textproduct: San Joaquin Valley

AVIATION UPDATE

MVFR/VFR conditions are expected at all sites with ISO -RA showers until 26/05Z when a more organized line of moderate rain is expected to arrive near KMCE/KMER bringing IFR/MVFR conditions. As the line moves southeast it will disorganize around 26/07Z leading to ISO to SCT -SHRA across the San Joaquin Valley. There will be a break in showers around 26/11Z then ISO to SCT -SHRA return around 26/15Z and will continue through tomorrow afternoon.

Lowered CIG will likely persist for several locations as additional showers and thunderstorms are possible. VIS will be determined by the timing of the showers and thunderstorms. South to southeasterly SFC winds generally less than 12 kts expected.

KEY MESSAGES

1. High confidence of flooding across the foothills with continued rainfall. Soils are saturated, resulting in excessive runoff.

2. High Wind Warning and Wind Advisory areas impacted by downed trees or power outages, moderate confidence. Travel will be difficult, especially for high profile vehicles. 40% probability of winds gusting to 30 mph from Merced to Hanford through this afternoon, will also topple trees and move loose objects.

3. High confidence of heavy snow above 7000 feet, with snow load impacts on roofs in the high Sierra continues. Travel will be impossible with 60 mph winds and whiteout conditions.

4. Thunderstorms are expected today (Christmas Day) and Friday across the Joaquin Valley.

DISCUSSION

River and areal flooding will be highest concern as recent heavy rainfall saturated soils has triggered initial rises at some river gages. A higher than normal snow level near 6,000' with enhanced low level moisture flux resulting in 100% HREF probabilities above 2,000'+ for 1.5"+ of QPF over the next 24 hours. While confidence is high for hourly and subhourly QPF thresholds remaining below burn scar thresholds, the last few runs of the HRRR, one of the more moist members of the 25.00Z HREF, tease near threshold values for the Salt 14-2 burn scar this morning although less so at the Garnet, Basin, and Coffee Pot burn scars through the early afternoon. Besides storm chances, steep low level lapse rates and associated instability may also provide increased subhourly rainfall amounts. Therefore, will be additional forecast detail to monitor. Otherwise, strong winds and heavy snow with HREF probabilities for 30" of snowfall in 24 hours hours over Tioga Pass, Mammoth Peak, and John Muir will cause whiteout conditions in the high Sierra.

Flood Watches remain in effect across most of the forecast area through Friday evening as a Winter Storm Warning continues for the High Sierra into Friday night. High Wind Warning and Wind Advisory areas impacted by downed trees or power outages, moderate confidence. Travel will be difficult, especially for high profile vehicles. 40% probability of winds gusting to 30 mph from Merced to Hanford, will also topple trees and move loose objects.

The Upper level low driving today's increased rainfall seen occluding off the coast on GOES upper level water vapor imagery persists meager moisture flux advection through Friday, lingering mostly orographically-driven precipitation chances through Friday. Eventually the global pattern progresses the trough east into the Desert Southwest through the weekend, reducing higher precipitation chances locally. Subsequent widespread colder temperatures with mid to upper 30 overnight lows Sunday into Monday and Monday into Tuesday.

AVIATION

MVFR/VFR conditions are expected from KFAT southward until 26/03Z when an organized line of heavier rain is expected to arrive near KMCE/KMER bringing IFR/MVFR conditions. As the line moves southeast it will disorganize around 26/07Z leading to ISO/SCT -SHRA across the San Joaquin Valley. ISO/SCT -SHRA will continue through tomorrow afternoon.

Lowered CIG will likely persist into the afternoon for several locations as additional showers and thunderstorms are possible. VIS will be determined by the timing of the showers and thunderstorms. At this time, have left thunder out of the TAFs as lightning probabilities are generally <20%, but there could be a period this afternoon where thunder gets added.

Windy southeast SFC winds expected at KMER, KMCE and KBFL sustained 8 to 17 kts gusting to 30 kts until 26/00Z. SFC winds then lower to generally less than 12 kts.

CERTAINTY

The level of certainty for days 1 and 2 is low. The level of certainty for days 3 through 7 is low.

Certainty levels include low...medium...and high. Please visit www.weather.gov/hnx/certainty.html for additional information and/or to provide feedback.

HNX WATCHES/WARNINGS/ADVISORIES

Flood Watch through Friday afternoon for CAZ300>339. Wind Advisory until 4 PM PST this afternoon for CAZ300>304-308- 309-313-314-316>318-324-332. Winter Storm Warning until 10 PM PST Friday for CAZ323-326>331. High Wind Warning until 4 PM PST this afternoon for CAZ333>336.