Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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377 FXUS64 KHGX 182253 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 553 PM CDT Sun Apr 18 2021 .AVIATION... Mid/high level cloudiness will continue to stream over the region for the next 24 hours. Winds will decrease quickly this evening at all TAF sites except KGLS. Winds will veer to the east on Monday as surface high pressure moves east. Some "lower" mid level cloud cover possible after 00z Tuesday but conds still expected to remain VFR. 43 && .PREV DISCUSSION... /ISSUED 233 PM CDT Sun Apr 18 2021/ DISCUSSION... SHORT TERM [Through Monday Night]... Zonal flow aloft will continue to bring different impulses of energy across the region, producing enough forcing for some passing showers. Most of the saturated layer remains at 750 mb or above with sfc dewpoints in the 40s; therefore, drier air at low levels should keep this activity as sprinkles or virga. The exception will be over the Gulf waters with scattered light rain but impacts are minimal. Surface high pressure over west TX pushes off any remaining precipitation early this evening and leads to decreasing cloud cover overnight. Light winds and mostly clear skies will allow for radiational cooling into Monday morning. Overnight lows will range in the 40s and 50s. Sfc high slides eastward into the region by Monday, resulting in northeast to easterly flow by Monday afternoon. Low-level moisture and warmer airmass will filter in, though temperatures will still remain slightly below normal for this time of year. Highs will generally be in the low to mid 70s. Another chilly night is expected Monday night into Tuesday, though lows will be a few degrees warmer than tonight given onshore flow and increasing cloud cover. Overnight lows will mainly be in the 50s to low 60s. LONG TERM [Tuesday Through Sunday]... The bulk of the long term period will feature fair weather and relatively dry conditions until onshore flow returns later in the week, increasing humidity and moisture ahead of a weekend front that will bring the only real chance of showers and storms in this period. Now, despite the "lack" of weather, that doesn`t actually mean there`ll be a lack of weather. Indeed, on Tuesday morning, a front will be heading through North Texas on its way to the coast. Driven by high with a central pressure in the 1025-1030 range, and some decently cold air for mid-late April, it should make its way through the area pretty easily. Of course, the deep moisture just won`t be there, with progged precipitable water values maybe almost sorta but probably not reaching an inch, so this will be a dry frontal passage when the front reaches our northern border late Tuesday afternoon, and makes its way to the coast through the night. While already dry to begin with, conditions will get even drier for the midweek stretch as the Plains high takes command. Don`t be surprised to see very low humidity on Wednesday, with dewpoints falling into the low 30s inland. The dry air and northerly winds should also make for an unseasonably cool night on Tuesday night, with lows in the northern reaches of our area falling to around 40 degrees. Even at the coast, lows in the 50s are expected. Enjoy it while you can get it. Winds gradually shift back to become more onshore on Thursday as the high moves off to the east. It`s probably no surprise that a gradual warming trend to temperatures better matching the seasonal average will follow. Meanwhile, an upper trough will be digging into the Desert Southwest, and will induce lee cyclogenesis in eastern New Mexico on Friday. That low will make its way towards the Ohio Valley this weekend, and we can look forward to another frontal passage when the attached cold front sweeps through the area very late Friday night and/or Saturday morning. With the trough and surface low so far south, the pressure gradient should allow for moderate to gusty winds to develop ahead of the front, and should really amplify moisture return Thursday and Friday, and precipitable water looks to be in the ballpark of 1.9 inches when our rain potential is maximized. Though there`s a pretty solid consensus on the big picture here, there are still a lot of details that are fuzzy, and could potentially have a decisive impact on sensible weather this weekend. We continue to see deterministic models fall into one of two typical springtime scenarios: the Euroish solution, which more definitively drives a front through, allowing for rain to wrap up more quickly as high pressure settles in. The other idea is to develop a coastal trough, which will hang up the front and keep it lingering in the area, dragging rain potential almost entirely through the weekend. The GFS in particular continues to show heavy rain bullseyes on this surface trough, though there`s not a lot of consistency in exactly when or where this would occur. Taking a more holistic look, the NBM now has almost a 1 in 3 chance of exceeding an inch of rain on Friday, with a half-inch now just a shade more likely than not. The deterministic QPF for IAH on Friday is 1.5 inches, decidedly higher than the half-inch amount of the median value. While a big difference, this is probably not a huge surprise when considering potential for convection on the Gulf Coast. We can take this as a signal that if we do get a convective burst on this front, we should expect more rain than if it does not happen. AVIATION [18Z TAF Issuance]... VFR conditions are expected through the TAF period. Spotty showers will be possible this afternoon, but coverage/intensity should be minimal. Cloud cover decreases tonight, with only FEW to SCT high clouds prevailing through the period under light and variable winds. MARINE... Moderate northerly to northeasterly flow continues to prevail in the offshore waters with seas persisting around 6 to 7 feet through the afternoon hours. Winds and seas will gradually diminish going into the evening/overnight hours. Surface high pressure will move over the area by Monday afternoon and bring a brief transition to easterly flow for the coastal waters. By Monday night, the flow becomes southeasterly as the high pushes off to the east. Onshore flow will persist through Tuesday night when the next cold front will push through the coastal waters. Moderate offshore flow will return in the wake of this front with caution to advisory conditions likely through Wednesday. FIRE WEATHER... The early part of this week looks to feature some unseasonably dry air for inland Southeast Texas. The brief period of onshore winds Monday night into Tuesday isn`t likely to help things much either before a dry frontal passage Tuesday evening. The timing of the front will certainly help mitigate fire weather concerns, but the wind shift with a lack of precipitation will be something to keep an eye on Tuesday into Wednesday. While RH will recover Tuesday night after sunset, the influx of dry air behind the front will make for max RH numbers considerably lower (60-70 percent) than typical in our area (frequently near 100 percent). In the wake of the front, we can expect a surge of dry air not commonly associated with the spring. Wednesday may well see critically low humidity for much of the inland area, and uncommonly dry air all the way to the coast. Again, the timing of the front overnight will help mitigate concerns as winds will not be quite as strong on Wednesday than they could be. Still, the relative uncommon forecast should be a cause for alertness with this frontal passage. && .PRELIMINARY POINT TEMPS/POPS... College Station (CLL) 46 74 50 81 42 / 0 0 0 0 0 Houston (IAH) 50 74 55 81 48 / 0 0 10 0 0 Galveston (GLS) 61 72 63 76 56 / 0 0 10 0 0 && .HGX WATCHES/WARNINGS/ADVISORIES... TX...NONE. GM...NONE. && $$ Discussion...99 Aviation/Marine...99

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