Eastern Pacific's Invest 91E A Threat to Mexico

An area of disturbed weather (Invest 91E) is approaching tropical depression status in the record-warm Pacific waters off the south coast of Mexico, about 425 miles south of Acapulco. Satellite images on Monday morning showed that 91E had a large and expanding area of heavy thunderstorms with a pronounced rotation. The 7 am EST Monday run of the SHIPS model predicted that 91E would be over record-warm ocean waters near 30°C (86°F) and under light wind shear around 10 knots through Friday, which should allow the storm to develop into Tropical Storm Sandra by Wednesday. In their 7 am EST Tropical Weather Outlook, NHC gave 91E 2-day and 5-day odds of development of 90%. The HWRF and GFDL intensity models have consistently shown the potential for this storm to become a hurricane by late in the week. Steering currents favor a path to the west or west-northwest parallel to the coast and far enough offshore to prevent heavy rains in Mexico through Thursday. On Friday, a trough of low pressure passing to the north of 91E will turn the storm to the north and northeast, and 91E will likely make landfall in Mexico this weekend, passing near the tip of the Baja Peninsula. Those of you planning to spend Thanksgiving weekend in the Cabo San Lucas area you should anticipate the possibility of a tropical storm affecting your weather.


Figure 1. Latest satellite image of Invest 91E.

If Sandra does form this week, it will join last week's Tropical Storm Rick as one of the latest-forming tropical storms in the history of the Eastern Pacific. Since accurate records began in 1949 (with higher-quality satellite records beginning in 1971), the Eastern Pacific has seen only five tropical storms form after November 18: December 5, 1983 (Winnie), November 27, 1971 (Sharon), November 27, 1951 (Unnamed), November 20, 2011 (Kenneth), and November 19, 2015 (Rick). None of these storms hit land. If 91E becomes a tropical storm and hits land, it will be the latest landfalling Eastern Pacific storm on record. This year has also beaten the onset year of the 1997-98 El Niño in terms of the span from the earliest to latest tropical cyclone in the Northeast Pacific. The season’s first storm of 2015, Andres, formed on May 28, four days earlier than Andrea in 1997. The last NE Pacific storm of 1997, also named Rick, formed on November 7, although it was followed on December 2 by Tropical Storm Paka in the Central Pacific. (Thanks to WU member Mark Cole for this statistic.)


Figure 2. MODIS image of Typhoon In-fa taken at 02:00 UTC November 23, 2015. At the time, In-fa was a Category 2 storm with 105 mph winds. Image credit: NASA.

Typhoon In-fa no threat to land
In the Western Pacific, Category 2 Typhoon In-fa is tracking to the north on a path that is expected to keep the storm from striking any land areas as a typhoon. On Saturday, In-fa topped out as a Category 4 storm with 130 mph winds, becoming the record-shattering 24th Category 4 or stronger tropical cyclone in the Northern Hemisphere in 2015. According to wunderblogger Dr. Phil Klotzbach's Twitter feed, the previous record was eighteen such storms in 1997 and 2004. On Sunday, In-fa  brought the number of typhoon days in the Northwest Pacific to 104.5, the most on record for the basin to date (previous record: 103 in 1997.)

The Atlantic is quiet with no tropical cyclone formation expected for the next five days.

Potent mid-autumn snowstorm socks Midwest
Parts of the Chicago area saw their biggest snow on record for so early in the season on Friday and Saturday, courtesy of a fast-moving storm that left a compact but focused swath of snow from South Dakota to Michigan. It was the parting shot of a powerful sequence of upper-level lows that caused a variety of extreme weather throughout the week, from tornadoes in Kansas and Texas to destructive winds in Washington.

The tight focus of the snow-making machine led to big contrasts in accumulations over fairly small areas. Northwest of downtown, Chicago’s O’Hare International Airport picked up 11.2”, which made it the heaviest snowstorm ever recorded for Chicago so early in the year and the second-heaviest November storm behind 12.0” on Nov. 25-26, 1895. However, the city center got far less snow: only about 1” was recorded downtown, according to the National Weather Service. Likewise, the Rockford area west of Chicago saw totals ranging from less than 9” to 18”. As in Chicago, Rockford’s official storm total of 8.8” ranked as the heaviest on record so early in the year and the second-heaviest for November, behind 9.5” on Nov. 6-7, 1951.


Figure 3. Except for a few obscuring clouds over southern Michigan, this weekend’s swath of snow remained clear in a GOES visible satellite image at 1630Z (11:30 am EST) Monday, November 23, 2015.


Figure 4. Clear skies atop the snow swath shown in Figure 3 allowed temperatures to plummet on Saturday night. Shown are the low temperatures for the 24 hours ending on Sunday morning, November 22, 2015. Widespread lows below 10°F occurred across the snow-covered areas, with a few below-zero readings evident (purple dots). Image credit: Midwestern Regional Climate Center.


Figure 5. A band of more than 12” of snow stretched across southern Michigan and northern Indiana, with 15.5” falling at the NWS Detroit/Pontiac office--the heaviest total recorded at that office since it opened in 1994. At my backyard weather station at Dunham Lake, Michigan, we had 15" inches of snow in 15 hours on Saturday--a pretty shocking way to get our first measurable snow of the season! This is one of the top-five snowstorms of my life here in Michigan, and the second one this year. On February 2, we had another 15" snowstorm here. --Jeff Masters

El Niño notches another record-high weekly value
The record-setting El Niño event of 2015 continues to impress. NOAA reported today that last week’s departure from average sea-surface temperature across the benchmark Niño3.4 region was +3.1°C, the highest weekly value on record in NOAA data going back to 1990. The 1997-98 El Niño maxed out at a weekly Niño3.4 anomaly of +2.8°C for the week centered on November 26, 1997. There’s an increasing westward focus to this El Niño event, as the current anomaly over the Niño4.0 region (which straddles the international date line, from 160°E to 150°W longitude) is up to 1.8°C, almost as warm as the Niño1+2 region (just off the South American coast), which currently sits at 2.1°C. Since the Niño1+2 region remains well above average, this event does not qualify as an El Niño Modoki, which can produce quite different U.S. effects than a “classic” El Niño. For more background on El Niño science and impacts, see our dedicated WU microsite.

Wunderblogger Steve Gregory has a new Monday afternoon post, Stormy Thanksgiving / Potential Flash Flooding and Blizzard Conditions for Texas.

Jeff Masters (tropics), Bob Henson (snow and El Niño)