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 212324 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 624 PM CDT Wed Apr 21 2021 .AVIATION [00Z TAF Issuance]... VFR conditions are expected tonight with light easterly winds as a surface high moves east of the region. Some low-end VFR ceilings to occasional MVFR will begin to filter in from the west-southwest by Thursday morning, becoming BKN/OVC during the afternoon and evening. With increasing pressure gradient in response to an approaching area of low pressure, gusty southeasterly winds from 15 to 20 knots are expected in the afternoon. && .PREV DISCUSSION /Issued 343 PM CDT Wed Apr 21 2021/... .DISCUSSION... Much of the attention for this forecast period focuses on Friday into Saturday morning, as showers and storms - the strongest with potential for severe weather beginning Friday afternoon, and continuing until the front clears the area. Early on, the most threatening severe hazard is likely to be (very) large hail as storms fire off the dryline. For the most part, this will be west of our area, but how far east the dryline gets before convective initiation is expectedly fuzzy this far out. The farther east it can propagate earlier in the day, the more that threat for big hail bleeds into our west. And while hail may be the primary threat, damaging winds and even a tornado are also on the table with these storms. As we push deeper into Friday evening, the storms will grow upscale as a cold front overtakes the area, and that will push a line of showers and storms across our area through the night. Expect the threat to shift more from hail to wind gusts - but again, at three days out, there are still important details that need to be worked out. As usual when discussing a Day 3 outlook - stay tuned, be aware. .SHORT TERM [Through Thursday Night]... Hanging on to the last bit of CAA as northeasterly winds will continue to persist through the evening hours. Afternoon temperatures today will run 10-15 degrees below normal with highs in the mid-to-upper 60s. Enjoy the sunny skies while you can! As surface high pressure scoots off to the east, winds will transition to easterly tonight through Thursday morning before becoming southeasterly. Since WAA will not have taken full effect, temperatures tonight will only be a smidge warmer with lows ranging from the mid 40s to low 50s. Onshore flow kicks in Thursday morning and with that comes the WAA and increasing moisture. Cloud cover will gradually increase from the southwest to the northeast throughout the day. High temperatures for Thursday will top out in the low-to-mid 70s. As an area of low pressure develops near the TX/OK Panhandles, the pressure gradient will tighten over the area which will increase our low-level moisture transport with PW values increasing to 1.2"-1.4" by Thursday afternoon. Hi-res model guidance is in agreement on scattered rain shower development beginning late in the afternoon and persisting into Thursday night with higher PoPs in the western portion of the CWA. Not expecting any of these rain showers to be heavy since they will be the result of low-level moisture that will be relegated to the lowest 2km above the surface due to a subsidence inversion around 800-750 mb with plentiful dry air aloft in the mid-levels. With low-level clouds in place and steady WAA, temperatures on Thursday night will run a little above normal with lows ranging from the low 60s to upper 60s. .LONG TERM [Friday Through Wednesday]... Friday into early Saturday morning look to be the impact stretch of this forecast period, as we find the entirety of our area of responsibility in the Day 3 slight risk area from SPC. There are still many details that need to be worked out, and the evolution of the severe weather outlook will be interesting to see over the next couple of days. However, for what we can reliably predict at this point, the slight risk seems to describe our area well. One of the big questions here will be - as it so frequently ends up - the strength of the cap. While an upper trough moving through will eventually erode this cap, a sufficiently strong capping inversion Friday afternoon may largely choke off convection, and I expect meso-beta/gamma features may not be significant enough to trigger convection. What will likely do the deed is the dryline, but there remains uncertainty about just how far east it will propagate before initiation. Too far west, and storms might not make it to our area before they get overtaken by the cold front and line out. If initiation is farther east, locations like Caldwell, B/CS, and the like could be looking at a potential significant hail threat. This will be an important factor to be teased out in the next couple of days. There is greater confidence in what happens later in the night, as the front rolls through and storms will likely become linear, shifting the greatest severe threat from hail to damaging winds. The good thing for us here is that the timing on this activity crossing our area will be during the overnight hours, which tends to mitigate things some. Not enough to remove the severe threat entirely, but at least enough that our primary hazards will be more "typical" severe weather, rather than a higher-end threat. The downside is that people like to be asleep late at night, so you`ll definitely want to have an idea of what the weather setup is looking like before you go to bed on Friday night, and be prepared with alerting systems that can wake you from sleep if need be. Even "low end" severe weather can be deadly in the wrong circumstances. Once the front is through, we`ve got a good long stretch of fair weather as high pressure dominates most of the stretch from this weekend and early next week. Towards the middle of next week, however, we`ll be looking for the next big upper trough and front to bring another chance of showers and storms. There`s a good bit of uncertainty in even synoptic scale things this far out - on one end, the upper trough is pretty positively tilted, and the enhanced southwest flow ahead of the front would likely keep things very strongly capped. On the other end, there is at least some potential for a more neutral to negatively tilted trough, and more backed flow would allow for stronger convective activity. There`ll be more time to focus on sussing these things out in the days to come, fortunately. .MARINE... Moderate northeasterly winds will continue to persist in the Gulf waters through the afternoon hours. Surface high pressure moving off to the east will induce an easterly flow tonight through Thursday morning. Offshore seas will rise as a result with wave heights increasing to 6-8 feet. Southeasterly flow returns by Thursday afternoon. A developing area of low pressure in North Texas will strengthen this southeasterly flow to moderate from Thursday afternoon through Friday morning with winds bordering the advisory threshold. Will have to keep an eye on the potential for coastal flooding on Friday afternoon/night during high tide. As this low tracks eastward, it will push a cold front through the waters along with showers and thunderstorms on Friday night/Saturday morning. Seas will begin to subside following the frontal passage. Offshore flow in the wake of the front will be temporary, only lasting through Sunday morning before transitioning back to onshore flow by Sunday afternoon. && .PRELIMINARY POINT TEMPS/POPS... College Station (CLL) 45 73 62 79 61 / 0 20 30 90 40 Houston (IAH) 49 73 64 79 67 / 0 10 40 90 50 Galveston (GLS) 60 72 69 78 70 / 0 10 40 70 30 && .HGX WATCHES/WARNINGS/ADVISORIES... TX...High Rip Current Risk until 8 PM CDT this evening for the following zones: Bolivar Peninsula...Brazoria Islands...Galveston Island...Matagorda Islands. GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 7 PM CDT this evening for the following zones: Waters from Freeport to Matagorda Ship Channel TX from 20 to 60 NM...Waters from High Island to Freeport TX from 20 to 60 NM. && $$ DISCUSSION...Luchs SHORT TERM...Batiste LONG TERM...Luchs AVIATION...05 MARINE...Batiste
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