Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 04 2024 08:23:20 FOUS30 KWBC 040823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 322 AM EST Sun Feb 04 2024 Day 1 Valid 12Z Sun Feb 04 2024 - 12Z Mon Feb 05 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Central to Southern California... The deepening low off the Central to Northern California coast will continue to strengthen the Atmospheric River event into large portions of Central to Southern California day 1. The strongest IVT...600 to 800+ kg/m/s will be impacting the Central California coast range, south into the Transverse Range and inland into northern portions of the San Joaquin Valley and Sacramento Valley during the first half of day 1, with decreasing IVT during the second half. Models remain consistent in showing widespread heavy totals from the northern portions of the Peninsular Range through the Transverse Range and northward along the Central California Coast Range. A high risk was maintained through the Transverse Range where the best upslope component will occur. Changes to the previous outlook were to extend the high risk area farther to the east into the eastern portions of the Transverse Range where the upslope flow lingers longer than areas farther the west. The high risk was also expanded south to cover all of Los Angeles county to the south of the Transverse Range as per coordination with WFO LOX. Life threatening flash and urban flash flooding possible in the high risk area. Areal average day 1 precip totals of 3-5" likely from the Central CA Coast Range south into the Transverse Range, with isolated 6-10" totals in the best upslope areas of the Transverse and Central California Coast Range. The high risk area fit well where the HREF neighborhood probabilities are high for 3" and 5"+ totals day 1. HREF neighborhood probabilities are also showing a fairly large 40-60% probabilities for 8"+ totals across central portions of the Transverse Range.=20 ....Southeast, eastern Gulf Coast and Florida... The broad upper vortex over the Lower Mississippi Valley will be sinking slowly southeastward into the northeast Gulf of Mexico day 1. The previous marginal risk area on the north and northeast side of this vortex was narrowed by approximately 50-75 miles to better match where the latest HREF hourly probabilities are high for .50"+/hr amounts. This will be during the period of strengthening southeasterly low level flow ahead of compact surface wave rotating northeastward off the northeast Gulf and into the Florida Panhandle. Heaviest totals ahead of this low likely along the GA/Lower SC coast where the low level onshore east southeasterly flow persists the longest day 1 and corresponds to the area of most defined boundary layer convergence.=20 On the south side of the northeast Gulf vortex, a more defined area of instability and associated organized convection may push eastward into Southwest Florida early day 1, reaching the east central to southeast coast of FL by late morning/early afternoon.=20 This convection will be very fast moving but still may produce totals of .50-1"+ in an hour, resulting in potential for isolated urban issues both on the Southwest coast and Southeast coast. A marginal risk was re-introduced for the urban areas on the southeast coast and maintained for the urban areas of Southwest Florida. Oravec Day 2 Valid 12Z Mon Feb 05 2024 - 12Z Tue Feb 06 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Southern California... While the AR event from day 1 will be weaker during day 2, the inflow of anomalous PW values..2 to 2.5+ standard deviations above the mean...will become focused more into southern California, northeastward into southern Nevada as the front pushing through southern California slows. This will support widespread heavy precip totals for day 2 across southern California from the Peninsula Range west to the coast and westward through much of the Transverse Range. There will be overlap of the day 1 heavy precip with the day 2 heavy precip across eastern portions of the Transverse Range, south into the Los Angeles metro area. A high risk was added for this overlap area where additional 2-4"+ totals are possible, bringing the day 1 and 2 totals to the 5-10" range.=20 The slight risk area was also extended farther west from the previous issuance into the western portions of the Transverse Range. While the strongest inflow is expected farther to the southeast of this, additional heavy rains on top of the heavy totals from day 1 will continue a flash flood threat. Inland into the deserts of southeast California, toward the Nevada border, model consensus is for an axis of moderate to heavy precip totals in the above mentioned anomalous PW axis the persists across this area day 2. A slight risk was extended northeastward across the southeast California desert regions into a portion of far southern Nevada. 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. Oravec Day 3 Valid 12Z Tue Feb 06 2024 - 12Z Wed Feb 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL SOUTHERN CALIFORNIA, SOUTHWEST TO WEST CENTRAL ARIZONA, FAR SOUTHERN NEVADA AND FAR SOUTHEAST CALIFORNIA... The axis of anomalous PW values that are expected to persist across Southern California day 2, will begin to push eastward during the second half of day 3 as the longwave trof off the California coast begins to push inland. Additional moderate to heavy rainfall totals are possible during the first half of day 3 across eastern portions of the Transverse Range, south through the Peninsular Range and west to the southern California coast.=20 Similar to the day 1-2 period, there may be overlap of heavy precip areas in the day 2-3 period over portions of southern California. At the moment, the threat level was kept at slight given the overall downward trend in qpf amounts during day 3, but an upgrade to moderate for the eastern Transverse, into the Peninsular Range and west to the coast is possible on subsequent issuances.=20 With the inland push of the longwave trof through Southern California day 3, the anomalous PW axis and 850-700 mb moisture flux axis will be pressing inland into the Southwest, but also lingering across portions of southeast California into southern Nevada. The anomalous PW and 850-700 mb moisture flux will support heavy precip totals of 1-2"+ in the upslope of the western to central Arizona, and potential for overlap of locally heavy day 2 rains across southeast California and far southern Nevada with locally heavy day 3 rains. The previous slight risk area was expanded northwestward into far southern Nevada/southeast California to account for the potential day 2/day 3 precip overlap. The slight risk was also expanded into southwest Arizona to better match the CSU first guess field. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5e0p3HadHS1xFSmJX3DIRZ--GidmoowN-XTyhEe9N8gC= kahf9Z8-TXgcMN2-e6E0ZAElki5URgND6GFLlgBrJzkRP78$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5e0p3HadHS1xFSmJX3DIRZ--GidmoowN-XTyhEe9N8gC= kahf9Z8-TXgcMN2-e6E0ZAElki5URgND6GFLlgBr8kdSK4g$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5e0p3HadHS1xFSmJX3DIRZ--GidmoowN-XTyhEe9N8gC= kahf9Z8-TXgcMN2-e6E0ZAElki5URgND6GFLlgBr9xq5Q7I$=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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