THE SANTA CLAUS STORM...

If you've been playing along, you are well aware that Santa's big night will have some unique weather challenges this year. Already, word is out at the pole that the rain slicker will be required, as well as the lightweight suit due to temperatures 35 degrees above normal. The sleigh will also be equipped with lightning detection gear, as some strikes are possible in parts of the Midwest. Comet had a close call many years ago with a bolt out of the blue that singed an antler. Goggles with lightning rods are now standard attire.

This unusual weather pattern is due to a potent upper level low which is currently located off the coast of California.

The potent upper air low is spinning spokes of energy (vorticity) into the 500mb jet stream flow and directing them into the nation's mid-section. Not only does that bring deep moisture and rain chances, it tugs in exceptionally mild air by late December standards, creating much above normal temperatures. Again, note the parent low off the California coast that is being closely watched by the Meteorological Elf Association at the North Pole Observatory.

Breaking it all down, the bottom line is warm air advection begins in earnest today, driving significant cloud cover into the Midwest through the Christmas holiday and beyond. While a few sprinkles or some drizzle is possible Thursday, the first round of rain looks to hold off until Friday as a mid-level short wave ejects out of the Plains. This lead short wave shows up nicely in the GOES satellite imagery.

Rainfall amounts with this wave should be light, generally around 1/4 inch or less. More important, it begins to seed the atmosphere with deep moisture that leads to heavier rain potential with a far stronger system Christmas Eve and Christmas day. This is the "Santa" storm.

Leading up to the event, Saturday should be the best day of the holiday weekend. The region will be in between forcing, promoting dry mild conditions. Christmas Eve day is also showing signs of being drier with the slower arrival of the primary energy. Hopefully, rain will hold off until late evening, but it will show at some point that night. The big man will have to deal with it as he makes his rounds. By Christmas day, we are right in the heart of the warm moist conveyor belt of the storm and occasional rain (and possibly a thunderstorm) is likely. With the slow movement of the cut-off low, pockets of light rain or drizzle may linger into parts of Tuesday before diminishing.

I did want to point out a few things I feel are important about the holiday forecast out by way of the NWS IDSS grid forecast, centered in the Quad Cities.

Under the max temperature column, notice highs of 56, 56, and 55 the 23rd-25th. Those are not records, but will be good enough to make this a top 5-10 warm Christmas period going back some 130 years. Notice too, lows Christmas Eve morning and Christmas morning are 51 and 49. That would give many of us a very good chance of breaking the warmest low temperature records for those dates. In fact, temperatures are not shown below freezing for at least 7 consecutive days!

In the dew point category, they are shown in the 40s and 50s for 6 consecutive days. The only other time anything similar was noted by the NWS was in 1998. A 53 dew point Christmas day could be a first. I don't have the data available to confirm that. Most certainly it will lead to at least some small CAPE, which is why a thunderstorm is a possibility. In the maximum RH (relative humidity) column, levels of 100 percent are shown 5 consecutive days. That level of saturation will most likely lead to periods of fog later in the weekend. In some areas, it could be locally dense if we get the right set-up. That is the only factor that could lead to some travel issues near Christmas.

I also need to address the available water vapor with the Santa storm. The EURO maxes PWAT levels out at 1.25 inches, which is in the 99th percentile of NWS soundings on Christmas Eve. It's about 400 percent higher than normal.

Such high levels of moisture have historical analogs that point to rain total potential of 1–2 inches over a 2-day period. We could sure use that, especially in eastern Iowa, where major drought conditions exist. Here's what guidance is suggesting for rain totals. This includes what falls Friday. We still see some variability as to where the heaviest core sets up.

The EURO

The GFS

There's just no available cold air for snow, which is a real bummer, especially with the amount of precipitation depicted. No doubt there would have been a snow band in excess of a foot somewhere in the Midwest had the cold air been available. As it is, the EURO shows this for snowfall through December 26th. Gosh, that's disappointing!

These are the areas that are shown with a white Christmas by the EURO Christmas day. That is beyond pathetic. Bad sledding all around!

THE NAUGHTY LIST...

I will say that after the 1st of the year, some very cold air is shown departing eastern China on its way to the northern Pacific. It is likely to play havoc with the pattern as it crosses a body of water that is historically warm. The impacts of the interaction are hard to foresee due to the unprecedented warmth of the sea surface temperatures and the lack of reliable analogs. I do think there is the possibility of a significant Midwest storm around January 5th. That is on the back burner and certainly on mother nature's naughty list. Ho. ho, ho.

That's enough for now. Have a terrific day as the clock ticks toward Christmas. Roll weather...TS

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Carolyn