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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 171234

Area Forecast Discussion...COR
National Weather Service Houston/Galveston TX
458 AM CDT Wed Apr 17 2019

A (Tues)day of brisk onshore flow is evident in this morning's 
middle 60 dew points and lower overcast skies. GOES satellite PWAT 
product shows 1.5 to 1.7 inch pw air hugging the lower Texas 
coastline. This air mass will be advected north into southeastern 
Texas upon today's 25 to 45 knot mid-layer south to southwest 
steering flow. A 30 to 40 knot low level jet will aid in 
saturating up the lower few hundred millibar near-surface layer 
underneath a fairly stout 700-800 mb inversion. The combination of 
an unseasonable pwat air mass (slightly under the 99th percentile 
from the Coastal Bend source region), a pronounced low level jet 
and warming into the upper 70s to lower 80s should allow for the 
formation of widely scattered west-to-east showers. There is a 
late afternoon chance for elevated thunder as lapse rates above 
the warm nose are forecast to reach or exceed 8 deg C/km. 

As the western U.S. trough evolves into a broad upper low over the 
southern Rockies this afternoon and moves into the OK-TX 
Panhandles this evening, the chance for south central Texas 
convection along the dry line boundary will be on the rise. The 
highest confidence of this highly advertised scenario remains the 
same; that of clustering thunderstorms forming west of the San 
Antonio area and advancing eastward with discrete cells moving up 
ahead of this clustering and impacting those counties immediately 
to our CWA's west and northwest through the evening hours. These 
discrete cells may come together as a ragged QLCS and move across  
the northern third of the forecast area from around midnight 
through a few hours before sunrise Thursday. While the southern  
two-thirds of our CWA will be under stout capping, there may be 
enough frontal forcing or lingering upper exhaust from a northeast
exiting right rear jet quad to advance a few strong storms across
the Houston metro area around or just after sunrise. If storms do
impact the city, they will center around the height of the morning 
rush hour(s). 

The main threat mode has transitioned from large hail (last night) 
to more of a locally heavy rain threat. While CIPS analogs keep 
all of the severe weather to our north over areas that have 
recently been hammered with large hail and long track tornadoes, 
the thinking that storm cells could train is taking root in this 
morning's messaging. As storm motion vectors become more parallel 
to the mean environmental flow, clustering storms could repeatedly
travel across the same areas over a relatively small amount of 
time. High pw air in the 1.6 to 1.7 inch range that will be lifted
by either decent frontogenesis or upstream convective outflow 
lift (or both), in tandem with a splitting jet structure, boosts 
confidence into the high moderate range that those far interior 
communities stand a good chance of picking up 6 hour 2 to near 4 
inch rainfall amounts. This behavior could provide a rapid rise to
quick response waterways and their tributaries. Towns and rural 
areas generally along and north of a Columbus to Conroe to 
Livingston line will be under the gun for locally heavy rainfall 
leading to overnight flash flooding. Large hail will be a secondary
threat. Another mode to consider would be damaging downburst 
winds if these discrete cells or clusters form into a quasi-organized
squall line as they advance into the (north)eastern CWA after 2 
or 3 AM.

West winds kick in behind this system and relatively cooler air
behind a Thursday cold frontal passage scours out mid to late
morning lingering showers and isolated storms. West-to-east
clearing with afternoon middle 70s across the more wet ground
northern counties to the lower 80s over the relatively drier 
southern CWA. Friday will sport stout offshore winds with a 
mostly sunny day warming into the slightly below normal lower to 
middle 70s (per the efficient mixing brought on by moderate to 
strong northerlies). A very nice Easter weekend with onshore flow
returning Saturday and strengthening on a partially cloudy Easter
Sunday. Lower 60 sunrise temperatures Easter Sunday will be 
around 10 degrees warmer than Saturday's min Ts...both days will
warm into the average lower 80s. 

Early next week's precipitation chances will be on the rise each 
subsequent day from Monday through Wednesday. The impetus to
return showers/isolated storms will be the slow deepening of a 
western upper trough. the associated northeastern Mexico surface 
low may allow an emanating lower level trough/boundary to develop 
across south central Texas. This lower level focus, in tandem with
a series of shortwave disturbances rounding the bend of the western
Texas 5H trough axis, will keep at least moderate POPs in the
extended. 31


Winds and seas slightly diminished overnight. Have cancelled the 
high risk of rip current statement for the time being and will let 
the dayshift re-evaluate with beach patrol officials. The gradient 
tightens this evening ahead of the incoming storm system and onshore 
winds and seas will increase to small craft advisory criteria. May 
see some minor overwash at Highway 87/124 on the Bolivar Peninsula 
tonight around the high tide (~4am). 

Look for showers and thunderstorms to push east-to-west across the 
waters Thursday morning along with a westerly wind shift. A cold 
front will follow during the afternoon and nw winds will increase to 
20-30kt overnight. Will need to keep an eye on possible low water 
conditions late Thurs night & Fri morning at low tide across 
northern bay locations.

Winds diminish late Fri as high pressure builds in. Onshore flow 
resumes on Saturday as it moves off to the east and persists into 
early next week. 47


MVFR conditions will persist across the region for the next 24 hours 
or so. Expect some streamer shra generally north of a Matagorda-
Livingston line today, with slightly better coverage around CLL, UTS 
& CXO terminals this afternoon. Main focus will be timing of precip 
expected to arrive tonight (likely a line of shra/tstms) between 6-
12z. Most favorable locations for strongest & training heavy storms 
look to be situated just north of the metro airports...say north of 
a Columbus-Livingston line. But it is subject to 
changes.  47


College Station (CLL)      80  63  76  53  75 /  40 100  30  10   0 
Houston (IAH)              79  67  80  55  76 /  30  90  60  10   0 
Galveston (GLS)            75  70  77  62  75 /  20  60  70  10   0