Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 1:57 PM GMT on September 23, 2010
A tropical wave (Invest 95L), moving westward at 15 mph though the south-central Caribbean, will bring gusty winds and heavy rain squalls to the northern coasts of Colombia and Venezuela this morning, as well as the Netherlands Antilles Islands. This disturbance will bring dangerous flooding rains to the countries bordering the Western Caribbean this weekend, and may also be a threat to the U.S. Gulf of Mexico coast late next week. A hurricane hunter aircraft is in 95L, and found a large region of surface winds of 35 - 45 mph to the east of 95L's center. If they find a closed circulation, 95L will be upgraded to Tropical Storm Matthew today. Satellite loops show that heavy thunderstorm activity has increased markedly in recent hours, with a solid curved band of intense thunderstorms growing to the northeast of 95L's center. Low-level spiral bands are also developing to the southeast of the center, but a closed surface circulation is not yet obvious from satellite imagery.
Figure 1. Morning satellite image of 95L.
Forecast for 95L
An upper-level high pressure system lies to the west of 95L, near the coast of Nicaragua. The clockwise flow of air around this high will bring a moderate 10 - 15 knots of wind shear over 95L today. By Friday, 95L will move more underneath this upper-level high, causing the shear to decline. The SHIPS model forecasts that shear will fall to the low range, 5 - 10 knots, by Friday afternoon. This drop in shear may allow for rapid intensification of 95L as it approaches landfall near the Nicaragua/Honduras border on Friday night and early Saturday morning. NHC is giving the disturbance an 80% chance of developing into a tropical depression by Saturday.
Figure 2. Forecast track of 95L from an ensemble of runs of the GFS model done at 2am EDT this morning.
The future of 95L depends critically upon the storm's interaction with land over the coming days. If 95L misses making landfall in Nicaragua and Honduras, and instead skirts the north coast of Honduras, the storm is likely to intensify into a hurricane by Monday, as predicted by last night's 00Z (8pm EDT) run of the GFDL model. However, if 95L spends significant time over Honduras, as predicted by the latest 06Z (2am EDT) run of the GFDL model, the storm will likely stay below hurricane strength. 95L is being forced just north of due west by a strong ridge of high pressure. This ridge will keep the storm moving at 15 mph through Saturday. On Sunday, a trough of low pressure diving southwards over the Eastern U.S. will weaken the steering currents over the Western Caribbean and cause 95L to slow and turn more to the north. 95L will begin a period of slow and erratic movement on Sunday that may last many days, as the storm wanders in the Western Caribbean and over Belize, Honduras, and Mexico's Yucatan Peninsula. If the center of 95L spends significant time over water, the storm could easily develop into powerful and dangerous Hurricane Matthew. If the center remains mostly over land, 95L will still generate extremely heavy rains over Central America, but remain below hurricane strength. By late next week, the trough of low pressure over the Eastern U.S. may lift out, allowing a ridge of high pressure to build in and force 95L westwards across the Yucatan Peninsula and into the Gulf of Mexico's Bay of Campeche. Alternatively, the trough of low pressure over the Eastern U.S. may amplify, drawing 95L northwards across Western Cuba and into Florida. The uncertainties in the long-range fate of 95L are high, and depend strongly on slight variations in its track that determine how much time the storm spends over land. One measure of the uncertainty in 95L's future track can be gained by viewing the ensemble forecast from the GFS model. An ensemble forecast is generated by taking the initial conditions in the atmosphere and making slight variations in the temperature, pressure, and humidity fields. Twenty or so tweaks of the initial conditions are made, and the GFS model run twenty separate times for each new set of initial conditions. The resulting ensemble of model runs gives one an idea of how sensitive the future track of the storm might be to errors in characterizing the initial state of the atmosphere. As one can see from this morning's GFS ensemble run (Figure 2), there are a wide range of possibilities for where 95L might go. The main thing I am confident of at this point is that 95L will generate very heavy rains over Nicaragua, Honduras, Belize, and Mexico's Yucatan Peninsula Friday through Tuesday that will likely cause dangerous flooding rains and life-threatening mudslides.
Tropical Depression Lisa
Tropical Depression Lisa continues to churn the waters of the far Eastern Atlantic. By Saturday night, upper level winds out of the west are expected to increase, bringing high wind shear of 20 - 45 knots over Lisa. The high shear may be capable of destroying the storm by early next week. It appears unlikely that Lisa will affect any land areas.
Elsewhere in the tropics
The remnants of Hurricane Julia are still spinning over the far Eastern Atlantic, and are generating some sporadic heavy thunderstorm to the north of the center of circulation. Julia's remains are being given a 10% chance of re-developing by NHC. There are no other threat areas to discuss, and none of the models is calling for development (except for 95L) over the next seven days.
Hurricane Karl's moisture causing flooding in Minnesota, Wisconsin
A plume of very moist air associated with what was Hurricane Karl last week has surged northwards over the Central U.S. over the past two days. This moisture is now being lifted over a warm front draped over Minnesota and Wisconsin, and has generated flooding rains in excess of six inches in southern Minnesota this morning. Flood warnings and flood watches are posted for a wide swath of the upper Midwest today, including most of Minnesota, northern Wisconsin, and Michigan's Upper Peninsula. The latest forecast discussion from the NWS office in Minneapolis notes that the forecast amount of moisture in the air this afternoon is similar in magnitude to the all-time record for this time of year (set on September 2, 1953.) These kinds of rain events are called Predecessor Rain Events (PREs), because they typically precede the actual arrival of the rain shield of a tropical storm.
Figure 3. Radar estimated rainfall over Minnesota.
I'll have an update on 95L later today. The timing will depend upon what the Hurricane Hunters find.
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