Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 09 2023 16:01:18 FOUS30 KWBC 091601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM 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 The Day 2 outlook will be updated by 0830Z. Day 3 Valid 12Z Sat Mar 11 2023 - 12Z Sun Mar 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF CENTRAL/NORTHERN CALIFORNIA AND THE MID-SOUTH... ....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!_wCnGLQnYWMeVjCUf8gYyQAIKYL4cW6uQcumTLCGAFFw= TN4vFWcZ_rn2jEbR5Jn-gIkp8Hkv5lXYwcyAasmqJPe5vdk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_wCnGLQnYWMeVjCUf8gYyQAIKYL4cW6uQcumTLCGAFFw= TN4vFWcZ_rn2jEbR5Jn-gIkp8Hkv5lXYwcyAasmqPjbvTBk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_wCnGLQnYWMeVjCUf8gYyQAIKYL4cW6uQcumTLCGAFFw= TN4vFWcZ_rn2jEbR5Jn-gIkp8Hkv5lXYwcyAasmqQxpLysg$=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) .