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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 210922
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
422 AM CDT Tue May 21 2019

.DISCUSSION...
Many hazards to consider today; from strong gradient winds to
potential strong northern county thunderstorms. The east-to-west
synoptic pressure gradient between GA-FL high pressure and 
western Texas low pressure is the tightest across eastern Texas 
this morning. This is producing early day 20 to 30 mph sustained 
southeasterly winds with gusts approaching 35 to 40 mph over a 
good western 2/3rds of the forecast area including the immediate 
coastline. These winds have prompted a Wind Advisory that will be 
in effect through 6 PM this evening. This healthy long duration 
onshore fetch will push Gulf and Bay water onto local beachfronts 
and there will coastal flooding issues during high tide times
(please see Marine section below). A Coastal Flood Warning is in 
effect through 6 PM for the Galveston Bay area with a Coastal 
Flood Advisory for western coastal region down shore of the San 
Luis Pass. A line of Central Texas thunderstorms will be entering 
our warm and conditionally unstable environment this afternoon. 
There is a marginal chance that isolated thunderstorms could reach
strong to severe thresholds and produce short duration, limited 
coverage damaging downburst winds and large hail. If discrete 
cells do form just ahead of the main line late this morning, there
will be enough uni-directional shear energy with height to open a
small window of opportunity for tornadoes. 

The main line of thunderstorms will be encountering a more stable
air mass over the southern 2/3rds of the CWA. Prog and AMDAR
aircraft sounding reports depict quite the robust, deep warm 
layer with decent near 7 C/km lapse rates above 700 mb. If 
surface temperatures across the northern third of the forecast 
area can achieve the lower 80s before noon this could erode this 
warm nose enough for frontogenesis-induced lift to tap into this 
higher CAPE column. Early day overcast will inhibit warming 'but'
sunrise temperatures will be within 5 degrees of convective
temperatures. For what is it is worth, all models are showing 
strong enough frontal forcing to suggest enough ascent to sustain 
or initiate downstream thunderstorms north of the city early this 
afternoon. Higher confidence of strong thunderstorms to impact 
the far northern tier of counties from Brazos County over to 
Houston County with developing storms south of these communities 
struggling to maintain enough structure to pose much of a threat. 
As column winds veer more parallel with the mean flow this loss of 
bulk shear, in tandem with more stout southern CWA capping, has 
focused this early afternoon marginal severe threat north of a 
Brenham to Huntsville to Livingston line. 

Once today's storms travel east of the region, we will not be
visited by any significant precipitation for several days. 
Surface to upper ridging builds in from the east through the 
remainder of the week and this will shut off any mention of 
rain/convection. Other than early day streamer showers over our
southern/coastal counties, days will be sunny enough to warm he
region to around 90 F each subsequent day through early next 
week...minimum temperatures in the average middle to upper 70s. 
The only deviation in overnight temperatures will come from (late 
week) periods of slightly drier air advecting in on southeasterly
steering flow. Onshore winds will remain in the moderate realm as
mid-morning through late afternoon interior western forecast area
and coastal winds regularly strengthen to 15 to 25 mph through 
the remainder of the work week. These winds will aid in mixing 
down some drier mid layer air thus regulating afternoon apparent 
temperatures to the lower 90s, or a few degrees warmer than
ambient temperatures. 31

&&

.MARINE...
Winds should remain in the 20-35kt range into the mid morning hours. 
Frequent gusts to gale have been occurring in the Gulf so went ahead 
and expanded the Gale Warning into all Gulf waters. Speeds should
begin diminishing later this morning, but still remaining above 
Small Craft Advisory criteria all day. Considering strong onshore
winds, elevated tides and increased wave run up, the coastal 
flood threat remains fairly high along and east of Freeport and 
will maintain the coastal flood warning there thru a good part of 
the day until we get past the next high tide cycle. Low lying 
roads surrounding Galveston Bay will probably see some flooding as
well until winds and wave action subside enough to allow some 
water to retreat back into the Gulf. West of Freeport, elevated 
water levels will also prevail, but impact threshold isn't quite 
as low as areas to the east so think a coastal flood advsy will 
suffice. 

Moderate to occasionally strong onshore flow will persist for a good 
part of the work week. Minor coastal and beach flooding, similar to 
what we've seen in the past several days, can be expected around 
times of high tide. Small Craft Advisory/Caution flags can also 
be expected. Slight (but slow, gradual) improvements in overall 
marine conditions can be expected this weekend. 47 
  
&&

.AVIATION...
Gusty SSE winds will continue today. MVFR ceilings will persist 
into late morning as low level moisture continues to stream into 
the region beneath a capping inversion. A weak surface trough will
approach from the west thru the day...toward the HWY 69/59 corridor
later this afternoon. The line of shra/tsra is currently situated
along this boundary will encounter some capping as it approaches 
so would anticipate a slight weakening trend, but with daytime 
heating fcst soundings indicate there will be still enough 
instability and less capping generally north of I-10 for at least 
the possibility of some embedded TSRA thru the aftn & early evening
hours. Don't anticipate many changes to the ongoing 06z TAFs for 
the 12z package. Might throw in some tempo groups north of the 
metro sites. 47

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      86  74  91  75  92 /  60  10  10  10  10 
Houston (IAH)              88  77  89  76  90 /  30  20   0  10  10 
Galveston (GLS)            85  79  85  78  85 /  10  10  10  10  10 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Coastal Flood Warning until 6 PM CDT this evening for the 
     following zones: Brazoria Islands...Chambers...Coastal 
     Galveston...Coastal Harris...Galveston Island and Bolivar 
     Peninsula.

     Wind Advisory until 6 PM CDT this evening for the following 
     zones: Austin...Brazos...Burleson...Chambers...Coastal 
     Brazoria...Coastal Galveston...Coastal Harris...Coastal 
     Jackson...Coastal Matagorda...Colorado...Fort Bend...
     Galveston Island and Bolivar Peninsula...Grimes...Houston...
     Inland Brazoria...Inland Galveston...Inland Harris...Inland 
     Jackson...Inland Matagorda...Madison...Matagorda Islands...
     Montgomery...Walker...Waller...Washington...Wharton.

     Coastal Flood Advisory until 6 PM CDT this evening for the 
     following zones: Coastal Brazoria...Coastal Jackson...
     Coastal Matagorda.

     Coastal Flood Advisory from 6 PM this evening to 1 PM CDT 
     Thursday for the following zones: Brazoria Islands...
     Galveston Island and Bolivar Peninsula.

     Coastal Flood Advisory until 1 PM CDT Thursday for the following 
     zones: Matagorda Islands.

GM...Small Craft Advisory from 9 AM this morning to 7 PM CDT this 
     evening for the following zones: Galveston Bay...Matagorda 
     Bay.

     Small Craft Advisory from 9 AM this morning to 4 AM CDT 
     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.

     Gale Warning until 9 AM CDT this morning 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.

&&

$$

Discussion...31