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A Slowly Unfolding Saga: 90L Takes Its Time Developing

By: Jeff Masters and Bob Henson 4:49 PM GMT on November 16, 2016

An area of disturbed weather in the extreme southern Caribbean off the coast of Nicaragua (Invest 90L) is likely to develop into a tropical depression by this weekend as it moves little. Satellite loops on Wednesday morning showed that 90L had developed a modest amount of rotation, and heavy thunderstorm activity was sparse but on the increase. The disturbance has plenty of moisture to work with (about 70% relative humidity at mid-levels of the atmosphere), and water vapor satellite imagery showed no large-scale areas of dry air that 90L might have to contend with. Wind shear was favorable for development, a moderate 10 - 15 knots. Sea surface temperatures (SSTs) were very warm, near 29.5°C (85°F), which was about 1°C (1.8°F) above average.

Figure 1. Visible satellite image of 90L as of 1445Z (9:45 am EST) Wednesday, November 16, 2016. 90L is drawing on moisture from the Caribbean as well as from the Pacific Ocean south of Panama. Image credit: NASA/MSFC Earth Science Office.

Figure 2. Sea surface temperatures that are now even higher than the average readings for September in the Caribbean (about 2.5 standard deviations above the mean) will help support development of 90L. Michael Lowry (The Weather Channel) tweeted on Tuesday: “Caribbean waters are *supposed* to cool down by this point in the season but then again what’s “normal” nowadays?” According to Lowry, SSTs in the Caribbean are running just behind those in 2005 as the warmest on record for November. Image credit: @MichaelRLowry.

Track forecast: 90L a heavy rain threat to Central America
Steering currents are weak in the region, and 90L will not move much over the next five days. Heavy rains over Panama, Costa Rica and Nicaragua are a major concern from 90L. Even a weak cyclone parked in this area could cause significant flooding and landslides. During the hyperactive season of 2005—when we’d already run through the alphabetic list of Atlantic names by mid-November—the slow-developing Tropical Storm Gamma stalled just north of Nicaragua on November 18 - 20, causing at least 37 fatalities in Honduras and Belize.

In their 7 am EST Wednesday Tropical Weather Outlook, NHC gave 90L 2-day and 5-day development odds of 30% and 80%, respectively. The Hurricane Hunters are on call to potentially investigate 90L on Friday afternoon. The next name on the Atlantic list of storms is Otto. Ensemble model guidance from 00Z Wednesday supports the idea that 90L could become Otto. About two-thirds of the European model ensemble members develop Otto into at least a tropical storm by Sunday, including three of the four highest-probability members. About three-quarters of the GFS ensemble members produce a Tropical Storm Otto by this weekend. The ensemble guidance suggests only a slim chance that Otto could go on to become a hurricane. Any movement of 90L away from the extreme southwest Caribbean is likely to be at least a week from now.

The heat goes on
At last count, the U.S. had set 2820 record highs and 23 record lows for the month of November thus far, and this ratio is sure to become even more lopsided as the month rolls on. Dozens of record highs are likely to fall on Wednesday and Thursday from the Rockies to the East Coast, ahead of a sharp cold front plowing across the nation late this week. A blizzard watch was in effect for Thursday and Friday for parts of the eastern Dakotas and western Minnesota, with winter storm watches extending from the Northern Rockies to the Upper Midwest. This week’s front will bring temperatures down to near or slightly below average in many areas. However, it now appears that temperatures will surge back well above average for at least the western and central U.S. during most of Thanksgiving week, while the East Coast states are in line for seasonably cool conditions. It’s quite possible that a number of U.S. locations will end up with their warmest November on record, including Lincoln, NE (see embedded tweet below from Ken Dewey at the University of Nebraska-Lincoln).

We’ll be posting regularly on 90L as well as other topics over the next few days.

Jeff Masters and Bob Henson

Figure 3. Temperature anomalies (departures from average, in degrees C) at 18Z (1:00 pm EST) Thursday, November 17, 2016, as projected by the 12Z Wednesday run of the GFS model. Readings will be 15°F to 25°F above average on Thursday from the Central and Southern Plains to the Midwest and South, while temperatures will run below average over most of the intermountain West on Thursday behind a sharp cold front. Image credit: tropicaltidbits.com.

Hurricane Heat

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.