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 300014 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 614 PM CST Tue Dec 29 2020 .AVIATION [00Z TAF Issuance]... For the most part, MVFR ceilings overnight and into tomorrow. Our northernmost airports will receive VCSH and VCTS earlier than everywhere else with the frontal boundary hovering near CLL by the end of the TAF period. Overall, SSE winds and gusty conditions for most terminals especially with nearby showers and storms. Added TEMPOs during hours where precip activity has higher certainty, but expect additional amendments should conditions change since timing and location remain uncertain and variable. KBL && .PREV DISCUSSION /Issued 348 PM CST Tue Dec 29 2020/... .SHORT TERM [Through Thursday Evening]... The main focus of the short term period will be Wednesday into Thursday as a strong cold front and a deepening surface low could result in extended periods of showers and thunderstorms across SE Texas. Ongoing southerly flow will maintain the warm weather trend across SE Texas through Wednesday. Highs today were once again in the low to mid 70s and lows tonight are expected to be generally in the 60s, slightly lower in portions of the E-NE counties. Breezy conditions will prevail as the local pressure gradient remains tight in response to very broad surface high pressure moving across E-NE CONUS and a surface low deepening across the southwestern Gulf Coast region. Look for winds between 10 and 20 MPH with higher gusts on occasion. This evening into Wednesday morning, a negatively tilted upper level trough will extend from the northern Plains into Southern CA and dig southward into the NW Mexico region. Underneath the trough, surface high pressure will broaden across the W CONUS/Rockies and a low pressure system and associated cold front will then translate southeastward across the Texas Panhandle. The cold front will make its way into Central Texas by Wednesday afternoon and move into SE Texas during the late afternoon or early evening hours. As the night progresses, the front is expected to slow and possibly stall while approaching the southern counties as the surface low pressure that was building across the southwestern Gulf Coasts moves northeastward into our local area and eventually merges with the frontal boundary sometime Thursday morning. The low and cold front will eventually lift northeastward Thursday afternoon and exit our CWA sometime in the late afternoon to early evening hours. What could SE Texas expect with respect to the local weather? To begin, chance of rain through tonight are progged to remain on the lighter side (around 15-30%) and mostly confined to the western and northern counties with minimal or minor accumulations expected. But development of showers and thunderstorms will rise quickly early Wednesday morning. Given that SE Texas will still be influenced by southerly flow and ongoing low level moisture transport, a more widespread rain event with stronger storms is expected ahead and along the frontal boundary. Models continue in fairly good agreement with the weather scheme for Wednesday and Thursday. They still indicate PWs of 1.2 to 1.5 inches prior to the frontal passage and forecast soundings have kept MU and SB CAPE of around 1000-1500 J/kg, SFC-1km SRH between 200-300 m^2/s^2, SFC-6km shear between 40- 50 kts. Models also agree with the 100-120kt jet streak moving overhead Wednesday afternoon. Therefore, we are still expecting stronger storms to have the potential for producing heavy rainfall, strong gusty winds, and hail. There is also a low probability of weak rotation. The weather pattern remains rather complicated as the surface low to our southwest begins to shift northeastward Wednesday night and the front slows/stalls before reaching the coastal counties. The low will move into the CWA by early Thursday morning, pooling in additional moisture across SE Texas while merging with the front later in the morning and afternoon hours, which unfortunately can result in ongoing/training of showers and thunderstorms across portions of SE Texas well into Thursday afternoon. At this time, Storm Total Accumulations of around 1.0-3.0 inches with isolated amounts of up to 5.0 inches are anticipated through Thursday, with the highest amounts expected over areas along and west of I-45. Conditions improve quickly as the low and cold front progress northeastward Thursday evening/night. WPC Day 2 Excessive Rainfall Outlook continues a Marginal to Slight Risk for much of the region from 12Z Wednesday to 12Z Thursday. The Day 3 Outlook has a Marginal Risk for most SE Texas from 12Z Thursday to 12Z Friday. Meanwhile, SPC Day 2 Convective Outlook starts with a Marginal Risk along a line from Brazos County extending southeastward into Chambers County from 12Z Wednesday to 12Z Thursday and expands it across most of SE Texas (with a Slight Risk clipping the eastern Liberty/Chambers counties) for the Day 3 Outlook, valid from 12Z Thursday to 21Z Friday. Although this discussion encompasses a general overview of the upcoming weather features and expected rainfall, at this time, when/where the heavier rainfall will occur is not concrete, as any displacement of the location and/or timing of any of these features can change the time/location/duration of the rainfall for any given area. It is advised to keep up with the upcoming forecasts and monitor closely the radar and weather conditions throughout the day Wednesday and Thursday, especially if you plan to travel or spend time outdoors. In addition, strong winds that may accompany these storms could knock out or drag outdoor items/furniture or decorations. 24 .LONG TERM [Thursday Night through Tuesday]... On Thursday evening, a deep upper level low over Texas will get bumped NE as weak short wave energy drops SE toward the Big Bend. At the surface, low pressure will also get nudged N-NE and this will allow a cold front to push east and allow significantly drier air to move into SE TX. There should be a sharp cut off to the rain from the SW to the NE as the low pulls away. Moisture trapped beneath the capping inversion will keep skies mostly cloudy. Another weak cold front will cross the area Friday evening and usher in some slightly cooler and reinforce the dry air already in place. Moisture levels look too dry to offer any precip with the fropa Fri night and forecast soundings show a lot of dry air below 700 mb. High pressure will dominate SE TX weather Sunday and Monday with with seasonal temperatures. The high will push east of the region Sunday night allowing a return of onshore winds. Another cold front will cross the coast next Tues/Weds. 43 .MARINE... Moderate to strong onshore winds and building seas will continue tonight ahead of the approach of a surface cold frontal boundary on Wednesday. Small Craft Advisories remain in effect for the offshore waters and will be in effect for the nearshore waters by 6 PM CST today. Caution flags are also in effect for the bays for winds of 15-20 KTS. As the frontal boundary nears the coastal waters late on Wednesday, a prolonged period of showers and storms are expected with the potential for some strong storms. Stronger storms will be capable of producing strong gusty winds and locally higher seas, which can overturn small boats. Additional precipitation is expected on Thursday as a developing surface low along the southwestern Gulf coast pushes northeastward. Moderate to strong offshore winds and elevated seas are expected in the wake of the departing system with low water levels in and around the bays possible. Marine conditions should begin to improve by Friday with all Gulf waters below SCA threshold by Friday afternoon. Caution flags may linger across the offshore waters through Saturday afternoon. Light onshore flow and seas of 1-3 FT expected Sunday into Monday. 24 && .PRELIMINARY POINT TEMPS/POPS... College Station (CLL) 62 74 50 56 37 / 50 90 100 90 20 Houston (IAH) 62 75 62 66 40 / 20 80 90 90 10 Galveston (GLS) 63 72 63 67 45 / 10 60 80 100 10 && .HGX WATCHES/WARNINGS/ADVISORIES... TX...High Rip Current Risk through Wednesday afternoon for the following zones: Brazoria Islands...Galveston Island and Bolivar Peninsula...Matagorda Islands. GM...SMALL CRAFT SHOULD EXERCISE CAUTION through Wednesday afternoon for the following zones: Galveston Bay...Matagorda Bay. Small Craft Advisory until 6 PM CST Wednesday for the following zones: Coastal waters from Freeport to Matagorda Ship Channel TX out 20 NM...Coastal waters from High Island to Freeport TX out 20 NM...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. && $$
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