Developing Cyclone Arabian Sea / INVEST 94L Caribbean / Warm Weather Ahead

(By Steve Gregory - Substituting for Dr. Masters who is taking the weekend off.)


UNUSUAL CYCLONE DEVELOPING IN ARABIAN SEA

A disturbance (INVEST IO90) in the Arabian Sea has been slowly developing into a Tropical Cyclone over the last few days, and appears very close to reaching Tropical Depression intensity. The developing storm is located about 600NM SE of the Oman coast, and remains quasi-stationary. Most models are showing intensification to a CAT 1 storm over the next 72 hours, and is forecast to move towards the OMAN coast by Tuesday. After which, the storm should turn northeast and accelerate away from the coast and weaken quickly to minimal tropical storm intensity (or depression) by next Thursday. It’s worth mentioning that latest imagery loops suggest very dry air is approaching from the Northwest – and this may prevent the storm from intensifying beyond Tropical Storm (gale) force – AND prevent the system from even approaching the Oman coast before it turns Northeast and weakens. Tropical Storms in this region of the world are unusual – but they are not rare ,

INVEST 94L UNLIKELY TO DEVELOP ANYTIME SOON

The remnants of TD #9 that moved into the western CARIB earlier this week, continues to drift E/SE as a frontal system from the Bahamas to the southern Yucatan with much drier air behind it keeps shunting this system further towards the SW CARIB Sea. With high wind shear near the frontal boundary and drier air still advancing into the NW CARIB – this system is unlikely to develop anytime soon. Should the system manage to survive beyond 5 days (doubtful!) conditions could become more favorable for development in the SW CARIB at the end of next week.


NATIONWIDE WARMTH BUT STORMY WEATHER NEXT WEEK

A nice warm-up is sweeping eastward from the central US towards the east as high pressure and dryer conditions develop over the eastern US during the next couple days. A strong upper level TROF moving across the northern Rockies will deepen and dig southeastward towards the Mid Atlantic states by late this week, with cooler air moving south into the Midwest and eventually the eastern US. The development of another coastal storm in the east is also possible along the frontal boundary late in the week as well. The overall hemispheric pattern continues to feature strong TROFS moving from the eastern Pacific across the nation every 5-7 days, with warm temps ahead of, and cooler conditions behind, each storm/frontal system. This highly progressive
Pattern is likely to keep average Temperatures on the warm side of normal through mid November.




Fig 1: This true color image of the Arabian Sea region from overnight (US time) shows a large – but slowly developing system about 600 NM off the Oman coast. This unusual (but not ‘rare’) development was ‘helped along’ by a strong MJO signal that persists over Africa and the Indian Ocean which tends to enhance convective based precipitation in the tropics..



Fig 2: The color enhanced IR image from this morning shows some heavier, but not intense, convection near the low-level circulation center (LLCC). The system does, however, exhibit significant rotation and banding features.




Fig 3: The surface analysis from 11:30Z using ship, land based and satellite derived winds (including data from the last 2 ASCAT passes) shows a large and slowly deepening area of Low pressure, with MAX winds in the 25Kt-30Kt range – right on the cusp of being designated as a Tropical Cyclone.




Fig 4: The High Level wind pattern shows a very large and well developed outflow pattern above the cyclone, with anti-cyclonic flow providing a venting mechanism for the developing surface cyclone. That said – there are no strong outflow channels – so development will continue to be slow.




Fig 5: The Intensity forecasts from the specialized cyclone models that are a ‘sub-set’ of the global models are in very good agreement that this system will reach hurricane intensity in a few days. But this is far from certain based on both Climatology and real-time imagery that shows drier air moving towards the system - but such good agreement amongst all the models does suggest intensification is likely.




Fig 6: The VIS image INVEST 94L in the western Caribbean. The system has been shifting towards the E/SE over the past day as cooler and drier air from the NW pushes into the NW CARIB.



Fig 7: The surface analysis from this morning shows a broad area of Low pressure (INVEST 94L) with a small, non-tropical Low pressure system over the Bahamas which brought very heavy rains to the Florida Keys and extreme SE Florida yesterday. Note the strong northerly flow over the Bay of Campeche that reaches into the eastern Pacific and could lead to the dissipation of INVEST 94L if this drier, more stable air continues advancing into the southwest CARIB.




Fig 8: The upper level wind analysis shows ‘one of the best’ anti-cyclonic circulation systems of the entire season over the western CARIB! If this was AUG – we almost certainly would be looking at a significant hurricane formation. But the reality of the season is evident just to the north - with a large TROF digging into the deep tropics with high wind shear (not shown) reaching into the NW Caribbean. IF the dry air/high wind shear intrusion ends soon enough, and the deep moisture field and vorticity associated with INVEST 94L survives, we might see some kind of development late next week in the SW CARIB – though the chances of that happening remain quite low.


CLICK IMAGE to open full size image in new window

Fig 9: The above set of 200 mb (~40,000’) charts highlighting the jet stream - shows the current pattern over North America (Left Panel) and the forecast wind pattern late TUE (center panel) and then next Friday (right panel). Note the VERY Strong wind Max over the northern Rockies (180mph speeds!). This is associated with a strong short wave TROF that will dig southeastward this week, leading to a deeper TROF over the eastern US at the end of next week. The warm weather that will prevail well into the middle of next week will ease back to near normal reading late next week as the TROF and a possible surface storm system impacts the eastern third of the nation. By that time – the next big TROF over the eastern Pacific will be approaching the west coast.



Fig 10: The Temperature forecasts based on the GFS MOS model data calls for well above normal readings across the nation during the next 7-Days – but is a bit misleading in that very warm anomalies early in the week will be offset by near or even slightly below normal Temps at the end of the 7-day period over the eastern half of the nation..




Fig 11: The Week 2 Temperature anomaly outlook continues to be on the warm side of normal for most of the nation – though Temps closer to or a bit below normal on average will prevail in the northeastern quarter of the nation.


✭ MY next Weather Update will be issued THURSDAY under my own BLOG unless conditions warrant an earlier update. ✭

Jeff will have a new post on Monday, unless 94L shows show unexpected development.

Steve