Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 12 2023 05:44:23 FOUS30 KWBC 120544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1243 AM EST Sun Mar 12 2023 Day 1 Valid 0540Z Sun Mar 12 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 SOUTHERN U.S....=20 ....0530Z Special ERO Discussion... Amended the on-going Excessive Rainfall Outlook over the Southern U.S. to expand the Slight Risk area and the Marginal Risk area southward based on latest radar and satellite trends. The orientation of the convection and the short-term rainfall rates suggests the threat will linger over southern Arkansas and may reach northern Louisiana and portions of Mississippi through the remainder of the night...with at least some potential for heavy to excessive rainfall spilling out of the ERO that was issued at 12/01Z. Refer to WPC Mesoscale Precipitation Discussion #099 for more details about the mesoscale set up and resultant evolution. Previous Discussions Below... ....California...=20 0100 UTC Update -- Made few, minor modifications to the Day 1 ERO based on the latest observational trends. Per the 18Z HREF exceedance probabilities, it still appears that the 0.5"+ hourly rainfall rates will be most prevalent through 06Z.=20 =20 Hurley 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.=20 =20 ....Mid-South...Lower MS Valley...TN Valley...=20 0100 UTC Update -- Made some changes to both the Slight and Marginal Risk areas across the Mid South-Lower MS Valley and TN Valley based on both observational trends (radar, satellite, and mesoanalysis) as well as trends in the latest high-res guidance. Onset of focused, favorable upper divergence within the right entrance region from the upper jet streak over the mid Atlantic is noted with the cooling cloud tops and blossoming WAA convection across the eastern TN Valley. Over time, expect the activity to expand farther upstream toward the MS Delta region, ahead of the next jet streak approaching from the southern Plains (left exit region), where the degree/depth of instability and moisture transport will become most favorable. The Slight was oriented in an area where the best deep-layer QG forcing coincides with the highest PW advection (1.5+ inches) and instability (MUCAPEs climbing to 1000-2000 J/Kg along the surface warm front track/tight instability gradient). This is where the 18Z HREF exceedance probabilities (1-2+ inches/hr) were highest.=20 Hurley 0830Z Excessive Rainfall Outlook 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.=20 =20 Bann/Taylor=20 Day 2 Valid 12Z Sun Mar 12 2023 - 12Z Mon Mar 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA AND EXTREME SOUTHWESTERN OREGON... ....2030Z Update... ....California... The next atmospheric river will be ramping up on Sunday as a deep positively tilted trough approaches the coast of the Pacific Northwest Sunday night. Ahead of the trough, deep unidirectional WSW flow will continue into northern California. This in turn will continue to pump Pacific moisture with PWATs around 0.75 inches remaining consistent through the Day 2 period. This flow will favor upsloping not just along the coast, but right up into the northern end of the Sacramento Valley as well. Meanwhile, snow levels will be largely steady in this region, holding between 5,500 and 6,500 ft. This is plenty high enough to result in rainfall on all but the highest peaks of the mountains in Siskiyou County. Continued snowmelt will also contribute to rising river levels and the flash flood threat. As a result, the Slight Risk area was expanded to include essentially all of the Sacramento Valley. While lower elevations in the valley won't get as much rain, it will have to contend with runoff from all of the surrounding mountains to the west, north, and east, so the lesser rain amounts there will be made up for by the runoff. Rainfall rates of 0.25-0.5 inches/hour are expected. Expect 2 inches or more of rainfall in most of the higher elevations, with 1/2 to 1 inch expected in the middle of the valley. The Marginal Risk area was expanded well to the north to include all of coastal OR and much of western WA as well. Snow levels will rise to around 5,000 ft during the day into the overnight Sunday, which will allow for a period of snowmelt as well as rainfall peaking around 2 inches in the mountains. The further north you go, the less time snow levels will be that high, but nonetheless those rainfall amounts on snow could result in isolated flash flooding issues, particularly in urbanized areas and around burn scars. ....Southern AL/GA... The Marginal Risk area inherited was shifted south with this afternoon's update. The models are coming into better alignment that an axis of heavier rainfall will set up over far southern AL and GA. This is due largely to an increased possibility of training thunderstorms that will develop Sunday afternoon and move slowly south and east into Sunday night. Corfidi vectors are close to parallel to the axis of the stalling front, and guidance is slowly catching on to this. Thus, QPF has come up considerably compared with previous forecasts. Working strongly against flash flooding however are the antecedent bone dry conditions in this region, resulting in very high FFG values, that will be difficult to overcome even if the convection develops and trains over the same areas. In coordination with TAE/Tallahassee, FL, MOB/Mobile, AL, FFC/Atlanta, GA, and BMX/Birmingham, AL forecast offices, the Marginal Risk area remains in place (though shifted south as mentioned above) though it's certainly a high-end Marginal risk. Consideration of an upgrade to a Slight Risk will need to be considered with future updates if forecasted rainfall continues to increase. Wegman ....Previous Discussion...=20 ....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 AND EXTREME SOUTHWESTERN OREGON... ....2030Z Update... ....California... The threat for flooding rains across northern California continues to increase this afternoon for the Monday time period. A pair of lows will impact the area during this Day 3 period. The first will be a dying low weakening into a front as it moves ashore near the CA/OR border Monday morning. This low will increase precipitation rates particularly in the northern end of the ERO risk areas for far northern CA and into OR. The low/front will also draw the cold front behind it southward, which will once again direct the heavier rainfall south with it into CA. Rainfall rates could reach as high as 0.5 to 0.75 inches per hour, but will be steady and very persistent for much of the day, particularly into the middle of the Moderate Risk area of the northern CA coastal ranges and the far northern Sacramento Valley. By Monday evening, the cold front will have shifted all but some remnant shower activity into CA, and falling snow levels will end much of the flash flooding threat north of the border. Unfortunately for CA, the second in the pair of lows, and by far the stronger one, will approach the coast of northern CA Monday night, riding along that aforementioned cold front. This low and its trailing cold front will provide the surface forcing for heavy rains further south into central California Tuesday. But in the meantime, the higher rainfall rates will combine with upslope into the mountains and obviously very wet antecedent conditions along with fairly steady snow levels to result in numerous instances of flash flooding across much of the northern half of the state by Monday night. Snow levels will remain around 6,000 feet in northern California, though from San Francisco south, the slug of moisture will raise snow levels back closer to 9,000 feet, once again setting the stage for another round of very impactful rainfall on Tuesday. As the rain shifts south Monday night, the rain will impact increasingly sensitive portions of central CA that were hit hard by the rainfall yesterday. Thus, areas south of San Francisco's latitude will largely be spared the heavier rain until late Monday night, but it consequently won't take long once the steady heavy rain gets started for flooding impacts to resume. The snowpack across much of California's mountains has been primed due to yesterday's extreme rainfall event, so the combination of heavy rain in addition to a now saturated and primed snowpack and already full rivers will again result in impacts beginning much closer to the start of the heavy rains, and only worsen with time. ....Long Island and Surrounding Coastal Areas... Very little has changed from the previous forecast in this region. There remains the usual uncertainty as to where the rain/snow line will set up, but those on the rainy side of that line have potential to see isolated flash flooding as rainfall totals around 1.5 inches are expected. This storm will be far more impactful on the winter/snowy side of the low. Wegman ....Previous Discussion... ....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!5ZKvWJJjN1U8aJNDsJgFLxih1AAZ2BrF1A5lSa2P8GJg= K3nvbh77hyDfCztgi6vPCfwcMoXpq7bfyCFl_-ag4VzCG6s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ZKvWJJjN1U8aJNDsJgFLxih1AAZ2BrF1A5lSa2P8GJg= K3nvbh77hyDfCztgi6vPCfwcMoXpq7bfyCFl_-agH2coDrI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ZKvWJJjN1U8aJNDsJgFLxih1AAZ2BrF1A5lSa2P8GJg= K3nvbh77hyDfCztgi6vPCfwcMoXpq7bfyCFl_-agulcgnss$=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) .