14/202 PM.
Dry conditions and seasonable temperatures are expected this weekend. Then, a powerful storm system will affect the region starting Monday, bringing the potential for severe thunderstorms, burn-scar debris flows, flash flooding with rock and mud slides, damaging gradient winds, heavy mountain snow, and high surf with coastal flooding. Cold, blustery, and periodic precipitation will occur through at least middle parts of the upcoming week.
(tdy-Tue), 14/202 PM.
Dry conditions, seasonable to slightly above-normal temperatures, and increasing clouds are expected this weekend, as an upper ridge transits the forecast area and a major storm system approaches from the west.
A powerful storm system will bring multiple modes of potentially dangerous weather to Southern California on Monday -- possibilities ranging from severe thunderstorms and perhaps a couple of weak tornadoes, to burn-scar debris flows, to flash flooding with rock and mud slides, to damaging gradient winds, to heavy mountain snow, and very high surf with coastal flooding. Overall, the large-scale synoptic pattern, and already hints of the mesoscale environment, suggest that some of these hazardous- weather modes could even reach extreme or even destructive levels in a few isolated spots, though uncertain.
And before addressing details, it is important to note that not all locations will experience dangerous weather, as the fast- evolving, convective, and progressive nature of this system will keep dangerous weather fairly isolated. Nevertheless, interests across the local area are encouraged to remain at a high state of readiness for dangerous weather conditions on Monday. The convective environment will be characterized by a low-CAPE, high- shear parameter space, in which nonlinear feedbacks inherent to updraft-shear interactions influence the specific locations, timing, and magnitudes of the relatively-spotty extreme weather. These feedbacks are notoriously accompanied by low predictability, such that uncertainty in the details of extreme impacts -- if they were to occur -- could remain significant right up until the onset of such impacts, if they were to occur. In short, the synoptic pattern will favor the potential for dangerous, and even life-threatening, weather conditions to occur on Monday, anywhere from the Transverse Ranges southward to the Pacific Coast. Everyone is encouraged to monitor the latest information from the National Weather Service in Los Angeles/Oxnard, California. Follow the direction of Emergency Management and Law Enforcement officials as the situation quickly evolves on Monday. Flood Watches, Winter Storm Watches, High Wind Watches, and Wind Advisories have been posted for Monday. The conclusion of this short term section will be followed by a technical discussion concerning the potentially dangerous weather hazards for Monday, along with sources of uncertainty, details on impacts, and ranges of forecast scenarios.
Beyond Monday, Tuesday will feature relatively less precipitation as the forecast area is positioned between lobes of ascent traversing the base of CONUS-full-latitude troughing anchored over the West. Given the placement of a sub -20C 500-mb cold core over the area, diurnally enhanced convective precipitation and upslope-flow-favored precipitation will continue, and it will be a blustery, cool/cold day. Temperatures in most areas will remain at or below the 50s. While high-elevation snow will have tapered off temporarily by Tuesday, blowing/drifting snow and much- reduced visibilities will remain of concern and Winter Storm Watches will continue for the eastern San Gabriels and northern Ventura County Mountains, through the passage of an even colder precipitation-producing storm system Tuesday night into Wednesday -- addressed in the long-term section.
---Technical discussion for Monday weather hazards follows---
*Severe Thunderstorms and Perhaps a Couple of Tornadoes:
Within a highly amplified flow pattern in the mid and upper levels, impulses of significant upper jet energy digging southward from the Gulf of Alaska will be carving out a longwave trough from late this weekend and into next week. A lead shortwave trough will round the base of larger-scale broad cyclonic flow on Monday and eject east-northeastward with a trajectory right over the local area, becoming prominently negatively tilted during the day Monday. An accompanying 100-110-kt 500-mb, cyclonically-curved speed maximum will race east-northeastward toward the Channel Islands in the afternoon, positioning its left-exit region and highly diffluent upper flow over the Bight of Southern California.
The accompanying thermally indirect ageostrophic circulation in the exit region of the upper jet is expected to facilitate a relatively deep layer of low-level mass response, with an intense low-level jet forming and progressing across the region on Monday. Within the warm sector of the cyclone, 850-700-mb southerlies to southwesterlies in conjunction the low-level jet are forecast to reach 45-65 kt. Related thermal advection will augment the strong, deep ascent to foster intense cyclogenesis for a northeastward- tracking surface low toward the California Central Coast during the day on Monday, driving LAX-SFO onshore pressure gradients to around 10-13 mb. Deepening rates for this surface low could briefly approach 1 mb per hour for a short duration on Monday, resulting in a strong isallobaric response to the deep cyclogenesis N/NW of the forecast area, backing surface winds across the region. This mass response will also form a north-south confluence axis offshore from the Central Coast, which will progress eastward through the day in conjunction with the progression of the ejecting lead trough.
East of the confluence axis, a narrow corridor of weak surface- based buoyancy will spread across the forecast area, owing to the influx of modest Pacific moisture -- lower to middle 50s surface dewpoints -- being surmounted by rapid midlevel cooling and steepening midlevel lapse rates in advance of the midlevel cold pool characterized by 500-mb temperatures around -28C to -30C offshore. A shallow, convective squall line will form in the vicinity of the confluence axis, ingesting weakly buoyant inflow within the low-buoyancy corridor -- spreading onshore along the Central Coast by mid-day Monday and rapidly progressing eastward through the day while. The timing of the squall-line passage through the area is expected to be around 9AM-6PM, likely crossing Pt. Conception before 2PM and exiting Los Angeles by 6PM. The buoyancy, and resultant squall line, will likely be truncated north of the Transverse Ranges, and east of the Central Coast ranges, by significant boundary-layer theta-e deficits owing to upslope precipitation and related wet-bulbing effects.
Exact SB/MLCAPE values within the narrow corridor of buoyancy -- likely in the 100-500 J/kg range and aided by muted diurnal diabatic heating the moist axis -- are still uncertain, and will likely remain uncertain until right before the onset of the event. However, severe-weather impacts will be highly sensitive to the exact buoyancy developing onshore, to which this uncertainty extends. For example, early and/or widespread development of convection near or ahead of the confluence axis could significantly deplete buoyancy by the time more organized convection arrives. This could greatly weaken the intensity of updrafts and downdrafts in the squall line. Alternatively, slower ejection of the lead trough, and a lagging midlevel cold pool, could result in weaker phasing between steepening midlevel lapse rates and ascent with the confluence axis, boundary-layer moisture influx, and squall line -- resulting in a considerably weaker squall-line vertical circulation. Nevertheless, the aforementioned low-level mass fluxes offering ample convective momentum transfer, even in weak-magnitude or weakly-organized convection, combined with highly favorable vertical shear profiles, will favor the risk for localized damaging wind gusts and perhaps a couple of weak, brief tornadoes.
At least 35 knots of line-orthogonal low-level and effective shear will offer the potential for mesovortices, and mesocyclones with embedded low-topped, mini-supercells, to focus isolated, meso-gamma-scale corridors of enhanced wind damage and weak tornadoes -- within 10-20 miles of the Central Coast earlier in the day, and then especially along and south of a line from Santa Barbara to Ventura to Oxnard to Camarillo to Malibu to Beverly Hills and Hollywood to Montebello to Whittier. Locations farther north into the Santa Monica and San Gabriel Mountains may experience convectively enhanced wind gusts to severe levels, as convection augments the background strong gradient winds, even where snow is occurring at elevations above 6-6.5 kft. Local convectively-enhanced gusts could even briefly exceed 70 mph in these areas, though the tornado risk will be confined to areas farther south where surface dewpoints exceed around 50F modestly boosting low-level buoyancy.
As addressed above, regarding alternate scenarios, lesser coverage of showers ahead of the squall line and greater synoptic- scale phasing of the midlevel cold pool and low-level moist axis would boost buoyancy and the risk for severe wind gusts and tornadoes. Meanwhile, greater pre-squall line shower coverage in the warm-advection wing of the synoptic system, and less favorable upper-level phasing, could greatly limit the severe- weather risk. Impacts within the very-low-CAPE part of the thermodynamic parameter space are highly sensitive to small variations in buoyancy given the non-linear feedbacks in strongly sheared environments, and the coverage/intensity of severe- weather hazards may not become clear until the weather event is unfolding. This is a major factor, combined with the elevated conditional risk for severe storms, for interests to remain at a high state of weather readiness on Monday. Regarding preparedness, if winds greatly intensify, remaining indoors in the lowest part of a building and away from windows would greatly help mitigate impacts from such severe-weather hazards.
By 6PM Monday, the squall line will likely have pushed east and south of the forecast area. However, through the day Monday and continuing into Monday night, scattered showers and isolated thunderstorms will continuously re-generate behind the squall line across much of the region. This will occur as the cold pool aloft continues to overspread the area and weak-vorticity centers move across the area, and frictional convergence near the coast, terrain-circulations, and residual convective boundaries focus subsequent convective development amid the steepening midlevel lapse rates, which will offset the decreasing post- squall-line moisture. While the coverage of this activity will be lesser, updrafts will have a tendency to extend deeper into icing layers aloft owing to the midlevel cooling, and lighting strikes would be more likely with this activity, compared to the squall line that could contain no lightning. Surface winds will veer in the wake of the squall line, so hodographs will be less favorable for low-level mesocyclones to rapidly form, however the ample background vertical vorticity and continued intense momentum fields aloft would support both weak non-supercell tornado and strong to locally damaging convective wind-gust potential, as well as the risk for small hail, behind the squall line into Monday night. Such risk would be maximized before sunset when low- level lapse rates are diurnally steeper.
*Burn-Scar Debris Flows, Flash Flooding, and Rock and Mud Slides:
Precipitation rates with the aforementioned squall line will likely reach 0.75-1 inch per hour in many areas, with much of that rain falling in 15-30 minutes. As a result, there is an increasing chance (at least 50%) for burn-scar debris thresholds to be at least marginally exceeded for the burn scars. These rainfall rates may support at least minor debris flows on the burn scars, and there is a remote possibility (20% chance) for a more significant, damaging burn-scar debris flow to develop in Los Angeles County where moisture fluxes ahead of the squall line maximize with muted daytime heating. Outside of the burn scars, such rainfall rates will be conducive for flash flooding in and around roadways, and smaller creeks and streams -- potentially becoming significant in and around LA. In and around the mountains and foothills, especially in LA and Ventura Counties, the previously mentioned intense low-level jet will prolong the duration of more extreme rainfall rates by augmenting ascent through strong upslope flow amid 1+ inch precipitable water. In these areas, more significant flash flooding, along with dangerous rock and mud slides, will be possible, and these conditions could reach life- threatening magnitudes on a localized basis -- though event, timing, and location uncertainty for such conditions is currently significant. Flood Watches have been issued 9AM-9PM Monday along and south of the Transverse Ranges to address the increasing risk for dangerous flooding to develop on Monday. While flooding potential will slowly decrease following the passage of the squall line, more cellular convection in its wake will locally prolong flooding potential on a localized basis, given the saturated antecedent ground.
Also of note, with snow levels expected to remain in the 6-6.5 kft range during the duration of most intense rainfall rates. Above these elevations, heavy-snow accumulations will locally mitigate the burn-scar debris-flow potential. Thus, the burn-scar debris potential will be elevation-dependent, and will should quickly where there is an onset of high-elevation snow.
*Heavy Mountain Snow:
With the onset of intense precipitation rates previously mentioned, and snow levels holding around 6-6.5 kft on Monday, the onset of an extended duration of heavy mountain snow will accompany the north edge of the squall line and peripheral upslope-flow-enhanced precipitation over the Transverse Ranges on Monday. While the coverage of affected area will be limited by the warm-advection- boosted snow levels, at least several inches of snow could accumulate above 6 kft in the eastern San Gabriel and the northern Ventura County mountains by late Monday afternoon. And then through Monday night, as cooling aloft further overtakes the local area, snow levels will fall to around 4-4.5 kft while precipitation regenerates (albeit not as heavy) -- bringing a larger areal coverage of mountain snows across the higher elevations in LA and Ventura Counties. Snow will begin tapering off by late Monday night, but not before 1 to 2 feet of snow accumulate above 6 kft, with 3-6 inches of snow accumulating in many areas from 4 to 6 kft. And with the snow combining with the previously mentioned low-level jet intercepting the high terrain to bring 60-70 mph wind gusts producing blowing/drifting snow and reduced visibilities, a Winter Storm Watch has been posted for the eastern San Gabriels and northern Ventura County mountains starting at 6AM Monday. Even after snow temporarily ends by early Tuesday morning, blowing and drifting snow and reduced visibilities will be a concern, along with roadways remaining snow covered, and the Winter Storm Watch will continue through Thursday morning.
*Strong to Damaging Gradient Winds:
As addressed above, there will be a significant low-level jet crossing the region in conjunction with the ejecting midlevel shortwave trough. As the low-level jet intercepts the interior mountains -- especially those oriented west-to-east, gusts of 60-70 mph are anticipated to develop including through passes and canyons. While convection and the development of gravity waves could enhance these winds even further, the potential for several hours of gusts exceeding 60 mph warrant the issuance of High Wind Watches for the western San Gabriels, I-5 Corridor of NW LA County, the Antelope Valley Foothills, southern Ventura County mountains, and the Santa Barbara County Interior Mountains. These areas are outside of Winter Storm Watches, and although any snow in mountainous locations in these areas should be limited in coverage and magnitude given comparatively high snow levels, isolated spots of high-elevation snow and blowing/drifting snow reducing visibilities will be possible in the High Wind Warning areas.
Outside of the High Wind Warning areas, widespread 35-50 mph southerly wind gusts are anticipated for several hours -- provided mechanical and buoyant mixing into the deep layer of the significant mass response, characterized by 30-40 kt mean winds in the 0-1-km and 0-3-km layers, and the tightening surface pressure gradient and isallobaric flow response to the deepening surface cyclone. A Wind Advisory has been posted for the remainder of the forecast area for Monday. If pockets of severe convective winds were to become imminent on Monday, especially in the Wind Advisory area, then shorter-fuse convective warnings could become required.
Wind Advisories and High Wind Warnings will be in effect 6AM to 6PM Monday. This would be in conjunction with the strongest low- level mass response, and ahead of the expanding midlevel cold core that will prolong convective/flooding potential beyond cessation of stronger gradient winds.
*High Surf and Coastal Flooding:
High Surf Advisories have been posted, and isolated minor coastal flooding will be possible in locations most vulnerable to significant northwest swell. Please reference the Beaches section below for more information.
(Wed-Sat), 14/202 PM.
Looking further ahead, the large, upper-level longwave trough will remain anchored across the western CONUS throughout much of the remainder of the upcoming week. This will be reinforced by multiple impulses of compact vorticity and accompanying speed maxima pivoting through the western rim of the larger-scale troughing. It appears that Southern California will be beneath the trajectory of multiple impulses through the remainder of the upcoming week. Each impulse will have the potential to boost precipitation rates and winds within a rather long duration of on- and-off showers, cold, and blustery conditions. The progressive nature of each impulse following the Monday, more-favorable-for- flooding event may tend to mitigate subsequent flooding impacts. However, increasingly saturated ground conditions could offset the short-duration precipitation character to maintain, or even boost, flooding concerns through the week, and uncertainty regarding hydrological impacts grows quickly through the course of the week. Regardless, temperatures through the week in most areas will be quite cool, in the 50s and lower 60s as high temperatures, while snow levels gradually fall in response to cold advection with each passing impulse. Cold-weather impacts could become of increasing concern through the week.
The upcoming Tuesday night through Wednesday system is a more- likely period of stronger jet energy passing over the local area, providing more precipitation and gusty winds. In addition to the risk for at least minor flooding, cooling aloft and lowering snow levels will bring another round of heavy snow in the mountains but over a broader area given lowering snow levels. Heavy snow, with blowing and drifting snow, could impact elevations as low as 3000 or 4000 ft elevation including the Grapevine of the Interstate-5 corridor, if enhanced mositure can adequately phase with cooling aloft. This will depend on numerous mesoscale factors for which uncertainty is significant. The Winter Storm Watch expands starting Tuesday night to include Santa Barbara County Interior Mountains, southern Ventura County Mountains, and the Interstate 5 Corridor, remaining in effect for the eastern San Gabriel and northern Ventura County mountains. Before the system departs, across the Winter Storm Watch areas, total snow accumulations could reach 1 to 4 inches for 3500 to 4500 feet elevation possible, 6 to 12 inches for 4500 to 6000 feet elevation possible, and one to two feet above 6000 feet elevation possible. Winter Storm Watches continue through Thursday morning, when the snow tapers off.
While a drying trend may occur on Friday and Saturday, the potential for precipitation cannot be ruled out through late in the upcoming week and into next weekend, as embedded impulses track across the local area.
15/0251z.
At 2328Z at KLAX, there was no marine layer and no inversion.
High confidence in KWJF and KPMD. Moderate confidence in coastal/valley TAFs. For tonight, high confidence in return of MVFR CIGs, but low confidence in timing (could be +/- 3 hours of current forecasts). Additionally, cigs may scatter and reform at times.
KLAX, Moderate confidence in the 00Z TAF. Timing of flight cat chances may be off by +/- 3 hours. An easterly wind component of 6-7 kt will return between 12Z-20Z Sun.
KBUR, Moderate confidence in the 00Z TAF. Timing of flight cat chances may be off by +/- 3 hours. Cigs may scatter and reform at times.
14/252 PM.
For the Outer Waters, moderate to high confidence in current forecast. Small Craft Advisory (SCA) level seas will diminish this evening. Tonight and Sunday, southeast winds will increase to SCA levels as well as the seas. For Sunday night through Wednesday, high confidence in SCA level winds and seas. Additionally, there is a 50-70% chance of GALE force winds late Sunday night through Monday and again Tuesday night through Wednesay.
For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. Through tonight, high confidence in winds and seas remaining below SCA levels, except for locally in the far western portion. For Sunday through Wednesday, high confidence in combination of SCA level winds and seas developing. Additionally, there is a 50-60% chance of GALE force winds late Sunday night through Monday and again Tuesday night through Wednesay.
For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. Today through Sunday, high confidence in winds and seas remaining below SCA levels. For Sunday night through Wednesday, SCA level winds are likely. Additionally, there is a 40-50% chance of GALE force winds on Monday.
A squall line will likely develop in association with the cold front late Sunday night through Monday, which will result in hazardous marine conditions. These include: GALE force winds, dangerous Lightning, heavy rainfall with reduced visibility, and waterspouts.
14/226 PM.
A High Surf Advisory is in effect for the Central Coast, though it is set to expire this evening as the swell decreases.
A period of very large waves is expected to start on Monday, with surf generally peaking Tuesday and Wednesday across west facing beaches. High Surf Advisories and Beach Hazard Statements are in effect for all coasts, see the CFWLOX and SRFLOX products for more details.
Elevated high tides as the new moon cycle approaches will coincide with the arrival of the higher surf and swell. This will bring an elevated chance of coastal flooding, or at the very least, minor tidal overflows especially during the time of the high tides, particularly on Tuesday and Wednesday. A coastal flood advisory may be needed should swell guidance remain consistent along with wind- driven factors.
Ca, High Wind Watch in effect from Monday morning through Monday afternoon for zones 38-344-345-353-376-378-379-381-382. (See LAXNPWLOX). Wind Advisory in effect from 6 AM to 6 PM PST Monday for zones 87-88-340>343-346>352-354>358-366>375-383-548. (See LAXNPWLOX). Flood Watch in effect from Monday morning through Monday evening for zones 87-88-348>358-362-366>380-548>550. (See LAXFFALOX). High Surf Advisory in effect until 9 PM PST this evening for zones 340-346. (See LAXCFWLOX). High Surf Advisory in effect from 10 AM Monday to 6 PM PST Thursday for zones 340-346-362-366. (See LAXCFWLOX). Beach Hazards Statement in effect from Monday morning through Wednesday morning for zones 349-350. (See LAXCFWLOX). Winter Storm Watch in effect from Tuesday evening through Thursday morning for zones 353-376-378. (See LAXWSWLOX). High Surf Advisory in effect from 10 AM Monday to noon PST Thursday for zone 354. (See LAXCFWLOX). Winter Storm Watch in effect from Monday morning through Thursday morning for zones 377-380. (See LAXWSWLOX). PZ, Small Craft Advisory in effect until 9 PM PST this evening for zones 645-670-673-676. (See LAXMWWLOX). Gale Watch in effect from Sunday evening through Monday afternoon for zones 645-650-670-673-676. (See LAXMWWLOX). Gale Watch in effect from late Sunday night through Monday afternoon for zone 655. (See LAXMWWLOX).
