Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 11 2023 15:49:08 FOUS30 KWBC 111549 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1048 AM EST Sat Mar 11 2023 Day 1 Valid 16Z Sat Mar 11 2023 - 12Z Sun Mar 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL/NORTHERN CALIFORNIA SIERRA FOOTHILLS AND FOR PORTIONS OF THE MID-SOUTH... ....16Z Update... No big changes overall were made with this morning's ERO update (finally!). In California, the area is in as much of a lull as they're going to get, but definitely the "eye of the storm" in the sense that Round 2 is on its way for Monday and Tuesday. In some areas, it's tough to know the difference though. Rain continues along much of the length of the western Sierra Nevada range in California this morning, though significantly lighter in intensity than yesterday. This is largely being driven by simple unidirectional atmospheric flow throughout the column, slamming into the high peaks of the mountains and upsloping Pacific moisture in the form of steady (and unfortunately stationary) rainfall. This afternoon, diurnal heating is expected to rapidly increase MUCAPE values to as high as 600 J/kg. In this saturated environment, that kind of instability is certain to result in cellular development of stronger thunderstorms, which in turn will move along with the flow into the same hard-hit areas of the Sierras. Thus, expect localized increases in rainfall rates with any thunderstorms this afternoon. The rain is likely to continue into tonight, though gradually shifting northward with time, giving southern areas of the San Joaquin Valley a much needed (if depressingly brief) respite from the rain before Round 2 arrives Tuesday. Closer to the coast, generally north of San Francisco, the area wasn't nearly as hard hit from "Round 1" the past couple days. As such, the mountains are expected to better handle the steady (if usually light) rain expected in that area through tonight. The aforementioned increases in afternoon instability are not expected to occur that far north. In coordination with the EKA/Eureka, CA forecast office, have downgraded the area to a Marginal risk for this update. For more specific information, please refer to any Mesoscale Precipitation Discussions issued through the day. Turning towards the mid-South, very little has changed from 24 hours ago and the inherited forecast. A small expansion was made of the inherited Marginal and Slight risk areas eastward with the Slight now including far northwestern AL. Some of the CAMs guidance suggests the heavier thunderstorms that will develop along a cold front and associated shortwave trough tonight will be able to take advantage of the increasing MUCAPE as the storms move southeast to keep the heaviest rain going longer, over a portion of Alabama with saturated soils. This nudged my confidence for flash flooding in the area a bit higher into the Slight category, though the entire Slight risk category area in the mid-South is on the lower end. Wegman ....Previous Discussion... ....California... The significant and strong atmospheric river event continues to wane across California today with the bulk of the heaviest precipitation ending across the foothills of the Sierra Nevada. However, a persistent westerly flow thanks to a nearly stationary mid/upper level low positioned off British Columbia will allow for a stream of enhanced moisture to push onshore through tonight. A strong upper level jet streak reaching 130-150 kts off of southern California will also provide the large scale forcing for lift across the region. Precipitable water values are forecast between 0.5-0.75", which is just +1 to +1.5 sigma above the climatological mean. So overall the environmental ingredients aren't as impressive as the recent strong AR event, but nonetheless a prolonged period of light to occasionally moderate rainfall is expected across parts of central to northern California. The 00Z HREF shows the best signal for 0.5"+/hr rain rates across the foothills of the Sierra Nevada with a secondary area across the coastal areas of northwest California. Areal average precipitation totals of 1" to locally 3" are expected for the period. Snow levels will hover in the 5,500-7,000 ft range, which for the northern Sierras will be all but the highest peaks, while those higher values will persist further south, where there's still expected to be a melting snow pack. The combination of steady moderate rainfall over areas extremely saturated and a melting snow pack will lead to scattered additional flooding. ....Mid-South... A fast moving shortwave trough ejecting out of the Rockies today will lead to an area of low pressure quickly tracking through portions of the Mid-South and Tennessee Valley late tonight. Along and ahead the passing cold front, a surging low-level jet characterized by 850 mb flow approaching 40 kts will bring enhanced moisture northward. Precipitable water values are forecast to increase above 1". A strong line of showers and thunderstorms are expected to develop along/ahead of the advancing cold front. With upwards of 1000-1200 J/kg of MUCAPE available overlapping with the enhanced moisture, intense rain rates up to 1"/hr associated with the line of storms are likely. Storm motions are expected to be fast and the environment isn't favorable for training convection. However, the wetter antecedent conditions (NASA SPoRT soil moisture percentiles above 80 percent in places) due to recent heavy rainfall suggests some localized flash flooding will be possible. Bann/Taylor Day 2 Valid 12Z Sun Mar 12 2023 - 12Z Mon Mar 13 2023 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... ....California... Continued unsettled and active weather pattern for California with another round of rainfall approaching the state Sunday into Sunday night. Slightly anomalous moisture streaming onshore within the persistent westerly flow will work with broad large scale forcing for ascent to produce light to moderate precipitation. The focus for this period looks to be across northern California into far southwest Oregon with another 1-2" of precipitation likely. Rain rates approaching 0.25-0.5"/hr will be possible and this intensity over the expected duration may lead to additional flooding issues, particularly given the extremely saturated soil conditions and ongoing flooding. ....Southeast... Shortwave trough energy continues pressing eastward across the Southeast during the period. Convection is likely to be ongoing at the start of the period, lingering from the overnight/early morning nocturnal low level jet. That line of storms will likely dissipate mid to late morning while as low pressure shifts eastward, convection is expected to redevelop/refire along the boundary further south and east, though there remains some uncertainty in the placement of where the storms will cluster. The environmental ingredients look more favorable for stronger storms with greater instability (1500 J/kg) overlapped with slightly higher moisture (1.5"+ PWs) so some intense rain rates between 1-2"/hr will be possible. Mostly dry soils and antecedent conditions should limit flash flooding to localized/isolated instances. Taylor Day 3 Valid 12Z Mon Mar 13 2023 - 12Z Tue Mar 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA... ....California... A strong shortwave trough progressing through the eastern Pacific combined with a digging shortwave rounding the base of the larger scale closed low off British Columbia will direct another strong and potentially significant atmospheric river event at portions of central to northern California beginning late Monday through Tuesday. A deepening area of low pressure is expected to push onshore late Monday night into Tuesday morning with a surge of highly anomalous moisture associated with it. The 00Z model guidance shows PW values exceeding 1.25", approaching +3 sigma for this time of year while IVT values of 500-700 kg/m/s reach the coast after 06Z Tuesday. This should bring widespread precipitation to the region with increasing intensity Monday night with higher probabilities of exceeding 0.5"/hr rain rates along the coastal areas from far southwest Oregon through the Bay Area as well as for the favored upslope ares of the northern Sierra. Snow levels will once again be higher, around 5-7 kft during the bulk of the period, but beginning to rise slightly Tuesday ahead of the passing low pressure. Another 1-3"+ of precipitation is likely, highest amounts for the coastal ranges and Sierra. Given the repeated heavy precipitation events recently (many areas approaching 300-600 percent of normal precipitation for the last 14 days), soils are extremely saturated and flooding ongoing. The combination of the strong atmospheric river over areas that are very sensitive to additional heavy rainfall, the Slight and Moderate Risks were maintained for portions of northern California where new or worsening flooding is possible. ....Coastal Northeast... As shortwave energy approaching the Mid-Atlantic Monday night takes on a negative tilt and combined with the left exit region dynamics of a modest 110-130 kt jet off the Carolinas, low pressure is forecast to rapidly intensify as it nears the Northeast coast by early Tuesday morning. The intense forcing and lift combined with a modest inflow of enhanced moisture should yield very efficient producing precipitation. While there remains a lot of uncertainties with the storm track, thermal profiles, and sensible weather, the 00Z guidance shows potential for heavy rainfall along the coast from northern New Jersey through southern New England and when combined with the strong onshore flow, a risk of flooding and excessive rainfall exists so a Marginal Risk was maintained from the Day 4 ERO. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ZUteNcIfOMmK92i9DrRHJiTNEujhOEdgM1TORXX87yz= g1b_d0dtIw20Tql-uCAphpOmtWuA_nU2gBkoFon8UTIZSu0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ZUteNcIfOMmK92i9DrRHJiTNEujhOEdgM1TORXX87yz= g1b_d0dtIw20Tql-uCAphpOmtWuA_nU2gBkoFon8UVBK2GA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ZUteNcIfOMmK92i9DrRHJiTNEujhOEdgM1TORXX87yz= g1b_d0dtIw20Tql-uCAphpOmtWuA_nU2gBkoFon8Z3Cwpzk$=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) .