textproduct: San Diego

SYNOPSIS

Slight chance of showers over the mountains today. Otherwise drier and warmer through Saturday. Cooler and breezy Sunday. A winter storm will bring moderate to heavy rain, mountain snow, and gusty winds Monday and Tuesday along with a slight chance for thunderstorms. Snow levels fall through Wednesday, with elevations above 5000-6000 feet likely to see heavy snow and significant impacts next week. Additional chances of precipitation and cool weather continuing through Friday.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

A band of high clouds continues to lift northeast across the region ahead of a large upper level trough. The main trough axis will move across California late this morning into the afternoon. Forcing will be weak as the vort max passes well to our south, and moisture is generally confined to the 700-400 mb layer, so any precipitation generated by this system will be light. There is a 15% chance of showers over the mountains today, but with a very dry subcloud layer lower elevation locations should remain dry. Very weak offshore flow developing later this morning will contribute to minor warming today with highs a couple degrees above normal. Warm and dry on Saturday under weak transitory ridging.

An upper level low currently over the Gulf of Alaska will drop south this weekend, closing off just off the Northern CA coast on Sunday with troughing beginning to overspread Southern CA. This will bring cooler weather along with an increase in south to southwest winds over the mountains and deserts, most notably in San Bernardino County.

00Z ensembles are coming into better agreement with the timing of the initial short wave and associated Atmospheric River, which will bring widespread moderate to heavy rain, mountain snow, and gusty winds to the region. There is a slight (10-15%) chance of thunderstorms Monday evening as colder air aloft begins to move in. Based on current timing, the AR and heaviest precipitation would occur during the day Monday. The EPS and GEFS control runs are on the stronger end of the ensemble guidance, showing peak Integrated Vapor Transport (IVT) of just over 500 kg/m/s, though the probability of IVT of 500 kg/m/s from each of the ensemble systems is only in the 25-30% range.

Snow levels at the onset of precipitation Sunday night will be in the 5500-6000 ft range, rising to 7000-7500 ft Monday morning with the arrival of the AR, then falling to 6000-6500 ft Monday evening, and as low as 5000-5500 ft Tuesday morning.

Additional weak short waves will move through the mean trough on Tuesday, maintaining cool and unsettled weather with periods of light to moderate showers continuing. Snow levels will remain near 5000-5500 ft during the day Tuesday. The main trough axis begins to push east on Wednesday with snow levels falling to around 4000-5000 ft by Wednesday morning. There are still some timing uncertainties on how quickly this trough will eject late Wednesday or early Thursday. It is followed by another reinforcing short wave dropping south along the West Coast Thursday and Friday, maintaining chances of precipitation (20-40% Thursday lowering to 15-25% Friday). Snow levels could drop as low as 3500 ft Thursday morning across the San Bernardino Mountains. While precipitation is currently forecast to be light during this period, accumulating snow near Cajon Summit (I-15) can't be ruled out, which would make for hazardous travel conditions.

In terms of precipitation amounts, for the 48 hr period from 4 AM Monday - 4 AM Wednesday, the NBM chances for at least 0.50" are around 80-100% for the mountains westward, 40-70% over the high desert, and 15-35% over the low deserts, locally up to 50% near the San Gorgonio Pass. Chances of at least 1" are are around 40-75% for the coasts and valleys, 75-100% for the mountains, 10-15% for the high desert, and less than 10% for the low desert. At the 6500 ft level in the San Bernardino and Riverside County mountains, there is around a 75% chance of at least 12" of snow during this same 48 hr period, and around a 35% chance down at the 5000 ft level. Given the higher snow levels in San Diego County, chances of at least 12" at Palomar and Mount Laguna are around 20%.

For the 48 hr period from 4 AM Wednesday - 4 AM Friday, NBM mean precipitation accumulations are closer to 0.20-0.40" for the coasts and valleys, 0.30-0.70" in the mountains with isolated amounts near 1" in the eastern San Gabriel Mountains, 0.10-0.20" in the high desert, and less than 0.10" in the low deserts. Another 4-8" of snow (NBM mean) could fall in the mountains above 5000 ft.

As far as storm total rainfall/liquid-equivalent, the official forecast currently has around 2-3" for the coasts and valleys, 2-5" in the mountains with local amounts to 7", 1-1.5" in the high desert, and 0.40-1" in the low deserts with local amounts near 2" near the San Gorgonio pass, which could bring increased flows in the Whitewater River.

AVIATION

131030Z...High clouds will continue moving east out of the area by 16Z. Patchy low clouds have a 30% chance of impacting KSAN between 12-16Z. Any low clouds will be based between 1000-2000 feet MSL. Mostly clear conditions will prevail thereafter, except for BKN cumulus clouds around 9000 feet MSL over mountains and a 15% chance of -SHRA near the Mexico border 17-00Z.

MARINE

No hazardous marine conditions are expected through Sunday night. A storm system will generate hazardous winds and waves Monday through at least Tuesday. There is also a slight chance of thunderstorms near shore Monday evening.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.