Area Forecast Discussion
Issued by NWS Houston/Galveston, TXVersions: 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
000 FXUS64 KHGX 242340 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 540 PM CST Wed Feb 24 2021 .AVIATION [00Z TAF Issuance]... Trends are generally on track as a slow-moving surface boundary remains expected to stall over the area this evening, with persistent southerly flow along the coast and reduced water temperatures making conditions favorable for the development of sea fog overnight and into tomorrow morning. GLS will experience the greatest impacts with prolonged dense fog possible, though areas of fog could extend as far inland as the I-10 corridor. Cigs are expected to concurrently lower at all sites throughout the period, with MVFR conditions taking hold after midnight and IFR conditions developing closer to the coast by the morning. Shower coverage is expected to increase as the TAF period progresses and the surface boundary remains stalled, so have maintained VCSH wording in this package. Cady && .PREV DISCUSSION... /ISSUED 330 PM CST Wed Feb 24 2021/ DISCUSSION... Unsettled weather looks to prevail through the second half of this week and into the first half of next week as a weak front moves into the area tonight and stalls/washes out. While rain chances will remain somewhat elevated for the next several days, it will likely be in the form of on/off showers than a sustained rainout, and rainfall amounts are expected to be relatively light. Indeed,the most impactful weather is likely to be a prolonged threat of sea fog over the nearshore waters, bays, and immediate coastal areas as warm, humid air flows over waters cooled by the recent cold snap. The incoming front is not expected to be strong enough to clear the coast, meaning the threat of sea fog is likely to persist into next week. This general pattern should prevail until the middle of next week, when a more significant cold front is able to sweep through and at least scour out the moisture and give us some fair weather in its wake. SHORT TERM [Through Thursday Night]... Satellite and observations show a weak front is starting to drift in from the northwest, and the bleeding edge of this diffuse frontal zone may already be starting reach the area, though any functionally different airmass still has some distance to cover before reaching our area of responsibility. Meanwhile, we sit mired in a warm, moist pre-frontal airmass, especially near the coast where we clouds have stubbornly refused to lift for most of the day, and developing sea fog lurks over the nearshore waters. Given how weak the front is, and the difficulty in modeling such a shallow post-frontal airmass, there is considerable uncertainty in just how far south this front will push into Southeast Texas. Temperatures in North Texas certainly don`t indicate a solid push of cold air that will carry this front to the coast. For now, the forecast takes the front into the rough vicinity of US-90 by tomorrow morning, and even this may be somewhat generous. Wherever it stalls, the expectation is that the front will drift around, and generally back to the northwest as a quasi-warm front tomorrow. Whatever movement there is will be small, and probably will be counteracted by the fairly diffuse nature of the front. Ultimately, the coast will stay warmer, more humid, and with a continued threat for sea fog. Up north, modestly cooler air will cut high temperatures from today around 10 degrees - not a ton, but enough to notice. Additionally, upglide over the weak frontal surface will gradually build rain chances overnight, through tomorrow, and into tomorrow night. The best shot for rain looks to be north of the Houston metro tomorrow night, in the rough area of B/CS, Huntsville, and Onalaska northward. LONG TERM [Friday Through Wednesday]... Mostly cloudy skies and multiple chances for precipitation are the main features in the extended. Showers and a few thunderstorms will be ongoing on Friday as the front lifts back towards the north-northwest and becomes quasi- stationary. This will put our region under a persistent warm and humid southerly flow. While some breaks in precipitation are expected, showers and isolated thunderstorms cannot be ruled out into the weekend as different impulses of energy ride along the west/southwest flow aloft. The combination of upper-level forcing and strong southerly flow at the surface will keep a healthy moist airmass with PWAT values in the 1.0-1.6 inch range. The main challenge will be temperatures as it will depend on the amount of cloud cover and the location of the aforementioned front. For this forecast package, have placed the front roughly between Brazos and Harris counties by Friday morning, then, lifting northward throughout the day. With most of the area sitting on the warm side of the front, have increased temperatures a few degrees. Highs will range mainly in the low to mid 60s across the Brazos Valley and in the 70s elsewhere on Friday. Highs mainly in the 70s on Saturday and Sunday. Better and more widespread rain and thunderstorm chances arrive late Sunday into late Monday. A strong upper-level low will dive southeastward from the western CONUS Saturday night into Sunday, leading to increasing PVA on the nose of a southerly 25 to 35 kts LLJ and strong low-level moisture over southeast TX. Overall, instability looks weak, but enough elevated instability exists to leave some thunder mention. While guidance still diverges on the evolution and strength of this system, confidence is increasing that our region will see scattered showers and thunderstorms Sunday night into early Tuesday. At the moment, better chances look possible along the frontal boundary, across our far northern and northwestern counties. Upper-level flow shifts to the northwest by Tuesday, as the ridge aloft builds in over Mexico. While rain chances remain low, some spotty showers cannot be ruled out given persistent onshore flow at the surface. MARINE... Southeasterly flow is expected to continue through the weekend. Sea fog is lurking offshore on coastal webcams and observations as warmer air flows over the cold waters. While it does not appear to be dense yet, there are only a couple isolated observations beyond a few miles from shore, clouds mask much of the waters this afternoon. Some patches of dense fog may already exist in unobserved nearshore waters. Regardless, we should expect degradation in conditions overnight, and a dense fog advisory may become necessary. A weak cold front will approach the coast on Thursday and stall over land before moving back to the north. Latest model guidance indicates the front will drift - at best - only close enough to the coast to offer a very brief reprieve from the fog on the upper portion of the bays. Expect little to no help elsewhere on the waters. Fog and rain chances linger into early next week. At this point, a more significant front will arrive on scene, with a better chance of scouring out this dreary airmass. && .PRELIMINARY POINT TEMPS/POPS... College Station (CLL) 78 53 65 53 66 / 20 10 50 80 40 Houston (IAH) 77 61 70 60 75 / 20 30 40 50 30 Galveston (GLS) 68 60 64 58 66 / 20 30 50 30 20 && .HGX WATCHES/WARNINGS/ADVISORIES... TX...NONE. GM...NONE. && $$ Discussion...99 Aviation/Marine...99
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