Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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850 FXUS66 KLOX 190748 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1248 AM PDT Thu Sep 19 2024 .SYNOPSIS...18/824 PM. An unsettled weather pattern is on the way for the next couple of days, as a compact atmospheric disturbance crosses the region. It will turn much cooler across the area for Thursday into early Friday, when showers and thunderstorms will move across the region. Not every location will get rain, though many will, and thunderstorms producing possible flooding rains will impact interior portions of Ventura, Santa Barbara, and San Luis Obispo Counties for Thursday and Thursday evening. Some showers or thunderstorms could linger into Friday before the disturbance exits the region, and then it should be dry for several days starting this weekend when temperatures warm up again. && .SHORT TERM (THU-SUN)...18/845 PM. ***UPDATE*** Interesting weather pattern the next 24-36 hours as an unseasonable cold upper low moves through the area. 00z models continue to zero in on the interior part of SLO County through interior SBA/Ventura Counties as the focus of the most active weather beginning Thursday, then shifting southeast into the remainder of Ventura and LA Counties Thursday night into Friday morning. It won`t rain everywhere, but there could be periods of heavy rain and thunderstorms with hail in the most active areas, which again models are focusing in the interior areas bordering Kern County, west of LA County. It`s a very tricky and low confidence forecast but the potential is there for some significant impacts in those areas. For this reason a flood watch was issued earlier, emphasizing the potential for debris flows around the recent burn scars. For LA and southern Ventura County much of Thursday may remain dry with on and off sunshine. Hi res models do develop a fairly deep marine layer later tonight that could generate some morning drizzle across coast/valleys/foothills, but a weak inversion should allow for at least some sunshine in the afternoon. Overall, no significant changes to the forecast other than to decrease rain chances south of Pt Conception Thursday during the day (aside from the morning drizzle potential). ***From Previous Discussion*** A record-caliber, upper-level cold core low is headed to Southern California. Satellite loops show the tightly coiled upper vort center diving southward over the central California waters, about 100 miles west of San Francisco. Multiple mesoscale swirls are drifting southward beneath the synoptic cold core, suggesting that sub-grid-scale processes are probably influencing what is evolving into a very unseasonably deep, cut-off upper low that will affect the region during the next couple of days. Model solutions continue to catch up to this considerably deeper system and its disconnection from westerly steering currents at higher latitudes. What this means is that the potent upper system -- accompanied by record-cold upper-level temperatures based on sounding climatology data -- will take a more southern trajectory and achieve greater depth as it tracks across the bulk of the local area, with core 500-mb heights around 566 dam and core 500-mb temperatures around -19C. In response to these developments, confidence continues to grow in a potentially high-impact convective event taking place over interior portions of Ventura, Santa Barbara, and San Luis Obispo Counties -- where a Flood Watch has been posted for Thursday morning through Thursday evening. This does include the potential for debris flows on the Hurricane and Apache burn scars. However, this is not a particularly high-confidence event. As the strongest cooling aloft overlies diurnally-enhanced orographic ascent along the interior slopes across the Watch area, expect thunderstorms to quickly form by mid-day Thursday. Sub- cloud-layer humidities will be elevated by onshore flow, and DCAPE will be modest, so individual potential-temperature deficits in convective cold pools will be weak and permit fast cycles of upshear boundary-layer recovery. And with the slow-moving large- scale ascent, conditions will be optimal for regenerative and backbuilding convection that will facilitate convective training across the Watch area, as individual convective plumes slowly drift northward to eastward at 5-15 kt. So even though precipitable water will be modest -- on the order of 0.75 to 1.00 inch -- this convective scenario will be able to fully leverage any moisture available in advance of the upper low. Given surface- based CAPE around 500 J/kg and the convective training scenario, and low echo centroids maximizing rainfall efficiency (i.e., limited sub-cloud evaporation of precipitation), rainfall rates of one-half inch per 30 minutes and one inch per hour will be possible, capable of producing flash flooding in the Watch area. The most-likely time period for this activity will be 1 PM PDT through 7 PM PDT Thursday. Potential impacts across the Flood Watch include flooding of rivers, creeks, streams, and other low- lying and flood-prone locations. The thunderstorms will have the potential to impact the Hurricane and Apache burn scars during the period of the Watch. Residents near the Hurricane and Apache burn scars should prepare for the possibility of debris flows and flooding impacts. Of note, given the dominant convective character to precipitation on Thursday, and expected narrow convective plumes limiting the predictability of their evolution, confidence in related flood impacts -- if they were to occur -- could escalate rather quickly with limited lead-time prior to the onset of these impacts. In addition to potential flooding and debris-flow impacts, with low freezing-level heights around 9 kft AGL and ample upper- tropospheric flow offering strong ventilation to updrafts and a modest increase of effective shear to 20-30 kt, small hail -- perhaps accumulating in some areas -- will be possible with these thunderstorms, and local wind gusts to 45 mph will be possible as well. Moreover, stretching of ambient vertical vorticity around the deeply-stacked cold core low could support a couple of cold- air funnel clouds, as well, though tornadoes are highly unlikely (less than 2% chance). POPs have been increased to 100% across a large portion of the Flood Watch area, in conjunction with the lobe of strongest ascent forced by differential cyclonic vorticity advection ahead of the upper low favorably phasing with the strongest orographic ascent. Through Thursday evening, in locations where the heaviest rain falls for the longest duration, total QPF could locally exceed 2 inches in spots. However, there may be numerous sharp gradients in total precipitation amounts given anticipated narrow convective plumes. As a result, total rain amounts and related impacts may vary considerably over short distances. Nevertheless, instability created by the anomalously cold upper-level temperatures and sufficient ascent will provide at least slight chances for showers and thunderstorms across much of the remainder of the forecast area for Thursday into Thursday night, as activity gradually develops eastward through in tandem with the track of the upper low. Any locations that experience heavier showers could pick up a quick tenth or two tenths inch of rain. Similarly, there should be a relatively sharp gradient in convective coverage between the Watch area and elsewhere across the region for Thursday into Thursday night -- owing to the compact lobe of synoptic-scale ascent and lack of greater deep moisture. The exact coverage pattern of this convective precipitation will be heavily influenced by the precise track of the upper low, for which some uncertainty still exists owing to the aforementioned sub-grid-scale effects. Minor deviations in its track could result in sizable spatial shifts in higher convective coverage and related impacts. For example, the Lake burn scar is southwest of the most-likely area of Thursday thunderstorms. However, there is some potential for this activity to drift over the Lake burn scar Thursday afternoon and evening. If this were to occur -- around 20 percent chance -- flash flooding and debris flows could also impact the Lake burn scar. This area, and also other areas peripheral to the Flood Watch, will be monitored closely for possible expansions to flood headlines. All of the above is to say -- Thursday into Thursday evening has the potential to be a localized high-impact, low-confidence event. Shower and thunderstorm coverage will gradually lessen by late Thursday evening. However, this activity will not completely dissipate, owing to the very pronounced ascent and slight nocturnal cooling at the top of the moist layer that will maintain marginal boundary-layer-based buoyancy into the overnight hours. Isolated to widely scattered showers and thunderstorms will have the potential to be spreading across Ventura and Los Angeles Counties through the overnight hours. However, with enough nocturnal stabilization (though incomplete), the potential for flooding and strong thunderstorms will decrease with southward extent of a line from Point Conception to Gorman. Going into Friday, chances for showers and slight chances for thunderstorms will linger across the eastern half of the forecast area -- given an anticipated slower-moving and deeper upper low prolonging its effects over the local area. A low tracking even slower than expected could bring potential hydrological impacts to the Post and Bridge burn scars -- 10% chance. By Friday evening, showers and thunderstorms will have exited the region, giving way to several days of dry conditions. Significant erosion of the inversion surmounting the marine- influenced moist boundary layer -- owing to the large-scale ascent -- will greatly disrupt marine-layer maintenance and related low stratus and fog. As a result, cloud coverage, especially later tonight through Saturday, will likely be driven solely by convection and related convective debris. Once convection exits the region, skies will mostly clear by late Friday into Saturday. Temperatures will be coolest on Thursday, owing to the greatest influence of the cold core aloft, and will likely be running 10-15 degrees below normal -- i.e., high temperatures in the lower 70s in many areas, with several locations over higher terrain and near the beaches staying below 70 degrees. High temperatures are forecast to warm a few to several degrees on Friday, though slower motion of the upper low could considerably mute the warming trend (30% chance). Confidence is comparatively higher that rebounding midlevel heights in the wake of the departing upper low will facilitate a more significant warming trend by several degrees for Saturday, with highs topping out in the 80s for many locations. .LONG TERM (MON-THU)...18/225 PM. A much quieter period is currently expected in the extended forecast, as the general consensus amongst medium-range model solutions is for an upper ridge to remain nearly stationary over the southwest states from late this weekend into next week. Differences in the strength of this ridge are significant between the various model solutions, casting uncertainty regarding the amount of warming that takes place. High temperatures are expected to reach the 80s and 90s by early next week in many areas away from the coast, and some model solutions indicate even warmer temperatures materializing (30% chance). Pressure gradients trending somewhat offshore owing to surface ridging behind the departing upper low will have the potential to enhance the warming trend late in the weekend into Monday. Alternatively, some model solutions indicate additional wave amplifications over the central and northern CONUS carving out an upper low that retrogrades over the southwest CONUS around early next week. This would limit the degree of warming (10-20% chance). Overall, the upper flow over the CONUS will remain highly amplified into next week. While the most-likely outcome is for upper ridging to affect southern California, short wavelengths to the amplified pattern aloft tend to suggest limited predictability. Thus, current forecast temperatures are of only low to medium confidence at this time. However, there is strong consensus among model solutions that a dry air mass will prevail across the region, with an unfavorable flow pattern for deep-layer moisture to return -- keeping dry conditions in place. However, the marine layer should more significantly re-establish across coasts and coastal valleys by Sunday and into next week -- accompanied by night through morning stratus and fog. && .AVIATION...19/0747Z. At 06Z at KLAX, there was a 5000 ft deep moist layer and no inversion. Good confidence in TAFs for KPMD and KWJF. Low to moderate confidence in the remainder of the TAFs. Cigs heights will vary frequently due to presence of upper level low moving over the region. For KPRB, KSMX, KSBP, and KSBA -DZ is possible through the morning. Then there is a 20% chance of TSTMs with brief heavy rain and gusty and erratic winds 17Z through the period. Timing of flight cat changes and rain timing could be off by +/- 3 hours. For Ventura and Los Angeles County TAFs (except desert TAFs), Cigs will vary between MVFR and VFR through 16Z, then lift to VFR. -DZ is possible at all sites through the morning due to the marine layer rapidly deepening. There is a 10% chance of isolated TSTMs at KOXR and KCMA after 00Z Fri, and a 20% chance of VCSH at all sites after 04Z. There is a 20% chance of MVFR cigs at L.A. sites after 04Z Fri. KLAX...Low to moderate confidence in TAF. Cigs may vary between BKN015-035 frequently through 17Z as upper level clouds move overhead. -DZ is possible through 17Z Thurs. There is a 20% chance of BKN025-030 cigs after 04Z Fri. VCSH could occur as early as 06Z. No significant east wind component is expected. KBUR...Low to moderate confidence in TAF. There is a 20% chance of BKN022-030 conds through 15Z Thurs and after 04Z Fri. && .MARINE...18/938 PM. Low confidence in the current forecast for the period. Higher confidence exists in the forecast for seas. Lower confidence in the current forecast for winds and thunderstorms. Isolated to scattered thunderstorms will be possible over all of the coastal the marine zones Thursday afternoon through Thursday evening as a potent upper-level low pressure system drops from Northern California to near Pt. Conception by Thursday afternoon. The trough will move southeastward across the southern California bight Thursday night and onshore over southern California through Friday. A marine weather statement is in effect to communicate the hazards from thunderstorms to the marine community Thursday. Winds and seas diminished a little earlier than expected this evening and the Small Craft Advisory (SCA) was cancelled early. There is a moderate chance of localized wind gusts at SCA levels through midnight tonight. Winds and seas will likely remain below SCA levels through the period, except for a moderate-to-high (30-50 percent) chance of SCA level winds each afternoon and evening for the southern waters from Point Conception south to San Nicolas Island, in the vicinity of the Channel Islands, and inside the southern California bight. The highest chance will be on Friday afternoon and evening when marginal to widespread low end SCA level winds could develop. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Flood Watch in effect from 5 AM PDT early this morning through this evening for zones 38-344-345-377. (See LAXFFALOX). PZ...NONE. && $$ PUBLIC...MW/Cohen AVIATION...Lund MARINE...Hall SYNOPSIS...MW/Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox