Marine Weather and Tides
3/30/2021 -- ANZ330 has been split into ANZ331 and ANZ332. Click EDIT in Marine Forecast and select your new zone.
1/26/2021 -- The West Coast Satellite images havd been updated. They now use GEOS-17.
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|Sunrise 5:56AM||Sunset 8:23PM||Saturday July 24, 2021 3:16 PM CDT (20:16 UTC)||Moonrise 8:22PM||Moonset 5:17AM||Illumination 100%|
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Area Discussion for - St. Louis, MO  (on/off)  Help NOTE: mouseover dotted underlined text for definition
FXUS63 KLSX 241944 AFDLSX
Area Forecast Discussion National Weather Service Saint Louis MO 244 PM CDT Sat Jul 24 2021
SHORT TERM. (Through Late Sunday Night) Issued at 243 PM CDT Sat Jul 24 2021
Currently aloft, a broad trough is centered over the western United States, placing the Midwest under northwest flow. Within this flow, a shortwave can be seen passing through the Great Lakes region. At the surface, a cold front extends from the Great Lakes southwestward into Nebraska. South of the front, temperatures have reached the low to mid 90s for most locations over the CWA, and with dew points in the mid 70s, heat index values are around 100. With a few more hours of heating left, I expect nearly all locations to see heat index values of at least 100 with the St. Louis metro area seeing values around 105.
With this heat and high dew points, the environment over the CWA has become quite unstable with approximately 3,500 J/kg of SBCAPE noted in SPC mesoanalysis. This instability has lead to isolated showers developing over portions of southeastern Missouri and southwestern Illinois this afternoon, and I expect the chance for this isolated convection to continue through around sunset. While I can't rule out a rumble of thunder, warming in the midlevels should limit updraft strength and keep most convection as showers.
This evening and overnight, our attention turns to the aforementioned front to our north as it approaches the CWA. The main upper-level forcing from the Great Lakes shortwave is expected to remain northeast of the CWA, leading to the main forcing mechanism for convection being the front itself as it enters the CWA. Short- range guidance still differs on the timing and placement of the front and its associated convection, but despite some CAMs showing convection pushing into northeastern Missouri and west-central Illinois prior to 00z, confidence is greatest in the front and associated convection moving into these areas just after 00z. At that time, the 12z HREF has mean MUCAPE of 3,000 J/kg, but instability will quickly drop off with the setting sun to around 1,500 J/kg of MUCAPE overnight. Additionally, 0-6 km bulk shear is expected to be on the order of 15-20 kts. Given this setup, I'm not confident in severe storms occurring, but there is about a 2-3 hour window after sunset when I can't completely rule out an isolated damaging wind gust and brief hail with a storm. Forcing along the front and residual instability overnight will allow for isolated showers and thunderstorms overnight mainly north of I-70.
For tomorrow, the front will continue its slow southward progression through the CWA, reaching roughly the I-70 corridor by early afternoon. Here, MUCAPE of at least 2,000 J/kg per SREF guidance will be more than enough to support isolated to scattered convection. However, 0-6 km bulk shear of 15 kts or less and poor lapse rates will limit convection to showers or weak thunderstorms at best.
As for afternoon highs, temperatures will rise into the low to mid 90s once again, but be a degree or two cooler than what will have been experience on Saturday. This is due to the front and its associated cloud cover and convection, and is expected to be enough not to warrant an additional heat advisory.
LONG TERM. (Monday through Next Saturday) Issued at 243 PM CDT Sat Jul 24 2021
On Monday, the cold front is expected to continue southward. Deterministic guidance still shows a variety of solutions for frontal position relative to the CWA, but a slight decrease in the range of surface temperatures noted in ensembles gives me confidence the front will make it through much of the CWA. While this FROPA will only drop temperatures by a few degrees, it will push the higher dew points further south, reducing humidity and potentially the threat of dangerous heat index values. How much this threat is reduced is uncertain at this lead time, as spread among guidance dew points is still high at this time. This also leads to lower confidence in the chance of convection Monday. If the front is able to linger over or close to southern portions of the CWA, convection along or just behind the front may be possible, and the current forecast accounts for that.
As we approach the middle of the week, the western ridge will amplify as a notable trough develops over eastern Canada and the northeastern United States. As this pattern evolves midweek, 850 mb temperatures will begin to creep up. NAEFS and EPS guidance have come in a little warmer recently, with mean 850 mb temperatures over the CWA now around 22 degrees C by Wednesday. This translates to surface temperatures reaching the mid-90s by Wednesday afternoon, with some locations possibly seeing upper 90s. While these values are eye-catching and concerning for dangerous heat potential, the early week front looks to keep greater dew points still shunted south of the CWA. However, with the current temperature forecast, I still can't rule out some locations seeing heat index values approaching 105 degrees midweek. At the very least, areas of the CWA may need a heat advisory for duration (4 days of 100 heat index) by midweek depending on what happens earlier in the week.
For Thursday through the end of the week, ensembles begin to diverge on the phasing of the western ridge and eastern trough, which has varying implications on conditions at the surface. With the pattern amplifying, it seems likely that the Middle Mississippi Valley will be impacted either directly or indirectly by upper-level disturbances rounding the ridge or trough. This may lead to another cold front or two passing through the CWA at the end of the period, bringing at least a temporary reprieve from the heat. Given that, I'm confident temperatures will remain warm through the end of the week, but I can not say exactly how warm at this point.
AVIATION. (For the 18z TAFs through 18z Sunday Afternoon) Issued at 1236 PM CDT Sat Jul 24 2021
The main focus for this forecast period will be the chance of isolated showers and thunderstorms. Currently, a field of agitated cumulus has developed over portions of eastern Missouri and western Illinois within the vicinity of the all local terminals except for KUIN. Within this area, brief, small showers have developed. I expect any showers that do form this afternoon to remain isolated and brief, therefore, my confidence in them impacting KCOU, KJEF, KSUS, KSTL, and KCPS or occurring in their vicinity is low enough to not mention in the TAFs at this time. However, I can't completely rule out needing to add VCSH or direct impacts at some point this afternoon. I also can't rule out a brief thunderstorm at these terminals, but the threat of that seems even lower.
This evening, a southward moving cold front will bring the chance of storms to the KUIN terminal. Storm coverage is expected to be isolated to scattered, so confidence in direct impacts to the terminal is low at this time. Tomorrow morning, the chance of showers and storms will move over the remaining local terminals, including KSTL. However, shower and storm coverage is expected to be even less at that point, if they are even occurring. Therefore, I have left mention of this activity out of the TAFs for now. When showers and storms form along the front near the end of the period into the afternoon, they are currently expected to form south of all local terminals.
LSX WATCHES/WARNINGS/ADVISORIES. MO . Heat Advisory until 8 PM CDT this evening for Jefferson MO- Lincoln MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO.
IL . Heat Advisory until 8 PM CDT this evening for Calhoun IL-Jersey IL-Madison IL-Monroe IL-Saint Clair.
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|St. Louis, Spirit Of St. Louis Airport, MO||39 mi||22 min||WSW 10||6.00 mi||Partly Cloudy with Haze||93°F||72°F||50%||1013.1 hPa|
Link to 5 minute data for KSUS
Wind History from SUS (wind in knots)
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