Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 09 2023 20:30:56 FOUS30 KWBC 092030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 PM EST Thu Mar 09 2023 Day 1 Valid 16Z Thu Mar 09 2023 - 12Z Fri Mar 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL CALIFORNIA... ....16Z Update... Very few changes needed for both ERO risk areas for this morning's update. ....California... The forecast as described in the previous discussion below remains on track. The atmospheric river event is just starting at the coast currently and will continue strengthening and pushing inland through the day into tonight. A targeted High risk upgrade was considered given forecast raises in snow levels over 9,000 ft at the height of the event, with some 10,000 ft areas seen in southern CA. Snowmelt impacts will be felt even to pass level given these high snow levels. IVT values peak around 800 J/kg*s, which while very impressive for this time of year, are far from unprecedented. PWATs remain an astonishing 4 to 5 standard deviations above normal, though there is some concern as to how efficiently that will translate to heavy rainfall, given the upper level pattern is fairly progressive, which in turn will limit the duration of the heaviest rainfall, as described below. Overall, at this point today's forecast looks to be on the higher end of the Moderate category. It will take a few hours more before the heaviest rainfall really gets going across central and Northern California, which is now limiting the amount of time before the 12Z transition over to Day 2 that impacts would be realized. If High risk impacts were to happen, which is far from certain at any time, they would be most likely in Day 2/ Friday in the central CA Sierras and coastal ranges. ....Portions of Texas to Alabama... Few changes have been made here as well as the Marginal Risk area inherited covers the isolated flash flooding threat from TX to AL well. Light to occasionally moderate rain is ongoing across this area but there will be several hours break before either one or 2 more rounds of rain move across the same area in advance of a cold front. As discussed below, in isolated instances the storms may grow strong enough to cause locally heavy rain and a resulting isolated flash flooding threat if the heavy rain happens to fall over a particularly sensitive area, which works well with the ongoing Marginal risk area. Wegman ....West Coast... A strong atmospheric river is set to surge into the West Coast today, with rain overspreading northern California by about 18z and reaching central California by about 00z. Rainfall rates will also continue to steadily increase in intensity through 12z Friday. The updated ERO only incorporates very minor changes from the previous forecast. The upper-level setup for this event remains quite impressive as an upper-level low north of Hawaii and associated shortwaves rounding the parent low act to "scoop" subtropical and tropical moisture from this region northeastward towards the West Coast, while a separate upper-level low off the Pacific Northwest coastline reinforces this moisture surge and focuses the push into California. This setup has been well-advertised by global guidance over the last few days and remains consistent at the time of this forecast. IVT values of 800-1000 kg/m*s per the ECMWF and GFS are forecast to impact much of central California. This will lead to a rather widespread area of heavier QPF, but limit the potential for extreme rainfall rates due to no confined intense axis of convergence. However, this moisture surge will also last for 12 hours or so over a similar area and aid in the potential for impacts due to longer-duration excessive rainfall. The resulting PW values are also forecast to reach above 1.00" into the central valleys and 1.25" along the central coastline (a very impressive +4 standard deviations above climatology), which further amplifies the warm subtropical airmass to surge inland. For the day 1 period, widespread QPF amounts of 1-3" are forecast from southwest OR through much of central and northern California. Upslope enhancement along the California coastal ranges and foothills of the Sierra will support higher rainfall totals up to 4-5" and a greater threat for widespread rapid runoff potential. Snow levels will be much higher than recent storms given the tropical moisture source in play, quickly rising to about 8000 feet. These high snow levels are also likely to lead to rain east of the Sierra crest and near the Lake Tahoe region, which explains why the Slight Risk expands into far western Nevada. The high snow levels also mean coastal ranges and much of the Sierra foothills will receive heavy rain on top of snow, increasing the threat of runoff due to snowmelt. The most significant snowmelt is expected to occur below 5000 ft, where snowpack is shallower and temperatures will be warmer. NOHRSC depicts SWE of 2"+ for some of these lower ranges. Along with the snowmelt threat, soils remain overly saturated per the NASA SPoRT 0-40 cm moisture percentiles. USGS streamflow gauges also show numerous rivers running above to well above normal throughout central California. 00z HREF highlights the Santa Lucia Ranges and the southern Sierra Nevada foothills as two areas where intense rainfall rates greater than 0.5" per hour are most likely between 09z and 12z Friday. Here, more rapid water rises and immediate flooding concerns could arise early Friday morning. ....Southern Plains, Lower Mississippi Valley, and Mid-South... The prolonged localized heavy rainfall threat impacting the Lower Mississippi Valley and nearby surrounding regions will conclude today as a potent shortwave trough swings eastward towards the Ohio Valley. Initially, ongoing convection in response to a sufficient 40 kt low-level jet near the Red River of the South intersecting a draped stationary front is expected to be situated across parts of southeastern OK to central AR. This morning convection is likely to slide eastward to the north of a warm front stretching from FL to the central Gulf Coast in response to the eastward progressing shortwave. PWs greater than 1.25", MUCAPE up to 1000 J/kg, and a modest 30-40 kt 850 southwest inflow should help this convection maintain strength through the afternoon into central MS and AL. Another round of showers and storms are set to form upstream along and ahead of an advancing cold front, with PWs still greater than +2 standard deviations above climatology, supporting the potential for intense rainfall rates. This last round of rain will also occur throughout regions of the Lower Mississippi Valley, where several inches of rain are possible by Thursday. However, showers and storms have remained fairly progressive as of late and there is no reason to believe this will change today, which limits the flash flooding to a predominantly localized threat. The trend the last few days has also been for storms to develop farther southwest, which is likely to include parts of central to eastern TX this evening. The previous Marginal Risk was shifted towards the southwest to account for latest 00z guidance trends, which also matches the southern trend over the last few days. Rainfall totals over 2" are likely to be hard to come by and very localized, but the 00z HREF does show spotty 15-30% probabilities for exceeding 2" in 6 hours. The 00z HREF also depicts a pool of greater than 1500 J/kg MUCAPE over parts of central MS this evening that could advect into areas along the frontal boundary into southern/central Mississippi, leading to the chances for locally intense rainfall rates up to 2"/hr. This combined with the ample atmospheric moisture content in play and some more sensitive soils in southeast OK and central AR is enough to maintain a low-end Marginal Risk area. Snell Day 2 Valid 12Z Fri Mar 10 2023 - 12Z Sat Mar 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOOTHILLS OF THE CENTRAL CALIFORNIA SIERRAS AND COASTAL RANGES... ....2030Z Update... Portions of the southern Sierras and the central CA coast ranges south of Monterey were upgraded to a High Risk with this afternoon's update in coordination with the HNX/Hanford, CA, MTR/Monterey, CA, and LOX/Los Angeles, CA weather forecast offices. A few things have changed...guidance continues increasing snow levels across central California during this heavy rainfall event, with most guidance now between 9,000 and 9,500 ft during the bulk of the heaviest rain. Further south, levels may get as high as 11,000 ft. This is roughly 1,000 ft higher than 24 hours ago. Secondly, PWATs are somehow continuing to increase, with PWATs approaching 1.5 inches along the coast, which is as much as 5 sigma above normal! Coordination with the aforementioned offices confirm concerns for potentially unprecedented flooding in areas where reservoirs are already near full. Isolated major flooding is possible over Monterey County. Antecedent conditions remain incredibly favorable with well above normal snowpack in the Sierras that has incredible amounts of water stored there. Soils remain near saturated area-wide via NASA Sport imagery, and the abnormally warm and wet conditions moving in are expected to cause rapid snowmelt. Combine this snowmelt with as much as 10 inches of rain in the 24 hours from 00Z this afternoon through 00Z Saturday, and the potential for widespread flooding is considerable, especially in the High Risk areas. Wegman ....Previous Discussion... The strong atmospheric river impacting California is anticipated to continue into Friday, while gradually shifting the focus southward. The greater IVT values (800-1000 kg/m*s) are forecast to occur between 12z and 18z per the ECMWF and GFS. This better moisture surge is also expected to be over central California and into parts of the Transverse Ranges of southern California. Similar to Thursday night, snow levels will remain very high relative to recent events this winter and into the 8000-10000 feet range. Snow levels will fall back to below 7000 feet by Friday night behind the cold front, but this will be as the heavy precipitation threat is waning. 24-hour QPF amounts of 2-4" are forecast along the central California Coast, Sierra foothills, and Transverse Ranges of Southern California, with locally higher totals possible. In total, the entire event ending Friday is forecast to produce upwards of 6-8" of areal average rainfall across the central California coast and Sierra Nevada (although the highest terrain above 8000 ft will see heavy snow). 00z HREF guidance has impressively high neighborhood probabilities for 24-hour QPF (ending 00z 03.11) exceeding 100 year ARI for the central California coast, central/southern Sierra, and nearby San Joaquin Valley. This alone raises alarm bells for a particularly intense excessive rainfall event. The Moderate Risk areas encompass parts of the central California coast/ranges and the Sierra foothills, where orographic enhancement should support a continuation and several additional hours of 0.5-0.75" per hour rainfall rates. This, combined with any additional snowmelt and severely saturated ground conditions, should support the potential for widespread runoff concerns and the potential for significant flooding impacts. These potential impacts will likely be ongoing by the start of Day 2 and only become more widespread into the first half of Friday within the Moderate Risk. The only notable change to the current day 2 outlook was coordinated with the local forecast office in Elko, Nevada (LKN) to expand the Marginal Risk into parts of northern Nevada. The strong atmospheric river is forecast to push IVT values greater than 400 kg/m*s past the Sierra Nevada and into the central Great Basin. While rainfall rates should remain meager and under 0.5" per hour, the combination of rainfall amounts up to an inch and snowmelt could lead to ponding of water and rises to creeks, streams, and rivers. 00z HREF 24-hour QPF probabilities for exceeding 100 year ARI are also elevated for this area. Snell 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!5Jz0eSadSQwyvlYTxxi6ghnBlqi2RmQKbGwVa5SM8vz7= MbJ8zZsugHr3yYSPpjQpGp6WT3FsmPmAUaIa-xtRDf90yGU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Jz0eSadSQwyvlYTxxi6ghnBlqi2RmQKbGwVa5SM8vz7= MbJ8zZsugHr3yYSPpjQpGp6WT3FsmPmAUaIa-xtRl4-3I08$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Jz0eSadSQwyvlYTxxi6ghnBlqi2RmQKbGwVa5SM8vz7= MbJ8zZsugHr3yYSPpjQpGp6WT3FsmPmAUaIa-xtRuEhyNRM$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .