Issued by NWS Houston/Galveston, TX
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000 FXUS64 KHGX 192347 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 547 PM CST Tue Jan 19 2021 .AVIATION [00Z TAF Issuance]... A messy aviation forecast with MVFR to occasional LIFR conditions through through most of the period. A weak/shallow front, currently between KCLL and KCXO will slowly move southward, before washing out around 06-09Z near the Houston Metro area. Behind the front, IFR to LIFR conditions are expected along with some light rain/sprinkles. Ahead of the frontal boundary, south to southeast winds around 04-07 knots are expected, leading to areas of fog overnight for terminals south of KIAH. Areas of fog will gradually diminish by mid-late morning; but MVFR ceilings may persist through the TAF period at all sites. 05 && .PREV DISCUSSION /Issued 336 PM CST Tue Jan 19 2021/... .SHORT TERM [Wednesday Through Thursday Night]... The day started rather warm for the southern half of the CWA with temperatures in the mid 60s. As a result, temperatures today will end up higher than guidance is indicating with highs in the mid-to- upper 70s, especially with the cloud cover breaking up over much of the CWA as dry air pushes in from the west. This drier air has also steepened the low-level lapse rates so we`re getting moderate wind gusts getting mixed down to the surface. For the far northern counties, high temperatures today will depend on the speed of the cold front currently in Central Texas. Latest high-res guidance shows the front being on the Brazos Co./Washington Co. border around 00z. This will likely relegate high temperatures north of this to the low 70s. The movement of the cold front is a key part of the forecast for the next couple of days. It is expected to make it as far as the coastline by 09z before stalling out. Winds will transition to northeasterly after frontal passage except along the coast where winds will be more easterly. As a result, temperatures tonight will be a bit cooler than last night with lows in the low-to- mid 50s. A weak area of low pressure will develop in Southern Texas early on Wednesday and push a weak warm front through most of the CWA by Wednesday afternoon. Southeasterly winds will make a return at this point and so will some suitable WAA. Current thinking is that the warm front will not move in early enough to drive afternoon temperatures up on Wednesday, so most locations will remain under the 70 degree mark. Wednesday night will reflect the WAA with temperatures not dropping that much, only down into the upper 50s/low 60s. Low-to-mid 70s return for high temperatures on Thursday as the southerly winds continue to drag in warm, moist air from the Gulf of Mexico. Now that was the temperature/wind impacts of the cold front. Let`s get into its implications on the rain chances. PWATs for today are around 1.2" across most of the CWA, but the best chance for rain will be along the cold front since this is the only source of lift. Jet dynamics are not favorable as we have upper-level convergence aloft. As the front slowly moves southward this evening/overnight, there will be less and less rain associated with it due to dry air aloft down in the southern portion of the CWA. This is evident around 700 mb with large dewpoint depressions and is accented by a subsidence inversion. All of the moisture will be trapped below 750 mb for today, so only looking at light rain/drizzle. This also creates an environment for low cloud ceilings overnight tonight. For Wednesday, most of the rain will stay to the west of the CWA as the weak area of low pressure develops in South Texas. Once the weak warm front pushes through, PWATs will shoot up to 1.5"-1.8" by Thursday morning. Theta-E values will be on the rise as well. This will help moisten the mid-levels and remove the subsidence inversion. As a result, moderate rain is more likely for Thursday afternoon with forecast soundings indicating a very saturated profile into the upper-levels. The highest chances of rain will be further north near the frontal boundary, which as of right now looks to be just north of the CWA. However, rain is likely to extend south of it due to the high PWATs so went ahead and added higher POPs for the far northern counties for Thursday afternoon. With the still cool sea surface temperatures and warmer temperatures (and dew points) being advected over the Gulf, sea fog will be possible beginning tonight and likely lasting into the weekend. 26 .LONG TERM [Thursday Night Through Tuesday]... Upper trof currently off the SoCal/Baja coast will steadily be filling and become absorbed in the wsw flow aloft over the Plains. South and east of this, there will be a corridor of moisture & embedded disturbances between that & the remnant Gulf ridging along the coast. Rain & showers should be ongoing across northern parts of the CWA Thurs evening within this particular area and ahead of where a weak sfc frontal boundary will be situated. PW`s will be quite high (1.6-1.8") for this time of year, though instability will be lacking. Models are showing some 0.75-2.5" accumulations across n/ne parts of the region, though it`ll likely come in the form of some steady/training light to moderate rains over several hours so am not anticipating much of a FF risk attm. Aforementioned frontal boundary will backdoor thru the remainder of the area and offshore Fri night. Slightly drier airmass will filter in Sat, but will quickly modify as the front returns north as a warm front late Sat and Sat night. This will put us right back in the soup...with periods of drizzle, light rain and fog heading into Sunday. Medium range solns showing decent agreement bringing a Pacific front into the region Monday. 47 .MARINE... Winds and seas will continue on a slow downward trend tonight as a weak frontal boundary sags into southeast Texas then stalls. Patchy sea fog may develop later tonight in advance. The front will dissipate and onshore winds will persist into Friday leading to a better/continued risk of sea fog at times. A weak cold front should move into the waters Friday night. Onshore winds will quickly resume by late Saturday then strengthen. This will bring a return of rain chances, sea fog and elevated seas into Monday when the next front should approach the coast. 47 && .PRELIMINARY POINT TEMPS/POPS... College Station (CLL) 49 61 55 70 60 / 40 30 30 60 60 Houston (IAH) 54 67 59 73 64 / 10 10 10 30 60 Galveston (GLS) 58 64 60 68 63 / 0 10 10 20 40 && .HGX WATCHES/WARNINGS/ADVISORIES... TX...None. GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 9 PM CST 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. &&
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