textproduct: San Diego

SYNOPSIS

Periods of rain, wind, and mountain snow are expected Monday through Thursday. There is a slight chance of thunderstorms Monday afternoon and evening, locally heavy rain and erratic winds possible where thunderstorms develop. Low elevation snow is possible Wednesday evening into Thursday. Chances of precipitation decrease Thursday into the weekend.

Previous Discussion (150 PM Saturday)

Key Points: *A winter storm will bring widespread, heavy snow to area mountains creating hazardous travel conditions Monday through Thursday.

*Gusty southwest to west winds are expected Monday from the coast to the deserts. Additional rounds of gusty winds expected late Tuesday into Wednesday. *Moderate to locally heavy rain is expected below the snow level. Ponding of water on roads and minor street flooding in urban areas possible.

Highs on Sunday are expected to cool a few degrees compared to today, bringing high temperatures down to near or slightly below average. Additionally as onshore flow begins to increase ahead of the incoming low, southwesterly winds will begin to increase across the mountains and San Bernardino county deserts. Peak gusts Sunday afternoon will be around 35 mph.

As the upper level low drops south on Monday, a negatively tilted shortwave is expected to progress across Southern California. This will kick off a period of rain and mountain snow. Details on rain and snow can be found in the hydrology section below. In addition to precipitation, this shortwave will allow winds to increase from the coast to the deserts. Breezy south winds will develop Monday morning ahead of the cold front. The strongest winds will occur with and the few hours after the passage of the cold front. The front will move from northwest to southeast across the area starting in Orange County/Inland Empire early Monday afternoon, reaching San Diego county by late Monday afternoon. For the coast, widespread gusts of 30 to 40 mph expected, with some local gusts up to 45 mph possible with the frontal passage. For the deserts, gusts 40 to 55 mph expected. The mountains and desert mountain slopes are expected to see gusts 55 to up to 70 mph. Gusty winds could result in hazardous travel conditions, particularly for high profile vehicles; falling tree limbs; or uproot shallow rooted trees.

A second shortwave is expected Wednesday, which should set off another round of widespread rain, mountain snow, and gusty winds. This secondary system looks colder than the first, with the potential for snow levels to fall to 3500-4000 ft which would result in snow impacting major passes like the Cajon Pass (I-15) and near Pine Valley and Crestwood Summit (I-8). Most of the individual ensemble members of both the ECMWF and GFS are showing measurable precipitation, but there remains some uncertainty in amounts likely from the uncertainty in the track of the second shortwave. Overall, the ensemble means are indicating less overall precipitation for this second round compared to the first. Chances of showers continue into Thursday but any precipitation looks lighter. NBM chances of 24- hour rainfall exceeding 0.25" is 15-25%, highest in the mountains from Thursday morning to Friday morning.

Precipitation chances lower to around 10% for Friday as most ensembles have the short wave off to the east by then. High temperatures will be well below normal through the week, as much as 10-15 degrees below on Wednesday. Slow warming occurs Friday and Saturday.

AVIATION

150600Z...Areas of low clouds have continued to develop across the inland areas (generally along the I-15 corridor) but have been spreading westward and increasing in coverage. These clouds currently based around 2000-2500ft will eventually lift above 3000ft by about 15z before scattering out through 18-21z. MVFR CIGs will likely continue across most of the coastal basin and may bring CIGs to KONT/KSBD, though there is only about a 20-30% chance. Once CIGs lift above 3000ft later this morning, VFR should prevail through the remainder of the period with BKN high clouds in place AOA 20,000ft MSL.

MARINE

No hazardous marine conditions are expected through Sunday night. A incoming storm will generate strong winds with high and choppy seas beginning Monday, weakening a bit on Tuesday before increasing again on Wednesday. Seas expected to be around 9-13 feet with wind gusts 25-35 knots, with briefly higher to 40 knots. A Gale Watch is in effect and contains more information. There is also a slight chance of thunderstorms over the waters Monday afternoon/evening.

BEACHES

Increasing westerly swell (9-11 feet with an average period of about 10-12 seconds) will produce large breaking waves of 8-12 feet on most beaches across Orange and San Diego Counties from Monday afternoon through Wednesday. Widespread, strong rip currents are also likely with these waves and dangerous swimming conditions expected. A High Surf Advisory is in effect and contains more information.

HYDROLOGY

An atmospheric river is expected to bring a surge of moisture to Southern California on Monday. The increased moisture coinciding with a shortwave trough moving through the area will spark a period of rain and mountain snow. Periods of moderate to locally heavy rain are possible along the main cold front, along with a 15-20% chance of thunderstorms Monday afternoon. Peak hourly rainfall rates during the frontal passage will range from 0.40-0.70" per hour. Current forecast rainfall totals for Monday and Tuesday will be 0.90- 1.25" for the coast and valleys, 1-3.5" for the mountains, near 0.50" for the High Desert, and 0.10-0.40" for the low deserts. Much of this rain will fall on Monday, with light to locally moderate showers possible on Tuesday. A Slight Risk of excessive rainfall is forecast for Orange County into the lower slopes of the San Bernardino Mountains on Monday.

If rainfall amounts trend up for the northern Coachella Valley, increased flows on the Whitewater River could cause flooding concerns at some of the low water crossings. Latest river forecast ensembles have around a 15% chance of the San Diego River reaching Monitor Stage on Monday and Tuesday.

Snow levels at the onset of precipitation are currently forecast around 6500 ft for the San Bernardino Mountains and increasing to around 7000 ft in San Diego County. This will reduce the flash flood threat on upper portions of the burn scars and higher elevations of area mountains. The snow level falls to around 5000-5000 ft Monday night as precipitation turns lighter and more showery. For the first round of snow Monday, 3-9 inches expected at 6000-7000 ft, 9-14 inches expected at 7000-8000 ft, and up to 20 inches above 8000 ft.

More information on the forecast precipitation amounts and rainfall rates for the second round of precipitation (Wednesday into Thursday) will be provided in the coming days.

SKYWARN

Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Wind Advisory from 7 AM to 8 PM PST Monday for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Coastal Areas-San Diego County Valleys-Santa Ana Mountains and Foothills.

High Surf Advisory from 10 AM Monday to 4 AM PST Thursday for Orange County Coastal Areas-San Diego County Coastal Areas.

Winter Storm Watch from Monday morning through Thursday morning for San Bernardino County Mountains.

Winter Storm Watch from Monday morning through Thursday morning for Riverside County Mountains.

High Wind Watch from Monday morning through late Monday night for San Diego County Mountains-San Gorgonio Pass near Banning.

Wind Advisory from 6 AM to 10 PM PST Monday for Apple and Lucerne Valleys.

Wind Advisory from 10 AM Monday to 1 AM PST Tuesday for San Diego County Deserts.

PZ...Gale Watch from Monday morning through late Monday night for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm-Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.