Harvey Likely To Drop Excessive Rainfall Along Texas/Louisiana Coasts

Quick note: This is a forecast that I wrote specifically for a tropical meteorology class that I’m taking, which is why the format is a bit different than what you’re used to seeing from me. However, I figured all of you would find this information beneficial.

Discussion on Harvey (August 24, 8 am CT update):

As expected, Harvey re-strengthened back into a tropical storm last night and now has sustained winds of 50 knots. Harvey has picked up some forward speed and is currently moving north- northwest at 9 knots. Convection is now much better focused around Harvey’s center, and he does not have the elongated look that he did just last evening. At this time, no modifications/updates need to be made to last night’s forecast (below).

Hurricane warnings have now been issued for parts of the Texas coastline. Excessive rainfall is still expected, and at this point, it is not unreasonable to say that parts of Texas could exceed 20 inches of rainfall over the next 5 to 7 days.

Figure 1: Tropical Storm Harvey’s latest projected path from the National Hurricane Center

Discussion on Harvey (August 23, 7:00 pm CT update):

As of 7:00 pm CT, Tropical Depression Harvey is currently to the west of the Yucatan Peninsula and is moving very slowly to the northwest at 2 knots. After weakening and losing its closed circulation earlier in the week, Harvey finally made its transition back into a depression and will likely strength into a tropical storm by tonight or tomorrow morning. At this point, hurricane status may eventually be reached before landfall.

An upper-level low has been positioned just to the south of the Texas/Louisiana border over the Gulf of Mexico, which has resulted in an elongated region of convection firing from Harvey’s center of circulation extending northward away from this center in an area of heightened upper- level divergence. Harvey’s circulation has been quite elongated thus far, but convection should eventually become more confined to Harvey’s center as the upper-level low dissipates.

With time, vertical wind shear should decrease ahead of Harvey, as the upper-level low dissipates. There is currently moderate to strong vertical wind shear that extends south of the Louisiana coast to the northeast Gulf of Mexico. The southern extent of this stronger shear has likely played a role in Harvey’s disorganized and asymmetrical look so far this week, but over time, this should no longer be a significant hindrance to Harvey’s intensification, especially as this tropical system moves northwest.

Figure 2: Locations of vertical wind shear across the Gulf of Mexico and western/central Atlantic from CIMSS

Figure 3: Upper-levels wind across the Gulf of Mexico and western/central Atlantic from CIMSS

Sea surface temperatures are anomalously warm over much of the western Gulf with temperatures running around 30 to 31°C on average. Even more noteworthy, Harvey will be moving over a region of higher ocean heat content, so while some upwelling could occur due to Harvey’s slow forward motion, it appears that this system has plenty of warm, Gulf of Mexico water to enhance strengthening. In addition to warmer sea surface temperatures and a low-shear environment, there will be very little dry air to hinder further development.

Figure 4: Gulf of Mexico and western Caribbean ocean heat content map from CIMSS

Figure 5: GOES-16 water vapor image from early morning, August 24, 2017

Aside from the fact that Harvey could quickly intensify as he moves towards the Texas coast, one major concern at the moment is the significant flooding situation that could unfold across Texas and Louisiana. A region of high pressure is expected to build over the eastern Gulf of Mexico by this weekend, while ridging is expected to amplify over the western United States around the same time. The mean flow will generally be from the north on the east side of the western ridge and from the south on the west side of the eastern Gulf high pressure system, putting Harvey is a region of weak steering flow. Unless one of these high pressure regions becomes more dominant than the other, Harvey will likely meander along or just inland over Texas for quite some time. Broad troughing over the eastern U.S. could eventually pull Harvey northeastward next week, but at this point, that remains uncertain. Due to the complexity of this pattern, model guidance has had a difficult time determining where Harvey will go once he moves closer to the Texas coast. Given the moisture-rich environment that will be present along the Texas and Louisiana coasts and the frontal boundary that will remain stalled across that region, rainfall totals over the next 5 to 7 days could reach 10 to 20+ inches across those locations. Reaching such totals will be highly dependent on the overall track of Harvey, which will be dependent on the strength and exact placement of the two high pressure features.

Figure 6: 7-day rainfall forecast from the WPC

Discussion on the rest of the Atlantic (August 23, 7:00 pm CT update):

Aside from Harvey, the rest of the Atlantic is relatively quiet. There is a trough of low pressure that is currently located near the Florida peninsula, which is bringing unsettled conditions south of a frontal boundary that will continue to makes its way southward.
In addition to sea surface temperatures being anomalously warm across the Gulf of Mexico, temperatures are also warm across the main development region. This will be extra fuel for any African easterly wave that moves across that region. It is worth noting that there is quite a bit of dust that is now moving over the eastern Atlantic from Africa, which for the time being, will likely hinder the development of any wave in that region.

Figure 7: Weekly global sea surface temperature anomalies from NOAA

Figure 8: Saharan air layer map from NOAA

Discussion on the eastern and central Pacific (August 23, 7:00 pm CT update):

For now, the tropics in the eastern Pacific are quiet. Kenneth recently dissipated after moving northward into a region of lower sea surface temperatures. While sea surface temperatures are anomalously cool over the central and eastern equatorial Pacific, sea surface temperatures are above average north of that region, including near Hawaii. As the hurricane season continues, that region will bear watching.

Figure 9: Sea surface temperature anomalies across the central Pacific from Tropical Tidbits