely to result in isolated flash flooding. However, there is greater uncertainty with coverage by Monday in this region with lesser forcing than on Sunday. For the Carolinas, lingering moisture with the same front may cause yet another round of storms Monday afternoon into the evening. With greater potential that the bulk of the activity will be offshore by Monday afternoon, the previously inherited Marginal risk remains largely unchanged. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4cD4avvER0flxOwOqiaTolVahQdAVUHVbAQn8tt1wqL5= fJcu81MjTtnO43eVc_uVvdVB2KFW82S0If84rParUGa9co8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4cD4avvER0flxOwOqiaTolVahQdAVUHVbAQn8tt1wqL5= fJcu81MjTtnO43eVc_uVvdVB2KFW82S0If84rParwBQAY0k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4cD4avvER0flxOwOqiaTolVahQdAVUHVbAQn8tt1wqL5= fJcu81MjTtnO43eVc_uVvdVB2KFW82S0If84rParL4rmZ0I$=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. 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