Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 18 2023 20:35:08 FOUS30 KWBC 182035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 434 PM EDT Fri Aug 18 2023 Day 1 Valid 16Z Fri Aug 18 2023 - 12Z Sat Aug 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST ARIZONA INTO SOUTHWEST UTAH... ....16z Update... Minor changes with regards to the previous D1 ERO. Have expanded the SLGT and MGL on the western extent of the previous issuance across the southwest US given radar trends and latest hi-res guidance targeting the southern tip of NV later this afternoon and evening. Steady progression of convection across New England will continue to pivot to the north and east through the period with decreasing convective coverage expected across eastern NY state into the western half of VT, CT, and MA. Some light precipitation is still possible in the aforementioned areas, but a decay in overall coverage and intensity allows for a trimming of the western extent of the MGL. Remainder of the forecast is on track. Kleebauer ....Previous Discussion... Northeast...=20 A Marginal Risk remains across the region. Heavy rain may be ongoing at 12z Friday (a continuation of the previous day's rainfall)...with another round possible during the day Friday as destabilization occurs ahead of the approaching trough/closed low. The timing of the trough and how it interacts with the moisture/instability streaming up the coast will play a role in the magnitude of the eventual flash flood risk. As the guidance amounts still appear modest, on the order of 1-2", some portions of the area have been wet this past week. Can't rule out an upgrade to Slight Risk at some point should the signal in the guidance increase.=20 =20=20 =20 Portions of the West...=20 A Slight risk remains across northwest AZ into southwest UT, with some expansion into far southern NV and western AZ. Seeing a notable uptick in mid/upper level forcing by this time as the closed low off the CA coast shifts eastward and the upper jet west of the area intensifies. The precipitable water values in the monsoon surge up the Gulf of California are increasing this period with values perhaps breaching 2" by early Saturday morning and ~1000 J/KG of ML CAPE is expected during Friday afternoon and early evening. Enough moisture and instability around to support heavy rainfall rates, up to 2" where cells merge, backbuild, or manage to train. This would be a problem over the susceptible areas of southern NV, western AZ, and southwest UT.=20=20 =20=20 =20 Florida...=20 A Marginal risk over portions of central/southern FL. The moisture plume from the polar front gets entangled with an incoming easterly wave that is moving through the Bahamas. Flow should be becoming more easterly, though remaining light, so there is concern about slow moving cells, and with the incoming easterly wave, increasing amounts of effective bulk shear which could lead to some loose organization. The thinking is that outflow boundaries will shift activity enough to prevent overwhelming amounts, but hourly rain totals to 2.5" with local amounts to 5" cannot be ruled out. Since Central FL in particular has been so dry lately, kept the risk level as Marginal.=20 =20=20 Roth=20 Day 2 Valid 12Z Sat Aug 19 2023 - 12Z Sun Aug 20 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHWEST... ....2030 UTC Update... Only minor changes needed in the position of the Marginal and Slight Risk areas in the US. Overall...the potential for heavy rain is confined to a well-defined channel between a trough located off-shore and mid/upper level high over the Plains meaning any the along-track differences are more crucial to timing and arrival of heavy rain more so than east/west placement. Numerical guidance still focused on the potential for some heavy downpours over Florida and the South Carolina Plain...and the previous outlook had that well in hand. Bann ....Previous Excessive Rainfall Discussion... =20 Portions of the West...=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 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. Some of the guidance shows local amounts of 5"+. The most likely spot would be the eastern slopes of the Peninsular Ranges within Southern CA and a Moderate Risk was introduced for this area. =20 =20 Florida...=20 An easterly wave is expected to move through the Keys and southern FL into the central Peninsula by Sunday morning. 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.=20=20 =20 South Carolina Coastal Plain... A boundary near the coast along with ample moisture and instability is expected to lead to heavy rainfall in and near the SC coast. Hourly amounts to 2" with local amounts to 4" are anticipated. While some areas have received ample rainfall in this region this past week, the expected rainfall would be an issue most within urban areas, and would likely be isolated in nature, hence the new Marginal Risk area. Roth=20 Day 3 Valid 12Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023 ....A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR THE PENINSULAR RANGES OF SOUTHERN CALIFORNIA... ....2030 UTC Update... Made a slight westward expansion of the lower-end threshold mainly across southern California in response to a couple of the global models which started to spread rainfall that way. However...the track forecast from NHC has been consistent and the WPC outlook areas are right where they should be...conceptually. The biggest uncertainty becomes how much of an effect that terrain has on the storm...whether Hilary interacts with land before it ever arrives in the U.S. or exactly what happens to the storm as it crosses the complex terrain in the U.S.. Given an uptick in guidance...both model and WPC QPF...a second QPF maxima appeared with a magnitude as large as amounts which prompted a High Risk farther south. As a result...introduced a second but targeted High Risk there. The WPC storm total QPF rivals 200 percent of annual average rainfall in some places. Farther north...expanded the Slight Risk over portions of Idaho where the magnitude of anomalies has shown consistent model run to run increases. Few other changes were needed. Bann ....Previous Discussion... Portions of the West... The guidance is unanimous in merging Hilary with an upper low stuck near central CA. 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 turn it more north or north-northeast, but since Hilary should be strongly shearing while moving over cold waters and transitioning to a post-tropical or remnant low in the process, it should have diminishing convection in its vicinity, so this isn't the typical model bias (this time). A large area of precipitable water values of 1.75-2.5" will approach if not exceed all-time time records, 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 Range get heavy rainfall. The NAEFS is indicating IVT values 14.8 sigmas above the mean. Even assuming a non-standard distribution, this is extreme. There is a very real potential for 3" amounts in an hour in this environment. Some of the guidance shows local amounts of 7"+, 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. If a 7"+ maximum materialized over Mount Charleston Sunday into early Monday, it would challenge Nevada's 24 hour rainfall record, set in 2004. Given the overall uniqueness of this event, chose to upgrade to a High Risk for areas near and east of the Peninsular Ranges of Southern CA. Florida... An easterly wave is expected to move through the Keys and southern FL into the central Peninsula by Sunday morning. 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 cannot be ruled out, which would be most problematic in urban areas. Since the guidance QPF was so modest, left the risk level as Marginal.=20 Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4TfX26jlSwAqPkFF8jCGGzLDARR_E8rkwvM4VBvS3LJc= myBM83bGY7HAsk8XJoxvekIrdsW84LR7C8gOFSl1RUF_1gw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4TfX26jlSwAqPkFF8jCGGzLDARR_E8rkwvM4VBvS3LJc= myBM83bGY7HAsk8XJoxvekIrdsW84LR7C8gOFSl1j02HemU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4TfX26jlSwAqPkFF8jCGGzLDARR_E8rkwvM4VBvS3LJc= myBM83bGY7HAsk8XJoxvekIrdsW84LR7C8gOFSl1T_swjzM$=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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