Author: admin
Impactful storm to affect entire Bay Area by Tues morning
Flood Watch in effect from February 3, 04:00 PM PST until February 5, 04:00 AM PST
From MTR AFD:
.SHORT TERM... Issued at 1247 PM PST Mon Feb 3 2025 (This evening through Tuesday) Key Points: * Flood Advisory continues for portions of Sonoma County * Flood Watch goes into effect 4 PM Monday through 4 AM Wednesday across the Bay Area and Central Coast * Mon PM-Tue AR ontrack late Monday with moderate to at times heavy rain * Mon PM-Tue will also bring gusty winds and thunderstorms * Thur-Fri AR is expected to be weaker and aimed farther south * Cold morning temperatures return mid-week but temperatures will drop more significantly over the weekend Being upfront - lengthy AFD as lots of interesting and impactful weather headed our way. Near term - KMUX is showing an increase in shower activity off the North Bay coast and northern Sonoma county. The increase is a result of better forcing tied to a low pressure off the coast moving eastward. The approaching low has also allowed for gusty southerly winds across the region this afternoon. This is only a preview of things to come over the next 36 hrs. This evening through Tuesday: The broader meteorological picture still shows a quasi-stationary boundary to the north. This front extends west to a surface low near a triple point 37N 128W and farther west of that is an occluded low pressure 32N 140W also tracking eastward. Aloft is a little more interesting as split flow with a high amplified polar jet (digging SW thru BC and then recurving into NorCal) is showing some phasing with the sub-tropical jet arriving from the SW. These upper level jet features/phasing will aid in overall large scale ascent. On top of this features, PWAT satellite imagery shows a Pineapple Express (AR) plume just SE of Hawaii extending north along the surface boundaries to NorCal. So what does this mean and how will it impact the Bay Area and Central Coast? The upstream "triple point" low will move east and help to lift the stationary boundary slightly, but also allow for a more defined surface front. This surface enhancement will be aided by an upper level jet exit region over NorCal this evening. Large scale forcing, orographic enhanced, and PWAT transport will lead to heavier rain spreading over the North Bay this evening. Could see some decent rainfall rates of 0.5" per hr. Rainfall coverage and intensity will gradually move S and E over the rest of the forecast area by Tuesday morning. Not only will rain increase this evening through early Tuesday, but the winds will too. The winds will increase rapidly over the northern coastal waters this evening and then spread to the coastal areas Monterey Bay northward. A Wind Adv was issued to account for this increase in winds. Early Tuesday will feature a brief "lull" in activity as the "triple point low" fades with an exiting jet region aloft. The lull will be short lived as even better jet dynamics aloft take aim at the Bay Area and Central Coast Tuesday mid morning through Tuesday evening. Expect another rapid increase in winds and precip on Tuesday as a cold front pushes through the region. This will bring the most widespread and intense rainfall. Of particular concern is between 18 and 21 UTC Tuesday, flow through the column increases. In fact, examining the wind field at 300mb reveals a phased or coupled jet type pattern with a wind maxima approaching the CA/OR coastline with a broader field of +120 knot mid level flow. In response, the lowest 1 km wind field responds and 925mb flow ramping up to around 50 knots. This is particularly true along and south of the trough/cold frontal boundary for regions along the Central Coast and Big Sur Coastline. While instability values will be on the lower end of the spectrum with under 50 J/kg. Forecast hodographs indicate a large amount of low level curvature with 0-1 km shear in excess of 30 knots. These types of high shear, low CAPE regimes do support both the transfer of damaging 55+ mph winds aloft down to the surface and in some instances brief tornadoes. We`ll continue to monitor trends in high-resolution output, but as it stands right now, the favored region for enhanced winds of 55+ mph and a slightly higher tornado risk will be along the San Francisco Peninsula Coast (Pacific Ocean side) down to the Big Sur Coastline late Tuesday morning and through the evening hours. That being said, have included thunderstorms to the forecast for Tuesday with specific wording highlighting gusty convective winds. Heavy rain will transition late Tuesday to more shower activity from N to S behind the departing cold front. Rainfall amounts tonight through Tuesday night will 4-6" North Bay and coastal mts (jackpot up to 9" above Big Sur), 2-4" elsewhere, with lesser amounts 0.75 to 1.5" Santa Clara Valley/Interior Central Coast. It goes without saying, but these rounds of intense rainfall will lead to flooding concerns, especially in areas where rain fell over the weekend. Do expect Flood ADvisories to be issues for urban and small stream flooding concerns. If you encounter a flooded roadway or road closure barricades remember - turn around, don`t drown! && .LONG TERM... Issued at 240 PM PST Mon Feb 3 2025 (Tuesday night through next Sunday) Wednesday can be described as another lull in overall activity as one storm/moisture plume exits and another approaches from the west. Thursday and Friday...a surface low will approach the Central Coast. Unlike the Tuesday AR this system will have less support aloft and track a little farther south. Additionally, the moisture tap is less with IVT/IWV values solidly in the Weak AR realm. Thankfully, this low also appears to be a fast mover with highest rainfall totals North Bay and Coastal Mts around 1" with 0.25-0.75" elsewhere. Given the more southern track this system will be cold with lowering snow levels. Snow level will lower N to S, but by the time the lowest snow levels arrive the precip may be tapering off Thursday into Friday. Regardless, some wet snow will be possible over the higher peaks Thursday into Friday. Not expecting much in the way of accumulation. Speaking of colder, the precip threat will quickly transition to a temperature threat next weekend. Cold advection aloft behind the departing low combined with clearing skies will lead to some chilly overnight lows. Both Sat/Sun mornings will feature temps in the 30s (even toward the coast) with far interior in the 20s. Will likely need a mix of frost/freeze products. &&
Heavy rain expected late morning Tuesday across Bay Area
Flood Watch in effect from February 3, 04:00 PM PST until February 5, 04:00 AM PST
Moderate to heavy rain now expected to cover Bay Area by about 10am Tuesday.
From MTR AFD:
.SHORT TERM... (Today and tonight) Issued at 304 AM PST Mon Feb 3 2025 Key Points: * Flood Advisory continues for portions of Sonoma County into Monday * Flood Watch goes into effect 4 PM Monday through 4 AM Wednesday across the Bay Area and Central Coast * Second AR arrives late Monday bringing moderate to at times heavy rain * Third AR arrives Thursday-Friday but is expected to be weaker and aimed farther south * Cold morning temperatures return mid-week but temperatures will drop more significantly over the weekend Light showers continue to the north of our CWA but are expected to shift southwards again as a second, stronger, AR arrives later today. Forecast IVT values are in excess of 600 to 700 kg/ms which is supportive of a more moderate intensity AR arriving mid to late Monday and continuing into Tuesday. The first round of moderate to heavy rain will take place later in the day on Monday and a second round of heavier rain will take place on Tuesday. While the bulk of the moisture is still located to our north, there will be enough lingering moisture for light showers across the region Monday morning. By the late morning/early afternoon, showers will start to pick up in intensity across the North Bay as troughing deepens over the Pacific and a shortwave approaches the West Coast. This shortwave, in combination with the sub-tropical jet lifting northward, will push the main moisture plume southwards with shower activity subsequently increasing throughout the day across the Bay Area and Central Coast. At the same time, a stationary cold front over northern CA will gradually give way to a more progressive cold front as a surface low associated with this shortwave moves inland. The initial rain (on Monday) will primarily be concentrated over the North Bay through the late afternoon before we start to see more moderate intensity rain shift south of the Bay Bridge over the evening hours. Rainfall totals - for Monday - will be highest across the North Bay Mountains which will see between 1.5"-2". Elsewhere, initial rain totals of up to an 1" are possible on Monday with the bulk of the rain from this AR to fall late Monday into Tuesday. As this occurs, we will see a southwards shift in where the highest rainfall totals are located with an emphasis on the Santa Cruz Mountains and Santa Lucia Range (see the long term discussion for more information). Gusty winds ramp up along the coast and across elevated terrain Monday into Tuesday. While winds generally stay below Wind Advisory criteria, coastal areas may flirt with Wind Advisory criteria Monday afternoon/evening around the time of frontal passage. For now, continuing to hold off on a Wind Advisory due to the short duration and more isolated nature of higher gusts but will continue to monitor and update if things change. The WPC has issued a day one (Monday) slight risk (15%) of excessive rainfall for the North Bay and a marginal risk (5%) for portions of the Bay Area and Central Coast. A Flood Watch will go into effect at 4 PM PST on Monday for the Bay Area and Central Coast as rain starts to ramp up in intensity and becomes more widespread across the region. As has been talked about for the past few days, rainfall across the North Bay and coastal areas has acted to saturate soils and has essentially "primed" them for flooding to occur. For areas that have not received as much rain, moderate to heavy rain is expected to saturate soils fairly quickly Monday/Tuesday with increased potential for urban nuisance flooding as heavier rain rates occur. While the most recent CNRFC forecasts continue to limit mainstream river flooding to the North Bay, smaller creeks and streams across the rest of the Bay Area and Central Coast may see sharp rises as the bulk of the rain passes through. If you encounter a flooded roadway or road closure barricades remember - turn around, don`t drown! && .LONG TERM... (Tuesday through Sunday) Issued at 304 AM PST Mon Feb 3 2025 Heading into Tuesday, heavier showers will spread into the rest of the Bay Area and Central Coast overnight with the highest accumulations anticipated in the Santa Cruz Mountains and Santa Lucia Range. Rain may briefly lighten mid to late morning but widespread heavy rain will return Tuesday afternoon as a second pulse of moisture (higher IVT values in exceedance of 700 kg/ms) arrives. This second pulse will be associated with with the greatest flood risk as any areas not previously saturated by the Friday to Sunday rains will be at least partially saturated from the Monday to early Tuesday morning rains. Moderate to heavy rains will then persist through the evening hours before rains decrease in intensity overnight Tuesday into Wednesday. When all is said and done from Monday to Tuesday, the North Bay will (in total) see an additional 2"-4", an additional 1"-3" south of the Bay Bridge, 3"-5" in the Santa Cruz Mountains, 1"-2" across the Central Coast, and 5"-8"+ in the Santa Lucia Range. Given that the moisture plume is oriented almost perpendicular to the coastline, areas where orographic uplift is favored may see locally higher totals than previously mentioned. An additional factor is that over the last few grid updates the forecasted QPF totals have continued to increase for this event with particular emphasis on the southern portions of our CWA (Santa Cruz, Monterey, and San Benito counties). As such, it is important to stay up to date on the most recent forecast and be aware that there is elevated potential for flooding to occur. The WPC has issued a widespread day two (Tuesday) slight risk (15%) of excessive rainfall exceeding flash flood guidance across the Bay Area and coastal Central Coast and a marginal risk (5%) of exceeding flash flood guidance across interior Monterey/San Benito Counties. Gusty winds will continue through the day on Tuesday before weakening Tuesday night into Wednesday. Winds continue to look below Wind Advisory criteria on Tuesday but we will continue to monitor in case frontal passage slows down and the time period of stronger winds becomes more prolonged. Wednesday will be the lull between our Monday-Tuesday AR and our Thursday-Friday AR. Lingering moisture may be enough to support scattered, light showers but any totals from this will generally be below a tenth of an inch. Notably, morning temperatures will begin to drop Wednesday after cold frontal passage occurs Monday/Tuesday. This may necessitate the issuance of frost/freeze products for portions of the Bay Area and Central Coast. Thursday to Friday will see a third, albeit weaker, AR impacting our CWA with additional totals between 1"-2" likely across the region. This system will be spurred by the arrival of another upper level shortwave and a surface low pressure system with IVT guidance showing a subtropical moisture tap. The main thing to note about this system is that, so far, it looks fairly progressive and has started to trend more southwards (Central Coast into Southern CA) on recent model runs. This may result in additional fluctuations in forecasted QPF amounts Thursday and Friday as we get closer to this AR. Another cold front is expected to pass through the region on Friday bringing with it locally gusty winds and a more significant drop in morning temperatures on Saturday/Sunday. Current guidance suggests temperatures may drop into the 30s to upper 20s. The combination of prolonged wet conditions, gusty winds, and cooler temperatures will create hazardous conditions for anyone without access to adequate heating or shelter. The issuance of additional Frost and Freeze products are likely Saturday and Sunday. &&
Slight shift in timing of heavy rain and wind gusts to late Mon through Tues
The Mon/Tues storm forecast for the Greater Bay Area remains on track, with a slight shift to a later onset of heavy rain and gusty winds to later Mon night and lasting through Tuesday.
Keep track of current MTR-issued alerts: https://www.weather.gov/mtr/
This graphic shows the 6-hour precip forecast with increased rain intensity moving into the Greater Bay Area by approx 10pm Mon night.
From MTR AFD:
.LONG TERM... (Monday through Saturday) Issued at 248 AM PST Sun Feb 2 2025 Light showers continue Sunday into Monday with the next round of higher intensity rain returning late Monday morning/Monday afternoon. Taking a look at the synoptic pattern, an upper level trough off of the PNW and an upper level ridge over southern CA have in part kept our moisture plume tighter and pushed farther north this weekend. By Monday, the upper level trough will deepen over the PNW, pushing farther into northern CA, and ridging over southern CA will start to shift eastward. This will allow for the moisture plume to widen and shift southwards again late Monday morning/Monday afternoon with precipitable water values generally increasing to greater than 1" across the region. At the same time, the stationary front over the North Bay will give way to a more progressive cold front that will move through the Bay Area Monday into Tuesday. Rain will start to pick up in intensity by mid to late Monday morning with the initial heaviest rain located in the North Bay. High resolution models are starting to show precipitation lingering over the Bay Area (south of the North Bay) and Santa Cruz Mountains overnight Monday through Tuesday morning. Totals have trended upwards Monday into Tuesday with an additional 3"-4" possible across the North Bay, 2"-3" across the rest of the Bay Area, and 1"-3" across the Central Coast. Locally higher totals (4"+) are possible in areas where upsloping is favored (North Bay and coastal mountains). A slight chance (<10%) of thunderstorms continues to be monitored for Tuesday morning but the probability has been trending downwards recently. Wet weather will then continue Wednesday through the end of the week as troughing persists over the West Coast. Late week rain totals will be lower, however, with most areas seeing less than 1" of additional rain. The exception, again, will be where upsloping is favored within the coastal mountains where totals will be between 1"- 2". This second system will be colder with morning lows expected to drop into the mid 30s to low 40s across the region. The combination of damp weather and cooler temperatures may create conditions that are particularly hazardous for people without access to adequate heat or shelter. It is worth noting that the WPC maintains a slight to marginal risk of excessive rainfall for the Bay Area and portions of the Central Coast through Tuesday and again on Thursday. Flooding concerns will remain elevated through the week as our initial rainfall from Friday to Saturday have caused soils to saturate. This will lead to increased surface runoff and is likely to lead to additional flood related products as we move through our next two rounds of wet weather. In addition, gusty winds are likely to persist for coastal areas and elevated terrain Monday through Wednesday with the strongest gusts likely to develop ahead of/around the time of cold frontal passage Monday into Tuesday. This may result in a few trees being knocked over and increased potential for power outages to occur. Make sure to have multiple ways to receive warnings and stay up to date on the forecast as we move through the upcoming week.
Spotters self-activate; Rain next several days throughout the area
SF Bay Area Skywarn spotters in the North Bay should self-activate immediately. Spotters in the rest of the Bay Area should self-activate starting Monday afternoon / evening.
Flood Watch until February 2, 10:00 PM PST
Rain will continue throughout the Bay Area at least through Tuesday night. The North Bay will be mostly impacted through Sunday. Starting Monday and lasting at least through Tuesday night, the entire Bay Area will get significant rain and winds.
The SJSU WRF 2KM Ensemble Precipitation Forecast 12Z run on Feb 1 shows a rapid increase in the ensemble mean at approx midnight on Monday night / Tuesday morning at KSJC, which indicates a later start to the heavy rain in the South Bay, whereas at KSFO the increase starts in the early to mid afternoon.
This graphic shows the model ensemble 6-hour precip forecast intensifying over the Bay Area during the afternoon (4pm) on Monday Feb 3.
From MTR AFD:
.LONG TERM... (Sunday through Friday) Issued at 317 AM PST Sat Feb 1 2025 Showers are expected to continue through the remainder of the weekend and into next week as an AR continues to impact the West Coast. Sunday will act as a brief break between rounds of heavier rainfall on Saturday and Monday with lighter rain expected everywhere. After the passage of Saturday`s shortwave trough, the upper level flow becomes more zonal (reducing upper level forcing) and the surface low pressure system shifts northward, becoming stationary over the North Bay. This will shift the moisture plume northward as well and continue to train moisture over the North Bay with locally higher rainfall totals possible there. Comparatively, the amount of available moisture will be reduced farther south, with fairly light rain totals anticipated outside of the North Bay. Given this, an additional 1"-2" of rain is possible over the North Bay, up to 1" across the Bay Area, and up to 0.25" across the Central Coast on Sunday. Heading into Monday, zonal flow will give way to persistent upper level troughing across the West Coast, kicking off the next round of moderate rain. The surface low pressure system will become less stationary as the upper level trough develops with the associated moisture plume widening and spreading southwards on Monday. A cold front associated with this low will progress through the Bay Area and Central Coast late Monday night into Tuesday morning. This system will bring an additional 2"-4" of rain to the North Bay, 1"- 2" to the rest of the Bay Area, up to 3" across the Santa Cruz Mountains and Santa Lucia Range, and 1"-2" across the Central Coast. At the same time, cold frontal passage will increase atmospheric instability and bring a 10-20% chance of isolated thunderstorms over the coastal waters and along the coastline Tuesday morning. Thunderstorm chances will peak early Tuesday morning before decreasing through the remainder of the day. Additional rounds of light rain (generally totaling to between 0.5"- 1.0") are expected Wednesday through Friday as persistent upper level troughing continues over the West Coast. Gusty winds persist throughout the week with cooler morning temperatures returning Wednesday through the extended forecast. The combination of gusty winds, cold mornings, and continued wet weather will create hazardous conditions for those without adequate access to heat or shelter. Cold weather products are likely to be evaluated for mid week and beyond as we get closer in time to them. For snow lovers, it is worth noting that, in the wake of cold frontal passage, snow levels will drop to between 2500-3500 feet for portions of the region. This, in combination with the continued wet weather, means there is a non- zero chance for light snowfall on the highest peaks of the North Bay Mountains and in the Santa Lucia Range. While confidence remains low, probabilistic guidance does show a 30-40% chance of trace snowfall across the peaks of the North Bay Mountain mid to late week. This is still several days out and the forecast is likely to change, but, it may be worth keeping an eye on the Cal Alert Cameras late week to see if you can spot any mountain snowfall.
Rain today through Tuesday; Flood Watch in effect
Flood Watch in effect from January 31, 12:00 PM PST until February 2, 10:00 PM PST
Currently the most impactful rain and winds are expected to occur on Monday into Tuesday. The graphic below shows the expected precip (mm) at 4pm PST Tuesday according to GFS. This shows the strongest signal for rain over the Bay Area on the 24-hour GFS runs.
From MTR AFD:
.LONG TERM... (Saturday through Thursday) Issued at 302 AM PST Fri Jan 31 2025 If you`ve been sorely missing the rain for the last month, this upcoming week is for you! Continuing from our Friday system, more widespread, moderate rain moves in on Saturday with the heaviest showers expected Saturday morning into the afternoon. Current guidance suggests 1.0"-2.0" are possible across the North Bay, 0.5"- 1.5" across the rest of the Bay Area, and up to 0.5" for the Central Coast on Saturday. A slight risk (at least 15% chance) of excessive rainfall is in effect for the North Bay and portions of the Bay Area (including the city of San Francisco) for Saturday with a marginal risk of excessive rainfall in effect for portions of the South Bay and Central Coast. Heading into Sunday, guidance suggests rainfall will be lighter and more concentrated over the North Bay as the moisture plume shifts slightly northward. As such, the WPC has issued a day 3 marginal risk of excessive rainfall for the North Bay on Sunday. The next round of widespread moderate showers begins Monday as another upper level low and associated surface low pressure system move through the Bay Area. The day four (Monday) WPC excessive rain outlook shifts the North Bay into a slight risk of excessive rainfall and portions of the Bay Area into a marginal risk. By day five (Tuesday), a more widespread marginal risk of excessive rainfall is expected across the entire Bay Area and Central Coast. When all is said and done, from Friday to Thursday, the North Bay will see roughly between 5"-8" of rain, the remainder of the Bay Area will see between 3"-5", and up to 2.5" is possible across the Central Coast. Locally higher totals are possible throughout the elevated terrain of the Santa Cruz Mountains and the Santa Lucia Range with overall totals between 4"-5" possible by late next week. Gusty winds are expected to continue through the week with NBM and local WRF guidance highlighting Tuesday and Wednesday for particularly windy conditions. Daytime temperatures will be in the 50s to low 60s through midweek while morning lows hover in the 40s to low 50s. Cold mornings return again Wednesday through the end of the forecast period, with lows to drop into the 30s again. This combination of wet weather, gustier winds, and cold mornings may create a setup hazardous for those without adequate heat or shelter mid to late next week.
Heaviest rain expected on Saturday and Monday
Friday’s storm will be mostly impacting the North Bay, but the whole Bay Area should expect some rain. Monday’s storm will likely be stronger across the entire Bay Area, and will include potentially heavy rain and strong westerly winds. This graphic shows precip forecast for Saturday 4pm PST according to the GFS model.
From MTR AFD:
.LONG TERM... (Friday night through next Wednesday) Issued at 248 PM PST Thu Jan 30 2025 Confidence continues to increase for widespread impactful rainfall to persist through the remainder of the extended forecast. The first system arrives on our doorstep late tonight. Much of the North Bay will have already seen between 0.50-1.00" of rain by late tomorrow night and early Saturday. The focus of higher rainfall amounts will then shift south into the Bay Area, East Bay and Santa Cruz coastal range throughout the day Saturday. A relatively strong upper level trough axis will shift onshore late Friday into early Saturday when increased rainfall rates can be expected from the North Bay into the Bay area. While the parent low reconsolidates well to our north, zonal flow with a series of embedded shortwaves will continue to drive widespread light rain across most of our area, with enhanced upsloping producing periods of moderate to heavy rain along our coastal ranges through the weekend. The main upper low then dives south late Sunday night into Monday. Deterministic guidance is in decent agreement for the evolution of the primary low`s shift south, but this part of the forecast will continue to be watched closely. Any change or retreat north for the beginning of next week, will result in big changes for rain totals outside of the North Bay. The rainfall totals for tonight through early Monday morning should only lead to minor impacts with respect to flooding across the North Bay. However, by the time the second round arrives for Tuesday and Wednesday, we`ll see rivers and streams begin to respond quicker to the steady fetch of rainfall expected. Even with the higher end totals anticipated, there is roughly a 10% chance of flooding for normal trouble spots along the Russian Rive in Sonoma County and other locations into Napa County and Marin County. As we monitor the rainfall this weekend, and the evolution of the larger scale pattern for the beginning of next week, keep an eye on subsequent forecast updates as these river forecasts could change considerably. Along with the potential for periods of moderate to heavy rain leading to flooding, there will also be moderate to strong gusty winds. The strongest winds will be across higher elevations, beginning Saturday morning with the passage of the aforementioned trough axis moving onshore. As some might expect, the strongest gusts should be expected over higher elevations like Mt Helena, up to 60 MPH after sunrise Saturday through much of the day. We`re anticipating the need for a Wind Advisory beginning at some point early Saturday, with elevated wind speeds continuing through the middle of next week. While strong gusty winds at higher elevations may approach 60 MPH, lower lying areas along the coast and inland valleys can expect max wind gusts between 35-45 MPH at times.