Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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096 FXUS66 KLOX 230638 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1138 PM PDT Sun Sep 22 2024 .SYNOPSIS...22/153 PM. Warm and dry conditions can be expected through Monday as high pressure aloft builds over the region. The warmer air mass will likely remain in place through much of the week away from the coast, with the hottest daytime temperatures forecast for Monday and Tuesday. It should turn cooler late this week into next weekend. Mostly clear skies can be expected through the period except for night and morning coastal low clouds and fog. && .SHORT TERM (SUN-WED)...22/813 PM. ***UPDATE*** Temperatures today were able to warm up to the high 80s to mid 90s across the interior areas, including the San Fernando Valley. Coastal valleys reached 80s to 90s, while the beaches were largely in the 60s thanks to the the persistent marine layer clouds. A combination of very weak offshore flow setting up and slight increase of upper level highs overnight will lead to the potential of patchy dense fog, and less expansive marine layer clouds overnight into tomorrow morning. Low temperatures tonight will generally be a few degrees higher than last nights` due to very weak offshore flow setting up overnight, though areas at the coast will remain pretty similar. As previously mentioned, very weak offshore flow will set up due to a broad trough over the Great Basin and a low pressure system developing a few hundred miles off of the Central Coast. The weak offshore flow and slight rise in 500 mb heights will allow for better afternoon clearing of the marine layer tomorrow, and in turn result in up to 10 degrees of warming compared to today. The interior areas (including the interior valleys such as SCV and SFV) will be in the mid to high 90s tomorrow, the remaining valleys will be in the 80s to low 90s, and as you get closer to the beaches, temperatures will be in the high 60s to high 70s. Temps at the beaches will be fairly normal for this time of year, however most areas will be a few degrees to up to 10 degrees above normal. Forecast is in good shape, and no updates were needed at this time. ***From Previous Discussion*** The upper level low will edge slowly N Mon night thru Wed, and by Wed will weaken and become a broad upper level trof. H5 heights over SW CA will be in the 585-586 dam range thru Tue then fall to 581-584 dam by Wed with the upper level flow turning to the S and SW. The marine layer will shrink through tonight thanks to significant warming between about 900 mb and below. By early Mon morning, the inversion should be around 800 ft or so S of Point Conception and possibly near surface-based on the Central Coast. Little change in the inversion depth is expected thru Tue, then it should increase only slightly for Tue night into Wed and be about 1000 ft or less. A marine layer pattern with night and morning low clouds and fog mainly along the coast followed by clearing back to or off the coast during the late morning and afternoon hours can be expected tonight through Wed. Due to the shallow marine inversion developing, patchy dense fog is possible with the low clouds especially later at night into the morning hours thru Tue. Elsewhere and otherwise across the forecast area, mostly clear skies are expected thru Wed, with breezy onshore winds each afternoon. Pressure gradients to the N and E are forecast to trend offshore tonight into Mon morning which will allow temps to warm considerably. Highs on Mon are forecast to be 5-10 deg above normal except along the immediate coast. Weak pressure gradients will support continue significantly above normal temps into Tue, especially away from the immediate coast. Also of note, overnight lows will be quite a bit above normal for many areas, with temps likely remaining in the 70s for the foothills and lower mtns. By Tue night and Wed, temps should start to trend cooler as onshore flow increases. Highs on Wed are expected to be a few degrees below normal to near normal for the coast and vlys, and 3-7 deg above normal for interior areas including the mtns and deserts. .LONG TERM (THU-SUN)...22/152 PM. The GFS and EC ensembles and deterministic are in generally good agreement thru the extended period. An upper level low is forecast to develop about 500 miles SW of the fcst area by Thu then meander well off the coast thru next Sun. Upper level ridging will linger generally over the southern Great Basin into SE CA thru the period. H5 heights over the forecast area should be in the 586-588 dam range thru the extended period altho the EC deterministic does increase the H5 heights to 590-592 dam next weekend. The marine inversion should remain relatively shallow Thu and Fri then increase some to about 1000-1500 ft or so Sat and Sun. Night thru morning low clouds and fog mainly for the coast can be expected Thu thru Sat, then expand some inland on Sun. Otherwise and elsewhere, mostly clear skies can be expected thru the period. Temps Thu and Fri should be from a few degrees below normal for the coast and vlys to a few degrees above normal for interior areas, then cool several degrees over the weekend as onshore flow increases. By Sun, temps should be several degrees below seasonal norms for most areas, but remain a few degrees above normal in the far interior vlys and deserts. && .AVIATION...23/0638Z. At 0515Z at KLAX, the marine layer was 1100 feet deep. The top of the inversion was 2600 feet with a temperature of 23 degrees C. High confidence in TAFs for KPRB, KPMD, and KWJF. Good confidence in KBUR and KVNY. There is a 15% chance of LIFR conds 12Z-15Z. Moderate confidence in coastal TAFs. VIS will likely vary frequently but LIFR cigs will hold steady. There is 30 percent chance that cigs will linger an hour or two later than fcst. Low clouds may also return 2 hours earlier than fcst Monday evening. KLAX...Moderate confidence in TAF. Vis will likely vary frequently through 15Z and TAF will reflect the sfc vsby not the tall tower vsby. There is a 20 percent chc that VFR conds will arrive at 18Z and another 20 percent chc that it will be delayed until 21Z. Cigs Monday even could arrive as early as 02Z. Good confidence in no significant east wind component through the period. KBUR...Good confidence in TAF with only a 15 percent chc of OVC004 conds 12Z-15Z. && .MARINE...22/635 PM. High confidence in the current forecast through Wednesday, then moderate confidence thereafter. Winds and seas will remain below Small Craft Advisory (SCA) levels through at least Wednesday, except for 30-40% chance of local low end SCA level wind gusts near Pt Conception, around Malibu, and the San Pedro Channel thru this evening. There is also a 20-30% chance for SCA level winds in the inner waters off the LA and OC Coasts in the afternoon thru Wednesday evening. Then, moderate to high confidence in winds and seas increasing to SCA level Thursday thru Friday, with highest confidence in the Outer Waters (Central Coast south to San Nicolas Island) Friday. There is also a 20-30% chance for Gale Force winds Friday afternoon thru late night in the Outer Waters, with highest confidence from Pt Conception north. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. && $$ PUBLIC...Sirard AVIATION...Rorke MARINE...Lewis/Schoenfeld SYNOPSIS...Sirard weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox