Ana a Tropical Storm; Complicated Severe Threat for U.S. Plains

Tropical storm conditions are expected beginning Saturday evening along much of the South Carolina and North Carolina coasts as slow-moving Tropical Storm Ana plods to the northwest at 3 mph. Ana gained enough heavy thunderstorms near its center on Saturday morning to be classified as a tropical storm, but long-range radar out of Wilmington, North Carolina and satellite loops on Saturday afternoon showed that these thunderstorms were of only modest intensity and areal coverage, and were only slowly increasing. Strong upper level winds out of the north were creating an increasing amount of wind shear over Ana on Saturday morning, and the shear had reached a high 20 knots by 8 am EDT. This shear was driving dry air into the northern side of Ana, and keeping heavy thunderstorm activity limited on that side. Ocean temperatures were near 25°C (77°F), which is just at the limit of where a tropical storm can sustain itself. The Frying Pan Shoals Buoy 41013, located off the coast of North Carolina about 40 miles northeast of the center of Ana, recorded sustained winds of 38 mph, gusting to 40 mph, at 9:50 am EDT. Wunderblogger Steve Gregory pointed out to me this morning that significant wave heights in Ana have not been very impressive over the past few days, less than ten feet, suggesting that the advertised peak winds of Ana of 60 mph were either too high or were affecting a very small portion of the ocean.


Figure 1. Subtropical Storm Ana as seen by the International Space Station on Thursday afternoon, May 8, 2015. At the time, Ana had top winds of 45 mph. Image credit: Scott Kelly.


Figure 2. Radar image of Ana taken at 12:03 pm EDT Saturday May 9, 2015, from Wilmington, North Carolina. A modest rainband was affecting the coast of North Carolina, but the main activity was still out to sea.

Forecast for Ana
The 8 am EDT Saturday run of the SHIPS model predicted that wind shear will remain a moderately high 20 knots until the storm makes landfall early Sunday morning, and ocean temperatures beneath the core of the storm will cool to 22°C (72°F) by Sunday morning. These two effects should act to weaken Ana before landfall, and the storm should have sustained winds no stronger then 50 mph at landfall. The 0Z Saturday morning runs of our two top models for predicting tropical cyclone tracks, the European and GFS models, both showed the system making landfall on Sunday between 1 am - 5 am EDT near the North Carolina/South Carolina border. With winds of 50 mph, rainfall of 1 - 3 inches, and a storm surge of 1 - 2 feet likely the worst impacts that Ana can deliver, the main danger of the storm may be rip currents that can drown unwary swimmers.


Figure 3. Typhoon Noul as seen by the International Space Station on Thursday, May 8, 2015. At the time, Noul was a Category 3 storm with top winds of 115 mph. Image credit: Terry Virts.

Category 4 Typhoon Noul headed for the northern Philippines
Category 4 Typhoon Noul (called Dodong in the Philippines’ naming system) continued a slow intensification process on Saturday, with winds estimated at 135 mph as of 8 am EDT. Satellite loops on Saturday afternoon showed that Noul was a medium-sized storm with a 14-mile diameter eye and a modest area of heavy thunderstorms. Landfall of the center is expected to occur in northeastern Luzon Island near 2 am EDT Sunday (06 UTC), according to the 0Z Saturday run of the GFS model. Noul is encountering a trough of low pressure as it approaches the Philippines, which is inducing a more northwesterly track. This may cause the storm's eye to miss the Philippines, as predicted by the 0Z Saturday run of the European model. In either case, Luzon will be on the weaker left front side of Noul, which will make heavy rains the main threat from the storm. I do not expect this to be a major disaster for the Philippines; loss of life should be limited and damage in the tens of millions of dollars even if Noul makes a grazing landfall as a Category 4 storm. With wind shear still a light 5 - 10 knots and warm ocean waters ahead of it, Noul should be able to hold on to Category 4 strength until just a few hours before landfall.

Widespread rains complicate severe potential for this weekend
Round after round of heavy rain has triggered flooding in communities across the southern Plains while also denting the atmosphere’s potential to product tornadic supercells. Once again on Friday, a large convective complex developed by midafternoon before more classic, discrete supercells had a chance to form. The complex spun out several brief tornadoes along its southern fringes while dumping several inches of rain from north Texas across much of Oklahoma. The sense of deja vu was especially strong in northern parts of Norman, OK, where the same areas struck by flooding and an EF1 tornado on Wednesday experienced hail larger than golf balls, winds to 73 mph, and 1” - 2” of rain on Friday, knocking out windows and knocking down power lines. At the other end of the storm complex, softball-sized hail was reported near Slaton, TX (southeast of Lubbock). Rainfall totals for Tuesday through Saturday morning ranged from 5” to more than 10” across parts of the southern half of Oklahoma and adjacent north Texas.


Figure 4. Pastures near Chico, TX, northwest of the Dallas-Fort Worth area, were inundated by Friday, May 9, after multiple days of heavy rain. Image credit: wunduerphotographer Madermade.

A challenging severe weather forecast
The upper low that's driven the persistent storminess is finally on the move, pushing into the western Plains. Upper winds will be much stronger later today, raising the potential for a few significant tornadoes if the atmosphere manages to recover, but that’s a big “if.” With little in the way of a warm layer at midlevels to inhibit thunderstorm development until peak afternoon heating, a sprawling area of heavy showers and thunderstorms developed overnight from the Texas Panhandle into western Kansas and northeast Colorado. These storms are cooling and dampening a wide part of the region that would otherwise be at risk of late-day supercells. In its 1:00 am CDT Saturday outlook, NOAA's Storm Prediction Center called it "another messy/complex forecast." The 1:00 am outlook had placed southwest Kansas under a moderate risk of severe weather, but the 8:00 am CDT update eliminated that area, leaving an enhanced risk from eastern Colorado to north Texas. Behind this area, some afternoon warming may still occur, with redevelopment possible along a dry line/cold front. Further north, instability will be marginal but wind shear very favorable for relatively small rotating storms near the surface low and warm front pushing north across western Kansas and eastern Colorado. Toward the south, another large complex of storms may emerge by Saturday night, again heading toward the hard-hit region from the Dallas-Fort Worth area to the Red River. Heavy precipitation is also walloping northeast Colorado, where flash flooding is considered likely on Saturday afternoon and evening by the NOAA Weather Prediction Center. Heavy snow will envelop the mountains and higher foothills of northern Colorado and southeast Wyoming, as well as the Black Hills of South Dakota, with winter storm warnings covering much of this region. Even the Denver-Boulder area could see several wet inches of snow overnight Saturday night.

Severe weather on Sunday will expand into a larger swath of the central United States, from Iowa to Texas, although the areas of peak intensity and coverage will again depend largely on how storms evolve through Saturday night and Sunday morning. Any substantial tornado threat would most likely be centered in Iowa. Severe storms will again be widespread on Monday toward the Mississippi Valley, but that should be the last day of this prolonged, complex episode, as the upper low lifts out and wind shear relaxes.


Figure 5. More than a foot of snow is possible across the higher terrain of the central Rockies and adjacent Black Hills from Saturday to Sunday morning. Image credit: NOAA Weather Prediction Center..

Wunderground will be running a live blog Saturday afternoon from our main page to track the severe weather.

Jeff Masters (tropical) and Bob Henson (severe)