Winter Storm Warning

Late yesterday the track of the storm did turn south, and last night we were upgraded to a Winter Storm Warning. There are a lot of caveats here, but the potential for sticking snow has increased dramatically. The Warning is in effect from 9pm tonight (Friday) through 3pm Saturday for the entire area north of I-80. (Thankfully the evening commute should have no effects.)

For starters, do not fall victim to the "OMG we're going to get a foot of snow" hysteria some people will undoubtedly start spewing. The 7-11" totals referenced in some forecasts, including from NWS, are both somewhat unlikely (in my opinion) and not applicable to the vast majority of people in the forecast area. 

What is clear is that there will be a sharp gradient from the NW to SE along which snow totals will decrease rapidly. Areas near DeKalb or even as close as Kane and McHenry counties have a decent shot at 6+ inches of snow. For the vast majority of the area's population it gets trickier and snow totals will almost certainly be well below that. 

Clouds will thicken this afternoon and precip will start as rain before potentially switching to snow tonight as we approach midnight. The best bet for now:

• 6+ inches of snow for far western and northern suburbs (Kane, McHenry, Lake)
• 3-6" for Dupage and far NW Cook, decreasing rapidly to the SE
• Trace to 2" for areas in Cook and Lake that are within 10 miles of Lake Michigan, including the entire city of Chicago. (I think for the core of the city-- within 2-3 miles of the lake -- there is a 30-40% chance that the snow never really piles up. It is hard to see much more than a couple of inches even if it does accumulate. )

For context, the difficulties with this type of storm are worth noting.

• An earlier or later shift from rain to snow -- a transition that is very hard to pinpoint -- can dramatically alter snowfall totals.
• The ground is still warm after a very mild fall. Even getting the atmospherics right might not be enough to get the snowfall totals right if the snow melts before accumulating. 
• The warm ground effect is amplified in the city. 
• The surface water of Lake Michigan is still near or above 50 degrees (10-12 degrees warmer than at this point last year), and winds initially blowing off the lake could well keep the precip in liquid form over much of the city. 

Miscellaneous:

• Should a 6" snow be recorded officially at ORD, it would be very rare -- it's only happened twice in Chicago (and four times in Rockford) since the late 19th century. 
• It has been 238 days since our last measurable snowfall in Chicago, which is slightly above average. 
• Temps will rebound into the 40s and 50s next week, so any snow won't last long. 
• No major storms are on the horizon here for holiday travel next week. 

snow possible

A storm lifting out of the Plains could impact the area beginning tomorrow (Friday) night through Saturday afternoon. A Winter Storm Watch has been posted for areas well west and north of Chicago, where 5-8" of snow could fall. I think far less than that is likely in and around Chicago, and I'll generally take the under on everything I've seen so far. A wind off the 50-degree lake water is likely to keep the precip as mostly or all rain in Cook County and south of I-80. Additionally, the ground is still warm which won't help accumulations. Western and northern suburbs could see a dusting to maybe 1-2" if things come together, with the switch to snow only happening on the backside of the storm Saturday morning/afternoon. A slight shift to the south or an unexpected intensification could change things, but for now the accumulations and impact look very modest for the city of Chicago, moderate at worst for most suburbs, but potentially substantial for areas well west and north (e.g., Rockford). I will only send an update if something changes and significant accumulations become likely for the Chicago area. 

And to pre-answer the question, this potential snow is behind schedule. The average date of first flakes is November 1st, and the average date of measurable snow is November 15th -- we've had zero snow so far this year. The season overall has been very mild, and beyond this cold over the weekend things look seasonal (although not as balmy as they have been). And for comparison, last year on this date the high temperature was 19, with a low of 9. 

Heavy rain possible

After a glorious day, clouds will fill in tomorrow (Monday) and rain will develop by Monday night or Tuesday morning. Some projections are pointing to 3" of rain, with more than that possible under some isolated thunderstorms. There is a chance that we'll need to back off those totals, but it's still a good idea to check gutters and downspouts for leaf blockages. Tuesday is likely to be a washout. By late week we'll be back toward more seasonal highs in the 40s.

In other news, enjoy these 50s and 60s -- a year ago right now we were going into one of the coldest ever weeks in November history with highs were in the 20s and low 30s. On Wednesday we have a good chance to crack 60; that date last year saw a high of 19.

Severe storms

We are under a severe thunderstorm watch until 11pm and a line of severe-warned storms is approaching rapidly from the west. Warnings may be issued for our area between 8:30 and 10:00, give or take. The main threat is winds gusting 60-70. These storms have a history of tornadoes and wind damage but they are weakening and not very impressive overall.

High winds 30-45 mph will continue on the back side of this massive system through tomorrow. Temps will fall into the 40s but the weekend should be nice with daytime temps in the 50s to near 60 and lots of sun.

Rain

Since a couple people have asked, please pardon this non-severe update regarding the Halloween rain.

The bad news is that light to moderate rain is very likely for the whole area through most of the day.

The good news is that it is likely to taper to light sprinkles or drizzle late afternoon toward sunset and it will end completely at some point after sunset (clearing from west to east). So, with patience most kids should get a shot at some trick-or-treating today.

The best news is that tomorrow through next week looks spectacular. Temps will warm into the 60s tomorrow and spend most of the first half of the week climbing into the low or mid 70s with lots of sun.

Hurricane Patricia

This has nothing to do with Chicago, so forgive me. But this situation is just astounding and potentially catastrophic.

Hurricane Patricia was a tropical storm wandering off the coast of Mexico just over 24 hours ago. Now it is the strongest hurricane ever recorded in the Atlantic or Eastern Pacific basins, and pretty much identical to the strongest hurricane ever recorded anywhere on the planet. Sustained winds are 200 mph with central pressure of 880 mb, and the storm is taking dead aim at the coast of Mexico. If NHC is right and it actually strengthens to a landfall wind speed of 205 it will be the strongest landfalling hurricane ever recorded. Beyond the EF5-tornado-level winds across a large area, there could well be a storm surge of 20+ feet. Given the short notice and poor communication, numerous bays and inlets, mountainous terrain inland...the potential exists for a horrendous human catastrophe. 

HURRICANE PATRICIA DISCUSSION NUMBER  14  NWS NATIONAL HURRICANE CENTER MIAMI FL       EP202015  400 AM CDT FRI OCT 23 2015    Data from three center fixes by the Hurricane Hunters indicate  that the intensity, based on a blend of 700 mb-flight level and  SFMR-observed surface winds, is near 175 kt.  This makes Patricia  the strongest hurricane on record in the National Hurricane Center's  area of responsibility (AOR) which includes the Atlantic and the  eastern North Pacific basins.  The minimum central pressure  estimated from the aircraft data, 880 mb, is the lowest ever for  our AOR.  It seems incredible that even more strengthening could  occur before landfall later today, but recent microwave imagery  shows hints of a concentric eyewall developing.  If the trend  toward an eyewall replacement continues, it would cause the  intensity to at least level off later today.  The official forecast  shows only a little more strengthening before landfall.  Given the  very mountainous terrain that Patricia should encounter after  landfall, the cyclone should weaken even faster over land than  predicted by the normal inland decay rate.    Recent center fixes show that the hurricane is gradually turning  toward the right, and the initial motion estimate is 340/10 kt.  The  track forecast scenario remains about the same.  Patricia should  continue to move around the western periphery of a mid-level  anticyclone today and turn north-northeastward ahead of a trough to  the northwest tonight and Saturday.  The official track forecast is  somewhat slower than the latest model consensus and lies between  the GFS and ECMWF solutions.    The global models continue to depict the development of a cyclone  near the Texas coast over the weekend.  Based on the predicted  upper-level winds, this system should be non-tropical in nature.  However this cyclone is expected to draw significant amounts of  moisture from Patricia's remnants, and could result in locally  heavy rainfall over portions of the northwestern Gulf of Mexico  coastal area within the next few days.  Refer to statements from  local National Weather Service forecast offices for details.    We would like to acknowledge deeply the Air Force Hurricane Hunters  for their observations establishing Patricia as a record-breaking  hurricane.  Clearly, without their data, we would never have known  just how strong a tropical cyclone it was.    KEY MESSAGES:    1.  Confidence is high that Patricia will make landfall in the  hurricane warning area along the coast of Mexico as an extremely  dangerous category 5 hurricane this afternoon or evening.  Preparations to protect life and property in the hurricane warning  area should have been completed, or rushed to completion, as  tropical storm conditions are beginning to affect the area.  Residents in low-lying areas near the coast in the hurricane warning  area should evacuate immediately, since the storm surge could be  catastrophic near and to the east of where the center makes  landfall.    2.  In addition to the coastal impacts, very heavy rainfall is  likely to cause life-threatening flash floods and mud slides in the  Mexican states of Jalisco, Colima, Michoacan and Guerrero continuing  into Saturday.    FORECAST POSITIONS AND MAX WINDS    INIT  23/0900Z 17.0N 105.5W  175 KT 200 MPH   12H  23/1800Z 18.8N 105.4W  180 KT 205 MPH   24H  24/0600Z 21.7N 104.2W   60 KT  70 MPH...INLAND   36H  24/1800Z 24.5N 102.5W   20 KT  25 MPH...POST-TROP/REMNT LOW   48H  25/0600Z...DISSIPATED    $$  Forecaster Pasch

flash flood watch

The whole area is under a Flash Flood Watch until tomorrow morning. Rain is approaching from the SW, and the heaviest rain will arrive after sunset into the early AM. Another 1-2" of rain can't be ruled out, with higher amounts in certain locations. After some places -- particularly in the northern suburbs -- saw 3" or more of rain last night the ground is already saturated and the flooding risk is elevated. 

The good news is that the rain should clear out tomorrow morning. The temps should climb into the 60s with a drier north wind and some sunshine. Sunday looks really nice with sun and temps near 70.

In other news, I've had more questions about El Niño -- or El Grande Niño as this one is being called -- and the effects on our weather. The answer remains that El Niño looks like to persist if not strengthen into the winter, and that usually correlates with a mild winter in Chicago. But the correlation is not perfect, and this year's El Niño is being affected by "The Blob" -- a massive layer of warm water in the North Pacific. See below if you want to read more. 

So, we're still favored for a much wilder winter this year than the prior two just based on reversion to the mean -- those were two pretty harsh winters. And the building El Niño gives us a further likelihood of a mild winter. But it's too early to write that in stone, and The Blob may well throw a monkey wrench into the system. 

http://www.nws.noaa.gov/com/weatherreadynation/elnino_article.html#.Vfx5r99VhBf

http://ww2.kqed.org/science/2015/08/09/possible-spoiler-for-el-nino-a-battle-of-the-blobs/

https://en.wikipedia.org/wiki/The_Blob_(Pacific_Ocean)