Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 20 2024 20:25:35 FOUS30 KWBC 202025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EST Sat Jan 20 2024 Day 1 Valid 16Z Sat Jan 20 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-k69pTkUTHt8Ie_g8sPoS9v_JTYnYNpUA2CVUbjnBTvt= iLiutas_ec6JtVFrClBh2rSV0YJpYnABfTRGnieOrk4zBVg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-k69pTkUTHt8Ie_g8sPoS9v_JTYnYNpUA2CVUbjnBTvt= iLiutas_ec6JtVFrClBh2rSV0YJpYnABfTRGnieOOO3OzZo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-k69pTkUTHt8Ie_g8sPoS9v_JTYnYNpUA2CVUbjnBTvt= iLiutas_ec6JtVFrClBh2rSV0YJpYnABfTRGnieOpZIOzsg$=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) .