Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 14 2023 20:06:30 FOUS30 KWBC 142006 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Tue Mar 14 2023 Day 1 Valid 16Z Tue Mar 14 2023 - 12Z Wed Mar 15 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... ....16Z Update... No big changes of note, as the well-advertised atmospheric river (A.R.) has arrived in northern and central California and will gradually shift southward with time throughout the day. Thus, the 2-4 inches forecast for the L.A. Basin is still on track, due to start over the next few hours, then continue into the night. Multiple CAMs models show a one-two punch of sorts for L.A. proper, as the first round impacting the city and points north and west briefly wanes but a second round focused from L.A. south and east follows behind it a couple hours later. There are indications in the guidance that this second round will have convective elements to it, especially as the rainfall interacts with the terrain of the southern Transverse and Peninsular ranges. This instability could increase rainfall rates to occasionally exceed 1 inch per hour at times. A somewhat unusual QPF signature is evident with the upcoming rainfall in the L.A. Basin in that populated areas of the lower elevations, downtown and coast may see similar rainfall totals as the adjacent mountains. Extensive street flooding is likely. One silver lining for this area is that the first round a couple days ago did a number on the snowpack in the Transverse ranges, so what snowpack is left, which is primed to melt significantly as snow levels rise to 8,500 to 9,000 ft is not expected to contribute too much to overall runoff except in the local valleys immediately downstream of the snowpack. A smaller change made with this update in coordination with the HNX/Hanford, CA forecast office was to expand the High risk area in the San Joaquin Valley westward to include more of the lower elevations that were walloped a couple days ago. Many of the impacts were felt in the rivers that had collected the combination rain and snowmelt, and the resulting runoff traveled a fair distance from the mountains. These areas are unfortunately set to feel more impacts due to the additional rainfall in the Sierras along with additional snowmelt as snow levels rise to around 8,000 ft. Fortunately, snow levels will come down quite a bit as colder air moves in behind the atmospheric river during the day Wednesday. The falling snow levels will help limit runoff from the remaining higher elevation snowpack. Unfortunately, until then, much of the lower elevation snow pack has been primed by the previous atmospheric river event, which may make up for the somewhat lower (but still significant) rainfall totals forecast for this area with today's A.R. Wegman ....Previous Discussion... ....California and the Great Basin... An ongoing atmospheric river event is forecast to continue through today into Wednesday, producing widespread heavy precipitation across California, with considerable to locally catastrophic flooding impacts expected for portions of the coast, Central Valley, and the Sierra Nevada foothills. A cold front that pushed east across eastern Washington and northeastern Oregon yesterday remains draped across southwestern Oregon and northwestern California this morning. This boundary will likely remain a focus for moderate to heavy precipitation as an approaching shortwave directs moisture into the boundary. This shortwave is forecast to amplify through the morning, supporting a strengthening surface low that is expected to move northeast along the boundary and bring additional periods of moderate to heavy rains across far northern California later today. These additional rains on top of already saturated conditions and high snow levels, prompted the continuation of a Moderate Risk along portions of the Northern California coast to the northern Sierra foothills today. A more isolated threat is expected farther east as moisture spreads along the boundary into northern Nevada, where locally heavy rains along with melting snow may produce flooding. Meanwhile, farther to the south as the previously noted shortwave continues to amplify and approach the coast, strong southwesterly flow ahead of the wave will direct a long-fetch of deep tropical moisture into Central and Southern California. A powerful low level jet will take aim at the region, driving this anomalous plume of moisture across the region. Guidance continues to show PW anomalies climbing in excess of 4 standard deviations above normal across much of Central and Southern California by the afternoon. This moisture, along with strong upper forcing supported by a coupled upper jet, is expected to support the development of heavy precipitation across Central California this morning before shifting south into Southern California by the afternoon. While some of the 00Z guidance did back away some, it still appears that the coastal ranges and the southern Sierra foothills will take the brunt of the heavy rain today. Overnight HREF guidance indicates that accumulations of 3-6 inches, with locally heavier amounts are likely for these areas. The HREF even shows some high neighborhood probabilities for amounts of 8 inches or more centered along the Santa Lucia Mountains and southern Sierra foothills. Given the likelihood for heavy rain, along with the already saturated soils, a High Risk was maintained along the much of the Central California Coast, with a Moderate Risk extending farther to the south into Southern California, including the Los Angeles Metro. A High Risk was also maintained along the southern Sierra foothills where in addition to the heavy rain, melting snow is likely to contribute to flooding concerns.=20=20=20=20=20=20 Pereira Day 2 Valid 12Z Wed Mar 15 2023 - 12Z Thu Mar 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA AND THE SOUTHWEST... ....2030Z Update... No noteworthy changes were made with this update. ....California... The tail end of today's atmospheric river (A.R.) will continue impacting far southern coastal California and the Peninsular Ranges early Wednesday morning. CAMs guidance is in excellent agreement that the heaviest rain will end by around 15Z. Thus, we are only looking at the final 2-3 hours of the event overlapping the Day 2 period. That said, being the tail end of the event, much of the storm's rain will have fallen prior to this time. However, the area may see as much as 2 inches of additional rainfall into the mountains. Fortunately, this area was largely missed by the previous A.R. event, and very little of the Peninsular Ranges have any snow cover on them, so snowmelt here will be insignificant. However, 2 inches in 3 hours at the tail end of tonight's rainfall may still cause flash flooding issues. Since this area of the California coast is the least flood-prone portion of the coast (which isn't really saying much), impacts are expected to be limited to flood prone creeks, dry washes and low-lying urbanized areas. Small areas immediately adjacent to the mountains may see more widespread impacts, but this area is not large or certain enough to draw a Slight risk. Thus, the Marginal risk remains a high-end on and a short-fused upgrade remains possible tonight as the impacts in this area are better known. ....Southwest... As the A.R. impacting California moves inland on Wednesday, its overall strength will diminish as the plume of moisture shifts south. However, enough atmospheric moisture will be available in combination with diurnal heating that widely scattered convection is forecast to break out Wednesday afternoon across the Marginal Risk area. With the convection taking advantage of anomalous moisture and MUCAPE as high as 500 J/kg, some of the stronger cells may produce localized heavy rain, which if that occurs over particularly sensitive slot canyons, burn scars, and dry washes, could result in isolated flash flooding. There is some uncertainty as to how numerous the convection will be, with more robust development likely causing enough convection to also potentially need a localized upgrade to a Slight risk. Since there is not good agreement on this scenario, however, the area remains in a high-end Marginal with this update. In a similar manner to the SoCal mountains, significant snow coverage across northern AZ is very limited, so snowmelt is unlikely to contribute much to any flooding concerns. Wegman ....Previous Discussion... ....California and the Southwest... While drier conditions are expected across much of California, a heavy to excessive rainfall threat may continue through Wednesday morning across portions of Southern California. The mid-to-upper level shortwave and associated surface low impacting California on Tuesday are expected to move east into the Intermountain West by early Wednesday. Onshore flow and deeper moisture back along the trailing cold front may continue to impact portions of Southern California, with additional heavy rains possible, especially in the upslope regions of the Peninsular Ranges. As the shortwave moves east, its associated frontal boundary and accompanying axis of deeper moisture are expected to move across the Great Basin and Arizona. This moisture interacting with favorable upper jet forcing may produce isolated runoff concerns, especially in the upslope areas of Arizona into portions of southern Nevada and southwestern Utah. Pereira =20 Day 3 Valid 12Z Thu Mar 16 2023 - 12Z Fri Mar 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EAST TEXAS AND THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... As the same shortwave trough causing today's A.R. tracks east into the Arklatex region, it will gather some Gulf moisture along with it. The air mass with the Gulf moisture will also feature significant instability, increasing to up to 2,000 J/kg by Thursday afternoon. Thus, showers and thunderstorms, some severe, are likely to break out across this region. Soils across much of the Slight risk area aren't saturated, but wetter than normal due to recent rains. At the surface, a strong cold front will rapidly dig southward in association with the upper level shortwave trough. Thus, the primary threat in this region will be severe weather. However, due to the Gulf moisture increasing PWATs ahead of the front to over 1.5 inches and the aforementioned instability, the strong to severe thunderstorms will be capable of locally producing very heavy rainfall, which in turn could result in widely scattered flash flooding concerns, particularly in the Ozarks, where topography could locally exacerbate flooding concerns. Fortunately, with the convection likely to track perpendicular to the motion of the front, training convection appears highly unlikely. Any flooding concerns further south and west over southeast TX will likely be in areas where prefrontal convection moves over the same area as frontal convection (likely with little to no break in between). Stronger storms may produce as much as 3 inches of rainfall, which may exceed 1 hr FFGs in northern areas. In the Marginal risk area across coastal southeast TX and southern LA, FFGs are quite a bit higher, and so convection able to produce rainfall rates that exceed FFGs are less likely, despite a high likelihood of periods of heavy rain with any storms. A small westward expansion of the Slight Risk was made with a bit of a slow-down (westward shift) in the guidance as to where heavy convection is expected to initiate. Wegman ....Previous Discussion... ....Eastern Texas to the lower Mississippi Valley... The mid to upper level trough moving east across the western U.S. Tuesday and Wednesday is expected to amplify over the northern Plains on Thursday, with a deep closed system developing over the Upper Midwest Thursday night into Friday. Strong southerly flow will begin to tap moisture moisture from the Gulf of Mexico, drawing it north ahead of the system's trailing cold front as it advances east from the southern Plains into the lower Mississippi Valley. Both the GFS and ECMWF show PWs climbing to around 1.5 inches along a southwesterly 40-50 kt low level jet centered across eastern Texas into Louisiana. This moisture is expected to interact with favorable mid-to-upper level forcing to support convection, with heavy rain rates developing across the region on Thursday. As the front begins to reorient west-to-east, training storms along the front may bolster the threat for heavy to excessive amounts across portions of the region. The overnight guidance showed a fair amount of spread regarding the location of heavy amounts and therefore forecast confidence is limited.=20 However, the overall consensus generally supported a southerly shift which is reflected in this outlook. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-I3vADlY1it13letjwToTNcTbJGZhSrBqz27bZOrkrED= N6-0msvLquupLxvleID3D_P_beyZ6x1JHIblsz8HOZy0B1o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-I3vADlY1it13letjwToTNcTbJGZhSrBqz27bZOrkrED= N6-0msvLquupLxvleID3D_P_beyZ6x1JHIblsz8HK04H7xs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-I3vADlY1it13letjwToTNcTbJGZhSrBqz27bZOrkrED= N6-0msvLquupLxvleID3D_P_beyZ6x1JHIblsz8HUFcGGY4$=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) .