Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000 FXUS64 KHGX 211735 AFDHGX Area Forecast Discussion National Weather Service Houston/Galveston TX 1135 AM CST Tue Nov 21 2017 .AVIATION...
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A combination of an upper level shortwave trough and the left- front quadrant of the upper level jet stream was helping to kick off showers along the coastal areas at 11:30 AM. Expect these to move eastward over the next couple of hours. An outside chance for MVFR ceilings with any showers that occur, mainly at KGLS through 20Z. Otherwise, VFR this afternoon. Chances for showers return from KCXO southward after 00Z this evening with another upper level shortwave trough moving overhead. Actual front will move through KCLL after 06Z and then quickly through the coast the area starting at 08Z at KCLL and KUTS and through to the coast sometime toward 12Z. Timing has been tough to figure given that winds should pick up but with ceilings probably lagging behind the frontal passage. MVFR conditions should lift to VFR by or after 15Z. Breezy conditions continue after ceilings lift. 40
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&& .PREV DISCUSSION...
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/ISSUED 952 AM CST Tue Nov 21 2017/ DISCUSSION... Just some extremely minor tweaks for the forecast update - mainly to match obs/grids/trends. There`s currently a brief lull in the precip, but suspect we`ll see additional development stretching along an moisture axis situated across the se part of the CWA south southwest into the offshore waters later this morning and afternoon. 47 PREV DISCUSSION... /ISSUED 416 AM CST Tue Nov 21 2017/ Evening upper air analysis showed Southeast Texas located in between two mid-level disturbances, one located over Louisiana and a second located over the Texas Panhandle. Mid to high clouds associated with this secondary disturbance are evident streaming over the region on early morning GOES-16 infrared imagery with a second, lower level of clouds building into the region as moisture gradually returns from the Gulf. In addition to these lower clouds, signs of this moisture return is also evident with the isolated to scattered showers developing across the coastal waters this morning along a coastal trough/ axis of surface convergence stretching from Freeport southwest towards Brownsville. As southwesterly low-level flow pushes this surface boundary up the Texas coast through the day and nudges inland, shower coverage is expected to expand across the coastal waters and move into areas east of a Palacios to Huntsville line this morning. Increasing instability as a result of mid-level cooling from the approach of the Panhandle shortwave trough as well as a 60-80 knot upper level jet nosing into the northwestern Gulf today should provide favorable conditions for thunderstorms to also develop along the boundary. 00Z Convection Allowing Models (CAMs) have a fairly consistent signal for scattered showers and thunderstorms to develop this morning with what appears to be a thunderstorm complex developing somewhere across the eastern waters later this afternoon. Have increased rain chances for the marine areas as a result and pulled the isolated thunderstorm mention farther inland east of a Freeport to Houston to Livingston line. 0-6 km bulk shear values over the waters increase to 25-30 knots this afternoon and have concerns that this may result in enough organization for a few strong or severe thunderstorms over the coastal waters today. Marine forecast soundings show a fairly persistent dry layer of air between 700-500 MB with evaporative cooling within this layer promoting lower freezing levels and at least a small hail threat within stronger thunderstorms over the waters this afternoon into this evening before the thunderstorms push east towards coastal Louisiana. If the thunderstorm complex the CAMs have shown does develop, rain chances farther inland today may be much lower than the 20-40 percent currently advertised as the coastal storms rob Southeast Texas of better moisture inflow. A cold front located across the Northern Plains early this morning will continue to trudge south during the day today, reaching the Brazos Valley this evening, Houston metro around midnight, and pushes off the coast by sunrise Wednesday. Weak convergence along the front may produce a few showers as it sweeps across the region this evening and tonight with relatively drier and cooler air being ushered back into the region behind it. Highs today in the 70s will be replaced by highs in the 60s tomorrow with dry conditions again resuming across the region on Wednesday as north flow aloft becomes established in the wake of today`s shortwave trough. The weather on Thanksgiving will certainly be something to give thanks for as dry north flow aloft persists and results in sunny skies and highs in the mid to upper 60s. North flow aloft becomes more of a northwest flow aloft on Friday as a northern stream shortwave trough dives across the Great Lakes towards the Mid- Atlantic states and upper ridging builds in from the west. Temperatures will gradually warm back into the mid 70s to near 80 by Saturday as ridging spreads eastward, but the northern stream disturbance looks to send a weak cold front into the region late Saturday into Sunday and moderate temperatures a few degrees by the end of the weekend. With the region remaining closed off from the Gulf through the end of the week, expect the frontal passage to be dry. Upper ridging continues to build across Texas during the late weekend and into early next week with temperatures again warming back into the mid to upper 70s. Huffman MARINE... Light onshore flow will persist today with high pressure over the eastern US and low pressure over the Southern Plains. A cold front will cross the coastal waters this evening and usher in a strong offshore flow. NNE winds will increase to 20-25 kts with gusts approaching gale force over the Gulf waters. A Small Craft Advisory is in effect for tonight into early Wednesday and a Gale Watch/Warning may yet be required for the offshore waters as gusts may frequently gust in excess of gale force. A moderate offshore wind will persist Wednesday night and a SCEC will be needed. Winds will decrease on Thursday as high pressure settles over S TX. The high pressure will move east of the area on Friday and onshore winds will return. Another cold front will cross the region Saturday night with moderate offshore winds in the wake of the front on Sunday. 43/22
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&& .PRELIMINARY POINT TEMPS/POPS...
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College Station (CLL) 51 61 36 66 42 / 20 10 0 0 0 Houston (IAH) 56 65 40 66 44 / 20 10 0 0 0 Galveston (GLS) 61 67 50 64 50 / 30 10 0 0 0
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&& .HGX WATCHES/WARNINGS/ADVISORIES... TX...NONE. GM...Small Craft Advisory from 11 PM this evening to 11 AM CST Wednesday for the following zones: Coastal waters from Freeport to the Matagorda Ship Channel out 20 NM...Coastal waters from High Island to Freeport out 20 NM...Galveston Bay...Matagorda Bay...Waters from Freeport to the Matagorda Ship Channel from 20 to 60 NM...Waters from High Island to Freeport from 20 to 60 NM. && $$ Discussion...47 Aviation/Marine...40

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