Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 10 2023 01:00:29 FOUS30 KWBC 100100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Thu Mar 09 2023 Day 1 Valid 16Z Thu Mar 09 2023 - 12Z Fri Mar 10 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL CALIFORNIA COAST... ....West Coast... A strong atmospheric river is surging into the West Coast, with rain overspreading California currently with increasing intensity. Hourly rainfall rates totals will steadily increase in intensity through 12z Friday, potentially reaching/broaching 1" an hour by the end of the period. This update adds a targeted High Risk category for the southern portion of the MTR/Monterey CA and far northwest portion of the LOX/Oxnard CA forecast areas in and near their coastal ranges. The upper-level setup for this event remains quite impressive as a Pineapple Express brings moisture aloft 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 as well as cools off the atmosphere enough aloft to help cause MU CAPE to rise to 500+ J/kg across central CA. IVT values of 800-1000 kg/m*s are forecast to impact much of central California. A warm front moves ashore, bringing precipitable water values to 1.5", which is over 4 sigmas above the mean, and implies that freezing levels could flirt with 10,000 feet, which is unusual in this region and supported by some of the guidance. Inflow at 850 hPa is forecast to be 50-60 kts through 12z. This moisture surge will last for the next 11 hours over a similar area and aid in the potential for impacts due to longer-duration excessive rainfall. 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.=20 Some areas along the coastal ranges also show a 20% of reaching 8" between now and 12z, hence the High Risk upgrade. 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, particularly under 5000 foot elevation 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 now 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... As a potent shortwave trough swings eastward towards the Ohio Valley, precipitation under its base is slowly edging south and southeastward. Initially, ongoing convection across northeast TX in response to a sufficient 40 kt low-level jet. Precipitable water values greater than 1.5" and MUCAPE of 1000-2000 J/kg. 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 tonight, 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 south to account for the 18z HREF. Recent RAP runs depict that CAPE should slowly decrease from here on out, but a Marginal Risk still appears to be justified. Roth 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 and the National Water Center confirm concerns for considerable flooding in areas where reservoirs are already near full. Isolated major flooding is possible over Monterey County. Antecedent conditions remain incredibly favorable with a 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 Valid 12Z Sat Mar 11 2023 - 12Z Sun Mar 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/NORTHERN CALIFORNIA AND THE MID-SOUTH... ....2030Z Update... No significant changes were made to either of the Marginal Risk areas on Saturday. ....California... The main event by far is expected to be on Friday. By Saturday, expect lingering shower activity to continue in the Sierra mountains of central and Northern California especially, but to nowhere near the degree as on Friday. The challenge will be what lingering impacts there will be as the area gets mostly light rain on Saturday in an area that will have very little tolerance for any additional rainfall. With that said, there is certainly potential for even the 1-2.5 inches of new rainfall Saturday to make flooding concerns worse, despite that little amount of rain alone being a typical day. Thus, there remains some potential for a Slight Risk upgrade in the central and northern Sierras as snow levels rise a little bit through the 6-7k feet range during this period. Note that the primary driver of flooding will be antecedent conditions in this area, and not the additional rainfall...which will only serve to make the ongoing flooding situation in this area worse. ....Mid-South... There is a lot of spread in the guidance as to where the heaviest rainfall will be seen in this area on Saturday. For the most part, the significant rainfall will hold off until late Saturday night, after 06Z. But amounts will remain light, barely reaching an inch of rain except from any heavier thunderstorms. Thus, there is a higher than normal likelihood the Marginal Risk area will need to be shifted from its present location, which represents a broad average of the guidance. However, at this point the chances of needing an upgrade are low. Wegman ....California... The previous atmospheric river will have ended by the Day 3 period, but a stagnant upper-level pattern with troughing in the eastern Pacific will support continued onshore flow into northern/central California. Most of the heavy rainfall will be tied to orographic ascent near the northern coastal ranges/Sierra Nevada and not have a great deal of IVT to work with. However, extremely saturated ground conditions due to several inches of rain the prior two days will lead to particularly sensitive terrain and the isolated chance for flooding impacts associate with additional rainfall (it's possible flooding impacts will still be ongoing from Thursday and Friday's rain). QPF amounts under 5000 feet (the expected snow level) are anticipated to reach 1-2" and add up steadily over the entire day, limiting the overall rapid runoff concern. Additionally, snowpack at the lower levels should be greatly reduced by Saturday and limit any extra water source to add to any immediate excessive rainfall threat. ....Mid-South... Shortwave trough traversing the central/northern Plains on Saturday and responding height rises over the Mid-South will allow for moist return flow out of the western Gulf of Mexico. PWs around 1-1.25" are forecast from the GEFS and ECENS to reach into the Mid-Mississippi Valley ahead of a developing surface low pressure near OK by Saturday night. A warm front extending to the east will help focus shower and thunderstorm activity in response to a 50 kt southwesterly 850 mb jet. The exact latitudinal location of this warm front remains somewhat uncertain, with the GFS slightly to the north into the lower Ohio Valley and ECMWF over western Tennessee. This leads to QPF uncertainty and thus, uncertainty in the excessive rainfall threat. Leaned the outlook towards the ECMWF given it is a typical model bias to focus heavier QPF north and away from the better instability, which in this case should be located over the Lower Mississippi Valley. MUCAPE values per the GFS reach over 1000 J/kg by Saturday night and should aid in the potential for 1"/hr rainfall rates along and possibly just to the south of the frontal boundary. Much of the highlighted Marginal Risk area also includes sensitive terrain from central AR to western KY, with NASA SPoRT 0-40 cm moisture percentiles in 70-90% range. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8hZHsoztS5I_6cQZWfOMimRx2RzRoI23vyPGHHWZtR0u= 9vD75cWxmHW5uLXtKcPt4o79je8p4x51eAcFGaydVYMC6V8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8hZHsoztS5I_6cQZWfOMimRx2RzRoI23vyPGHHWZtR0u= 9vD75cWxmHW5uLXtKcPt4o79je8p4x51eAcFGaydgC-jn4o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8hZHsoztS5I_6cQZWfOMimRx2RzRoI23vyPGHHWZtR0u= 9vD75cWxmHW5uLXtKcPt4o79je8p4x51eAcFGaydRChmga4$=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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