Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 03 2024 15:50:28 FOUS30 KWBC 031550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1049 AM EST Sat Feb 03 2024 Day 1 Valid 16Z Sat Feb 03 2024 - 12Z Sun Feb 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL CALIFORNIA COAST RANGES AND ACROSS SOUTHERN LOUISIANA AND FAR SOUTHERN MISSISSIPPI... ....Central Gulf Coast into the Lower Mississippi Valley... A surface low that recently emerged into the western Gulf of Mexico and its related inverted trough to its north and warm front to its east are expected to be the main drivers for heavy rainfall along the Gulf Coast. Well inland, a TROWAL is expected to enhance moderate to heavy rainfall across portions of OK/KS within a moisture regime near the 99th percentile for this time of the year, per standardized anomalies. While the KS/OK convection should pivot slightly in place around a pair of mid- to upper-level lows, convection should remain fairly progressive across the Gulf Coast as the low moves to the east, continuing to be a detriment to very heavy totals. This continues to be illustrated in the latest HREF hourly probabilities for .50" and 1"/hr amounts. Left the Slight Risk area across southeast LA and southern MS which stand the best chance for raising MU CAPE to 500-1000 J/kg and by extension heavier rainfall. We continue to believe the slight risk is a low end slight for southeast LA and southern MS, outside of any urban issues. Changes made to the previous outlook area were to shrink the Slight Risk area with some dropping of the western sides of the risk areas due to the eastward progression of moderate to heavy rainfall. ....Central California... The beginning of the next atmospheric river event for central to southern California will arrive Saturday night into early Sunday with IVT values of 600+ kg/m/s impacting coastal central California. HREF neighborhood probabilities for 3"+ amounts ending at 1200 UTC Sun are high in the max onshore flow/IVT area, due to both 50-75 kt 850 hPa inflow into the region and MU CAPE occasionally approaching 500 J/kg, which could allow hourly rain totals to approach 1" early Sunday morning/right at the end of the forecast period. Some expansion was made to the east into the Sierra Nevada to account for the onset of heavy rainfall mostly due to low- to mid-level frontogenesis moving in from the west and some snow melt and a little bit of contraction made along the western sections of the Transverse Ranges, using 1"+ in 24 hours as a guide to keep the threat area most consistent with continuity. For the entire next AR event, the risk is much greater than depicted Saturday into early Sunday morning across this region, but the timing is such that only a slight risk is warranted through 1200 UTC Sunday, with much heavier precip and excessive rainfall risks thereafter. Roth/Oravec Day 2 Valid 12Z Sun Feb 04 2024 - 12Z Mon Feb 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR COASTAL SECTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... ....Central to Southern California... The latest models are in good agreement on the evolution of the deepening low off the central California coast early Sunday as this low pushes northward day 2 along the central to northern California coast. The strong IVT values that begin to impact the central California coast late day 1, 600-800 kg/m/s, will persist into central California coast range, pushing inland into the northern portions of the San Joaquin Valley and Sacramento Valley and southward into the Transverse Range. There is agreement that the strongest onshore flow/IVT values will be during the first half of the day 2 time period, with weakening during the second half. HREF neighborhood probabilities for the first 12 hours of day 2 are high for 2 and 3"+ totals (70-90%+) from the Transverse Range into the central California Coast Ranges. Probabilities drop off somewhat for 5"+ amounts in the first 12 hours of day 2, 40-80% in the Transverse Range and 30-50% in the Central California Coast Range. 3 to 5" areal average amounts likely in both the Transverse and Central California Coast Ranges, with maximum totals of 8-10" possible in areas of most persistent training and upslope flow. Not much in the way of changes made to the previous outlook areas, with a large moderate risk area depicted across the Transverse and Central California Coast Ranges.=20 ....Southeast, eastern Gulf Coast and Florida... The broad closed upper low over the Southern Plains at the end of the day 1 period will be pushing east southeastward into the north central to northeastern Gulf day 2. Strong southerly to southeasterly low level flow will press across the eastern Gulf coast/FL and the Southeast. This will support a broad region of moderate to heavy precip totals across the Southeast, eastern Gulf Coast and Florida. Heaviest totals likely where this precip area pivots and low level east southeast to easterly flow persists over the same region the longest. Model consensus is for this to be from the Lower coast of South Carolina, across south Georgia , North Florida into southern Alabama and far southern Mississippi.=20 Observed precip amounts have been below average over the past few weeks across this region, warranting just a marginal risk at the moment. The best area of instability and potential for organized convection is across Southwest to South Florida late Sunday afternoon/evening. Any associated precip moving off the eastern Gulf and across Southwest to South Florida likely will be very progressive, which will keep precip totals from being very heavy.=20 This potential convection, however, will be moving over the more urbanized regions of Southeast Florida, which will keep some threat of urban runoff issues, although precip totals are expected to be less than areas farther to the north. A marginal risk was reintroduced for the urban areas of southeast coastal Florida to account for this threat. Oravec Day 3 Valid 12Z Mon Feb 05 2024 - 12Z Tue Feb 06 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR COASTAL SECTIONS OF SOUTHERN CALIFORNIA... The AR event will be weakening late day 2 into day 3, with the best onshore flow and IVT values pushing southeastward into portions of the Transverse and Peninsula Ranges, westward to the Southern California coast. While the IVT values do weaken day 3, the eastward push of the trailing frontal boundary from the deepening low moving along the central to northern California coasts day 2 will be relatively slow moving across Southern California day 3. This will support the likelihood of additional heavy totals through the western portions of the Transverse Range day 3, albeit much lighter than day 2 given the weaker onshore flow/IVT, and heavy totals moving into the eastern portions of the Transverse Range, south into the Peninsular Range and westward to the Southern California coast. Additional 1"+ totals possible across the western Transverse Range, with areal average totals of 1-3" and maximum 5" across the eastern Transverse, through the Peninsula Ranges and west to the southern California coast.=20=20 Changes to the previous outlook for this period were to trim the moderate risk area across the western portions of the Transverse Range to better match the current timing and heaviest day 3 totals. No changes made to the marginal risk area extending northward through the San Joaquin and Sacramento Valleys. Some isolated heavy precip totals possible here where slow moving or training of cells may occur in the persistent southwesterly mid to upper flow. Moderate to locally heavy precip totals also possible farther inland into portions of southeastern California, southern Nevada, far northwest Arizona and far southwest Utah where an axis of anomalous PW values, 3+ standard deviations above the mean will persist across. .50-1"+ totals possible here with no changes to the previous marginal risk across this area.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-uM0ztCI7aUDgS405SE3aPbdlKrCt6fFS_bEv6LARoIk= DJeOXAN4O11Xg7WxtSMIvCXLfQ68p1f0kXJLGm0dmW67fyE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-uM0ztCI7aUDgS405SE3aPbdlKrCt6fFS_bEv6LARoIk= DJeOXAN4O11Xg7WxtSMIvCXLfQ68p1f0kXJLGm0diWBpWS0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-uM0ztCI7aUDgS405SE3aPbdlKrCt6fFS_bEv6LARoIk= DJeOXAN4O11Xg7WxtSMIvCXLfQ68p1f0kXJLGm0d4wGhlzA$=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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