Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 21 2024 00:27:06 FOUS30 KWBC 210027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 726 PM EST Sat Jan 20 2024 Day 1 Valid 01Z Sun Jan 21 2024 - 12Z Sun Jan 21 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON... ....16Z Update... No big changes were made with this morning's update, but the Marginal Risk was expanded to include all of the southern California coast, including adjacent Los Angeles and San Diego, with this update. Rainfall is not expected to be as steady in this region as across northern California. However, there is some elevated instability that is beginning to enhance rainfall rates in the showers just off the southern California coast. This instability will persist as the waves of showers move into the coast, resulting in locally heavy rains. Given the dry soil conditions which in this area of the country makes them quite hydrophobic, and the extensive urbanization of the L.A. Basin, think much of this rainfall will convert to runoff, resulting in highly localized flash flooding concerns through this afternoon. A few small expansions to the Marginal to include more of the coastal ranges south of the Bay Area were also made with this update. High snow levels keeping most of the orographically enhanced precipitation in the form of rain should result in more runoff with only the summits of the highest peaks likely to see any snow. Wegman ....Previous Discussion... Overall, no large-scale changes to the previous forecast, with rainfall rates and the threat for excessive runoff expected to increase later this morning across portions of northern California and far southwestern Oregon. As expected, a leading plume of deeper moisture associated with an upper trough and its frontal band moved inland, with precipitation spreading across much of northern California late Friday into this morning. The IVT associated with this leading band is expected to become more diffuse as it pushes further inland this morning. However, amplified westerly winds south of the parent low are expected to tap a secondary surge of deeper moisture, directing it inland as it lifts along the northern California and Oregon coasts later this morning. This moisture along with favorable upper forcing and orographic effects will support heavy rainfall amounts within the highlighted area. HREF neighborhood probabilities continue to indicate locally heavy amounts of 3 inches or more are likely within the Slight Risk area between 12Z Sat and 00Z Sun. These short-term heavy rains, along with the antecedent wet conditions can be expected to produce localized runoff concerns. Pereira Day 2 Valid 12Z Sun Jan 21 2024 - 12Z Mon Jan 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... ....2030Z Update... ....California... No significant changes were made to the outlook areas with this update. A series of disturbances over the Pacific will make their way into the northern California coast, with a nearly stationary 150 kt northwesterly jet lurking just off the coast. The left exit region of this jet streak will be just off the coast of southern California, which will allow the rainfall from these disturbances to persist and move into the coast as far south as the Mexican border. Thus, the Marginal Risk area was expanded to the Mexican border in the same way as on Day 1/today, as occasional convective showers move into the coast with each disturbance. The heaviest rains will be Sunday morning, with gradually decreasing chances of rain through Southern California through the overnight. Into northern California, expect occasional light shower activity until the mid-to-late afternoon when a strong shortwave moves into the coast, resulting in a period of steady rain with heavy embedded convective elements. The heaviest rain will continue through about 10Z. The axis of heaviest rainfall may be aligned a hair south of where it was today, focused from the Bay Area through to about Eureka, including the mountains ringing the northern Sacramento Valley. Regardless, since many of the same areas will be hit 2 days in a row, the flooding concerns will persist. ....Southern and Central Texas... A few minor tweaks in the form of a slight northward shift in the Marginal Risk area were made. A strong southerly jet of tropical moisture will develop up the Texas coast ahead of a strengthening negatively tilted shortwave trough moving across the Four Corners region. Enhanced lift ahead of the trough running into the plume of moisture will take much of the very light rainfall over Deep South Texas for much of the day and rapidly enhance the rainfall rates after 06Z, so much of the day's rainfall will occur overnight Sunday night. Despite the recent dry spell and drier soil conditions across southern Texas as one moves towards the Rio Grande, the prolonged period of heavy rainfall may locally overwhelm streams and creeks resulting in isolated flash flooding. Wegman ....Previous Discussion... ....California and southwest Oregon... An unsettled period is forecast to continue with widespread precipitation and the potential for locally heavy amounts continuing as a broad upper trough with a series of embedded shortwaves moves across the region. Overall, models continue to indicate the potential for organized heavy amounts to be less than previous day. However, there remains a fairly good deterministic model and ensemble signal for additional locally heavy amounts redeveloping across portions of northern California early Monday.=20 Some models, including the GFS, show another plume of deeper moisture moving inland ahead of an amplifying shortwave Monday morning. This deeper moisture and better forcing is expected to line up across much the same area where heavy rainfall is expected the previous day and may increase any ongoing runoff concerns. .....Southern and central Texas... As a broad upper trough moves across the Southwest into the southern Rockies and High Plains, southerly low level inflow is expected to increase across southern to eastern Texas. PWs are forecast to increase to 1.25-1.5 along a 40-50 kt low level jet extending from the northwestern Gulf of Mexico through eastern Texas. This moisture, interacting with increasing ascent afforded in part by favorable upper jet forcing, will support the development of rainfall across southern to central Texas. Despite ample moisture and forcing, limited instability is expected to help keep rainfall rates and the threat for widespread excessive runoff concerns in check.=20=20 Pereira Day 3 Valid 12Z Mon Jan 22 2024 - 12Z Tue Jan 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND FROM EASTERN TEXAS TO WESTERN LOUISIANA... ....2030Z Update... No major changes into the West on Monday. The Slight Risk for the Los Angeles and San Diego Basins remains in place as the heaviest rain of this 3 day period occurs on Monday. After today's and Sunday's lighter rains, soils may become a bit better able to absorb the rainfall by Monday as they'll be sufficiently moistened by the shower activity. However, the heavier amounts of rain into highly urbanized areas should result in more flash flooding. A widespread inch of rain remains forecast for much of southwestern Arizona as well, which may also result in isolated flash flooding, but with most of today's and Sunday's rains likely to remain into California, this will be the first rainfall for Arizona in a while. For the Arklatex Region, rainfall amounts continue to increase, though not too much from inherited. This area has recently been dry as well, except southern Arkansas where soil moisture conditions are about average. However, with PWATs getting over 1.5 inches at the Gulf Coast, and a strong longwave trough digging over the Southwest the rainfall concerns amount to nearly stationary forcing with a steady stream of Gulf moisture resulting in an all day event across east Texas, including Houston. Overall, the dry antecedent conditions may delay the start of any flash flooding, but there's increasing confidence that flash flooding will occur into east Texas and western Louisiana. Since the forecast has not changed all that much, the Slight was maintained, but continued scrutiny of a possible Moderate, especially around Houston will be needed. Wegman ....Previous Discussion... ....Western U.S.... For much the much of the West, unsettled weather is expected to continue for another day as a series of shortwaves continue to impact the region. Confidence in the details is limited, however additional locally heavy amounts may fall across portions of far southwestern Oregon and northern to central California, further raising runoff concerns across the same areas impacted days 1 and 2.=20 The potential for heavy to excessive rainfall is expected to extend farther south into southern California this period. A Slight Risk was maintained along the coast into the coastal ranges where an amplifying shortwave accompanied by deeper moisture and stronger forcing will support the increased threat for heavy precipitation. The ECMWF, GFS, CMC, and their ensemble means are in generally good agreement, indicating 1-2 inches, with locally heavier amounts within the Slight Risk area. These amounts are expected follow at least light to moderate accumulations falling across this same area the previous periods. As the trough continues to amplify and move inland, rain will spread across the lower Colorado Basin into southern Arizona.=20 While expected to be beneficial for most areas, this is likely to be the second of back-to-back days of widespread precipitation impacting the area, which may raise localized runoff concerns. ....Eastern Texas to Louisiana and southern Arkansas... An axis of strong southerly flow and deep moisture will shift east along with the associated shortwave. In addition to ample moisture (PWs 1.25-1.5 inches) and strong upper forcing, instability will be on the increase as well, raising the threat for heavy rainfall rates and excessive runoff. Model QPF spread remains significant, however the overall consensus indicates that amounts of 1-3 inches, with locally heavier amounts can be expected across the region. Models also indicate that this is likely the start of a multi-day heavy rainfall event and an update to a Moderate Risk may be required in this, or subsequent periods. However, given the current model spread and uncertainty, opted to maintain just a Slight Risk for now. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9KUyLRs0Lyb28hP77-uPQrb2Z8JSyMdHtrOYHD-Oykew= 6gNfhu5f3pzEKMLjYz9F-whqvUq3oZttNwwlPUXu9f354qQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9KUyLRs0Lyb28hP77-uPQrb2Z8JSyMdHtrOYHD-Oykew= 6gNfhu5f3pzEKMLjYz9F-whqvUq3oZttNwwlPUXuYT6Hue0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9KUyLRs0Lyb28hP77-uPQrb2Z8JSyMdHtrOYHD-Oykew= 6gNfhu5f3pzEKMLjYz9F-whqvUq3oZttNwwlPUXue2EWOf8$=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) .