Wednesday early morning briefing from Jeff:
***Hurricane/ Tropical Storm landfall along the TX coast likely Friday evening***
***Increasing potential for widespread excessive rainfall and flooding***
Discussion:
Satellite images show that the remains of Harvey are gradually becoming better organized over the waters of the southern Gulf of Mexico just WNW/NW of the NW tip of the Yucatan. Convection has been developing this morning around the broad low level circulation. A USAF mission is currently enroute and should begin to provide better data on if a tropical depression has formed around 630-700am this morning. NOAA high altitude mission yesterday evening dropped 30 sondes across a large portion of the Gulf of Mexico including some just west of the system. Some of the data just west of the circulation shows a large mass of dry air that may be helping to inhibit near term development some. Large amounts of moisture are moving into the central Gulf of Mexico along a trough axis that extends northward from Harvey into the northern Gulf resulting in numerous showers and thunderstorms well north of the circulation.
Track:
All high altitude data and special balloon launches from southern NWS offices went into the 00Z model data. There has been a very slight shift eastward (right) in the model consensus with a track near the western edge of Matagorda Bay possibly near Seadrift for a landfall. Looking at the last 4 cycles of models there is not been that much shifting of the consensus track of the major global models so confidence is starting to increase for a landfall between north of Corpus Christi to Matagorda Bay
Weak and complex steering begins to develop as Harvey nears the coast and moves inland Friday evening. High pressure builds into NW TX from the SW US blocking the continued NW motion of Harvey while at the same time high pressure builds across the central US Gulf coast with a trough extending from the NE US WSW to eastern TX. Harvey will become trapped in the weak steering flow regime over TX by Saturday where the forward motion will slow and possibly the system will stall somewhere between Austin and Victoria. It appears the system will then turn ENE along the tail end of the trough over east TX and slowly drift over SE TX and into SW LA. There are a few (including some of our major global models) that slow the system near the coast and then move it ENE/NE along the middle and upper TX coast which is possible. This would keep a much stronger storm in play versus one that would move inland.
Intensity:
Conditions appear favorable for intensification of Harvey up to landfall on the TX coast. All parameters appear to be satisfied including warm SST's, 200mb high pressure over the center of the system, potential dual outflow channels to the NE and SW of the center. It is possible that a period of rapid intensification could occur as Harvey nears the middle TX coast on Friday. Intensity forecast skills are poor especially when dealing with rapid intensificationthis aspect will need to be monitored closely on Thursday and Friday as Harvey approaches. General consensus currently is for a strong tropical storm or weak hurricane into the middle TX coast Friday evening.
Impacts:
Below impacts are based on the consensus model tracks and a blending of major model QPF and WPC guidance. There has been little change in the storm surge or wind portions of this since yesterday afternoon. Rainfall has been increased to better match WPC grids
It should clearly be noted that the potential for widespread excessive rainfall over multiple days is becoming increasing likely and that significant flooding may result over a large area of SE TX and the coastal bend region.
Rainfall:
Major global models have not backed away from some very impressive rainfall totals and the overall pattern with a slow moving/meandering tropical system certainly support these totals. WPC has raised the QPF for the entire region into the 10-12 inch range and certainly isolated totals will be higher. Rainfall of this magnitude will result in widespread and potentially dangerous flooding. River, creek, and bayou flooding along with flash flooding will all be possible late Friday through the weekend
Widespread: 10-12 inches (all areas)
Isolated: 20 inches +
A quick look back this morning does show that tropical systems in weak steering pattern have produced devastating rainfall over TX before:
TS Allison: 28.5 inches in 12 hrs (NE Harris County)
TS Claudette: 43.0 inches in 24 hrs (Alvin, TX)
TS Amelia (1978): 50.0 inches (72 hrs) over Medina and Kerr Counties
Unnamed (1921): 36.40 inches (18 hrs) Thrall, TX
Storm Surge:
Will start to see increasing tides Thursday afternoon and possible we may hit 3.0 ft total water level Thursday afternoon/evening especially Matagorda Bay southward toward Mustang Island. Will go with total water levels Friday rising to 4.0-5.0 feet along all of the upper and middle coast and likely nearing 5.0 feet around Matagorda Bay. 4.5 feet is our critical warning threshold along much of the upper coast for the start of impacts and think we will hit those levels on Friday especially Matagorda Bay. Overwash and coastal flooding appears likely Friday from Palacios southward including Matagorda Island and along the west side of Matagorda Bay.
Tides will increase Friday night across the coast NE of Palacios as SE winds pile water and wave action onto the coast. Will build to 4.0-5.0 feet Friday night along Brazoria, Galveston, and SE Harris coast with overwash likely on the west end of Galveston and Bolivar. With the continued onshore flow well into Saturday and Sunday tides will only worsen over the weekend as water becomes trapped in the baysespecially the western side of Galveston Bay. Certainly looking at potential coastal flooding around Clear Lake, Kemah, Seabrook, Galveston Island, Bolivar on Saturdaybut don't want to go much more than 4.5-5.5 feet total water level at this time as a lot of factors are in play.
Matagorda Bay: 4-5 ft (Friday/Saturday)
Galveston Bay: 4-5 ft (late Friday/Saturday)
Gulf Beaches: 4-5 ft (Friday/Saturday)
Onset of 3.0 ft total water level rise Thursday afternoon/evening
Note: Should the system slow or meander near the coast tides would possibly need to be raised some from Matagorda County NE as this would allow a greater period of time for onshore winds to pile water over several tide cycles.
Winds:
Still some uncertainty with the winds as to how strong the system becomes and how far inland it moves. If Harvey gets trapped closer to the coast it will not weaken as fast as if it moves further inland
Tropical storm conditions (40mph sustained) will begin to reach the middle coast on Friday around midday and spread inland into the evening hours. Will bring Jackson, Matagorda, Calhoun, Victoria, Aransas, Refugio, San Patricio, and Wharton Counties into the TS force winds by Friday evening and build to hurricane force (75mph sustained) over Calhoun, Aransas, and Refugio Counties Friday night. Could be very near hurricane conditions into much of Matagorda Bay Friday night and possibly Matagorda County with tropical storm conditions up to Galveston by Saturday morning.
Wind forecast moving forward into Saturday and Sunday will greatly depend on if the system moves inland over the western portions of SE TX or remains closer to the coast. A system closer to the coast would likely weaken much slower and keep stronger winds for a longer period of time along the coast and spread gradually ENE up the coast over the weekend.
Actions:
Hurricane plans should be fully prepared to be enacted today and Thursday including fully stocked hurricane kits.
Ride out teams should be notified and activation of COOP's is recommended.
Preparations should include the potential for loss of power and prolonged excessive rainfall and flooding along with prolonged high coastal tides. Some areas near the coast may become cut-off as early as Friday.
Significant rises on area rivers, creeks, and bayous is a real threat and residents living in flood prone areas should be prepared for rising water and potentially significant flooding.
Note:
Tropical storm, hurricane, and storm surge watches and/or warnings may be required for portions of the TX coast today or Thursday.