Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
010 FXUS66 KMTR 311900 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1200 PM PDT Fri May 31 2024 ...New AVIATION... .SYNOPSIS... Issued at 339 AM PDT Fri May 31 2024 Thermal ridging will bring a minor warming trend to the region through today. A trough will move into the Pacific Northwest this weekend though, which will help strengthen the onshore flow and deepen the marine layer, bringing a slight reprieve to the warmer conditions. However, high pressure will build back across the region by the middle of next week, bringing a much more pronounced warming trend to the region, especially across the inland areas. Inland areas could be upwards of 10 to 20 degrees above normal, with a moderate heat risk by Wednesday. && .UPDATE... Issued at 849 AM PDT Fri May 31 2024 Satellite imagery is showing coastal stratus across the Central coast with a series of offshore eddies visible in satellite imagery, one south of Santa Cruz and three others south of the Big Sur coast. Coastal stratus generally clears through the day. Main forecast concern continues to be the warming event in the early part of next week, which is covered in the previous discussion. DialH && .SHORT TERM... (Today and tonight) Issued at 359 AM PDT Fri May 31 2024 There`s been a slight decrease in 500 mb heights since last evening, however 500 mb geopotential heights are still near 580 decameters, a little greater than late May Oakland upper air climatology (578 decameters). Despite weak cyclonic curvature at the 500 mb level this morning there is still plenty of thermal ridging evident on recent upper air sounding(s) and the Bodega Bay and Fort Ord profiler data. The marine layer temperature inversion is compressed to near sea level, the marine layer depth between 300 and 600 feet per latest profiler data. Dry air through a great depth of the troposphere is allowing for nocturnal radiative cooling, helping to bring about a narrow channel of stratus & fog along the Big Sur coast to the Monterey Peninsula. Recent wind swept and lofting of sea salt is also helping to form stratus/fog. A 2.2 millibar SMX-SFO pressure gradient (light southerly wind) is helping to slowly transport stratus/fog northward along the immediate coast. Otherwise the sky is clear, leading to another sunny day most places today. Highs today will warm up to similar levels compared to Thursday, 60s/70s coastside, 80s/90 inland. Lows tonight cooling to the 50s, mixing with upper 40s especially across the southern two thirds of the forecast area where late night cool air advection is forecast to develop. Coastal stratus redeveloping tonight may also produce spotty light drizzle. && .LONG TERM... (Saturday through Thursday) Issued at 359 AM PDT Fri May 31 2024 Expect coastal stratus and fog Saturday morning, possibly leading to additional spotty light drizzle, and a repeat of coastal stratus, fog and patchy light drizzle redeveloping Saturday night and Sunday morning. Saturday-Monday daytime highs cooling a few to several degrees mainly inland, while near and along the coast daytime highs will be about the same as today. The rate of warming next week will then depend at least on several synoptic factors, including what occurs upstream across the mid-high latitudes of the North Pacific basin. A deep long wave trough (cold air) north of a strong long wave ridge (warm air), plenty of strong zonal jet stream wind including water vapor transporting eastward from the western Pacific (also running across positive anomaly sea surface temps with the Japan Current in the central North Pacific) will bring about late season low pressure system development, an eastward progression and zonal pattern to southwestern Canada/the Pacific NW. Even as far south as our forecast area the long wave trough off the Pacific NW may briefly pierce/lower broad ~ 580 decameter 500 mb heights early in the week. Disagreement exists in the global model forecasts as to the earliest stages of 500 mb high pressure system development, more specifically the geographic location of building air mass, will it initially be west (or east) of our forecast area? There`s good global model agreement on the large scale with the exception of this, and this is within the usually reliable 120 hour (5 day forecast). But, it matters and it`ll influence whether we quickly or slowly warm up next week, the depth of the marine layer, inland extent of maritime influence, how hot it`ll get inland in our forecast area. By early next week we`ll also see much greater water vapor move across our forecast area with precipitable waters near early June max moving average 1.25". Not enough dynamics for late season rain, however with water vapor riding over chilly sea surface temps expect good chance of coastal stratus and fog. Otherwise remember we`re nearing peak sun angle and heating, we are about three weeks from the summer solstice, continental surface heating over the CONUS almost always couples to a warm core high pressure system. With subsequent global model output we should see whether or not the warm up will be quicker or slower to develop next week. In the meantime, it`s a good idea to plan on daytime highs in the 90s to lower 100s inland, at least a moderate heat risk next week. Highs becoming well above early June normal, and so far looks below record highs at the long term stations. We`re going into this warm up next week with good potential for immediate coastal cooling sea- breeze even if limited as a shallow marine layer. The strength of the northerly pressure gradient ACV-SFO will matter too, which goes back to what was discussed in the previous paragraph in the global model handling in the early stage of high pressure development. && .AVIATION... (18Z TAFS) Issued at 1200 PM PDT Fri May 31 2024 Widespread VFR lasts into the evening. Moderate to breezy winds build this afternoon, with stronger winds along the immediate coast and over the ocean. Expect winds to reduce in the evening and overnight hours as stratus begins to move inland from the coast as well as filter into the SF Bay. This will give OAK and the Monterey Bay terminals IFR/MVFR CIGs that will last into the mid morning. Pockets of mist and drizzle will be possible along the coast as well. Widespread VFR returns in the mid to late morning on Saturday, with stronger winds expected to arrive that afternoon. Vicinity of SFO...VFR through the TAF period. Breezy to gusty westerly winds arrive this afternoon with peak gusts around 21 kts. Gusts taper off into the late night, but stay moderate through Saturday afternoon before becoming strong and gusty. Overnight winds look to prevent cloud cover from filling over SFO, but cloud cover will be filling over OAK and to the north of SFO terminal. SFO Bridge Approach...Scattered to broken clouds from 800 to 1400 ft AGL will be possible around 10z to 18z Saturday. Monterey Bay Terminals...VFR and moderate west winds last into the evening. MVFR CIGs arrive into the evening as winds reduce. IFR CIGs and pockets of drizzle and mist will push inland in the late night. CIGs and drizzle chances reduce into the mid to late morning, followed by VFR and breezy winds for Saturday afternoon. && .MARINE... (Today through Wednesday) Issued at 849 AM PDT Fri May 31 2024 Breezy to gusty northwest winds will continue over the outer waters into the weekend with gale force winds expected late Friday and into Saturday. This will cause a hazardous marine environment. Significant wave heights will reach up to about 11-12 feet Friday afternoon, but will begin to reduce today through the remainder of the weekend. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory from 3 PM this afternoon to 9 AM PDT Saturday for Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm. Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena to Pt Reyes 10-60 NM. Gale Warning from 3 PM this afternoon to 9 AM PDT Saturday for Pt Arena to Pt Reyes 10-60 NM. Small Craft Advisory until 9 AM PDT Saturday for Pigeon Pt to Pt Pinos 10-60 NM. && $$ SHORT TERM...Canepa LONG TERM....Canepa AVIATION...Murdock MARINE...Murdock Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea