| Tuesday afternoon briefing from Jeff: ***Hurricane landfall along the TX coast increasing likely Friday or Saturday*** Review hurricane response plans and be fully prepared to enact those plans starting tomorrow. Discussion: Satellite images show an increasingly well defined surface circulation across the NW Yucatan currently that will be moving over the southern Gulf of Mexico in the next several hours. The system currently lacks any deep convection near the center with thunderstorms currently displaced to the north along the northern coast of the Yucatan. The circulation is moving toward the NW around 10-12mph and this motion is expected to continue. Track: There has been little change in the forecast track reasoning today with a general NW track toward the TX coast expected Wed-Fri. Major global models continue to be in decent agreement moving Harvey into a weakness over the TX coast between a building ridge of high pressure over the SW US and high pressure over the SE US and a trough across the Great Lakes. Landfall locations continue to shift around with each model run, but the main consensus continues to be in the region between Corpus Christi and Matagorda Bay. The steering flow nearly collapses as Harvey reaches the coast and expect a decrease in forward speed as the system moves inland over the coastal bend on Friday night. This is where the forecast becomes greatly complicated as Harvey becomes trapped in a weak steering regime across SC/SE TX into the early portion of next week. The system is effectively left to meander and loop somewhere in the region bounded by Matagorda Bay, Austin, and Galveston. Eventually it appears Harvey will slowly track E to ENE either along the coast or inland of the coast across SE TX Saturday-Tuesday. 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 over on Thursday and Friday as Harvey approaches. Current coordinated NHC/WPC positions bring s minimal hurricane into the coast in the reach between Corpus and Port O Connor Friday evening. Impacts: Without any sort of NHC track guidance the following impacts are based on the TCVN (consensus model track) with storm surge forecast off ET surge, but I have cut it back some based on the latest GFS run. Have also taken a good look at the cat 1 SLOSH output for a NW tracking hurricane into just W of Matagorda Bay and merging those values with ET surge seems to be a good compromise at the moment. Additionally, there has been some coordination with the storm surge unit at NHC via the local WFO today. Rainfall has been coordinated with local WFO's and WPCmainly to encourage WPC to raise expected totals. Rainfall: I have never seen the GFS and ECWMF models forecasting such a large amount of rainfall over such a large area. Widespread rainfall amounts of 8-12 inches is likely with isolated totals of 20 inches or more. River, creek, bayou, and flash flooding would be a given with totals of this magnitude. Unlike so many of the flood events we deal with across this area where isolated areas get the very high totalsthe potential here is for a very large area to see excessive amounts of rainfall. Widespread: 8-12 inches (all areas) Isolated: 20 inches + 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 6.0 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 4.0-5.5 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 5.5-6.0 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: 4.5-5.5 ft (Friday/Saturday) Onset of 3.0 ft total water level rise Thursday afternoon/evening Winds: A lot of uncertainty with this aspect. 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, and Wharton Counties into the TS force winds by Friday evening and build to hurricane force (75mph sustained) over Calhoun County southward to near Corpus 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. If the system does in fact turn NE/ENE just inland this will bring tropical storm and potential hurricane conditions across a larger portion of SE TX. Should get better details on this tomorrow if NHC starts advisories. Actions: Hurricane plans should be fully prepared to be enacted on Wednesday including fully stocked hurricane kits. Ride out teams should be notified and activation of COOP's is recommended starting Wednesday. 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: GIV upper air mission is underway over the Gulf of Mexico which should help firm up model guidance for the 12Z runs on Wednesdaysome of the samples may get into the 00Z runs this evening. USAF/NHC have several C-130 53rd missions planned into Harvey starting tomorrow and this will continue up to landfallso there will be nearly constant checking on intensity trends. Tropical storm, hurricane, and storm surge watches and/or warnings may be required for portions of the TX coast on Wednesday |