Wednesday, July 30, 2008

Nevada Supercells Case Study -- Part I I

The synoptic environment was supportive of organized thunderstorms over much of Western Nevada. At the same time, the thermodynamic environment supported the threat of severe thunderstorms, including supercells. Since the reports of the tornadoes/funnel clouds occurred between 2300-0000 UTC, this will be the primary time frame of focus in this study.

The surface chart at 2300 UTC showed a moisture discontinuity across Western Nevada. The dew point at Reno was only 46 F compared to 52 F at Fallon. The dew point at 0000 UTC at Fallon actually spiked to 55 F when storms were in the vicinity. The lower moisture profile across the extreme western portion of the state was probably the result of a dry, downsloping wind out of the Sierra Nevadas. Moisture was able to pool around the Fallon area, and this played a crtical role in storm initiation and intensification.

Given that the area around Fallon is at a high elevation (~4,000 ft ASL) and the high dew points observed, this contributed to theta-e values in excess of 350 K! For comparison purposes, note that this theta-e max in Nevada is higher than the 348 K value observed in Southeast Mississippi. This just hammers the point home for forecasters in higher elevations to pay close attention to theta-e. What may seem like a meager amount of moisture may actually be quite substantial when elevation is considered. That's another discussion for another day, but if you are interested, do some research on the Cheyenne Ridge tornado in 1960.

The environment was moderately unstable given the combination of warm surface temperatures, the pooling of surface moisture in the Fallon area, and the presence of a trough (cold air aloft) just to the northwest. In fact, mixed layer CAPE values were running between 1000-2000 J kg-1 at the time the supercells were ongoing. This degree of instability balanced by moderate deep layer shear (35-40 knots) resulted in an environment supportive of supercells.

The threat of tornadoes appeared quite small given the lack of low-level shear and the high dew point depressions. The 0000 UTC sounding from Reno showed an inverted-V type of sounding which is indicative of strong evaporation potential below 600 hPa. Keep in mind that the environment east of Reno was a bit more moist, but the sounding still would have shown an inverted-V sounding given the dew point depression in Fallon at 2300 UTC was 38 F. The lack of low-level moisture resulted in high downdraft CAPE values between 1000-1200 J kg-1. This is typically too high for tornadoes to develop. The reason this inhibits tornadogenesis is because the cold, dry air descending from the RFD will be ingested back into the mesocyclone. This disrupts the low-level mesocyclone and prevents vortex stretching. Tornadoes need air that is less dense (warm and moist) in order to enhance vortex stretching.

In addition to the high DCAPE, the LCL and LFC were also a bit high. The LCL height was around 1,750 m AGL and the LFC height was around 2,000 m AGL. This is somewhat high, but tornadoes have occurred in environments of high LCL and LFC heights. In addition to the low-level dry air, the lack of low-level wind shear likely played a significant role in inhibiting tornado development. The 0000 UTC sounding from Reno showed minimal speed and directional shear in the lowest 1 km AGL. In fact, the 0-1 SRH was less than 50 m^2 s^-2.

Even with the lack of low-level shear, some mechanism(s) must have been responsible for the funnel clouds and wall clouds observed. It's tough to accurately pinpoint one or more factors with the data deficient observation network in Nevada. The very steep low-level lapse rates may have accelerated parcels fast enough to promote brief low-level rotation and weak vortex stretching (i.e. funnel clouds). It's also possible that the supercells may have traversed a mesoscale or microscale boundary that would have been undetectable in the observation network. This boundary would have enhanced low-level shear and aided in rotation.

This case study has shown that the dynamic and thermodynamic environment certainly supported the threat of severe weather, including supercells. However, a dry boundary layer and weak low-level wind shear precluded the threat for tornadoes. However, it's unclear what exactly generated the rotation observed in the supercells at various times during their lives.

Friday, July 25, 2008

Nevada Supercells Case Study -- Part I

Two tornadoes were reported in Nevada on Monday, and it appeared that the state's four-year tornado "drought" had ended. Not so fast, said the National Weather Service in Reno. After surveying three suspicious damage areas in Western Nevada, the NWS could not confirm the damage caused by the storms was tornado-related. Regardless, it was quite amazing to watch supercells moving through Western Nevada, an area that typically only experiences 13-15 thunderstorm days per year!

The first part of this case study will break down the synoptic levels. The 300 hPa pattern was chracterized by a trough over Northern California and a moderately strong jet streak (65 knots) moving into Northwest Nevada. The speed divergence and diffluence associated with the jet streak entering Northwest Nevada resulted in an environment supportive of rising air. Make note of the SSW flow at this level and compare it to the wind direction in the lower levels.

The 500 hPa pattern was somewhat similar to the 300 hPa and showed the base of the trough just offshore of California. There were relatively strong winds (45 knots) at this level, and the mid-level jet streak provided the deep layer shear needed for supercells. A strong lobe of vorticity was also associated with this trough. Given the nearly southerly flow at 500 hPa, positive vorticity advection occurred across Northwest Nevada and also enhanced rising air across the area.

The flow at 700 hPa was weak and from the west. There was some mid-level moisture that was pooling along the eastern slopes of the Sierra Nevadas. The key ingridient on this chart was probably the warm tempearture aloft. The warm temperatures at 700 hPa acted to cap the lower level energy that was building throughout the day. When it was released (due to the upper level dynamics -- jet streak and vorticity advection), strong to severe thunderstorms developed quickly.

Extremely warm conditions were prevalent at 850 hPa across a large chunk of the Western U.S. Granted, many locations in the west begin near 850 hPa, but there was ample warmth (26-28 C) at this level. A very weak convergent flow can also be seen across a broad area of Nevada, California, and Oregon. One thing to note on this chart is the lack of low-level moisture in Northwest Nevada, which may have inhibited the tornadogenesis process. We'll discuss this in more detail in Part II.

It's quite evident that the synoptic environment was supportive of organized thunderstorms. In Part II of this case study, we'll look at the thermodynamic environment in order to better assess the potential for supercells and tornadoes. I'll probably post it early next week.

Wednesday, July 23, 2008

Dolly Makes Landfall

Dolly is currently making landfall on South Padre Island, TX to the northeast of Brownsville. Since the pressure bottomed out at 964 mb a few hours ago, it has risen to 967 mb. The temperature difference between the eye and outside of the eye has dropped to 4 degrees instead of 7 degrees observed earlier with the deepening phase. That's not surprising as Dolly is probably beginning to feel the increased frictional efects and should be on a weakening trend from this point forward. Maximum sustained winds are near 100 mph and no further increases in strength are likely since Dolly will soon be entirely over land.

After landfall, the main focus will quickly turn to the flood potential. Already, radar has estimated over 4-10" of rain has fallen to the east of Brownsville in Cameron County. Hourly rainfall rates has recently been as high as 5" per hour in some part of Cameron County. If Dolly slows down or stalls, look for some rainfall amounts to exceed 15"!

Dolly Near Landfall

Hurricane Dolly underwent a deepening phase overnight as the pressure dropped from 982 mb to 967 mb. Correspondingly, maximum sustained winds have increased to 95 mph as of 8 a.m. It appears that Dolly moved away from a cooler pocket of water and is now moving over a very small increase in heat potential associated with a loop eddy. Also, it can be argued that the outflow in the southern quadrant of Dolly has improved slightly and may be assisting the current deepening phase. Dolly is moving slowly (8 mph) and is still a few hours from making landfall. Therefore, some additional strengthening is possible up until landfall, and Dolly could be packing winds of 100-105 mph at landfall.

Visible satellite this morning indicates the presence of an eye in Dolly, and reconnaissance reports indicate that the eye is about 20 nautical miles wide. Brownsville's base reflectivity shows intense convection surrounding the eye wall, especially in the north and southwest quadrants.

Given the slow speed of Dolly and the possibility of Dolly moving slower after landfall, flooding will become a very large concern. The largest concern may be the rain produced in the mountainous terrain of Mexico that will runoff into the Rio Grande valley. It wouldn't be surprising to see some rainfall totals between 10-15" after all is said and done. I'll post another update later this morning.

Tuesday, July 22, 2008

Dolly Update

It sure does look like Dolly is a hurricane now. Although Dolly has not yet been upgraded to a hurricane, I would expect this to happen later this afternoon or this evening at the latest. The beginning stages of an eye wall cycle have been visible at times on visible satellite imagery this afternoon. The eye has been blocked mostly by the development of a central dense overcast that has become apparent in infrared imagery. This is fairly common with weaker hurricanes. Additionally, the center of circulation has been observed on Brownsville's radar for several hours now. Convection around the eye wall has increased for most of the day, and it appears as though as Dolly is finally developing a true eye wall.

The latest data from the Hurricane Hunters indicates that Dolly is indeed a hurricane. Taking 90% of the flight level winds measured by the reconnaissance plane (72 knots) would yield surface winds at 75 mph. I also note the pressure has also fallen another 4 mb to 986 mb. I would be very surprised to see Dolly remain a tropical storm on the 4 p.m. advisory. If the pressure continues to drop over 1 mb per hour, expect to see a 5 mph wind speed increase in each advisory.

My thinking still remains unchanged about the track and intensity of Dolly. See the previous post for my thoughts on that.

**Update** The NHC has upgraded Dolly to a hurricane as of the 4 p.m. advisory.

Dolly Near Hurricane Strength

Tropical Storm Dolly strengthened overnight and is on verge of becoming a hurricane. The latest reconnaissance report indicated the pressure has dropped to 991 mb and surface winds measured by the SFMR were 74 mph. It's likely Dolly will be upgraded to a hurricane later this morning or early this afternoon. The recon report also indicated that an eyewall is forming and is only open on the west side of Dolly. When this fills in, which it should happen later today, expect for Dolly to strengthen at a faster rate.

Satellite imagery shows excellent outflow in the north and east quadrants of Dolly, but improving and non-existent outflow exists in the west and south quadrants, respectively. An upper low (located near the NOAA stamp in the linked image) over Mexico is hindering outflow in these quadrants, particularly in the southern quadrant of Dolly. Until this low weakens or moves away from Dolly, significant strengthening is going to be hampered. Still, the warm waters and relatively weak shear should support a modest gain in strength until landfall. I now expect Dolly to be a strong Category 1 or weak Category 2 hurricane at landfall.

The exact landfall of Dolly is uncertain, but model guidance is in agreement with a landfall in northern Mexico. Dolly moved a little more west than expected yesterday and this resulted in the track being shifted south from what it was yesterday. Still, the exact landfall is very difficult to pinpoint due a multitude of reasons, and any wobbles within Dolly could shift the track north or south. However, I do like the NHC's current forecast track. To sum up, I think Dolly will make landfall near Brownsville with winds of 95-100 mph tomorrow morning.

The strong tropical wave that moved off the coast of Africa encountered cool SSTs (~23 C) and development doesn't appear imminent for a few days. This wave will probably develop, but it seems likely to re-curve out to sea given that it is already at a high latitude (16 N).

If you would like to read an excellent tropical weather discussion, I highly recommend Dr. Jeff Masters' blog. You won't find any rants or excessive objections about the NHC, but you will get a well-informed and scientific discussion.

Interesting weather in Nevada yesterday with two tornadoes and other funnel clouds reported east of Reno. This ended a streak of almost 4 years without a tornado report in Nevada! I will post a case study of these tornado at some point in the future.

Monday, July 21, 2008

Dolly Targets Texas

The last thing residents of South Texas wanted to see was the center of Dolly re-form to the north. That's exactly what happened overnight, and Dolly is now back over very warm water in the Gulf of Mexico. The quick strengthening I expected to happen just before landfall in the Yucatan never materialized because Dolly's inner core was never fully developed. That won't be the case much longer, and I expect a steady strengthening phase over the next couple of days. Wind shear is weak (5-10 knots) and SSTs are quite warm (mid 80s). Additionally, Dolly will be traversing deep, warm water and over loop eddies in the western Gulf of Mexico that may enhance intensification before landfall. The only factor that may inhibit a rapid intensification phase is the deficiency of the tropical cyclone heat potential. This needs to be a bit higher to support the rapid intensification seen with hurricanes in the last several years. Regardless, the thermodynamic and dynamic environment in the Gulf of Mexico is supportive of a hurricane and possibly a major hurricane. I think that Dolly will be a strong Category 2 hurricane at landfall.

The next question is where will Dolly make landfall? The last couple of computer model runs have shifted the track of Dolly north into Texas. The 12Z NAM takes Dolly near Corpus Christi Wednesday morning. The 06Z HWRF also takes Dolly near Corpus Christi but on Thursday morning. The 00Z NOGAPS is targeting Brownsville Wednesday night. The Canadian, UKMET, and European models are progging a landfall in Northern Mexico Wednesday night. The GFDL sides with the other American models and places landfall near Corpus Christi Thursday morning. The American models tend to have a right bias (no, not political) with tropical cyclones, but I feel the non-American models are a tad too far south. I am leaning toward the NOGAPS solution and think landfall will occur just north of Brownsville Wednesday evening.

Sunday, July 20, 2008

No, It's July

Scanning this afternoon's Atlantic basin satellite, you might think that the calendar month is September. After all, there were three named tropical systems earlier today. Bertha has since become extratropical, Cristobal is located just off the North Carolina coast, and Dolly looks poised to strengthen quickly before landfall in the Yucatan late tonight or early Monday morning.

There's nothing left to say about Bertha. She was classified as extratropical this morning and should pack a wallop on Iceland on Monday and Tuesday. She will be remembered as the longest lived tropical cyclone in the month of July (17 days).

Cristobal is very near the Outer Banks of North Carolna and has weakened a little today. It appears as though dry air may have been entrained into the center and this will hinder any strengthening until the air moistens beyond current levels. It's common for tropical systems near a large landmass to entrain dry air. Cristobal shouldn't pose a large threat to the U.S. and will begin to move away from the mainland late tonight.

The bigger story is Tropical Storm Dolly, which bypassed tropical depression stage altogether before being upgraded this morning. The Air Force Reconaissance finally found a closed circulation this morning. Since then, deep convection is clustering near the center of Dolly and this could be a sign of a quick intensification period before landfall in the Yucatan Peninsula late tonight or early Monday morning.

It's a good thing Dolly does not have a long period of time over the very warm, deep waters of the Carribean because she could easily become a major hurricane within a short time given the thermodynamic and dynamic environment over the Western Caribbean. SSTs near Dolly are near 85 F, wind shear is very weak (5-10 knots), there is sufficient moist air surrounding the tropical cyclone, the tropical cyclone heat potential is high, and Dolly will be crossing over the Loop Current before landfall. All of these factors favor rapid intensification, and it would not be surprising to see Dolly near hurricane strength at landfall.

After landfall, Dolly should spend almost a day over the Yucatan Peninsula, which will weaken the system. The biggest question mark, which will determine the strength at second landfall, will be how much of the inner core is destroyed over land. Typically, developing cyclones are not as affected by land as mature hurricanes. If this holds true, Dolly should still have a good inner core when she emerges into the Bay of Campeche. When this happens, Dolly should strengthen and could easily become a hurricane again before reaching the Mexico or Texas coast late next week.

The thermodynamic environment over the Southern Gulf of Mexico is not as impressive as it is over the Western Caribbean. There is warmer water, but it is a little more shallow than the Caribbean. The warm water is still deep enough to support a quickly strengthening tropical cyclone. Look for Dolly to be a strengthening system at landfall, whether she is a strong tropical storm or weak hurricane. I think she could possibly be a Category 2 hurricane at the second landfall. I think this is entirely possible if Dolly's inner core remains intact after passage over land and given an expected residency time of nearly 3 days in the Gulf of Mexico.

Saturday, July 19, 2008

Tropical Outlook

T.D. 3 formed last night off the coast of South Carolina and continues to spin about 100 miles east of Charleston. This was a classic case of tropical development in the proximity of a dying frontal boundary. For the past few days, thunderstorms persisted in advance of the decaying front over the warm waters of the Gulf of Mexico and the Atlantic. With wind shear relaxing, this was an ideal breeding ground for tropical development, and it certainly doesn't surprise me to see this system spin-up.

The model guidance suite agrees that this system stays just offshore of the U.S. The GFDL, however, forecasts T.D. 3 to flirt with the Outerbanks of North Carolina Sunday evening. It appears that the NHC is placing the track between the other computer models and the GFDL. I actually like their track given the position of the Bermude High. Given this track, most of the heavier squalls and strong winds should be confined to the immediate coastline and/or remain just offshore.

With shear remaining low (<10 knots) and warm water temperatures (82-84 F), this system should easily acquire tropical storm status. In fact, this system will probably be named by the end of the day. My biggest disagreement with the NHC's forecast is the conservative intensity forecast for T.D. 3. I feel that this system could wind up being a fairly strong tropical storm within a couple of days. Afterwards (Tuesday and on), increased wind shear associated with a trough will likely limit further strengthening.

A menacing looking tropical wave is moving through the Western Caribbean this weekend. This is the same wave that has been monitored by the NHC for several days but has yet to develop. It moved through the graveyard of the Eastern Caribbean a few days ago and survived. Air Force planes found a broad center of circulation a few days ago but not tight, closed circulation. As it is, it still remains a strong easterly wave. It's currently being impacted by wind shear associated with an upper low over Cuba. The northern part of the wave is experiencing about 20 knots of shear while the southern end is experiencing about 15 knots of shear.

Until this upper low weakens and moves out of the way, development should be inhibited. However, models are in agreement with weakening the upper low and moving it out of the path of the tropical wave by Sunday. When this happens, development will become increasingly likely with very warm water and low shear in its path. Given the upper low in its path right now and the eventual breakdown of the Bermuda High next week, this system does pose a threat to the U.S. late next week.

I think there is a strong likelihood this system will develop and could strengthen rapidly once in the Gulf of Mexico. A possible caveat does include any interaction with land, which would weaken the system. Still, I think there is a real possibility of Texas or Mexico dealing with a strengthening system at landfall late next week.

These slow organizing systems that develop closer to the U.S. worry me much more than the long-track Cape Verde systems. Why? Most importantly, the water is usually much warmer closer to the U.S. and increases the potential for rapid intensification. Two, these systems don't have a week for complex interactions to disrupt the circulaton. They usually intensify quickly and are usually intensifying at landfall. Three, when they develop close to the U.S., they are almost guaranteed to hit some land mass. A large majority of the quickly developing Cape Verde systems usually re-curve out to sea and only pose a threat to shipping. I'll be keeping a close eye on this system over the next several days.

What is left to say about Hurricane Bertha? Wow! She continues to persist in the open Atlantic and is still a tropical cyclone. She has been around for 16 days and she shattered the old record for longest July storm (12 days). Her time is almost up, and she should transition to an extratropical cyclone by the end of the weekend. It even looks like she may take aim at Iceland early next week!

Elsewhere in the tropics, things are quiet, but that may not last long. A very strong wave is poised to merge off the coast of Africa next week with several other waves to follow. It's still a bit early to be concerned about Cape Verde systems, but Bertha showed anything is possible this year. Any system that emerges off the coast at a latitude lower than 13 degrees N should have warm enough water to develop. I still think the Cape Verde systems will be more of a threat in a couple of weeks.

Tuesday, July 15, 2008

National View

The active weather will be found in a stronger band of westerlies that will migrate across the Upper Midwest today. Severe storms are possible from Kansas to Wisconsin with large hail and damaging winds being the primary threats. Heavy rains will be possible again in advance of a cold front across Florida. More thunderstorms will also be possible in the Desert Southwest, especially for Arizona and New Mexico. I'm sure they will provide some picturesque views for photographers in that part of the country. The heat will be cranked up for much of the nation today with just about everywhere experiencing 80s and 90s. There should not be many extremes, but an expansive area of the country will be quite warm. Even parts of Alaska will get in on the warmth with 70s expected in several areas of the state. With the lack of a significant trough, cool air is going to be difficult to find today. In fact, you'll have to look along the extreme northern fringe of the lower 48 to find comfortable temperatures.

The tropics remain more active than normal. Tropical Storm Bertha has eclipsed the old record for longest-lived July storm (12 days). Bertha continues to churn in the Atlantic to the northeast of Bermuda, and she will continue for several more days. It's likely Bertha will regain hurricane status again as she moves southeast over warmer ocean water.

Elsewhere in the tropics, a healthy looking tropical wave is about 930 miles east of the Lesser Antilles. SSTs in the lower 80s and a lack of significant dry air are positive environmental factors for strengthening. The latest Dvorak intensity estimate was a 2.0, which places this wave on the borderline for being upgraded to a depression. However, wind shear is on the moderate side (15-20 knots). Still, the environment appears favorable for continued development as the system moves off to the east over the next few days. If this wave does remain intact and develops, it's possible that it could threaten the U.S. at some point next week.

Another healthy looking wave is behind it, but it's still at a relatively low latitude for development. This may bear watching if it can move slightly more northward because it would have warm water and low shear in its path. The easterly wave train doesn't appear to be settling down anytime soon with other waves poised to moved off the coast of Africa in the next week.

Thursday, July 3, 2008

Tropical Storm Bertha

The second named storm in the Atlantic has formed in the far eastern Atlantic. Despite the cool SSTs beneath Bertha, weak wind shear and a lack of dry air may allow for some intensification over the next couple of days. However, the cool water will likely inhibit significant intensification for several days. It's possible that Bertha could also entrain some dry air in a few days, which may counteract the warming waters. Regardless, Bertha should persist for quite some time.

Rain Shadow Storms

I notice the SPC has outlooked a large chunk of Washington state and central Oregon in a slight risk of severe weather. That's out of the ordinary in itself, but the large part of Washington that is outlooked is in the rain shadow of the Cascades. The rain shadow doesn't mean no rain is ever measured, but most of eastern Washington is a desert climate with annual rainfall totals less than 15". Contrast that with the windward side of the Cascades that receive well over 100" of rain every year!

The images below compare and contrast the vegetation and scenery of both sides of the state. I took these last summer while instructing an 8-day field course, and Washington quickly earned one of the top spots in my favorite U.S. states.

Eastern Washington

Western Washington

Upslope flow in central Washington combined with a Pacific Northwest trough will provide the needed lift for thunderstorm development later this afternoon. Moderate to strong instability (for this area) will be present across the slight risk area with MLCAPE values up to 2,000 J kg-1! Additionally, deep layer wind shear will enhance the potential for organized thunderstorm development and the possibility of damaging winds and hail. The tornado threat looks very slim, but it is not zero. Did I really just type that while referring to Washington? As crazy as it sounds, given 0-3 km SRH values between 150-300 m^2 s^-2, some low-level rotation is possible in north-central Washington with any storms that develop. It should be interesting to watch this event unfold today.

T.D. 2 Forms

The NHC has begun issuing advisories on Tropical Depression 2 that is located 230 SSE of the Cape Verde Islands. The discussion on T.D. 2 from the NHC is excellent and jives with my thoughts on the evolution of the depression. This is a remarkable feat for a system to be classified this far east and this early in the season. Cape Verde activity typically ramps up in the first couple of weeks of August and lasts through early October. The good news is that most systems that form near the Cape Verdes usually recurve before posing a threat to the Caribbean islands or the U.S. mainland.

Wednesday, July 2, 2008

Tropical Update

Sad news over the weekend involved the death of a 5-year old boy at the Huntsville Airshow. A localized microburst occurred at the event and blew over several tents and injured 12. The storm was produced by the deep trough and strong cold front that swept through the region. My thoughts and prayers are with the family and friends of the young boy.

The tropics are beginning to get heat up. The Eastern Pacific has been active for the last couple of weeks with Tropical Storms Boris and Douglas currently churning. Both storms only pose a threat to shipping at least point. The Atlantic basin looks get in on the action this week with two tropical disturbances showing some potential for further organization.

The first disturbance moving through the Lesser Antilles is producing heavy rains and gusty winds. Hewanorra, St. Lucia measured a little over 0.50" of rain with sustained winds up to 28 mph out of the E and ENE. Thunderstorm activity increased today, but the system is currently experiencing 20-30 knots of southwesterly wind shear due to an upper-level trough to its west. This is a bit too high for significant development, but if the upper trough dissipates or moves further west, this would allow the shear to relax and strengthening would be more likely. Water temperatures are supportive of tropical development with SSTs in the lower 80s in the eastern Caribbean. In fact, the SHIPS model calls for slow strengthening of this system over the next 24 hours, and it even develops the wave into a named storm within 48 hours. I think that may be a bit optimistic given the current wind shear in place, but a lessening of the shear would allow for development. Regardless, this system bears watching as it moves to the WNW. If the system does develop, the forecast upper-level pattern would more than likely keep the main threat to Mexico.

A larger and much healthier looking tropical wave is centered about 220 miles southeast of the Cape Verde Islands. It's climatologically early to be looking this far east for tropical development, but this system appears impressive on satellite with deep convection clustered the center. SSTs near and to the west of the wave are marginal (~79 deg F) for development, but some slow strengthening is possible as the system moves quickly to the west along the southern periphery of the subtropical high. The marginal SSTs may be compensated by weak/moderate wind shear (10-15 knots) and an absence of Saharan dust/dry air. The Saharan air is well to the north of the system, and wind shear will become very weak as the system moves to the west. These two factors argue for continued strengthening, but the marginal SSTs should prevent significant development for the next several days. However, SHIPS calls for the wave to be a named system by 48 hours and up to hurricane intensity within 72 hours. I think this is overly aggressive, but I wouldn't be surprised to see a tropical depression and possibly a weak tropical storm form within 48 hours. Computer models are unanimous at this point in recurving the system in about 5 days.