Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 19 2023 20:28:27 FOUS30 KWBC 192028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Sat Aug 19 2023 Day 1 Valid 16Z Sat Aug 19 2023 - 12Z Sun Aug 20 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....16z Update... Only minor adjustments made to the previous ERO. Current sat/radar composite shows a core of heavier rainfall across the CA/AZ border as the northern extent of the moisture flux ahead of Hilary has created a more pronounced region of heavier precip. Have moved the eastern extent of the Moderate about 15-20 miles to the east to reflect the latest trends, otherwise the remainder of the outlined area remains untouched. Extended the slight on the northwest fringe across CA to adjust for guidance trends on an uptick in precip totals across area mountains. Also expanded the slight on the northeast side to account for expected rainfall across SW UT over areas that have seen flooding rains the past few days. The rest of the forecast remains on track. Kleebauer ....Previous Forecast...=20 =20 Portions of the West...=20=20 An upper level low stuck near the central CA coast, the approach of Hilary from the south, a deep southerly flow across the Great Basin keeps precipitable water anomalies up to 4 sigmas above the mean to mid to late August from the Desert Southwest northward into Montana. For areas from Las Vegas southward, the 1.75-2.25"+ precipitable water values will be approaching all-time records, so there will be moisture to spare. While there is concern that cloudiness from Hilary, already moving into southeast AZ, will present some problems in building up instability across portions of the Southwest and Great Basin, the appearance of a low-level jet out of the Gulf of CA right at the end of the period early Sunday morning right near the apex of the monsoonal surge is of significant concern and could lead to convection during the late night of Saturday into early Sunday morning at an atypical time.=20 Some of the guidance shows local amounts of 5"+. The most likely spot would in and near the eastern slopes of the Peninsular Ranges within Southern CA and where the Moderate Risk area saw some expansion into the neighboring deserts due to input from the 00z HREF and the local forecast offices.=20 =20=20 =20=20 Florida...=20=20 An easterly wave is expected to move through the Keys and southern FL by Sunday morning, though it looks less amplified in the guidance when compared to previous days. Inflow at 850 hPa is expected to increase to 15-20 kts, which should increase the effective bulk shear enough to lead to greater convective organization than seen on Friday. Probably due to the progression of the feature, amounts weren't overly high in the guidance.=20 However, should any convective training or mergers occur, 3" an hour totals and local amounts of 5" cannot be ruled out, which would be most problematic in urban areas. Since the guidance QPF was generally modest, left the risk level as Marginal. As the wave appears less amplified, shaved some of the northern portion of the Marginal Risk area away. South Carolina/Georgia coasts... The guidance signal has become a bit weaker with the heavy rain signal since this time yesterday, likely due to the low-level inflow into the region becoming more divergent and weaker, so removed the Marginal Risk from the outlook.=20 =20 Roth=20=20 Day 2 Valid 12Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023 ....A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA... ....2030Z Update... Made a slight expansion northward of the High Risk area into portions of Nevada...in part due to an uptick in amounts and expansion of the areal coverage both by in the WPC deterministic QPF and model guidance. Based on the latest WPC QPF...40 km neighborhood probabilities had a swath along the axis of the High Risk where Annual Recurrence Intervals in excess of 70 percent at the 100 year ARI. Admittedly...there could be some blossoming of areal coverage resulting for being a 40 km neighborhood but the magnitude of signal at that interval is impressive. Latest QPF placement was good for a typical land falling tropical cyclone followed by expansion and growing areal coverage during the day given the moisture stream interacting with the terrain and the increasing dynamics. Moisture streaming northward should lead to increasing coverage of convection capable of producing heavy rainfall amounts/rates which spreads into parts of the Northwest U.S.. Remainder of the outlook was in good shape and largely unchanged. Bann ....Previous Excessive Rainfall Discussion... =20 Portions of the West...=20 The guidance is unanimous in merging Hilary with an upper low stuck near central CA this period, and has picked up the pace on its acceleration into/across the area. Normally for a tropical cyclone this would be a problem as convective lows would circle the periphery parallel to 1000-500 hPa thickness lines, which would otherwise=20 turn it more north or north-northeast, but since Hilary should be strongly shearing while moving over cold waters and interacting with the Peninsular Ranges of the Baja California Peninsula and southern CA, it should be transitioning to a post-tropical or remnant low in the process and have diminishing convection in its vicinity; the guidance probably isn't displaying its typical model bias (this time). A large area of precipitable water values of 1.75-2.25" will approach if not exceed all-time records across portions of the Southwest, so there will be moisture to spare. In the Southwest in particular, flow at 750 hPa is expected to reach or exceed 65 kts, so heavy upslope rains on the atypical sides of the southern Sierra Nevada and Peninsula Ranges of CA are anticipated. If the flow is more southerly than expected due to a slightly more westerly track of Hilary, there's a chance that both sides of the Peninsular Ranges of southern CA could get heavy rainfall. The 19/12z NAEFS is indicating IVT values 19.1 sigmas above the mean; it should be noted that it is using a dataset that does not include the rash of tropical cyclones that impacted the Southwest in the 1970's, so this value is likely a bit too high.=20 Even assuming a non-standard distribution and standard deviations half as large, this is extreme. There is a very real potential for 3" amounts in an hour in this environment should sufficient instability be present. Even if instability was completely eroded, 0.5" an hour totals would be possible; heavy rain appears inevitable. The 00z Canadian Regional shows local amounts of towards 10", which would be exceeding rare for the region from a tropical cyclone, potentially unique for Nevada. The 100 year ARI is forecast to be exceeded. Some locations within this arid region are slated to get 1-2 years worth of rain in one day. If a 7"+ maximum materialized over Mount Charleston Sunday into early Monday, it would challenge Nevada's 24 hour rainfall record, set in 2004.=20 The heavy rainfall combined with high winds expected at elevation could lead to mudslides and landslides, which would be exacerbated where trees uproot within saturating soils. Debris flows and rock slides are a given considering the volume of rainfall expected.=20 The overall combination of effects could block and undermine roads, particularly sensitive areas such as sections of U.S. 50 in NV. Towns could get cut off. Given the overall uniqueness of this event and expected impacts, the High Risk for areas of southern CA remains justified. The main change was the joining of the two separate High Risk areas and some slight westward shift of the risk areas in CA, NV, UT, and AZ and some northward stretching of the threat areas to account for the slightly accelerated guidance.=20 =20 =20 Florida...=20 An easterly wave is expected to move out of FL into the Gulf of Mexico by Monday morning. Even though precipitable water values are coming down in its wake, they're still above 1.5" across western and southern FL. Inflow at 850 hPa is expected to increase to 25 kts, which should increase the effective bulk shear further and potentially lead to training convection off the Bahamas across southern FL and otherwise greater convective organization than usual. Probably due to the progression of the feature and briskness of the low-level flow, amounts weren't overly high in the guidance. However, should any convective training or mergers occur, 3" an hour totals cannot be ruled out, which would be most problematic in urban areas. Issues would likely be isolated in this scenario. Since the guidance QPF was so modest, left the risk level as Marginal.=20=20 =20 Roth=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!4_Ejjm81qfhidfilsdvsJkh5tryVZE97dLzhyRD9jeWq= cx-nyy20SEkeCP90fJqCX1f7MlgGHKTA101_r2Nhk6U1IAI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4_Ejjm81qfhidfilsdvsJkh5tryVZE97dLzhyRD9jeWq= cx-nyy20SEkeCP90fJqCX1f7MlgGHKTA101_r2Nhu5ZN7bs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4_Ejjm81qfhidfilsdvsJkh5tryVZE97dLzhyRD9jeWq= cx-nyy20SEkeCP90fJqCX1f7MlgGHKTA101_r2NhQu3FO8U$=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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