A Pacific storm system will bring strong winds and low elevation rain to much of the West Coast, and heavy high elevation mountain snow to the Sierra-Nevada through Wednesday. Fire weather concerns will continue into Tuesday across the Mid-South and Southeast as gusty winds and dry conditions persist. Read More >
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727 FXUS66 KMTR 210719 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1219 AM PDT Tue Apr 21 2026 ...New SHORT TERM, LONG TERM... .KEY MESSAGES... Updated at 1214 AM PDT Tue Apr 21 2026 - Steady rain this morning gives way to isolated to scattered thunderstorms in the afternoon and evening, lingering showers through Wednesday - Seasonal temperatures and dry conditions for the later part of the week - Monitoring a passing shower this Saturday and potential rainfall early next week && .SHORT TERM... Issued at 1214 AM PDT Tue Apr 21 2026 (Today and tonight) As of midnight tonight, the main frontal rain band continues to make its way into the interior regions of Monterey and San benito Counties, with the steady rainfall extending across the majority of the San Francisco Bay Area, the Monterey Bay region, and the Santa Lucia range. The back edge of the steady rainfall has passed through the western half of Sonoma County and the Point Reyes area, behind which some scattered showers have developed offshore. The steady rain band should continue to pass through overnight, and should move east of the SF Bay Area and Monterey Bay regions by the morning commute. Low temperatures tonight range from the middle 40s to the middle 50s, while today`s highs range from the upper 50s to the middle 60s in the lower elevations, down to the middle 40s to lower 50s in the higher elevations. The main forecast issue today is the chance for convective activity across the Bay Area and Central Coast. To discuss this, let`s take a look at the ingredients for convection, namely instability, moisture, and lift. At the 200-250 mb level (around 35000-40000 feet above sea level), the polar and subtropical jet streams are phasing, meaning that they are aligned so that their troughs are roughly placed along the same line of longitude. This enhances the upper- level low and enhances its cold pool, and this system will pass over the region, while scattering skies behind the frontal rain band will enhance surface warming, both effects destabilizing the atmosphere for the afternoon and evening hours. Most unstable CAPE values from the models are peaking around 500 J/kg, near the 99th percentile value for pretty much any time of year in the long-term records for the OAK radiosondes. Some moisture will continue to be advected into the region through the morning on the back side of the front, but most of it will be left behind from the passing of the front earlier in the day. That leaves lift, and there are a couple of sources for lift across the region. First, and most intuitively, the wind can force air to condense and form clouds or storms when it flows across our mountain ranges (we call this "orographic" lift). The second source requires a fair bit of explanation. Back at the 200-250 mb level we see a jet streak, or some hints of one, passing over the state around the upper level low. Some of the model output hints that we may very well see the left exit region of the jet streak, where due to the dynamics of the atmospheric flow, divergent flow aloft will induce convergent flow at the surface, thus promoting atmospheric lift across the region in question. Chances for thunderstorms top out around 30 to 40% across the region late this morning into the early evening, while K-Index values of around 25-28 degrees Celsius suggest a possibility for isolated to widely scattered thunderstorms. If thunderstorms do develop, lightning, locally heavy rainfall, and small hail are the primary threats. Not expecting waterspouts or weak tornadoes with this system. Model output values for the 0-6 km shear look impressive, but most of that shear is confined to the 3-6 km layer, whereas you would want higher shear between 0-3 km to note a higher risk of rotating cells. We will keep monitoring the forecast evolution through the day. The SPC continues to mention a general risk of thunderstorms across the region, but is likewise backing off of any severe thunderstorm chances. && .LONG TERM... Issued at 1214 AM PDT Tue Apr 21 2026 (Wednesday through Monday) Some lingering showers may hang on through Wednesday afternoon as a shortwave trails behind the upper level trough as it departs into the Intermountain West. As the frontal system passes, temperatures will warm slightly into a rather dry latter part of the week, close to or slightly below the seasonal averages as highs in the lower elevations range from the lower 60s to lower 70s. Ensemble model cluster analysis points to continued troughing across the state which will help modulate the warming trend through the 7-day outlook. On Saturday, another passing trough will bring a chance for a passing shower to the region, and although the ensemble members continue to show more scattered precipitation total forecasts, most members continue to show some rainfall coming into the region. Model output is even putting rain chances in the early part of next week and extending beyond that, but it is still too early to say anything definitive at this point in the forecast. && .AVIATION... (06Z TAFS) Issued at 939 PM PDT Mon Apr 20 2026 The cold front has reached the coast, with prefrontal rain ongoing across the majority of the terminals. This light to moderate steady rain will continue through the night before a break in the early morning hours. This won`t last long, however, as a band of stronger showers and possible thunderstorms is expected to roll through late morning through early afternoon. After this finale, showers will become much more isolated as the sun comes back out. Vicinity of SFO...Prefrontal rain will continue through most of the night with moderate southerly winds. There will likely be a break in the rain from around 12-18Z as the southerly winds start to increase. The main band of showers and possible thunderstorms is expected to reach the terminal around 18Z and continue for 2-4 hours. In addition to the heavy rain, lower visibility, and lightning potential, winds will be gusty and erratic during this window. Conditions will improve in the early afternoon, but there is a chance for the atmosphere to recharge for another round of showers in the late afternoon or early evening. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Rain has finally arrived at the southern terminals. Impacts will be similar to what`s described above, just delayed by an hour or two. && .MARINE... (Tonight through next Sunday) Issued at 939 PM PDT Mon Apr 20 2026 Moderate to fresh SW breeze will continue through the night before gradually shifting to westerly through the day Tuesday. The first band of light to moderate rain is ongoing, with a break expected in the early morning before showers return later in the morning. Some of these showers will be strong, and there is a 20-30% chance of thunderstorms as well. As the weather breaks, winds will gradually shift back to the northwest Wednesday before increasing to a strong northwest breeze by Thursday. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory until 3 AM PDT early this morning for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm. Small Craft Advisory from 9 AM this morning to 9 PM PDT this evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm. Small Craft Advisory from 9 AM this morning to 3 PM PDT this afternoon for Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Flynn MARINE...Flynn Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, X, and YouTube at: www.facebook.com/nwsbayarea x.com/nwsbayarea www.youtube.com/nwsbayarea |
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