Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 19 2023 20:27:08 FOUS30 KWBC 192027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Sun Mar 19 2023 Day 1 Valid 16Z Sun Mar 19 2023 - 12Z Mon Mar 20 2023 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. However, an area of interest today turns once again to California... A widespread area of rain is ongoing across nearly all of California this morning. A cold front associated with a low tracking north up the coast is spreading the rain across the state. So far anyway, the storm is all bark and no bite. While the radar shows rain ongoing across the whole state, those radar returns are resulting in very light rainfall totals. As of the time of this writing, the highest 24-hour rainfall total thus far is less than 0.2 inches anywhere in the state. Meanwhile, IR satellite shows an area of much drier air approaching the northern half of the state. The drier air will help cut off what light rain is ongoing in that area, further reducing any flash flooding risk. Just like the most recent atmospheric river (A.R.) event, the drier air associated with the leading edge of a 130 kt jet will rapidly advect inland, pushing the area of rainfall ahead of it. This will ensure a progressive evolution to the rainfall, preventing it from hanging up in any one area of CA before pushing east. Snow levels remain lower with this event than the last A.R., which means the colder temperatures will allow a greater percentage of the expected QPF to fall as snow in the mountains. The primary factor favorable for flash flooding are the antecedent conditions. Soils remain fully saturated over nearly the entire state from previous A.R. events. However, once again the fast duration and very light rainfall rates over most of the state should make any flash flooding isolated, and most likely to occur near burn scars, urbanized areas, and where any locally heavy rain occurs over other particularly flood-sensitive areas.=20 Wegman Day 2 Valid 12Z Mon Mar 20 2023 - 12Z Tue Mar 21 2023 ....2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS COASTAL AREAS OF SOUTHERN CALIFORNIA AND CENTRAL ARIZONA... ....2030Z Update... ....Southern California... No significant changes were made with this afternoon's update. The forecast remains on track that another atmospheric river (A.R.) will track across CA and into AZ Tuesday through Tuesday night. There are some notable similarities with this upcoming event Tuesday and the last one which happened last Tuesday. Both events appear to be progressive (meaning the A.R. driving the rainfall will keep moving and not get stuck in any one place for very long), and are forecast to produce similar amounts of rainfall across the L.A. and San Diego areas, the adjacent mountains, and along the Mogollon Rim of central AZ. Much of the flooding that occurred last Tuesday was river flooding, with flash flooding somewhat limited. It's worth noting that the snowpack in the Transverse Ranges is very limited to small areas of the highest elevations, so snowmelt is not expected to be a factor. Further to that, snow levels are likely to be even lower with this upcoming event than the last one. In Southern California, the warmest air associated with the A.R. is only over the area for 6-12 hours at most before it shifts east. Prior to the A.R. passage, snow levels are around 6,000 ft, rise to 8,000 ft with the warmest air, then crash to 4,000 ft behind the cold front. This will greatly limit how much runoff in the mountains will drain into the lower elevations with this A.R. The rain (and mountain snow) will continue lightly behind the cold front with continued onshore flow, so the 24 hour forecasted rain is likely overestimating the amount of rainfall that has potential to be impactful in the form of flash flooding. ....Central Arizona... Unlike areas further west across Southern California, expect the higher snow levels (around 9,000 ft) associated with the A.R. to persist for much longer, which will allow the limited snowmelt from the higher elevations along the Mogollon Rim to contribute more to any flash flooding in the area. Rainfall amounts are likely to be similar, with up to 3 inches of rain expected for the higher elevations. Far less rain is expected further south and west, including the Phoenix metro, so the greatest chances of flooding will be north and east of Phoenix, where topography will exacerbate any flooding impacts due to the fast flowing water. This area appears to have the greater likelihood of needing future upgrades as compared with CA, though again the atypical consistency with total expected rainfall, with amounts not changing too much run-to-run, still makes this scenario unlikely. Wegman ....Previous Discussion... The latest model guidance is in good agreement with a strong closed low diving southeast day 2 across the Northeast Pacific and inland into central California and the Great Basin on day 3.=20 Strong 850 to 700 mb moisture flux expected on the south side of this closed low, impacting areas from the central California coast range , southeast through the Transverse and Peninsular Ranges and into the southern Sierra during the day on Tuesday, spreading inland into the Southwest Tuesday night into early Wednesday. The National Water Model streamflow anomaly map continues to show high flows to most of the streams across the central California coast range, southeast into coastal southern California, including the Transverse and Peninsular Ranges, which coincides with anomalous soil moisture values as per NASA SPoRT maps. There is fairly good model agreement on heavy precipitation potential across the slight risk areas of California, with widespread 1.25 - 2.5"+ values. Given this the previous marginal risk area across these regions was upgraded to slight. No changes made to the marginal risk area over the southern to central Valley/foothills of the Sierra with the heaviest precip here expected to fall as snow in the Sierra, with no significant contribution to runoff from snow melt expected.=20 Inland across central Arizona, the previous marginal risk areas was also upgraded to slight. 850-700 mb moisture flux values expected to become very anomalous late Tuesday into early Wednesday, 4 to 5 standard deviations above the mean, over the Southwest as the strong upper trof moves inland from Central California into the Great Basin. Snow level expected to be high with much of the precip into the Mogollon Rim area falling as rain. Stream flow and soil moisture anomalies are not as great across central AZ as points farther west across California, with values at or slightly above normal. However, given the strong south southwesterly upslope flow into the Mogollon Rim region and resultant heavy precip potential, model consensus 1 to 2"+ areal average totals, an upgrade to slight risk was warranted. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-v_bTqzKJNVDYBPwDpSLakTlHS8CIK3VehIPvlTmxPWf= 816b_aOJxQrsmAQhjRKMXN9bMaunsUR8cJFZNy0MeLnQrVA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-v_bTqzKJNVDYBPwDpSLakTlHS8CIK3VehIPvlTmxPWf= 816b_aOJxQrsmAQhjRKMXN9bMaunsUR8cJFZNy0MnWTom00$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-v_bTqzKJNVDYBPwDpSLakTlHS8CIK3VehIPvlTmxPWf= 816b_aOJxQrsmAQhjRKMXN9bMaunsUR8cJFZNy0MI2V7y6w$=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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