Will the temperature in San Jose drop below 40°F in December 2026?

- [2026-09-10] NOAA's Climate Prediction Center ENSO discussion states "El Niño is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27." Source: CPC ENSO Diagnostic Discussion. - [2026-09-17] NOAA's updated seasonal outlook (issued Sept 17, 2026, covering Dec 2026–Feb 2027) favors above-normal precipitation across much of the southern United States, including California, the Southwest, southern Rockies and Southeast , per Unofficial Networks' summary of the CPC release. - [2026-08-20] The CNN/Newsweek coverage of NOAA's Aug 20 DJF 2026-27 outlook notes that while warm signals dominate the northern tier, "above-normal temperatures are also favored across much of the southern U.S., although the outlook is less decisive in parts of California and the Southwest." California is not in a strong "above normal" temperature category for winter. - [2026-08-20] NOAA's precipitation outlook for coastal California shows "coastal California, above 50 percent through late winter" chance of wetter-than-normal conditions, with the wet pattern "peaking in January through March 2027" rather than December — implying December itself may see a more transitional, mixed pattern rather than persistent storminess. - [Sept 2026, ~2 weeks old] A California/Southern-Ops fire-weather outlook (GACC) states "The strong El Nino will likely bring below normal temperatures and above normal rainfall to the region October through December" — a competing regional forecast suggesting near-term (Oct–Dec) cooling, in some tension with the CPC's "equal chances"/slightly-warm national temperature outlook for California. - [historical baseline, through 2023] KSJC (San Jose Airport) December daily-average low temperatures over 2010–2023 ranged mostly 35–48°F, with several Decembers averaging a low right at or below 40°F (2020: 40°F, 2017: 40°F, 2013: 35°F, 2011: 38°F), per Current Results/NOAA GSOM data. Since these are *average* daily lows, individual overnight hours in nearly every December have dropped below 40°F. - [historical baseline] Extreme cold events are on record for the Bay Area in December, e.g. a December 2013 cold snap where "the lows of 29 in San Jose and 40 in San Francisco tied the marks in those cities" , illustrating that sub-40°F readings at KSJC are a normal, recurring December phenomenon, not a rare tail event. - [2026-09-29, today] Live 10-day/hourly forecasts for KSJC (Weather Underground, timeanddate) as of late September show nighttime lows in the mid-to-upper 40s°F, consistent with early autumn; no December-specific cold-snap forecast exists yet since skill for day-level forecasts does not extend to December from a Sept 29 vantage point. - [Site status] The designated resolution source, weather.com/kalshi (KSJC hourly feed), is operational but naturally shows no December 2026 data yet since the month has not begun; December 2026 hourly KSJC data will only populate starting Dec 1, 2026. - [2026-01-02] For context, NOAA's CPC noted that the prior winter (Dec 2025) saw "multiple atmospheric rivers resulted in above-normal precipitation and flooding across California and the Pacific Northwest during mid to late December" under a weak La Niña — illustrating that even wet/mild-leaning winter patterns in the Bay Area have not historically prevented cold, clear-night radiational cooling episodes from occurring in the same month. BOTTOM LINE: No December 2026 KSJC data exists yet (market resolves on data not yet generated), and short-range forecasts don't reach into December; the dimension therefore rests on seasonal outlook and climatology. NOAA's very-strong-El-Niño DJF outlook leaves California in an "equal chances"/mildly-above-normal temperature category (not a strong warm signal) while regional fire-weather outlooks even lean below-normal for Oct–Dec, and historical KSJC December records show sub-40°F hourly readings occurring in nearly every recent year — together these point toward a high likelihood of at least one qualifying cold hour in December 2026. NOT FOUND: An official NWS/NOAA point forecast or climate-normals table giving exact historical frequency of hourly (not daily-average) sub-40°F readings at KSJC in December; a December-specific (rather than DJF three-month) CPC temperature outlook; and any weather.com/Kalshi December 2026 data (none can exist yet as of today).

- [2026-01-01] The NWS San Francisco Bay Area office's official December 2025 climate summary for San Jose airport (KSJC) shows record high 79°F on 12/12/1958, record low 19°F on 12/23/1990, and the observed lowest temperature for December 2025 was 38°F on 12/29 , which is below the 40°F threshold used by this market. https://forecast.weather.gov/product.php?site=ARX&issuedby=SJC&product=CLM&format=CI&version=1&glossary=1 - [2026-01-01] That same official NWS report shows December 2025 average minimum was 45.0°F versus a 1991-2020 normal of 41.7°F (3.3°F above normal) , indicating December 2025 was milder than average yet still produced at least one sub-40°F hourly reading. https://forecast.weather.gov/product.php?site=ARX&issuedby=SJC&product=CLM&format=CI&version=1&glossary=1 - [2026-01-01] Despite the sub-40°F dip, the report shows days with max ≤32 = 0, days with min ≤32 = 0 for December 2025, confirming freezes (≤32°F) are rare even though sub-40°F readings still occur. - [2025-10-16] NOAA's 1991-2020 U.S. Climate Normals (the current 30-year reference period) are the officially used dataset for airport-station climatology, including KSJC. https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals - [2025-10-16] Using this NOAA 1991-2020 normals data, a secondary compilation (Current Results) reports that San Jose (KSJC) averages 14 days per December with a minimum temperature at or below 40°F, and about 1 day per December at or below 32°F, out of 31 days — indicating sub-40°F occurrences are a routine, near-certain feature of a typical December, not a rare tail event. https://www.currentresults.com/Weather/California/Places/san-jose-temperatures-by-month-average.php - [2025-10-16] The same normals-based table shows a 30-year (1991-2020) average December high/low of 60°F/42°F for San Jose, consistent with lows regularly approaching the high-30s to low-40s range on cold, clear nights. https://www.currentresults.com/Weather/California/Places/san-jose-temperatures-by-month-average.php - [1999-04] The NWS San Diego office's "Climate of San Jose" technical memorandum describes typical winter overnight conditions: "Mid-summer high temperatures... normally range from 80 to 85 degrees. Lows are consistently between 55 and 60" for summer, contrasted with winter, where the same document states nighttime lows away from the bay-moderated flats "average between 35 and 40 degrees," and the bay-flat/airport zone lows "average between 40 and 45 degrees" — placing the airport very close to the 40°F threshold on an average winter night, meaning any minor cold snap easily pushes readings below it. https://www.weather.gov/media/mtr/climate/SJC_CLIMATE.pdf - [2026-01-01] The NWS climate record period for KSJC spans 1893-2025, and the December record low of 19°F occurred in 1990, illustrating that San Jose winters can occasionally see much colder outbreaks than the typical mild "40s" pattern, adding further support that sub-40°F readings are common, not exceptional. https://forecast.weather.gov/product.php?site=ARX&issuedby=SJC&product=CLM&format=CI&version=1&glossary=1 - [2026-09-29] No explicit betting-market or expert forecast track record for this specific Kalshi contract (or its prior-year equivalent) was found in search results; only the underlying climate data and one resolved comparable month (December 2025) were available. NOT FOUND: A direct historical tally of how many of the last 10-15 individual Decembers had at least one qualifying hourly KSJC reading below 40°F (only the 30-year normals-based average of "14 days/month" and the single confirmed December 2025 data point were available); no Kalshi/other-venue sibling market prices or explicit meteorologist seasonal outlook for December 2026 were found. BOTTOM LINE: Historical NOAA climate normals and NWS station records for KSJC show that hourly/daily minimum temperatures dropping below 40°F in December are a routine, near-annual occurrence (averaging ~14 such days per December over 1991-2020), and the most recent comparable December (2025) — despite running 3+°F warmer than normal — still recorded a sub-40°F reading (38°F). This base rate strongly favors a "Yes" outcome for December 2026 under normal conditions.

YES means: at least one hourly KSJC (San Jose Airport) temperature observation between Dec 1–31, 2026 is strictly below 40°F, as published on weather.com/kalshi by market close. - [2026-08-27] The Southern California/South Ops (GACC) Monthly/Seasonal Outlook, issued August 27, 2026 (a newer edition of the "~2 weeks old" outlook colleagues cited), reaffirms that "the strong El Nino will likely bring below normal temperatures and above normal rainfall to the region October through December" , directly tying the cooling call to elevated Pacific coastal sea-surface temperatures shown in the accompanying SST anomaly figure — a mechanism (cooler, wetter storm-track pattern) that favors more clear-night radiational-cooling opportunities in December. - [2026-08-27] The same GACC outlook explicitly forecasts "Temperatures will likely be above normal in September and then below normal October through December" and "a below normal amount of Santa Ana wind events October through December" , indicating forecasters expect a pattern shift from the warm early-fall setup toward a colder, stormier winter exactly in the window that matters for this market. - [2026-09-17] NOAA's September 17 CPC update, as summarized by Unofficial Networks, shows the West's warm signal is uneven: "Warmer-than-normal conditions are also favored across parts of the West," but "Colorado, Utah and portions of the Southwest are closer to equal chances in the latest temperature outlook" — meaning inland/interior California, including the South Bay, sits in a weaker or "equal chances" temperature category rather than a strong warm lock, leaving room for cold nights. - [2026-09-17] The precipitation side of that same update states "California, the Southwest and southern Rockies: above normal, the strongest and most consistent signal on the map. This is the El Niño storm track doing exactly what it is expected to" . A more active winter storm track means more frontal passages, and cold post-frontal air behind Pacific systems is a classic trigger for below-40°F overnight lows at inland-influenced stations like KSJC. - [2026-08-10] An independent seasonal-outlook aggregator (El Niño Guide) frames California's winter as "Near normal to slightly above normal. Coastal areas see the strongest warming; inland valleys near baseline" — explicitly noting that inland valley locations (where San Jose/KSJC sits, away from the immediate coast) are expected to track near the historical baseline rather than sharing in the stronger coastal warm anomaly, supporting a normal-or-better chance of routine cold nights. - [2026-09-17] The same CPC-based summary also flags that "The Northeast has a particularly notable combination of above-normal precipitation and above-normal temperatures... the amount of snow that reaches the ground will depend heavily on whether individual storms can maintain sufficient cold air" , illustrating the general pattern this winter: even wetter, El Niño-driven storm tracks are not synonymous with uniformly mild temperatures, since individual storm systems still import cold air behind their frontal passages — the same dynamic applies to California's December cold outbreaks between storms. - [2026-08-26] NOAA's own official winter outlook has not yet been released; a secondary source notes NOAA's flagship Winter Outlook "has not yet been published as of late August 2026" and is expected around mid-October 2026, meaning the most authoritative month-specific (rather than 3-month DJF) guidance for December is still forthcoming and could sharpen either the warm or cold signal. Source: https://usahousinginformation.com/el-nino-snow-map/ - [historical corroboration] Combined with the base-rate report's independently sourced NOAA 1991–2020 normals showing San Jose averages roughly 14 days per December with a minimum at/below 40°F, the mechanism above (increased storminess, post-frontal cold intrusions, inland valley temperatures near baseline) reinforces rather than contradicts that high historical frequency for December 2026. BOTTOM LINE: The most current (Aug 27) regional fire-weather/seasonal outlook explicitly calls for below-normal October–December temperatures driven by the developing very-strong El Niño's storm track, and even NOAA's own DJF outlook leaves inland/interior California in a weak or "equal chances" temperature category rather than a strong warm lock — a pattern of increased Pacific storminess with post-frontal cold air intrusions, layered on San Jose's near-universal historical occurrence of sub-40°F hourly readings in December, makes a qualifying cold hour at KSJC in December 2026 highly probable. NOT FOUND: NOAA's flagship October/November 2026-27 Winter Outlook (not yet published as of this research); any December-specific (single-month) CPC temperature outlook for San Jose/Bay Area; and any December 2026 KSJC hourly data (unavailable until the month occurs).

YES means: at least one hourly temperature reading at KSJC (San Jose Airport) during Dec 1–31, 2026 is strictly below 40°F. - [2026-09-17] A Bay-Area-specific seasonal forecast built directly from NOAA's Sept 17 CPC update states plainly: "Warmer-than-normal temperatures are more likely in October-December and December-February, while spring temperatures have equal chances of finishing above, near, or below normal." This is a more decisive, localized warm signal for the Bay Area than the "equal chances/less decisive" characterization colleagues found for California broadly, strengthening the case that December 2026 nights will run mild. https://www.sfbayweather.com/learn/el-nino-and-la-nina-what-they-mean-for-bay-area-weather - [2026-09-17] The same source frames the winter rain signal as moderate, not extreme, for December itself: "NOAA says above-normal rainfall is more likely in the Bay Area during winter 2026-27 and spring 2027, while October-December rainfall remains equally likely to finish above, near, or below normal." A near-normal-to-uncertain December rain signal (versus a clearly wet one) means fewer nights with insulating cloud cover, but combined with the explicit warm-temperature lean, the net effect argues for milder minimum temperatures rather than sharp radiational cooling events. - [2026-08-24] Local Bay Area reporting on this same El Niño confirms the baseline mechanism cited by forecasters: "Usually, El Niño brings warmer weather to northern and central California during fall and winter." This is the operative seasonal mechanism working against a sub-40°F reading occurring. - [2026-09-23] A national explainer on the winter outlook cautions that "warmer" categorization doesn't equal a mild month outright — "if January normally averages 35 degrees where you live, an El Niño winter might average 38" — but this example is from the colder half of the country; applied to San Jose's much milder ~42°F average December low, an analogous few-degree upward shift would push many nights further away from, not closer to, the 40°F threshold. - [2026-08-20] Newsweek's coverage of the Aug 20 CPC map notes California sits in a less decisive warm category, but crucially for the fall shoulder season, an Sept 17 CPC fall-specific product shows "equal chances for above-normal, near-normal or below-normal temperatures" only for the southern Rockies/Plains, not California — for California the September product leans warm into Oct-Dec, reinforcing the NO case's warmer-than-average expectation for the month in which December falls. - [2024-12, NWS Hanford region, an analog inland-California warm December] The December 2024 regional climate summary (a recent warm/dry December analog for interior/coastal-adjacent California, albeit not KSJC itself) reported the month "generally much warmer than average with below average precipitation" and zero freezing nights at multiple stations against a multi-day average — illustrating that in a warm/dry December pattern, cold-tail events can be sharply suppressed even well inland, where cold outbreaks are typically more common than at the bay-moderated KSJC site. - [2024-12, NWS Hanford region] That same December 2024 report tallied "Bakersfield 52.0 +2.8 15th highest... Fresno 51.4 +3.9 3rd highest... Hanford 51.5 +5.5 2nd highest" monthly average temperature departures, showing that a warm-leaning December (as forecast for 2026) can produce among the warmest Decembers on record even at inland stations that are climatologically colder than KSJC — suggesting KSJC, which rarely gets as cold as these inland sites, could plausibly stay above 40°F all month in an analogous warm year. - [2026-09-10 / 2026-09-17] NOAA's very-strong El Niño strength probabilities, as compiled from the CPC's Sept discussion, show the event near or above 90-98% strong-to-very-strong through the entire Oct 2026–Jan 2027 window (per the sfbayweather.com summary of NOAA CPC data), meaning the season-long warm bias (not just a brief early-winter blip) is expected to persist through all of December, increasing the plausibility of a full month without a qualifying cold hour. https://www.sfbayweather.com/learn/el-nino-and-la-nina-what-they-mean-for-bay-area-weather - [historical technicality] KSJC's official 1991–2020 normal average December low is only marginally below 40°F (colleagues cite ~41.7°F normal minimum), meaning the threshold sits extremely close to the average night; a modest, El-Niño-driven upward shift of just 1–3°F in nightly minimums — consistent with the "warmer than normal" DJF/OND categorical forecast — could be enough to keep every single hourly reading at or above 40°F for the entire month, unlike in colder-leaning or La Niña years. - [resolution-mechanics technicality] Because the contract requires strictly-below-40°F on an hourly (not daily-average) basis and disregards any later data revisions, any borderline cold night that only reaches exactly 40°F, or that isn't captured due to an hour with "no reported value" (per the rules' explicit "no qualifying value" clause), resolves NO for that instance — giving the NO side extra technical cover beyond pure climatology, since KSJC's normal December low sits right at the 40–42°F boundary where marginal warm bias or a missing-hour gap could avoid triggering. BOTTOM LINE: The most locally specific, most recent (Sept 17, 2026) seasonal-outlook read explicitly favors warmer-than-normal Bay Area temperatures for both October–December and December–February under the strengthening El Niño, a stronger and more decisive local signal than the generic statewide "equal chances" characterization; combined with historical analog Decembers (e.g., Dec 2024 inland CA) showing warm/dry El-Niño-adjacent patterns can suppress cold-tail nights even at colder stations, and with KSJC's climatological low sitting only marginally below the 40°F threshold, there is a coherent, evidence-based case that December 2026 could see every hourly KSJC reading stay at or above 40°F, resolving NO. NOT FOUND: A KSJC-specific December 2023 (the most recent comparable moderate/strong-El-Niño December) NWS climate summary confirming whether that month's minimum stayed above or dipped below 40°F; any explicit statement from NOAA CPC of a December-only (rather than seasonal DJF/OND) temperature category for San Jose specifically; and any analog-year statistic on the number of El Niño Decembers historically that produced zero sub-40°F hourly readings at KSJC.