Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 09 2023 01:02:06 FOUS30 KWBC 090102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 801 PM EST Wed Mar 08 2023 Day 1 Valid 01Z Thu Mar 09 2023 - 12Z Thu Mar 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR WEST-CENTRAL ARKANSAS, SOUTHEASTERN OKLAHOMA, AND PORTIONS OF NORTH TEXAS... A limited risk for excessive rainfall will continue overnight across portions of TX/OK/AR in association with a stationary front aloft (between 850-700 mb) that is resulting in strong to severe elevated convection, currently persisting and further organizing in the vicinity of the TX/OK border/Red River (see MPD 90 and any subsequent MPDs for more detailed mesoscale information). The Marginal Risk area was altered with this update to encompass the area that is most likely to realize any isolated flash flooding over night in relation to this elevated convection, as sufficient instability (MU CAPE as high as 500-1000 J/kg) and ample deep layer shear (0-6 km shear of 30-50 kts) will be plenty to keep deep convective activity going that remains rooted near the stationary boundary aloft. Farther north and east, instability is much more lacking which resulted in a substantial decrease in the total area encompassed by the Marginal Risk. While convective activity is rather impressive at the moment, the deep layer mean wind of 30-40 kts will generally keep convection moving which is capping hourly rainfall rates near 1-2"/hr. These rates will likely come down to 1"/hr or less as the night goes on, with instability largely being eroded by the ongoing deep convective activity. On the flip side, the low-level jet is expected to strengthen with the diurnal cycle overnight, which may help to spur additional convection upstream of where storms have already been realized. There is relatively low confidence in this occurring in an organized fashion, as the elevated nature of the front and accompanying instability is rather fickle with the 18z HREF suite and subsequent HRRR runs fairly unimpressed with overall totals (with even the HREF PMM only depicting 2-3" QPF maxima through 12z). While additional training and some isolated maxima closer to 3-5" are certainly possible (given the over poor CAM performance with ongoing convection), most likely these totals will remain rather isolated and fairly unimpactful across a fairly rural region. Churchill Day 2 Valid 12Z Thu Mar 09 2023 - 12Z Fri Mar 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL CALIFORNIA... ....2030Z Update... ....California... No big changes were made to the inherited risk areas across California with this afternoon's update. There was a small southward shift in where the heaviest rain is expected in the mountains. Snow levels are little changed from previous. In scrutinizing snow levels around the OR/CA border, much of the guidance suggests any raises in the snow levels above 5,000 feet will only persist for a few (up to 12) hours. Even with the expected rainfall during those 12 hours, following the passage of the cold front snow levels will drop back down to 3,000 feet and lower further north. Thus, the amount of time snowmelt will contribute to flooding concerns will be very limited from Siskiyou County north, so those areas were removed from the Marginal Risk area, with a sliver of coastline from Eureka northward left. Otherwise, the 2+ inches of precipitation in the mountains will either be absorbed by the snowpack or fall as snow following passage of the cold front. Otherwise, as mentioned, the risk areas were expanded southward a few miles down the coastal ranges and Sierras, and there's likely to be a sharp cutoff to where the heaviest rain will fall. The Marginal Risk area covers those areas where rain will fall in the mountains and drain, with little rainfall contribution in those areas. ....Mid-South... More substantive changes were made across the Mid-South with this afternoon's update. As on Day 1, the QPF over this area is meager, and there has been a notable decrease in widespread rainfall forecast values across this region, with only a few spots in AL now forecast to get over 1 inch of rain in the 24 hours encompassing Day 2. Further, this rain is expected to fall in 1-3 separate distinct waves of showers and thunderstorms, with potentially several hours "break" in between. While there certainly is some potential for training of these rounds of storms, the separation between them should be enough to allow what little rainfall happens to drain off before the next round. Thus, the flash flood potential in these areas continues to diminish, and it's unlikely that widely scattered flash flooding will occur given this atmospheric setup. This is despite the nearly saturated soils over AR. Further east across MS/AL, soils are a bit drier, which will make up for the somewhat heavier rainfall forecast for that region. Wegman ....Previous Discussion... ....West Coast... A strong atmospheric river is set to surge into the West Coast by the second half of the Day 2 period (Thursday night) and steadily increase in intensity through 12z Friday. The updated ERO only incorporates minor changes from the previous forecast. This surge of moisture has continued to trend somewhat faster, with rain now expected to enter northern California before 00z Fri. The upper-level setup for this event is 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 dropping south from off the coast of British Columbia to the Northwest coastline reinforces this moisture surge and focuses the push into California. IVT values of 800-1000 kg/m*s per the ECMWF and GFS are forecast to impact a broad region and not remain confined to a singular intense axis, but last for 12 hours during this forecast period. The resulting PW values are forecast to reach above 1.00" into the central valleys (a very impressive +4 standard deviations above climatology). This creates the potential for widespread 1-3" QPF amounts through 12z Friday across much of central/northern California and into SW coastal Oregon. 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 flash flooding. 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 prompted the expansion of the Slight Risk into far western Nevada. This means all coastal ranges and much of the Sierra foothills will receive heavy rain on top of snow, increasing the threat of rapid runoff due to snowmelt. The most significant snowmelt is expected to occur below 5000 ft, where snowpack is shallower and temperatures will be warmer. The Moderate Risk was drawn tightly to this elevation as a result. 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. The rather broad nature of incoming moisture surge may actually limit the magnitude of flooding impacts as showers with embedded heavy rates appear most likely, versus events with extreme rainfall rates along a confined axis. However, the upslope and snowmelt component (2"+ of SWE per NOHRSC), as well as the depth of the IVT values is widespread enough to support the Moderate Risk. ....Southern Plains, Lower Mississippi Valley, and Mid-South... The prolonged heavy rainfall threat impacting the Lower Mississippi Valley and nearby surrounding regions will conclude on Thursday 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 eastern OK to central AR. This may also potentially occur over areas forecast to receive widespread total rainfall amounts of 1-3" on Tuesday and Wednesday. 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 northern 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. The Slight Risk area over southeast OK and the ArkLaTex was maintained/stretched westward to account for morning convection occurring over areas that should be more sensitive due to prior rainfall. The heaviest 24-hour totals during this time frame are actually forecast outside of the Slight Risk and into northern MS and AL, but these areas have been relatively dry and early green-up should limit the flash flooding potential to more of a localized threat. Given southward trends in the guidance with better overlap of moisture and instability, the Marginal Risk was also shifted to the south and west. Snell Day 3 Valid 12Z Fri Mar 10 2023 - 12Z Sat Mar 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... ....2030Z Update... The atmospheric river event from Thursday will continue across central and southern California on Friday. As was the case for Day 2/Thursday, there has been a small southward shift on where the highest rainfall totals are expected to set up. A primary difference from Thursday is that on Friday the primary forcing will be upslope flow, with less upper level support. Thus, there are more notable upslope-downslope couplets in the QPF. Portions of the San Joaquin Valley may see no or very little rain at all on Thursday, despite being in the middle of the atmospheric river. Meanwhile, the Sierras (particularly central and southern Sierras) are expected to get even more precipitation on Friday than they will on Thursday. This will be offset somewhat by a bit lower snow levels (6-8k ft) on Friday, so more of the higher elevations will see all snow. Meanwhile the coastal ranges are likely to get about the same rainfall as Thursday again, averaging around 2 inches. With the southward shift in the rain, the inherited Moderate and Slight risks were expanded southward to about Malibu for the Moderate Risk, and including all of the Transverse Ranges for the Slight Risk. Meanwhile, with over an inch of rain expected on the Peninsular Ranges, also expanded the Marginal Risk down to the Mexican border to account for this. Also as on Day 2, the northern edge of the risk areas were also shifted southward. This was mostly due to lower expected rainfall into the coastal ranges, but also due to continually falling snow levels on Friday, which may approach 1,000 feet by Friday night. Thus, the Marginal Risk north of Point Reyes was trimmed with this update. 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 and Sierra foothills, with locally higher totals possible. 1-3" of rain is also likely into the San Rafael mountains of southern California. In total, the entire event ending Friday is forecast to produce upwards of 6-8" of rainfall across the central California coast and Sierra Nevada (although the highest terrain above 8000 ft will see snow). A 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. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-DMPHgDC9RBnEB79x-XGOeHFAM6THyHyW6lDKgSV0_rI= D14q4TRE4HJzWe07WD-nlolTQaOeFHYc2nkmx0oBXAyqiiA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-DMPHgDC9RBnEB79x-XGOeHFAM6THyHyW6lDKgSV0_rI= D14q4TRE4HJzWe07WD-nlolTQaOeFHYc2nkmx0oBPMWn_8M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-DMPHgDC9RBnEB79x-XGOeHFAM6THyHyW6lDKgSV0_rI= D14q4TRE4HJzWe07WD-nlolTQaOeFHYc2nkmx0oB8fm_KQE$=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) .